Questions For Electromagnetic Induction (Set 1)

A horizontal bar is rotating at an angular velocity v about a vertical axis through its centre in a region of constant magnetic field B directed parallel to its vertical axis.

What is the e.m.f. between the two ends of the bar?

1. It is zero.
2. It is proportional to the product Bv.
3. It is proportional to the product Bv2.
4. It is proportional to the product B2v2.

The induced emf on the arms of the rod on each side of the centre of rotation is directed towards the centre. Hence, the emf induced across the whole rod is zero.

A coil carries a current in an anti-clockwise direction while a wire located at the axis of the coil carries a current that is coming out of the paper, as shown below.

What is the effect of this set-up on the coil?

1. It does not experience any force.
2. The resultant force on the coil is zero.
3. It experiences an attractive force towards the centre.
4. It experiences a repulsive force away from the centre.

Both currents are parallel to the magnetic fields produced by their counterparts; hence the currents will not interact with the magnetic field and produce a force.

A 20-turn square coil of side 8.0 mm is pivoted at the centre and placed in a magnetic field of flux density 0.010 T such that two sides of the coil are parallel to the field and two sides are perpendicular to the field, as shown below. A current of 5.0 mA is passed through the coil.

What is the magnitude of the torque acting on the square coil?

1. 1.6 x 10-9 N m
2. 3.2 x 10-8 N m
3. 6.4 x 10-8 N m
4. 3.2 x 10-5 N m

Magnetic force on a single coil = BIL
= 0.01 x 5.0 x 10-3 x 8.0 x 10-3
= 4.0 x 10-7 N m
Torque acting on the coil = 20 x 4.0 x 10-7 x 8.0 x 10-3
= 6.4 x 108 N m.

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