If u are asking about electric dipole then answer is as below

 

The electric dipole moment for a pair of opposite charges of magnitude q is defined as the magnitude of the charge times the distance between them and the defined direction is toward the positive charge. It is a useful concept in atoms and molecules where the effects of charge separation are measurable, but the distances between the charges are too small to be easily measurable. It is also a useful concept in dielectrics and other applications in solid and liquid materials.

Applications involve the electric field of a dipole and the energy of a dipole when placed in an electric field.

 

Magnetic moment can be explained by a bar magnet which has magnetic poles of equal magnitude but opposite polarity. Each pole is the source of magnetic force which weakens with distance. Since magnetic poles come in pairs, their forces interfere with each other because while one pole pulls, the other repels. This interference is greatest when the poles are close to each other i.e. when the bar magnet is short. The magnetic force produced by a bar magnet, at a given point in space, therefore depends on two factors: on both the strength p of its poles, and on the distance d separating them. The force is proportional to the product , where describes the "magnetic moment" or "dipole moment" of the magnet along a distance R and its direction as the angle between R and the axis of the bar magnet.

Any rotating charged object, from quarks to galactic superclusters, has a magnetic moment.

For relation between magnetic moment and magnetization see magnetization.

Magnetism can be created by electric current in loops and coils so any current circulating in a planar loop produces a magnetic moment whose magnitude is equal to the product of the current and the area of the loop. When any charged particle is rotating, it behaves like a current loop with a magnetic moment.

The equation for magnetic moment in the current-carrying loop, carrying current and of area vector for which the magnitude is given by:

where

is the magnetic moment, a vector measured in ampere?square metres, or equivalently joules per tesla, is the current, a scalar measured in amperes, and is the loop area vector , having as x, y, and z coordinates the area in square metres of the projection of the loop into the yz-, zx-, and xy-plane

 

Well the answer is (b)

 

The explanation is as follows:

Number of ways of selection of 8 letters from 24 letters of which 8 are a, 8 are b, is given by following cases

 

1) 8a = 1

2) (7a,1b) ; (7a,1 letter other than a and b) = 1 + ⁸C₁ = 1+8 = 9

3) (6a,2b); (6a,1b,1 other than a and b); (6a, 2 other than a and b) = 1+⁸C₁+⁸C₂

                                                         = 9 + 28 = 37

4) (5a,3b) and so on = 1+⁸C₁+⁸C₂ + ⁸C₃ = 37 + 56 =93

5) (4a,4b)...and so on = 1+⁸C₁+⁸C₂ + ⁸C₃+ ⁸C₄= 93+70 = 163

6) (3a,5b)...and so on = 1+⁸C₁+⁸C₂ + ⁸C₃+ ⁸C₄1+⁸C₅ = 163+56 = 219

7) (2a,6b)...and so on= 1+⁸C₁+⁸C₂ + ⁸C₃+ ⁸C₄1+⁸C₅ 1+⁸C₆=219+28 = 247

8) (a,7b)....so on = 1+⁸C₁+⁸C₂ + ⁸C₃+ ⁸C₄1+⁸C₅ 1+⁸C₆+⁸C₇=247+8 = 255

 

Similarly we can interchange the role of 'a' and 'b' to get exactly eight more cases as above.

 

so total number of possible selections are = 2(1+9+37+93+163+219+247+255)

 

= 2 x 1024 = 2¹¹ = 8 x 2⁸

Another approach could be as follows:

let ABC be a triangle inscribed in the circle with center O and radius r.

If the area of this triangle is maximum, then vertex C should be at a maximum distance from the base AB i.e. CD must be perpendicular to AB.

Hence, ABC is an isosceles triangle.
If  BCD = , where D is the mid-point of BC, then BOD = 2

so, AB = 2 BD

= 2r sin 2

CD = CO + OD = r + r cos 2

If S be the area of the triangle ABC, then

S = (1/2) AB x CD

= (1/2) x 2r sin 2 (r + r cos 2)

ds/d = r²[sin2 (-2 sin2) + (1 + cos2)(2 cos2)]

= 2r²[cos²2 - sin²2 + cos2] = 2r²(cos4 + cos2)

For maximum and mimimum

ds/d = 0

or, cos4 + cos2 = 0

or, 2 cos3 cos = 0

so, Either cos3 = 0 or, cos =0

If cos = 0 or 3 = /2 or = /6

(d²s/d²) = -ve, for = /6

ACB = 2 = 2(/6) = /3 = ABC = BAC

so ABC is an equilateral triangle.

First of charge doesn't have units of micro farad it must be micro coulomb.

 

Now consider cube of side 10cm enclosing the charge of 10 micro coulomb at the centre which is at a distance 5cm from each of the sides.

 

Now applying gauss's law of electrostatics

 

Electric Flux passing through the cube = q/₀.

 

But by symmetry of the problem the flux is equidistributed through each sides.

 

Thus flux through the square plate of side 10cm = q/6₀

 

or flux = 10^(-6)/6₀.

 

I expect your answer on npn transistor to be more specific.

 

I think u want to enquire about

 

 How does a transistor work?

 

Answer) The design of a transistor allows it to function as an amplifier or a switch. This is accomplished by using a small amount of electricity to control a gate on a much larger supply of electricity, much like turning a valve to control a supply of water.

Transistors are composed of three parts ? a base, a collector, and an emitter. The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply. By sending varying levels of current from the base, the amount of current flowing through the gate from the collector may be regulated. In this way, a very small amount of current may be used to control a large amount of current, as in an amplifier. The same process is used to create the binary code for the digital processors but in this case a voltage threshold of five volts is needed to open the collector gate. In this way, the transistor is being used as a switch with a binary function: five volts - ON, less than five volts - OFF.

the derivation part is avilable in any standrd txt or ref. book for class 12. so above i have presented the law only

The Biot-Savart law is a physical law with applications in electromagnetics and, more generally, it is also useful in numerous other applications of vector field analysis. As originally formulated, the law describes the magnetic field set up by a steady current density.

 Mathematically, the Biot-Savart law provides an inverse to the curl operation; the result is unique up to gauge transformation. As such it has numerous additional applications.

The Biot-Savart law is fundamental to magnetostatics just as Coulomb's law is to electrostatics. The Biot-Savart law follows from the Lorentz transformations of the electric field of a point-like electric charge, which results in a magnetic field, and is fully consistent with Ampère's law, much as Coulomb's law is consistent with Gauss' law.

In particular, if we define a differential element of current

then the corresponding differential element of magnetic field is

where