Question

(a) Can a transformer step up or step down a dc power supply?

(b) Can a step-up transformer work as a step-down transformer?

(c) Does a step-up transformer violate the law of conservation of energy? Justify your answer.

Hint:

A transformer needs a changing flux, so it fails on dc. Swapping primary and secondary reverses its action, and the power out cannot exceed the power in.

Answer 1

Step 1: Principle of a transformer:
A transformer works on mutual induction. A changing current in the primary coil produces a changing magnetic flux in the iron core. This flux links the secondary coil and induces an emf in it.

Step 2: Part (a) - dc supply:
No. A dc supply gives a steady current. The flux in the core then stays constant, so \(d\Phi/dt=0\) and no emf is induced in the secondary. A transformer works only with ac.

Step 3: Part (b) - step-up used as step-down:
Yes. For a transformer \(\dfrac{V_s}{V_p}=\dfrac{N_s}{N_p}\). A step-up transformer has more turns in the secondary. If we connect the supply to this coil (many turns) and take the output from the coil with fewer turns, the output voltage is lower. So it works as a step-down transformer.

Step 4: Part (c) - energy conservation:
No. A step-up transformer raises the voltage, but it lowers the current by the same factor. For an ideal transformer: \[ V_pI_p=V_sI_s \]
So the output power cannot exceed the input power. In a real transformer, the output power is even a little less because of losses. Hence the law of conservation of energy is not violated.

Final Answer:
(a) No. (b) Yes. (c) No, because voltage is stepped up at the cost of current and the output power does not exceed the input power.