Magnetic Effects of Electric Current — NCERT Solutions

A compass needle is itself a small magnet. When it is brought near a bar magnet, the magnetic field of the bar magnet exerts a force on the needle. So the needle turns and aligns itself with the magnetic field, causing deflection.

Magnetic field lines around a bar magnet are drawn as closed curves. They emerge from the north pole and enter the south pole outside the magnet, and inside the magnet they go from south to north. The lines are closer near the poles and do not cross each other.

The properties of magnetic field lines are:

• They form closed, continuous curves.
• The field lines go from the north pole to the south pole outside the magnet.
• Inside the magnet, they go from south pole to north pole.
• The closer the lines, the stronger the magnetic field.
• No two field lines intersect each other.

If two magnetic field lines intersected, then at the point of intersection the compass needle would have to point in two directions at the same time, which is impossible. Hence, magnetic field lines do not intersect.

For a circular loop carrying current clockwise, apply the right-hand rule by curling the fingers of the right hand in the direction of current. The thumb then points to the direction of the magnetic field inside the loop.

• Inside the loop: magnetic field is into the plane of the table.
• Outside the loop: magnetic field is in the opposite direction, i.e. out of the plane of the table.

A uniform magnetic field is represented by parallel, equally spaced straight field lines with arrows all pointing in the same direction.

The magnetic field inside a long straight solenoid carrying current is uniform, so it has the same strength at all points inside it. Therefore the correct option is (d).

For a proton moving freely in a magnetic field, its mass does not change. But the magnetic force can change its direction of velocity, so velocity changes and therefore momentum also changes. Since a magnetic field does no work, the speed remains unchanged in the ideal case. Hence the changing properties are velocity and momentum; among the given options, the best choice is (c) and (d), and speed does not change. If the question expects all properties that can change while direction changes, then velocity and momentum are correct.

The displacement of rod AB will:

1. Increase if the current in the rod is increased.
2. Increase if a stronger horse-shoe magnet is used.
3. Increase if the length of the rod inside the magnetic field is increased.

This is because the force on a current-carrying conductor in a magnetic field becomes larger when the current, magnetic field strength, or effective length of conductor in the field increases.

For a positively charged particle, the direction of current is the same as the direction of motion. Using Fleming’s left-hand rule for the given motion and deflection, the magnetic field must be downward. So the correct option is (c).

Two safety measures commonly used in electric circuits and appliances are:

• Electric fuse
• Earth wire