磁與電磁作用：練習題

Which of the following actions will not increase the strength of the magnetic field produced by an air-core solenoid?

A beam of electrons enters a uniform magnetic field as shown in the diagram. The magnetic field is directed into the paper. What is the direction of the magnetic force acting on the electrons as they enter the field?

As shown in the diagram, a bar magnet is being pushed towards a stationary copper ring with its North pole facing the ring. What is the direction of the induced current in the ring (when viewed from the right side) and the nature of the magnetic force between the magnet and the ring?

Which of the following materials is most suitable for making the core of an electromagnet used in a scrap yard crane?

A step-up transformer has \(100\) turns in its primary coil and \(500\) turns in its secondary coil. If the input voltage is \(12 \text{ V}\) AC, what is the output voltage?

Identify two ways to increase the strength of the magnetic field produced by a current-carrying solenoid.

A strong cylindrical magnet is dropped through a long, vertical copper tube. Explain how the induced effects cause the magnet to reach a terminal velocity, and state whether this terminal velocity would be higher or lower if the tube were cooled to a very low temperature.

In a step-up transformer, the primary coil has \(50\) turns and the secondary coil has \(250\) turns. If the input voltage is \(12\text{ V}\) AC, calculate the output voltage and state the relationship between the voltage ratio and turns ratio.

A long insulated copper wire is wound around a soft iron bar to form a solenoid as shown in the diagram. The ends of the wire are connected to a DC power supply.
(a) When the switch is closed, current flows through the solenoid in the direction shown. Identify which end of the solenoid (A or B) becomes the North pole and explain your answer using the right-hand grip rule.
(b) If the connections to the DC power supply are reversed, state what happens to the magnetic poles of the solenoid.

Consider a simple DC motor consisting of a rectangular coil placed between the poles of a permanent magnet.
(a) Explain how the interaction between the current in the coil and the external magnetic field produces a turning effect (torque).
(b) Describe the function of the split-ring commutator in this motor and explain why it is essential for continuous rotation.