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CBSE Class 12-science Answered

Draw a labelled diagram of a moving coil galvanometer. State the principle on which it works. Deduce an expression for the torque acting on a rectangular current carrying loop kept in a uniform magnetic field.Write two factors on which the current sensitivity of a moving coil galvanometer depends. Or State Biot-Savart law.Use it to derive an expression for the magnetic field at the centre of a circular loop of radius R carrying a steady current I.Sketch the magnetic field lines for such a current carrying loop.
Asked by Topperlearning User | 21 May, 2015, 12:59: PM
answered-by-expert Expert Answer

Moving coil galvanometer

Principle: A current carrying coil suspended in a magnetic field experiences a torque.

 

Let I be the current through the loop PQRS.

a, b = Sides of the rectangular loop

A = ab = Area of the loop

N = Number of turns in the loop

According to Fleming's left hand rule, the side AB experiences a normal inward force, F1= IaB and side CD experiences a normal outward force, F2 = IaB. These two equal and opposite forces form a couple, which exerts a torque given by,

= Force × Perpendicular distance

= IaB × b sin

= IBab sin

τ = IBA sin

For N turns,

τ = NIBA sin

 

The current sensitivity of a moving coil galvanometer depends on

(i) number of turns in the galvanometer coil

(ii) torsional constant of the suspension fibre.

 

Or

Biot-Savart law states that the magnetic field dB due to the current element dl at any point P is

(i) directly proportional to the current I, dB I

(ii) directly proportional to the length dl of the element, dB dl

(iii) directly proportional to sin, where is the angle between dl and r, dBsin

(iv) inversely proportional to the square of the distance r from the current element AB,

From (i), (ii) , (iii), and (iv), we obtain

At centre, x = 0

Answered by | 21 May, 2015, 02:59: PM
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