Name: Magnetic Dipole and Dipole Moment
Uploaded: 2019-07-19
Duration: 4 min 53 s
Description: A magnetic dipole is an arrangement of two unlike magnetic poles of equal pole strength separated by a minimal distance. Let's learn more about them.

Question

Draw diagram of pivoted galvanometer and explain why is it used in laboratory. Derive expressions for current and voltage sensitivity.

Accepted Answer

Pivoted galvanometer: This is also a moving-coil galvanometer. It is less sensitive than a suspended-coil galvanometer but is more convenient. It consists of a coil having many turns of fine insulated copper wire wound on an aluminium frame. The ends of the axle of the frame of the coil are inserted in two pivots so that the coil may rotate about the axle.

At both ends of the coil, near the pivots, are attached two springs which produce torsional couple on the rotation of the coil and connect the coil to two terminal screws $T_{1}$ and $T_{2}$ . On both sides of the coil there are pole-pieces of a permanent strong horse-shoe magnet. The coil rotates in the magnetic field of these pole-pieces. To read the deflection of the coil, a pointer is attached to the coil which moves over a circular scale. The divisions on the scale are made at equal distances and the zero-point is in the centre.

To make the magnetic field radial, the pole-pieces are cut cylindrical and a soft-iron core is placed within the coil. A galvanometer is used in electrical circuits to detect current and in experiments to determine the null point. If some how heavy current happens to flow into the coil of the galvanometer, then due to very large deflection the pointer of the galvanometer may strike the ‘stop pin’ and be broken. To save the galvanometer from these possible damages, a thick wire or a strip of copper is connected in parallel with its coil. It is called the ‘shunt’. Its resistance is very small compared to the resistance of the coil. Therefore, most of the part of the current goes through the shunt, and only a very small part goes through the coil. Hence, there are no chances of the burning of the coil or the breaking of the pointer.

Deflecting torque produced is expressed as $τ=NIABsin 90^{0} = NIAB$ , Because the normal to the plane of coil remains perpendicular to the magnetic field.

If this torque deflects the coilthrough an angle $ϕ$ then a restoring torque is set up in the coil due to elasticity of the springs such as $τ=C ϕ$

Now in equilibrium position, Restoring torque = Deflecting torque

$\Rightarrow NBIA = C ϕ \Rightarrow \frac{ϕ}{I} = \frac{NBA}{C}$

Sensitivity of galvanometer: Galvanometer is said to be sensitive if it shows large scale deflection even if small current is passed through it or small voltage is applied across it.

Current sensitivity of galvanometer is expressed as $I_{s} = \frac{ϕ}{I} = \frac{NBA}{C}$

And Voltage sensitivity of galvanometer is expressed as $V_{s} = \frac{ϕ}{V} = \frac{NBA}{CR}$

Draw diagram of pivoted galvanometer and explain why is it used in laboratory. Derive expressions for current and voltage sensitivity.

Important Questions on Torque on a Current-Loop : Moving-Coil Galvanometer

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