Question

A domestic electrical appliance requires an alternating voltage of 15 V. If 220 V of alternating voltage is supplied to the house, then determine the device that helps in the functioning of that electrical appliance.
A. Induction coil
B. Step-up transformer
C. AC dynamo
D. Step-down transformer

Answer 1

Hint:- Here the voltage supplied to the house is much greater than the voltage required for the electrical appliance to work. So the device that helps the electrical appliance to function must essentially reduce the voltage supplied to it so that only the necessary voltage is received by the electrical appliance.

Formula used:
The ratio of the voltage in the transformer is given by, $\dfrac{{{V_s}}}{{{V_p}}} = \dfrac{{{N_s}}}{{{N_p}}}$ where ${V_s}$, ${V_p}$ are the amplitudes of the alternating voltages in the secondary coil and primary coil respectively and ${N_s}$, ${N_p}$ are the number of turns in the secondary coil and primary coil respectively.

Complete step-by-step solution :
Step 1: Describe the condition to be satisfied for the electrical appliance to function so that we can determine the device that helps in achieving it.
It is given that the electrical appliance needs an AC voltage of 15 V to function. But it is also stated that the AC voltage supplied to the house is 220 V. This means that any electrical appliance will receive an AC voltage of 220 V unless it has an inbuilt device that reduces this supply voltage.
A change in the level of an AC voltage can be attained using a transformer. The working principle of the transformer is mutual induction.
The device that helps the electrical appliance to meet this condition is a step-down transformer. As the name suggests, a step-down transformer reduces the voltage level as the number of turns in the secondary coil is less than that of the primary coil i.e., ${N_s} < {N_p}$.
The ratio of the voltage in the transformer is given by, $\dfrac{{{V_s}}}{{{V_p}}} = \dfrac{{{N_s}}}{{{N_p}}}$ where ${V_s}$, ${V_p}$ are the amplitudes of the alternating voltages in the secondary coil and primary coil respectively while, ${N_s}$, ${N_p}$ are the number of turns in the secondary coil and primary coil respectively.
When ${N_s} < {N_p}$, we have ${V_s} < {V_p}$ which implies that the voltage gets reduced.
So the correct option is D.

Note:- Both transformers, step-up and step-down transformers can change the voltage level in a circuit. A step-up transformer steps up or increases the voltage level while a step-down transformer steps down or decreases the voltage level. The action of the transformer depends on the number of turns in the primary coil and the secondary coil. This is because the strength of the induced emf in the secondary coil by the alternating voltage in the primary coil and that of the back emf induced in the primary coil by the voltage in the secondary coil both vary with the number of turns in the respective coil.