Chapter 12 Solutions — Magnetic Effects of Electric Current

A compass needle is a small bar magnet that is free to rotate. It has its own magnetic field with a north pole and a south pole.

When brought near a bar magnet, the compass needle experiences a force due to the magnetic field of the bar magnet.

The north pole of the compass needle is attracted towards the south pole of the bar magnet and repelled by the north pole. Similarly, the south pole of the compass needle is attracted towards the north pole of the bar magnet.

This torque (turning force) causes the compass needle to deflect and align itself along the direction of the magnetic field of the bar magnet at that point.

The properties of magnetic field lines are:

1. Magnetic field lines are closed continuous curves. Outside the magnet, they go from north pole to south pole. Inside the magnet, they go from south pole to north pole.

2. The tangent to a field line at any point gives the direction of the magnetic field at that point. This is the direction in which a north pole placed at that point would tend to move.

3. No two field lines can intersect each other. If they did, the compass needle placed at the point of intersection would point in two different directions, which is not possible.

4. The relative closeness (density) of field lines indicates the relative strength of the magnetic field. Crowded field lines represent a stronger field, while widely spaced field lines represent a weaker field.

5. The field lines are parallel and equidistant inside a solenoid, indicating a uniform field.

Fleming's left-hand rule:

Stretch the thumb, forefinger (index finger), and middle finger of your left hand such that they are mutually perpendicular to each other (at right angles to each other).

Then:

- The forefinger points in the direction of the magnetic field (B).

- The middle finger points in the direction of the current (I) in the conductor.

- The thumb points in the direction of the force (F) or motion of the conductor.

This rule is used to find the direction of force experienced by a current-carrying conductor placed in a magnetic field, or the direction of motion of the conductor.

Application: Fleming's left-hand rule is used in electric motors, where a current-carrying coil placed in a magnetic field experiences a force that causes it to rotate.