Question

If a current is passed through a spring, then the spring will:
A) Expand
B) Compress
C) Remains same
D) None of these

Answer 1

Hint: The idea is to think of a spring as a solenoid. A solenoid is nothing but an arrangement in which a wire of a particular length is wound in the form of circles. When current is passed through a solenoid, a magnetic field is generated in the solenoid.

Complete step by step solution:
Firstly, let us take an example in which two coils of wire are released from a stand in such a way that both the coils are parallel to each other. Let us connect them in such a manner that when current is allowed to flow through the coils, the current flows in the same direction in both the coils. In such an arrangement, when we switch on the power, both the coils tend to attract each other. This is because of a force of attraction between them.
Now, let us take the case of a solenoid. In a solenoid, a wire of a particular length is wound in circles. When current is passed through the solenoid, it is seen that a magnetic field is generated in the solenoid. This magnetic field is a consequence of Biot-Savart law, which describes the magnetic field generated by a constant current. It can be seen that the generated magnetic field is directed from the negative end to the positive end as shown in the following figure.

If a spring is considered as a long solenoid, each turn of wire in the solenoid produces a magnetic field. This magnetic field caused by each turn of the wire creates a force of attraction on its adjacent turn of wire. This can be explained using Lorentz force, which describes the combination of magnetic force and electric force on a point charge due to an electromagnetic field. When such a force of attraction happens between each turn and its adjacent turn in a long solenoid or a spring, the whole solenoid or the spring tends to contract or compress.
Therefore, option B is correct.

Note:
The experiment mentioned in the first paragraph can be used to explain Ampere’s force law. Ampere’s force law refers to the force of attraction or repulsion between two current carrying wires due to the generation of a magnetic field. If the directions of current in the wires are opposite, there is repulsion whereas if the directions of current in the wires are the same, there is attraction. Ampere’s force law can be referred to as a consequence of both Biot-Savart law and Lorentz force law. Biot-Savart law explains the generation of a magnetic field while Lorentz force law explains the creation of a magnetic force.