CBSE Class 12-science Answered

(i) Consider the case r < a. The Amperian loop is a circle labelled 1.

For this loop, taking the radius of the circle to be r,

L = 2 Πr

Now the current enclosed I_e is not I, but is less than this value.

Since the current distribution is uniform, the current enclosed is,

Using Ampere's law, _

(ii) Consider the case r > a. The Amperian loop, labelled 2, is a circle concentric with the cross-section. For this loop,

L = 2 Πr

I_e = Current enclosed by the loop = I

The result is the familiar expression for a long straight wire

Figure shows a plot of the magnitude of B with distance r from the centre of the wire. The direction of the field is tangential tothe respective circular loop (1 or 2) and given by the right-hand

rule described earlier in this section

OR

The Principle: When a current flows through the conductor coil, a torque acts on it due to the external radial magnetic field. Counter torque due to suspension balances it after appropriate deflection which indicates the presence of the current in the circuit.

Two reasons why galnaometer cannot be used for measuring current:

(i) Galvanometer is a very sensitive device, it gives a full-scale deflection for a current of the order of A.

(ii) For measuring currents, the galvanometer has to be connected in series, and as it has a large resistance, this will change the value of the current in the circuit.

Current sensitivity of the galvanometer is given as:

It depends on:

i. Number of turns in the coil

ii. Area of the coil