Question

Complete the given statement, In order for a transformer to work.
A) The magnetic field must be changing.
B) The magnetic field must not change.
C) It must be connected to a direct current source.
D) The primary coils and secondary coils must have different number of turns.
E) The number of turns in the secondary coil should be greater than the number of turns in the primary coil.

Answer 1

Hint: Transformer works on the principle of electromagnetic induction. If the conditions for induction are fulfilled then the transformer will work.

Complete step by step solution:
Electromagnetic Induction (EMI):
When a conductor is placed in a magnetic field which changes with respect to time, or the magnetic field is stationary and the coil or conductor is moving, a potential difference opposing the change of magnetic flux is developed across the conductor. This principal is called Electromagnetic induction.
Transformer:
Transformer is a passive electrical device that transfers electrical energy from one electrical circuit to another circuit or multiple circuits. It works on the principle of electromagnetic induction. It consists of two conducting coils called the primary coil and secondary coil. These coils are wound around a core (mostly iron) on opposite sides. There is an AC voltage called the primary voltage is supplied across the primary coil, i.e. ac current flows through the primary coil. This AC current induces a magnetic field that changes with time in the area inside the primary coil. Now the iron core comes into action, it is the iron core’s rule to carry the magnetic flux produced inside the primary coil all the way inside the secondary coil. There thins changing magnetic flux induces an emf in the secondary coil and an AC current starts flowing through it. This emf is called secondary voltage.
The diagram above will give you a better understanding of how the components inside a transformer are arranged and how they work.

Transformer is classified into two categories:
Step-up:
In this type of transformer primary input voltage is less than secondary output voltage.
This transformer is used to amplify the voltage of a circuit. The number of turns in the primary coil is less than that of the secondary coil.
Step-down:
In this type of transformer primary input voltage is less than secondary output voltage.
This transformer is used to reduce the voltage in a circuit. The number of turns in the primary coil is more than that of the secondary coil.
The main application of a transformer is to change the voltage and transfer of electrical energy from one circuit to another, so it does not matter if the number of turns in each coil is the same or not, the transformer will work. Thus, the fact that the number of turns on each coil is different or the same does not affect the working of the transformer.

Thus, option D and option E are incorrect as they do not affect the working of the transformer.

Electromagnetic induction can only take place when there is a changing magnetic field or there is motion of conductor in a constant magnetic field.
As there is no relative motion of coil and core inside the transformer the only remaining factor is magnetic field. Hence, the magnetic field has to vary with time in order to induce an emf in the secondary coil.
DC current does not change with time and so, the magnetic field induced due to a DC current is a constant magnetic field as well.
Thus, if the only variable factor is constant then there will be no induced emf in the secondary coil of the transformer.
Hence, transformers do not work on DC current.

Thus, option C is incorrect.

As mentioned earlier, a transformer works on the principle of EMI and a changing magnetic field is vital for inducing emf in the secondary coil. There should be a changing magnetic field.
Hence, in order for a transformer to work, the magnetic field must be changing.

Therefore, option A is correct.

Note: Induced emf will always oppose the change in flux.
The number of turns of the coils is vital to determine the type of transformer (step-up/ step-down), but it has no role in determining whether the transformer will work or not.
DC current induces a constant magnetic field.