GALVANOMETER , AMMETER , VOLTMETER....
moving coil galvanometer (MCG)
1. use of radial magnetic field - the plane of coil in all positions remains parallel to the direction of magnetic field.so the torque is max. when suspended in a radial magnetic field
radial magnetic field is achieved by making the faces of magnet concave and inserting a soft iron core.
2. properties of material to be used as suspension strip in MCG :
1. non magnetic material
2. low temp. coefficient of resistance.
3. eg : phosphor bronze, quartz
3. a mirror is fixed on the phosphor bronze strip by means of which the deflection of the coil is measured with a lamp-and- scale arrangement.
4. angular deflection q = NABI/k
current sensitivity = q /I
Voltage sensitivity = q / V
galvanometer can be made sensitive by decreasing k (evident from the expression )
k depends on nature of material used as suspension strip. k is small for phosphor bronze and quartz..so it is preffered..
5. increasing the current sensitivity may not necessarily increase voltage sensitivity...
for eg . if no of turns of coil (n) ----> 2n
q /I -------> 2 q /I => current sensitivity doubles
but also resistance doubles , as R is proportional to length of wire...
thus q /V ----> q /V => voltage sensitivity remains unchanged.
6. resistance of ideal voltmeter = infinity
resistance of ideal ammeter = 0
7. ammeter needs to have a low resistance. it is connected in series in a circuit in which current is to be measured . if its resistance is high it will alter the actual value of current to be measured.
8. voltmeter needs to have high resistance.it is connected in parallel . if it has low resistance , it will decrease the value of p.d to be measured between two points
9. resistance order : ammeter < galvanometer < voltmeter
10. uses of shunt :
1. to increase the range of ammeter
2. protect the galvanometer from high current while converting it to ammeter.
11. ammeter of lower range has higher resistance than ammeter of higher range
12 . voltmeter of lower range has lower resistance than voltmeter of higher range.
13. we can increase the range of ammeter but we can't decrease its range
14. we can increase or decrease the range of voltmeter.
15. in order to increase the range of voltmeter n times , its total resistance should be icreased n times. the resistance that needs to be connected in series (R) = (n-1 ) G
16. in order to increase the range of ammeter n times , the shunt resistance to be connected in parallel = G/n-1.
17. An MCG can't be used to detect A.C . in a circuit since it measures avg. value of current and average value of A.C over a complete cycle is 0
18. hot wire ammeter is used to measure a.c. . it is based on the principle of heating effects of current .
heat produced = I2Rt . it depends on I2 which is always +ve .
19. scale of dc ammeter is uniformly spaced , scale of ac ammeter is non uniformly spaced.
20. VTVM ( vaccum tube volt meter ) has nearly infinite resistance and is used for accurate measurements of p .d.
Hope it proves to be useful...
Plzzzzz post ur comments .....
Back to Community Shelf
8 Nickels awarded!![[Post New]](/templates/default/images/icon_minipost_new.gif){kind=icon}
177
