S/HT/10/1 SOLUTION ? TEST X

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S/HT/10/1 SOLUTION ? TEST X

MAGNETISM, ELECTROMAGNETIC INDUCTION & ALTERNATING CURRENT

Induced e. m. f. = L dI/dt. Hence, we have, 1 = L (5/0.1) = 50 L. So, L = 1/50 = 0.02 H = 20 mH

Correct choice: c

2. Since the coil makes an angle 60º with the field, its area makes an angle 30º. Hence, flux = 3 m Wb = BAN Cos30 = B × 0.02 × 1 × ?3/2. So, B = ?3/0.01 milli tesla = ?3/10 T

Correct choice: b

When rate of change of current is unity, induced voltage = self-inductance.

Correct choice: b

The time for growth of current is governed by time constant = L/R.

Correct choice: c

For a step-up transformer primary to secondary turns ratio is less than 1, as secondary has more turns than primary.

Correct choice: b

Armature current is a maximum at the start and as the motor runs at a steady speed, there is a steady back e. m. f and hence the net voltage is less than the applied voltage and hence current is also less.

Correct choice: c

Given, ? = 3t² + 4t ? 5 m Wb. So, induced voltage, V = d?/dt = 6t + 4 = 16 m V, when t = 2 s.

Correct choice: a

Ideal inductors in series can be combined as we combine resistances. So here it is 2L

Correct choice: b

Induced e. m. f. = BLvsin30 = 1 times 0.5 times 10 times ½ = 2.5 V

Correct choice: b

Output voltage = ER/(R + r) = 440 times 4000/4400 = 400 V

Correct choice: b

Here flux = BANCos60 = 0.002 times 0.01 times 200 times ½ = 0.002. So, change in flux when the coil is turned through 180º is twice this = 0.004 Wb. Hence the induced e. m. f. = 0.004/0.1 = 0.04 V = 40 m V.

Correct choice: a

Induced e. m. f. = BAN?, in the case of a coil that is rotated. So, ratio of e. m. f. s = ratio of N? of Nf values which in this problem = 50 times 50/100 times 150 = 1/6

Correct choice: b

We know that M = k times square root of product of self-inductances. So, here, ?2 = k ?8. So, k = ½ = 0.5

Correct choice: c

When the loops approach each other, flux in each due to the other coil increases, and this will be opposed by the induced current. So the current in both the coils must decrease.

Correct choice: b

As current in the wire increases flux through the lop increases. So current will be induced in it such that flux is decreased. And hence current must be in the clockwise direction so that the field produced by the wire and that by the loop are in opposite directions.

Correct choice: a

Peak is ?2 times r. m. s. and is = 200 ?2 = 282.8 V

Correct choice: b

Power factor is the ratio of resistance to impedance = 4/8 = ½

Correct choice: a

Instantaneous voltage = 100 Sin ?t = 100 Sin (2? 50 × 1/600) = 100 Sin ?/6 = 50 V

Correct choice: a

Inductive reactance = L ? = 0.1 times 100 ? = 10 ?. So current = 314/31.4 = 10 A.

Correct choice: c

At resonance the whole voltage appears across the resistance and hence current = 240/R = 240/12 = 20 A.

Correct choice: d

Power in primary = 240 I = power in secondary 36. So, I = 36/240 = 0.15 A

Correct choice: d

Transformer works with only a. c. and so voltage in secondary in this case = 0

Correct choice: e

Here 0.9 times VI in primary = VI in secondary. So, voltage in secondary = 0.9 times 120 times 1.8/150 = 1.296 V

Correct choice: a

Here current is a sine function and voltage, cosine function, which means that phase difference between them is ?/2. So, power = 0.

Correct choice: d

Efficiency = power in secondary/power in primary = 11 times 90/220 times 5 = 0.9 = 90%

Correct choice: b

Inductance is ideal means its resistance is zero. Since the inductance is in parallel with the resistance, it short-circuits the resistance and hence current through resistance = 0

Correct choice: c

Since north pole is being introduced, induced current is such that a north pole is created on that face of the coil. And so, the current is in the anticlockwise direction.

Correct choice: b

Induced e. m. f. = B L average v gained in the first second (=10 m/s) = 0.0004 times 2 times 10/2 = 0.004 V = 4 m V

Correct choice: a

Vertical earth?s field = 0.0008 times sin30 = 0.0004. So. e. m. f. = 0.0004 times 20 times 100 m/s = 0.8 V

Correct choice: a

Induced e. m. f. is independent of resistance.

Correct choice: c

Self-inductance is proportional to square of number of turns and hence it becomes 4 times, in the present problem.

Correct choice: c

Charge induced = change in flux/resistance = 10 m Wb/10 ohm = 1 m C

Correct choice: c

Here voltage across inductance = ?(240² - 120²) = 120?3 V. Current in the circuit = P/V = 60/120 = 0.5 A. So, L ? = 120?3/0.5. So, L = 207.8/0.5 × 100? = 1.32 H

Correct choice: a

Growth of current in an inductance is governed by the equation, I = I₀ (1 ? e^{? R t/L}), where L/R is time constant (T). So, here, I₀/2 = I₀ ( 1 ? e ^{? 0.695/T}). So, ½ = e^{? 0.693/T}. Or, e^0.693/T= 2. Hence 0.696/T = ln 2 = 0.693. And hence T = 1 s.

Correct choice: b

Energy stored = ½ L I² = ½ 0.05 times 4 = 0.1 J

Correct choice: c

When current is maximum, frequency is resonance frequency = 1/(2??LC) = 1/2??[0.1 × 10^{? 3} × (1/?²) × 10^{? 6}] = 10 ⁵ / 2 = 50000 Hz = 50 k Hz

Correct choice: a

Current at resonance = V/R = 240/100 = 2.4 A

Correct choice: a

Since inductive and capacitive reactances are equal, the circuit is in resonance with the applied a. c. So the voltmeter connected across them together is zero.

Correct choice: a

Power is consumed only by resistance and is equal to V²/R = 200²/50 = 800 W

Correct choice: b

Phase difference is zero as there is resonance.

Correct choice: a

When a circuit contains only L and C, the current will be out of phase with voltage by ?/2

Correct choice: b

Current = e. m. f/reactance. Net reactance = L ? ? 1/C ? = 10^{? 3} × 100 ? ? 1/10^{? 3} × 100? = 0.314 ? 3.18 = - 2.87, negative sign need not be considered. So, I = 220/2.87 = 76.6 A

Correct choice: e

If coupling is 100%, in this problem L = square root of M = 2 m H.

Correct choice: c

Here, we have, L₁ + L₂ = 6 and L₁ L₂ / (L₁ + L₂) = 4/3, or L₁ L₂ = 8. So it is clear that the inductances are 4 and 2.

Correct choice: a

Here current = V/L? = 10/0.005 times 100 = 10/0.5 = 20 A.

Correct choice: d

It is, as you know, 50 Hz.

Correct choice: e

Given 2? f = 377. So, f = 377/2 times 3.14 = 60 Hz.

Correct choice: c

R. M. S = 2/?2 = ?2 A.

Correct choice: b

Here it is the square root of the sum of the squares of 5 and 10/?2 and is equal to 5?3 A.

Correct choice: b

Reactance = L ?. So, here 100 = L times 100 ?, or L = 1/? = 0.32 H

Correct choice: c

MAGNETISM, ELECTROMAGNETIC INDUCTION & ALTERNATING CURRENT

1. When the current in a circuit falls from 5 A to 0 A in 0.1 s, the emf induced is 1 V. Then the self inductance of the circuit is: a. 50 mH b. 10 mH c. 20 mH d. 30 mH e. 100 mH

A coil of area 200 cm² is placed at an angle 60º with a magnetic field, and the flux through it was found to be 3 m Wb. The magnetic induction of the field is: a. 20/?3 T b. ?3/10 T c. 20?3 T d. ?3/20 T

3. When the rate of change of current in a coil is unity, the induced emf in it is: a. total flux through the coil b. coefficient of self-induction of the coil c. number of turns in the coil d. current in the coil e. none of these

The time required for the current in an L-R circuit with D. C to reach maximum value depends on:

a. only R b. LR c. L/R d. only L e. L²/R

For a step-up transformer, the turns-ratio is: a. =1 b. <1 c. >1 d. ?1 e. ?1
The armature current is maximum in a motor, when it has: a. attained maximum speed b. minimum speed c. just started motion d. just when switched off e. current is the same irrespective of speed
Instantaneous magnetic flux through a coil, which is rotated in a magnetic field, is given by ? = 3t² + 4t ? 5 m Wb. Induced emf in it at t = 2 s is: a. 16 mV b. 16 V c. 11 mV

d. 11 V e. 15 mV

Two ideal inductors each of self-inductance L are joined in series and well separated from each other. The effective self-inductance of the arrangement is: a. L b. 2L c. L/2 d. L/4 e. 4L
A straight conductor of length 50 cm is moved with a speed 10 m/s, in a direction making an angle 30º with the direction of a magnetic field of induction 1 T. Induced e. m. f in it is:

a. 0 b. 2.5 V c. 5 V d.4.33 V

A generator produces an emf 440 V and has an internal resistance 400 ohm. If it is connected to a load of 4000 ohm, the output voltage is: a. 440 V b. 400 V c. 200 V d. 220 V e. none of these
A coil of 200 turns and area 100 cm² is placed with its plane making an angle 30º with a uniform field of magnetic induction 2 mT. If it is turned through 180º in 0.1 s, the induced emf in it is: a. 40 mV b. 80 mV c. 40?3 mV d. 80?3 mV e. none of these
Induced emf in a coil of 50 turns rotating at 50 Hz in a magnetic field is E₁ and that in another coil of 100 turns and same area as the first, rotating at 150 Hz in the same field is E₂ . Then, E₁ /E₂ is: a. 6 b. 1/6 c. 3 d. 1/3 e. none of these
Two coils of self-inductance 4 mH and 2 mH, when placed one above the other is found to have a mutual inductance ?2 mH. Then, the coefficient of coupling between the coils is: a. 1 b. 2 c. ½ d. 0.25
Two identical, coaxial circular coils carry a current of ?i? each circulating in the same direction. If the loops approach each other: a. current in each decreases b. current in each increases c. current remains the same in both the coils d. current in one increases while that in the other decreases
A current from A to B increases in magnitude. The direction of induced

e. m. f. in the loop is: a. clockwise b. anti clockwise c. there is no induced

e. m. f. d. the induced e. m. f. is a. c.

R. M. S. value of an a. c. is 200 V. Then, the peak value is: A B

a. 220 V b. 282.8 V c. 300 V d. 254 V e. 0

17. An a. c. circuit has resistance 4 ohm and impedence 8 ohm. Power factor is: a. ½ b. 2 c. ¼ d. 4

An a. c of frequency 50 Hz has peak value 100 V. Starting from zero, the e. m. f. after 1/600 a is:

a. 50 V b. 50?3 V c. 100/?3 V d. 100?2 V e. 100/?2 V

An inductance of 0.1 H is connected to an a. c. 314 V/50 Hz. Then, current is: a. 0 b. 1 A c. 10A d. 5A e. none of these
A 240 V a. c. is connected to a resistance of 12 ohm, an inductance of 4 mH and a capacitance of 8 ?F, in series. Current in the circuit at resonance is: a. 10 A b. 12 A

c. 15 A d. 20 A e. 0

A step down transformer, with its primary connected to 240 V, is used to operate a 12 V/36 W lamp. If the transformer is ideal, current in the primary is: a. 6 A b. 60 A c. 1.5 A d. 0.15 A e. 0.6 A
A step-up transformer has turns ratio 1:4. A cell of e. m. f. of 2 V is connected to its primary. Then, the voltage in the secondary is: a. 8 V b. 1/8 V c. 4 V d. ¼ V e. 0
When the primary of a transformer is connected to 120 V at a current of 1.8 mA, current in the secondary is found to be 150 mA. If the efficiency of the transformer is 90%, what is the voltage in the secondary? a. 1.29V b. 0.81 V c. 9.6 V d. 1.49 V e. none of these

24. Current in an a. c. circuit is i = 2Sin314t and voltage is v = 4 Cos314t. Power consumed by the circuit is: a. 4 W b. 8 W c. 2 W d. 0 e. none of these.

A transformer steps down the voltage from 220 V to 11 V. Current in the primary and secondary are respectively 5 A and 90 A. Efficiency of the transformer is: a. 100%

R

b. 90% c. 80% d. 40% e. 0%

L

An electric circuit with a D.C source of 2 V is show in the diagram.

The inductance is ideal. When the key is closed, current through the

20 ohm resistance is: a. .04 A b. 0.1 A c. 0 d. 0.067 A e. none of these

A bar magnet and a circular coil are held as in the diagram.

If the magnet is now moved towards the coil, the

current induced, as seen from the left hand side, in the coil is: key 2 V

S N

a. clockwise b. anticlockwise c. an a. c. d. 0

28. A conductor of length 2 m, held in the east-west direction is falling from rest, from a height 50 m. Voltage induced in it in the first second of its fall is (horizontal earth?s magnetic induction = 0.0004 T):

a. 4 m V b. 40 m V c. 0.4 m V d. 4 V e. 0.4 V

An aircraft is flying horizontally with a speed 360 km/h. Its wingspan is 20 m, earth?s total magnetic induction is 0.0008 T and the dip at the place is 30º. Induced e. m. f. across the wing is: a. 0.8 V b. 8 V c. 0.8 m V d. 8 m V e. none of these
The magnitude of induced e. m. f. in a coil, when a magnet is inserted into it does not depend on:

a. the number of turns in the coil b. resistance of the coil c. magnetic moment of the magnet d. speed of approach of the magnet e. area of the coil

The number of turns in a coil is doubled without changing the length of the coil. Then its self-inductance: a. becomes four times the initial value b. becomes twice the initial value c. remains unchanged d. becomes half the earlier value e. becomes one-fourth the initial value.
Charge induced in a circuit of resistance 10 ohm, when the flux through it of 10 m Wb is reduced to zero in 2 s is: a. 1 m C b. 0.5 m C c. 1 C d. 0.5 C e. 0
An electric lamp with rating 120 V/60 W is to be operated with a source of 240 V/50 Hz. The inductance to be connected is: a. 1.32 H b. 1.32 m H c. 0.132 H d. 0.132 m H
Current in an inductance falls by 50% from the maximum value in 0.693 s. Time constant of the coil is (ln 2 = 0.693): a. 0.693 s b. 1 s c. 1.44 s d. 0.1 s e. 6.93 s
A 50 m H coil carries a current 2 A. Energy store in it is: a. 1 J b. 2 J c. 0.1 J d. 0.5 J

e. 0.05 J

A series L ? C ? R circuit has L = 0.1 m H, C = 1/4?² ? F and R = 100 ohm. The current in the circuit is maximum when the frequency of a. c. connected to it is: a. 50 k Hz

b. 5 k Hz c. 100 k Hz d. 10 kHz

R

L

In the above problem, if the voltage connected is 240 V, current at resonance is: a. 2.4 A b. 24 A c. 240 A d. 0.24 A e. cannot be calculated.

In the circuit shown in the diagram, R = 50 ohm

X_L = 120 ohm and X_C = 120 ohm. Then the voltmeter

reading is: a. 0 b. 100 V c. 200 V d. 82.75 V V 200 V/50 Hz

In the above problem the power consumed is:

a. 0 b. 800 W c. 4 W d. 1.67 W e. none of the above

In the problem no. 38, phase difference between voltage and current is:

a. 0 b. ?/6 c. ?/3 d. ?/4 e. ?/2

Phase difference between voltage and current in a circuit with 1 m H and 1 m F in series with 220V/50 Hz a. c. is: a. 90º b. 0º c. 45º d. 30º e. 180º
In the above problem, current in the circuit is: a. 220 A b. 550 A c. 2.2 A d. 5.5 A

e. none of these

Two identical coils, one placed over the other has mutual inductance 4 m H. Assuming the coupling to be 100%, value of each inductance is: a. 4 m H b. 8 m H c. 2 m H d. 1 m H e. 16 m H
Two inductances while in parallel has effective value 6 m H and while in parallel, 4/3 m H. The value of the individual inductance is: a. 4, 2 m H b. 3, 3 m H c. 5, 1 m H d. none of the above
A coil of inductance 5 m H and negligible resistance is connected to an a. c voltage V = 10 Sin 100 t. The peak current in the circuit is: a. 2 A b. 5 A c. 10 A d. 20 A e. 0
Frequency of ac mains in India is: a. 30 Hz b. 60 Hz c. 100 Hz d. 120 Hz e. 50 Hz
An ac generator produces an output voltage E = 170 Sin 377t volt. Frequency of the ac is:

a. 50 Hz b. 110 Hz c. 60 Hz d. 230 Hz e. 75 Hz

If the instantaneous current in a circuit is I = 2 Sin (?t + ?) ampere, the r. m. s. value of current is:

a. 2 A b. ?2 A c. 2?2 A d. 0 e. 1 A

A direct current of 5 A is superimposed on an alternating current of I = 10 Sin ?t, flowing through a wire. The resulting current is: a. 7.5 A b. 5?3 A c. 5?5 A d. 15 A

e. 10?2 A

The reactance of a coil with 220 V/50 Hz ac is 100 ?. The self-inductance of the coil is:

a. 3.2 H b. 0.22 H c. 0.32 H d. 2.2 H e. 0.22 m H

SOLUTION

MAGNETISM, ELECTROMAGNETIC INDUCTION & ALTERNATING CURRENT

As you know, repulsion is the sure test for magnetism.

Correct choice: b

2. In this case, pole strength does not change but length becomes half. So moment also becomes half = 40 units

Correct choice: b

As force is inversely proportional to square of the distance of separation, force becomes one-fourth when the distance is doubled.

Correct choice: d

When distance is doubled, force becomes ¼, but as the pole strengths are doubled, force becomes 4 times and hence effectively force remains the same as before.

Correct choice: e

Heating a magnetic sample results in its loosing its magnetism.

Correct choice: b

Magnetic lines of force are closed curves, unlike electric lines of force.

Correct choice: b

Neutral point is that at which the resultant magnetic induction is zero. So, here the magnetic field due to the magnet and that due to earth are equal and opposite.

Correct choice: d

When a magnet is placed with its north pole facing north, its field on its axial line is towards north, i.e., in the same direction as the earth?s horizontal field. But along its equatorial line filed is directed towards south, i.e., opposite to earth?s field and hence there null point can be obtained.[magnetic field is directed away from north pole and towards south pole of a magnetic dipole]

Correct choice: b

When placed inside an iron ring (ferromagnetic material), magnetic fields outside cannot enter the region within the ring and there is effective magnetic shielding.

Correct choice: d

The instrument is called flux meter.

Correct choice: e

Magnetic susceptibility is the ratio of intensity of magnetization and the magnetizing field strength, both having the same unit A/m. So it has no unit.

Correct choice: e

Since the three magnets form a closed chain, the resultant moment is zero.

Correct choice: e

Field due to a short dipole is inversely proportional to cube of the distance.

Correct choice: e

Since the field due to a magnet is inversely proportional to cube of the distance of the point from the center of the dipole, and hence the fields must be in the ratio 8:1. But here the distances are not from the center and hence the ratio is not exactly 8:1.

Correct choice: c

For getting neutral point the condition is magnet?s field = earth?s horizontal field. So here, (?₀/4?) 2 M/r³ = 5 × 10^{? 5}. So, r³ = 10^{? 7} × 2 × 6.75 / 5 × 10^{? 5} = 0.027. So, r = 0.3 m [?₀/4? = 10^{? 7}]

Correct choice: c

Torque = M B Sin ?.

Correct choice: a

Torque ? = M B Sin ?. So, d?/d? = M B Cos ?, which is maximum when ? = 0º

Correct choice: a

Potential energy = - M B Cos ?. And hence the work done in turning through an angle 90º (i.e., from 0 to 90) = M B and that through 60º is = M B (1 ? cos 60) = M B (1 ? ½) = ½ MB. So, the required ratio = 2 or n = 2.

Correct choice: b

See solution 60 first. When broken into two halves, each half has half the mass and half the length. So moment is half and moment of inertia is 1/8 times. Look at an earlier similar problem, and you will see that period becomes half the initial value = 2 s.

Correct choice: b

Magnetic south pole, as you know, is near the geographic north.

Correct choice: a

It is around 11.5º

Correct choice: e

Earth?s field is totally horizontal at the equator and vertical at the poles. It is inclined to the horizontal everywhere else.

Correct choice: a

A magnetic needle aligns itself in the direction of earth?s magnetic field, which is usually inclined.

Correct choice: b

Dip is zero at the equator

Correct choice: a

Cos ? = B horizontal / total B. So, here, ?3/2 = 0.5/B. Hence B = 1/?3.

Correct choice: d

Such a line is called an aclinic line.

Correct choice: c

Diamagnetic substances tend to move away from magnetic fields.

Correct choice: a

It?s Curie temperature.

Correct choice: b

Soft iron has low hysteresis loss and high permeability. It has also high susceptibility and that is why they are used to make electro-magnets.

Correct choice: b

It gives loss of energy per cycle of magnetization and demagnetization.

Correct choice: a

Direction of induced current, as you know. As per the law induced current will oppose the cause, which produces it.

Correct choice: c

In accordance with Lenz?s law, the induced current in this case is such that a north pole is produced at the face that is being approached by the north pole of the magnet.

Correct choice: a

Inductances can be added as you add resistances.

Correct choice: c

As you know, in this case the induced e. m. f. is an a. c. and hence it changes direction every half rotation.

Correct choice: b

Self-inductance L = ? N² A/L.

Correct choice: c

CR has dimensions of time (called time constant of R ? C circuit) and hence 1/RC represents frequency.

Correct choice: a

Here as there is a cut in the ring, the induced e. m. f. does not send current through the ring and hence the ring does not become a magnetic dipole and hence the magnet moves downwards undisturbed, with acceleration = g.

Correct choice: a

Time constant of inductance is L/R

Correct choice: b

Since there is a condenser in the circuit there cannot be any steady current, except for the charging current in the beginning.

Correct choice: d

Inductance cannot affect steady current expect that at the time of switching on there is an opposition. So steady state current = V/R = 100/10 = 10 A.

Correct choice: a

Frequency of an L ? C circuit is f = 1/2??LC. So when L and C are both doubled, frequency becomes half the initial value.

Correct choice: d

Effective value or r. m. s. value of a. c. voltage = peak value/?2 = 423/1.41 = 299 V.

Correct choice: b

Here the inductance is an ideal one and hence current lags behind the voltage by 90º

Correct choice: c

Reactance of a capacitance = 1/C?. When frequency ?f? is doubled, ? = 2?f also doubles and hence reactance becomes half the initial value = 2.5 ohm.

Correct choice: c

Instantaneous voltage V = V₀ Sin ?t = 120?2 Sin 120 ?t = 170 Sin 120 ?/720 = 170 Sin ?/6 = 170/2 = 85 V.

Correct choice: a

Current becomes maximum at resonance frequency ? = 1/?LC = 1/?(0.5 ×0.0008) = 1/?0.0004 = 1/0.002 = 500

Correct choice: a

Current I = P/V = 50/100 = 0.5 A. Voltage across capacitance = ?(200² - 100²) = 100?3 = 172 V. So, reactance of capacitance (=1/C?) times current = 172. Or, (1/C 100?) 0.5 = 172. Or, C = 1/(100 ? × 344) = 9.2 × 10^{? 6} F

Correct choice: not there (such a problem is not worth attempting)

While in series Z = ?(R² + 1/C²?²) and it is = R/?1 + ?²C²R². Given, ?(R² + 1/C²?²) =

2 R/?1 + ?²C²R². Or, (R² C² ?² + 1)/ C²?² = 4R²/(1 + ?²C²R²). Or (1 + ?²C²R²)² = 4R² C²?². So, (1 + ?²C²R²) = 2 RC?, or, (1 ? RC?)² = 0 and hence ? = 1/RC, or f = 1/2?RC

Correct choice: b *** This problem is not worth attempting.

Voltage across L and C are always in opposite directions, or differ in phase by 180º.

Correct choice: c

Given ? = 1/?LC, which is resonant angular frequency. So the current is maximum.

Correct choice: a

At resonance current = 6 A. So, impedance ?Z? at resonance = R = 24/6 = 4 ohm. Hence when the same circuit is connected to a D. C of 12 V with r = 4 ohm, current = V/(R + r) = 12/8 = 1.5 A.

Correct choice: c

At resonance impedance = resistance = 20 ohm.

Correct choice: a

Since voltage across inductance and capacitance is the same, it is implied that the circuit resonates with the applied a. c. So impedance is resistance. Hence when resistance is short circuited, R becomes zero and hence current becomes infinity.

Correct choice: b

It is admittance.

Correct choice: d

At resonance frequency p. d. across inductance = p. d. across capacitance and opposite. So the total p. d. across inductance and capacitance together = 0, and hence A reads zero.

Correct choice: a

In this case a north pole has to be produced and hence the current induced is in the anticlockwise direction.

Correct choice: b

Tangent of dip = vertical component / horizontal component = 1/?3. So dip = 30º

Correct choice: a

There can be magnets without poles and with three poles.

Correct choice: d

Work done in this case = 2 M B = 2 times 2.5 times 0.2 = 1 J

Correct choice: b

Period of a magnet in a magnetic field T = 2? ?(I / M B), where I is moment of inertia = mass times length square divided by 12. Here in the first case, I = I and when cut into two pieces lengthwise, mass is half but length is the same, and hence moment of inertia becomes half. Also, moment is half the initial value. Therefore there is no change in period.

Correct choice: a

In this case it can be shown that the force is inversely proportional to fourth power of distance.

Correct choice: d

62. Here again the deduction of the result is a bit lengthy. Now, you simply accept that the result is ?a?.

Correct choice: a

63. If you look at the formula for inductance, L = ? N² A / l, we see that the unit of permeability is henry/meter, since A has unit m², length ?l? has unit m and N, number of turns has no unit.

Correct choice: c

64. This is somewhat similar to problem no. 62. True dip is 45. So tan 45 = 1 = B_V/ B_H. And if ? is the apparent dip, then tan ? = B_V / B_H cos 60 = 1/(1/2) = 2. So ? is a little more than 60º [since tan60 = 1.732] and so the correct value is 63º

Correct choice: a

65. When placed with northpole facing north, field on the axial line is in the direction of earth?s horizontal field and that on the equatorial line is opposite to it. So null point will be on the equatorial line.

Correct choice: b

66. Here the resultant moment is zero as the north and south together cancel each other.

Correct choice: d

67. When the dip needle, the instrument used to measure dip, is placed with its plane perpendicular to magnetic meridian, the field acting on the needle of the device is vertical earth?s field and hence the needle will stand vertical.

Correct choice: b

68. It is the characteristic of a diamagnetic substance.

Correct choice: d

69. Induced e. m. f. = ½ B L² ? = ½ 0.1 × 0.15² × 60 = 0.0675 V = 67.5 m V.

Correct choice: a

70. Since the current through the coils is in the same direction, the field between the coils is the sum of the fields produced by both of them. So, as they are brought closer, field strength increases and hence the induced current will be such as to reduce the field. In other words current in the coils decrease.

Correct choice: b

71. Current in the larger loop I = Voltage / resistance = (4 + 2.5 t)/(2? × 1 × 0.1 m ohm) = 6.37 + 3.98 t (m A). So, field on the axis of the larger coil = field on the axis of the smaller coil B = ?₀ I/ 2 a. Put the values of the quantities and get B. Now, flux through the smaller loop ? = B A. Induced e. m. f. in it = d?/dt. Induced current = induced e. m. f. /resistance, 2?×0.1×0.1×10^{? 3}. This will work out to 1.25 A. *** This is a problem, not worth attempting for our entrance examination.

Correct choice: b

72. Vertical component of earth?s field = horizontal component times tangent of dip = 0.02 tan 60 = 0.035 m T. So the induced e. m. f. = B L v = 0.035 millitesla times 20 times 250 = 0.175 V

Correct choice: d

73. When terminal velocity is reached, net force = 0. Or, weight of the rod = magnetic force. Or, 0.2 times 10 = B I L = 0.6 times I times 1. SO, I = 2/0.6 = 3.33 A

Correct choice: b

74. When the magnet is moved towards the loop, induced current in the loop will tend to oppose the relative motion and hence the coil moves backwards.

Correct choice: a

75. Time constant is time taken by current to become 63.2% of maximum.

Correct choice: e

76. Here the inductance becomes 2 times the initial value, since L depends directly on square of N and inversely on length.

Correct choice: a

77. Self-induced e. m. f. = L dI/dt. So, 2 = L (6/0.03) = 200 L. So, L = 0.01 H

Correct choice: d

78. Induced e. m. f. is directly proportional to L and hence the answer here is 8/2 = 4.

Correct choice: a

79. The quantity Planck?s constant ?h?/electronic charge ?e? has the same dimensions as magnetic flux.

Correct choice: c

80. Energy stored = ½ L I². So, when the current is doubled, energy becomes 4 times.

Correct choice: b

81. When the rate of change of current is steady, induced current will also be steady, but always opposing the inducing current.

Correct choice: b

82. When turned through 180º, flux through the coil changes from + ? to ? ?, and so the change is 2? = 2 B A N = 2 times 0.04 (m T) times 0.05 times 1000 = 0.004. So the induced e. m. f. = 0.004/0.1 = 0.04 V = 40 m V.

Correct choice: c

83. In this case the induced e. m. f = B L v = 0.02 milli times 1 times 20 = 0.4 m V

Correct choice: a

84. Look at the formula in solution no. 77, you can see that the unit of L is V s / A.

Correct choice: c

85. During switching on and off the inductance exhibits self-induction, while the resistance does not.

Correct choice: c

86. Here induced voltage = L dI/dt = 0.5 milli times 1.5 / 2.5 micro = 0.3 kilo volt = 300 V

Correct choice: e

87. So as to reduce eddy current as laminated core has high resistance.

Correct choice: d

88. As copper plate is pushed into the space between the poles, the induced eddy current will oppose this motion and hence the face of the plate nearer to the north pole, will attain north polarity, which means the current is in the anticlockwise direction.

Correct choice: b

89. From the equation we see that ? = 2? f = 377. So, f = 60 hz.

Correct choice: a

90. R. M. S. value of the given a. c. is = 10/?2 = 5?2. So the resultant of the a. c. and D. C. currents = ?[5² + (5?2)²= ?75 = 5?3 A.

Correct choice: e

91. Reactance = L ?. So, here 100 = L × 2?f. So, L = 1/? = 0.32 H, since frequency of our domestic a. c. supply = 50 Hz.

Correct choice: a

92. Peak current = peak voltage / L ? = 10/100 times 5 mH = 10/0.5 = 20 A

Correct choice: d

93. Battery provides D. C. which is unaffected by inductance and hence current = V/R = 10/10 = 1 A.

Correct choice: a

94. Hot wire ammeter can work on both a. c. and D. C, since heat is proportional to I².

Correct choice: a

95. D. C is 2 A and effective value of a. c. is 2/?2 = ?2 A. As heat is proportional to I², it is in the ratio 4:2 = 2:1

Correct choice: d

96. At steady state the voltage across the coil of the motor = 20 times 1.5 = 30 V. So, 200 ? 30 = 170 V is the back e. m. f.

Correct choice: b

97. Resonant frequency depends on L and C and not on R. So change in R will not affect it.

Correct choice: e

98. When an a. c. is passed through the wire the alternating magnetic field interacts with the magnet?s field and hence the string is set into vibration.

Correct choice: e

99. Peak current = peak voltage/resistance = 200?2/400 = 1/?2 A = 0.71 A

Correct choice: b

100. Induced current = induced voltage/resistance = B A N ?/r = 0.01 times pi times 0.3 square times 5 times 2 pi times 200/60 divided by 9.87 = 0.03 A = 30 m A.

Correct choice: a

MAGNETISM ELECTROMAGNETIC INDUCTION & ALTERNATING CURRENT

1. If a metal piece was thought to be a magnet, which one of the following observations would offer conclusive evidence? a. It attracts a magnet b. it repels a magnet c. it attracts a steel screw driver

d. it attracts a piece of copper e. none of the above

A bar magnet having dipole moment 80 units is cut into two halves of equal length. The moment of each half will be: a. 80 units b. 40 units c. 60 units d. 20 units e. 160 units
If the distance between two dissimilar magnetic poles held ?d? apart is doubled, the force of attraction between them: a. doubles b. quadruples c. becomes half d. becomes one fourth e. remains the same
The distance between two magnetic poles and also their pole strength are doubled. The force between them: a. doubles b. becomes half c. triples d. quadruples

e. same as before

All magnetic materials lose their magnetic properties, when: a. dipped in water b. dipped in oil c. brought near a piece of iron d. brought in contact with copper e. strongly heated
Magnetic lines of force: a. always intersect b. are closed curves c. tend to crowd together far away from the poles of a magnet d. do not pass through vacuum e. have none of the above properties
A magnet is placed in earth?s magnetic field with its south pole facing north. At the neutral point:

a. the earth?s magnetic field is zero b. the magnet?s magnetic field is zero c. the field of earth and the magnet are in the same direction d. the fields of earth and the magnet are equal and opposite e. none

If a bar magnet is place with its north pole pointing north, the neutral points of magnetic field are located: a. on the axial line of the magnet b. on the equatorial line of the magnet c. near the north pole of the magnet d. near the south pole of the magnet e. neutral points do not exist
A sensitive magnetic instrument can be shielded effectively from an outside magnetic field by placing it inside a box of: a. plastic b. wood c. copper d. soft iron e. none of the above
Magnetic field can be measured using: a. pyrometer b. hydrometer c. thermometer d. altimeter e. flux meter
Unit of magnetic susceptibility is: a. weber b. henry c. weber/meter d. ampere e. none of these
Three identical bar magnets, each of moment M are arranged as in figure.

The net magnetic moment of the system is:

a. ?3 M b. M c. 3/2 M d. 3M e. zero

The magnetic induction at point distant ?r? from a short dipole, on its axial line varies with ?r? as: a. r b. r² c. r³ d. 1/r² e.1/r³
Two points A and B are situated along the extended axis of a 2 cm long bar magnet, at distances ?r? and ?2r? respectively, from the pole nearer to the point. The ratio of the magnetic fields at A and B is:

a. exactly 4:1 b. about 4:1 c. about 8:1 d. exactly 8:1 e. none of these

A short magnetic dipole of moment 6.75 Am² produces a neutral point on its axial line. If the horizontal earth?s magnetic induction is 5 × 10^-5 T, the distance of the neutral point from the magnet is:

a. 10 cm b. 20 cm c. 30 cm d. 40 cm e. 5 cm

For a bar magnet placed at an angle ? with a field, the couple (torque) is proportional to:

a. sin ? b. cos ? c. tan ? d. ? e. 1/sin?

Rate of change of torque ? with angle ? is maximum for a magnet suspended freely in a uniform magnetic of induction B, when ? is: a. 0º b. 30 c. 90 d. 45 e. 60º
The work done in turning a magnet of moment M by an angle 90º from the magnetic meridian is ?n? times the work required to turn through an angle 60º. Then, ?n? is: a. ½ b. 2 c. ¼ d. 4 e. 1
A thin rectangular bar magnet suspended freely in a magnetic field has period of oscillation 4 s. If it is broken into two halves, each half having half the original length, and one of the pieces is suspended in the same field, the period of oscillation will be: a. 4 s b. 2 s c. 0.5 s d. 8 s e. 0.25 s
The magnetic south pole of earth is: a. near the geographic north pole b. near the geographic south pole c. vertically above the equator d. none of the above
The angle between the earth?s magnetic axis and geographic axis is about:

a. 12º b. 0º c. 23º d. 90º e. none of these

Earth?s magnetic field always has a horizontal component except at: a. the magnetic poles b. the magnetic equator c. the geographic poles d. a latitude of 45º
A magnetic needle pivoted about a horizontal axis makes an angle with the horizontal because: a. the gravitational force of earth attracts one pole and repels the other b. one pole is heavier than the other c. magnetic needle cannot be in equilibrium d. due to earth?s magnetism one pole dips e. none of these
The angle of dip at the magnetic equator is: a. zero b. 90 c. 45 d. 60 e. 180º
At a certain place the angle of dip is 30º and the horizontal component of earth?s magnetic induction is 0.5 oersted. Then, earth?s total magnetic induction in oersted is: a. 0.5 b. 1 c. ?3 d. 1/?3 e. none
An imaginary line passing through all the places having zero dip is called: a. isoclinic line b. isogonic line c. aclinic line d. agonic line e. none of these
A small piece of an unmagnetised substance gets repelled when brought near a powerful magnet. Then the substance is: a. diamagnetic b. paramagnetic c. ferromagnetic d. non magnetic
The temperature above which a ferromagnetic substance becomes paramagnetic is called: a. critical temperature b. curie temperature c. Boyle temperature d. Debye temperature e. none of these
Soft iron is used in many parts of electrical machines because it has: a. low hysteresis loss and low permeability b. low hysteresis loss and high permeability c. high hysteresis loss d. high susceptibility
The area enclosed by a hysteresis loop is a measure of: a. retentivity of the material b. coercivity of the material c. permeability d. susceptibility e. energy loss.
Lenz?s law gives the direction of: a. magnetic field b. motion of conductor c. induced current d. none
When a magnet is moved towards a coil with its north pole facing the coil, the nearer end of the coil acts as a: a. north pole b. south pole c. positive charge d. negative charge
Two pure inductors of self-inductance L each are joined in series, the net inductance is: a. L b. L/2 c.2L d. L/4
When a wire loop is rotated in a magnetic field, the direction of induced e. m. f. changes once in every:

a. rotation b. ½ rotation c. ¼ of rotation d. 2 rotations

If N is the number of turns in a coil, its self-inductance varies as: a. Nº b. N c. N² d. 1/N²
Among the following the combination having dimensions of frequency is: a. 1/RC b. RL/C c. R/LC d. C/L e. RC
A copper ring having a cut so as not to form a loop is held horizontally and a bar magnet is allowed to fall through it with its length along the axis of the ring. The acceleration of the magnet is: a. g b. g/2 c. 2g d. 0
The time constant of inductance is: a. LR b. L/R c. ?(L/R) d. R/L
A circuit has capacitance 10 mF and resistance 5 mega ohm. It is connected to a d. c. of 100 V. The value of steady current is: a. 20 ? A b. 10 A c. 20 mA d. 0.
A circuit has inductance 10 mH and resistance 10 ohm. It is connected to a source of e. m. f. (D. C.) 100 V. The value of steady state current is: a. 10 A b. 10000 A c. 0.1 A d. none of these.
A circuit with L and C has natural frequency f. If the capacitance and inductance are doubled, the frequency becomes: a. f/4 b. 2f c. 4f d. f/2.
In an a. c. circuit maximum value of voltage is 423 V. Its effective value is: a. 400 V b. 300 V c. 323 V d. 340 V.
An inductive circuit has zero resistance. When an a. c. voltage is applied across this circuit, the current lags behind the voltage by: a. 30º b. 45º c. 90º d. 0º
The reactance of a capacitance is 5 ohm at 50 Hz. If the frequency is increased to 100 Hz, the new reactance is: a. 5 ohm c. 10 ohm c. 2.5 ohm d. 125 ohm.
An a. c. source produces 120 V at 60 Hz. The value of voltage after 1/720 s from start is: a. 84.8 V b. 42.4 V c. 106.8 V d. 20.2 V.
An a. c. circuit has an inductance of 0.5 H and capacitance of 0.8 mF in series. The current in the circuit is maximum when the angular frequency of the a. c. is: a. 500 Hz

b. 100 Hz c. 250 Hz d.50Hz e. none of these.

A 50 W/100 V lamp is to be connected to an ac mains of 200 V/50 Hz. The capacitance of the capacitor to be connected is: a. 4.6 b. 2.9 c. 1.5 d. 19.2 mF
For a resistance R and a capacitor C in series, the impedence is twice that of a parallel combination of the same elements. The frequency of the applied ac is: a. 2?/RC

b. 1/2?RC c. 2?/(RC)^½ d. 1/2??(RC) e. none of these

In a series L-C circuit, the voltage across L and C are: a. in phase b. out of phase by 90º c. out of phase by 180º d. none of these
An alternating voltage of angular frequency ? is applied to an L-C circuit. If for the circuit, ?² = 1/LC, then: a. the current is maximum b. the current is minimum c. the voltage is minimum d. the impedence is maximum.
A coil, a capacitor and an ac of 24 V r. m. s. are connected in series. By varying the frequency of the source, a maximum current of 6 A is observed. If this coil is connected to a battery of e. m. f. 12 V and internal resistance 4 ohm, the current through it will be: a. 2.4 A b. 1.8 A c. 1.5 A d. 1.2 A.
In a series combination of R, L and C, in resonance with an ac, if R = 20 ohm, the impedence of the combination is: a. 20 ohm b. zero c. 10 ohm d. 400 ohm.
In a series L-C-R circuit, the voltage across the resistance, capacitance and inductance is each 10 V. If the resistance is short circuited, the current in the circuit will be: a. zero b. infinite c. 10 A d. 5 A.
Reciprocal of impedence is called: a. reactance b. inductance c. conductance d. admittance A B
In the circuit, when ? is resonant frequency,

R

L

which voltmeter will read zero? C

~

a. A b. B c. both A and B d. none

56. When the north pole of a magnet is moved towards a metal ring:

A. a clockwise current is induced in it

B. an anticlockwise current is induced in it.

C. no current is induced in it

D. an alternating current is induced in it

E. only a voltage and no current is induced in it

57. At a place the horizontal component of earth?s magnetic field is ?3 times the vertical component. The value of dip at the place is:

a. 30º B. 45º C. 60º D. 90º E. 15º

58. Which of the following statements is true?

A. There can be magnets with no poles

B. There can be magnets with two similar poles (or three poles)

C. Attraction is the sure test of polarity

D. both A and B

E. both A and C

59. A bar magnet having dipole moment 2.5 Am² is placed in a magnetic field of 0.2 T. Work done in turning the magnet from parallel to anti-parallel position relative to the field is:

A. 0.5 J B. 1 J C. 2 J D. 5 J E. none of these

A thin magnet suspended freely in a magnetic field has period of oscillation 4 s. If the magnet is broken into two halves, with each half having the original length, and one of the pieces is oscillated in the same field, the period of oscillation will be:

A. 4 s B. 1 s C. 0.25 s D. 8 s E. 2 s

Two magnetic dipoles are placed co-axially, separated by a distance ?r?. The force between them depends on ?r? as:

A. r B. 1/r C. r⁴ D. 1/r⁴ E. 1/r³

If A and B are the angles of dip in two vertical planes at right angles to each other, and C is the true angle of dip, then:

A. cot² C = cot² A + cot² B B. tan² C = tan² A + tan² B C. cot C = cot A + cot B

D. tan C = tan A + tan B E. none of the above is correct

The S. I (or Sommerfeld) unit of magnetic permeability is:

A. Wb/m B. Wb/m² C. H/m D. H/m² E. tesla

The true dip at a place is 45º. If the plane of the dip circle is turned through 60º from the magnetic meridian, the apparent dip will be:

A. 63º B. 90º C. 60º D. 55º E. 45º

If a bar magnet is placed with its north pole facing north, the neutral point will be on:

A. the axial line of the magnet B. on the equatorial line C. north-eastern side

D. on the south east of the magnet E. null point does not exist

Three identical bar magnets, each of moment M are placed to form an equilateral triangle, with the north pole of one touching the south pole of the other. The resultant moment of the system is:

A. 3 M B. ?3 M C. 1.5 M D. 0 E. M/3

The dip needle in a plane perpendicular to the magnetic meridian will be:

A. horizontal B. vertical C. at 60º with the horizontal D. 60º with the vertical

E. none of these

If the net dipole moment of the atom of a substance is zero, the substance exhibits:

A. ferromagnetism B. anti-ferromagnetism C. paramagnetism

D. diamagnetism E. none of these

A metal rod of length 15 cm rotates about an axis passing through one end of it, with uniform angular velocity 60 radian/second, in a magnetic field 0.1 T, directed parallel to the axis of rotation of the rod. The e. m. f. induced between the ends of the rod is:

A. 0 B. 67.5 m V C. 6.75 m V D. 67.5 V E. 6.75 V

Two identical, co-axial circular loops carry equal current in the same direction. When the coils are moved towards each other:

A. the current in one of them increases while that in the other decreases

B. the current in both of them decreases

C. the current in both the coils increases

D. the current in them remains the same as before

E. the current in them may increase or decrease, depending on the speed of motion

Two concentric co-planar circular metal loops each having resistance 0.1 m?/m have diameters 0.2 m and 2 m. A voltage that varies with time, V = 4 + 2.5 t volt is applied to the larger loop. Then, the current in the smaller loop is:

A. 0.125 A B. 1.25 A C. 12.5 A D. 0 E. 2.5 A

An airplane with wingspan 20 m is flying horizontally at a speed 250 m/s, towards south. Earth?s horizontal component of magnetic field at the place is 0.02 m T and the angle of dip is 60º. The e. m .f. induced across the tip of the wings is:

A. 0 B. 17.3 V C. 1.73 V D. 0.173 V E. 0.5 V

A horizontal metal rod of length 1 m and mass 0.2 kg falls down in a uniform magnetic field 0.6 T, directed perpendicular to the length of the rod. The current in it when it attains terminal velocity is:

A. 0 B. 3.27 A C. 0.327 A D. 3.27 m A E. none of these

A metal ring is kept suspended by a thread. If one pole of a magnet is moved towards it:

A. the ring will move away from the magnet

B. the ring will move towards the magnet

C. there will be no movement of the ring

D. the ring will move vertically upward

E. the ring will move vertically downwards

Inductive time constant of a circuit is the time taken by the current in the circuit:

A. to reach the maximum value

B. to become 25% of the maximum value

C. to become 50% of the maximum value

D. to become 36.8% of maximum value, from zero

E. to become 63.2% of maximum value, from zero

When the number of turns and the length of a solenoid are both doubled, its self-inductance becomes:

A. double the initial value B. half the initial value C. four times the initial value

D. one-fourth the initial value E. 8 times the initial value

When the current in a coil changes from 8 A to 2 A in 0.03 s, the self-induced back e. m. f. in it is 2 V. The self inductance of the coil is:

A. 1 H B. 10 H C. 1 m H D. 10 m H E. 0.1 H

Two coils have self-inductances 8 m H and 2 m H. At a certain instant, the current in both of them is increasing at the same rate. Then, the ratio of the induced e. m. f. in them at that instant is:

A. 4 B. ¼ C. 2 D. ½ E. 1

Which of the following statements is true?

A. the dimensions of electric and magnetic flux are the same

B. when a coil is kept stationary in magnetic field an e. m. f. is induce in it.

C. the dimensions of h/e are the same as that of magnetic flux

D. both A and C

E. both B and C

When the current in an inductor is doubled, by what factor does the energy stored in it change?

A. 2 B. 4 C. ½ D. ¼ E. 8

When the current through a solenoid increases at a constant rate, the induced current:

A. is a constant and is in the same direction as the applied current

B. is a constant but opposite to the direction of the inducing current

C. increases with time and is in the direction of the inducing current

D. increases with time and is opposite to the applied current

E. decreases and is in the same direction as the applied current

A coil of area 500 cm² having 1000 turns is placed in a magnetic field of 0.04 m T, with its plane perpendicular to the field. If it is turned through 180º in 0.1 s, the induced e. m. f. in it is:

A. 20 m V B. 8 m V C. 40 m V D. 60 m V E. 80 m V

Two rails of a railway track are insulated from each other and from the ground. A millivoltmeter is connected across them. If a train travels over them at a speed of 20 m/s, the reading in the voltmeter is (given: vertical component of earth?s field is 0.02 m Wb/m² and distance between the rails is 1 m):

A. 0.4 m V B. 4 m V C. 40 m V D. 10 m V E. 80 m V

The unit of inductance is:

A. volt/meter B. joule/ampere C. volt × second/ampere D. volt × ampere

E. volt/coulomb

A bulb ?A?, with a resistance in series with it and another one, ?B?, with an inductance in series, are connected in parallel to a power supply. When the supply is switched off:

A. both of them die out together promptly

B. both of them die out together with some delay

C. A dies out first and B with a delay

D. B dies out first and then A, with a delay

E. both of them die out together, but after a long time

A fully charged capacitor is allowed to discharge through an inductance of 0.5 m H. The initial current through the inductance rises from 0 to 1.5 A in 2.5 microsecond. The instantaneous voltage across it is:

A. 1000 V B. 125 V C. 75 V D. 1875 V E. 300 V

The core in the transformer is laminated so that:

A. ratio of voltage in secondary to that in the primary may be increased

B. residual magnetism in the core may be reduced

C. weight of the transformer may be reduced

D. there may be no eddy current

E. energy loss due to eddy current is minimized

A horseshoe magnet is placed with its north pole on the upper side. If a copper plate is pushed into the space between the poles, on viewing from above, the induced eddy current in the sheet will flow in:

A. the clockwise direction

B. the anti clockwise direction

C. the vertically upward direction

D. the vertically downward direction

E. there will be no eddy current

The ac produced in an alternator is represented by V = 170 Sin 377 t. The frequency of the voltage is:

A. 60 Hz B. 100 Hz C. 50 Hz D. 110 Hz E. 120 Hz

A direct current of 5 A is superimposed on an alternating current I = 10 Sin (314 t) ampere. The resultant current is:

A. 15 A B. 10.6 A C. 7.5 A D. 5?5 A E. 5?3 A.

The reactance of a coil connected to our domestic power supply is 100 ?. The self-inductance of the coil is:

A. 2.2 H B. 0.22 H C. 3.3 H D. 32 H E. 0.32 H

A ideal coil of inductance 5 m H is connected to an alternating voltage V = 10 Sin(100t) volt. The peak current in the coil is:

A. 5 A B. 10 A C. 2 A D. 20 A E. 1 A

A solenoid having resistance 10 ? and inductance 5 m H is connected to a battery of e. m. f. 10 V. The current through it in the steady state is:

A. 1 A B. 0.7 A C. 0 D. 1.41 A E. none of these

Hot wire ammeters are used to measure:

A. only D. C. B. only a. c. C. neither a. c. nor D. C. D. both a. c. and D. C.

E. there is no such device called hot wire ammeter.

A direct current of 2 A and an alternating current with peak value 2 A are allowed to pass through two identical resistances. The ratio of heat produced in them is:

A. 1:1 B. 1:2 C. 1:4 D. 2:1 E. 4:1

The armature of a D. C. motor has resistance 20 ohm. It draws 1.5 A when run by a 200 V D. C. supply. The back e. m. f. induced in the motor is:

A. 200 V B. 170 V C. 250 V D. 220 V E. 180 V

In a series L-C-R circuit, L = 10 m H, C = 5 m F and R = 10 W. For a particular ac frequency, the circuit is found to resonate. Now, if the resistance in the circuit is doubled, the resonant frequency will be:

A. halved B. doubled C. one-fourth D. quadrupled E. the same as before

A conducting wire is kept stretched between the pole pieces of a powerful magnet, so that it is perpendicular to the lines of force. If an alternating current is passed through the wire, it will:

A. remain stationary B. be pulled towards the south pole C. pulled towards north pole D. tend to move away from the field E. vibrate

An electric heater of 400 W is connected to a 200 V, 50 Hz mains. The peak value of current through the heater is:

A. 0.5 A B. 0.7 A C. 2 A D. 2.8 A E. none of the above

A circular coil of 5 turns and radius 30 cm is rotated about its diameter in a uniform magnetic field of 0.01 T, perpendicular to the direction of the magnetic lines of force. If the frequency of rotation is 200 rpm and resistance of the coil is 9.87 W, the amplitude of the current induced in the coil is:

A. 30 m A B. 6 m A C. 200 m A D. 100 m A E. none of the above

sorry

TRUE FRIENDS......

"Your friend is the man who knows all about you, and still likes you."

"Friends are like melons; shall I tell you why? To find one good you must one hundred try."

"Some people come into our lives and quickly go. Some stay for awhile and
leave footprints on our hearts. And we are never, ever the same."

"A friend loves at all times." -.

"In the sweetness of friendship let there be laughter, and sharing of pleasures."

"A real friend is one who walks in when the rest of the world walks out."

"A friend hears the song in my heart and sings it to me when my memory fails."

"A faithful friend is the medicine of life."

"Count your age with friends but not with years."

"Plant a seed of friendship; reap a bouquet of happiness." "No man is useless while he has a friend."

"A true friend is someone who is there for you when he'd rather be anywhere else."

"Good friends are good for your health."

"A true friend is one who overlooks your failures and tolerates your success!"

FRIENDS............
__________,-~-. _.--._.-~-, _______
_________/ .- ,'_______`-. \______
_________\ /`__________\'/______
_________ /___'a___a`___\ ________
_________|____,'(_)`.____ |______
_________\___( ._|_. )___ /_________
__________\___ .__,'___ /__________
__________.-`._______,'-.__________
_____,'__,'___`-'___`.__`._______
_______/___/____V_____\___\____
_____,'____/_____o______\___`.___
___,'_____|______o_______|_____`._
__|_____,'|______o_______|`._____|
___`.__,'_.-\_____o______/-._`.__,
"Most people come into our lives and quickly leave.
It is the special few that come in and leave a footprint in our hearts.
and we are forever changed.
These are our *F*R*I*E*N*D*S*!"

Jyothi Susan George

useful post........thank u ....:):)