Magnetic Effects of Electric Current: CBSE Class 10 Physics practice questions

A small compass is placed near a straight copper wire. The key is closed, and a current flows. The needle moves away from its north-south position. What does this show?

1. An electric current produces a magnetic effect (a magnetic field) around the wire
2. The copper wire has become a permanent magnet even without current
3. The current has heated the needle and made it turn
4. The compass needle has been given an electric charge by the wire

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A. An electric current produces a magnetic effect (a magnetic field) around the wire

This is Oersted's observation (1820): a current-carrying conductor deflects a nearby compass needle, so electricity and magnetism are linked. Copper is not magnetic, so it is not a permanent magnet. Heating is a different effect of current and cannot turn a needle in a definite direction.

Which statement about the magnetic field lines of a bar magnet is correct?

1. They start at the north pole and end at the south pole, so they are open curves
2. Outside the magnet they go from south to north
3. Outside the magnet they go from north to south, inside from south to north, so they are closed curves
4. They exist only outside the magnet and are straight lines

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C. Outside the magnet they go from north to south, inside from south to north, so they are closed curves

By convention the field lines emerge from the north pole and merge at the south pole. Inside the magnet the direction is from south to north, so each line is a closed curve. Lines outside are curved, not straight, and they continue inside the magnet.

A horizontal power line carries current from east to west. Point A is directly above the wire, and point B is directly below it. What is the direction of the magnetic field at A and at B?

1. A: towards the south, B: towards the north
2. A: towards the east, B: towards the west
3. A: towards the north, B: towards the south
4. A: towards the west, B: towards the east

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C. A: towards the north, B: towards the south

Point the right thumb west (the current). Over the top of the wire the fingers curl towards the north, and underneath they curl towards the south. Seen from the east end the field turns clockwise, as the textbook says. The field is never along the wire (east-west): it circles around it.