Magnetism and electromagnetism：練習問題

In an experiment investigating electromagnetic induction, a student moves a bar magnet into a solenoid connected to a sensitive voltmeter. Which of the following sets of changes will both increase the magnitude of the induced voltage?

A student drops a strong bar magnet through a vertical copper pipe. The magnet takes significantly longer to fall through the pipe than it would take to fall the same distance in a vacuum. Which of the following best explains this observation?

Why is soft iron preferred over steel for the core of an electromagnet?

In a simple d.c. motor, a coil is placed between the poles of a permanent magnet. What is the primary function of the split-ring commutator in this device?

A horizontal wire is placed in a uniform magnetic field that is directed vertically upwards. If a current flows through the wire from North to South, in which direction will the magnetic force act on the wire?

State two changes that would increase the magnitude of the force acting on a current-carrying conductor when it is placed inside a magnetic field.

Describe how a voltage is induced in a coil of wire using a permanent magnet and state two ways to increase the size of this induced voltage.

A simple d.c. motor consists of a coil of wire placed in a uniform magnetic field.

(a) Explain why the coil rotates when a current flows through it.
(b) Describe the purpose of the split-ring commutator in this motor.
(c) Predict and explain how the rotation of the motor would change if the magnetic field strength were doubled and the current were halved.