電磁與電磁鐵：練習題

Which of the following statements correctly describes the magnetic field lines around a straight wire carrying a steady direct current?

A horizontal wire carries a current directed from North to South. A uniform magnetic field is applied vertically upwards. According to Fleming’s left-hand rule, in which direction is the force on the wire?

An electron moves perpendicularly into a uniform magnetic field that is directed into the page. What is the direction of the initial magnetic force acting on the electron if its velocity is directed towards the right?

Which of the following describes the shape of the magnetic field lines around a long, straight wire carrying a steady electric current?

A horizontal wire carries a current from West to East. It is placed in a uniform magnetic field directed vertically downwards. What is the direction of the magnetic force acting on the wire?

What happens to the direction of the magnetic field lines around a straight current-carrying wire if the direction of the electric current is reversed?

According to Fleming's left-hand rule, identify what the first finger and the second finger represent when a wire carries a current in a magnetic field.

A straight wire carrying a constant current is placed in a uniform magnetic field. Explain why the magnetic force on the wire is maximum when it is perpendicular to the field lines and zero when it is parallel to them.

A student sets up a demonstration of the motor effect. A straight copper wire is placed horizontally between the North and South poles of two strong permanent magnets. The magnetic field is uniform and directed from left to right. A direct current (d.c.) flows through the wire in a direction 'into the page'.

(a) Describe the direction of the force acting on the wire. Explain how you determined this using a named physics rule.
(b) Explain how the magnitude of the force on the wire would change if:
i) The current in the wire is doubled.
ii) The wire is moved so it is parallel to the magnetic field lines.

The diagram shows a student investigating electromagnetic induction by moving a bar magnet into a solenoid connected to a sensitive voltmeter.

(a) Describe the change in the voltmeter reading when the North pole of the magnet is pushed into the solenoid and then pulled out at the same speed.
(b) Explain, using the concept of magnetic fields, why a voltage is induced in the solenoid only when the magnet is moving.
(c) Describe how the induced voltage would change if the student moved the magnet into the solenoid much faster.
(d) A bicycle dynamo uses a rotating magnet near a coil of wire to produce electricity. Explain why the frequency of the alternating current produced by the dynamo increases as the bicycle travels faster.