Explain working of moving coil galvanometer and write its uses.

Explanation

1. When a current flow through the coil, torque on it is `tau=NIABsintheta`.

2. The field is radial by design so angle between `vecAandvecB` always be `theta`. So, maximum torque is given by
`thereforetau_(max)=NIAB" "...(1)`
3. The magnetic torque NIAB tends to rotate the coil. A spring `S_(p)` provides a counter torque which is given by,
4. Restoring torque produce in spring G as,
`taupropphi`
`thereforetau=kphi" "...(2)`
5. A spring `S_(p)` provides a counter torque which balances the magnetic torque it indicate steady angular deflection `phi`.
6. At steady state,
`thereforetau=tau_(max)`
`thereforekphi=NIAB" "...(3)`
7. Here, k is the torsional constant of the spring i.e. restoring torque per unit twist.
8. Unit of torsional constant is
`("Joule")/("Radian")("From "k=tau/phi)`
9. The deflection `phi` is indicated on the scale by a pointer attached to the spring we have,
`phi=((NAB)/k)I" "...(4)`
The quantity in brackets is a constant for a given galvanometer.
`thereforephipropI" "...(5)`
Uses : (i) To detect the presence of current.
(ii) It is used to measure minute electric current.
(iii) By using galvanometer, ammeter and voltmeter can be constructed.