Question

A transformer is designed to work from a $240$ V a.c. mains and to give a supply of $8$ V to ring a house-bell. The primary coil has $4800$ turns. How many turns will be in the secondary coil?

• A. $240$
• B. $480$
• C. $160$
• D. $320$

Answer 1

Answer: (C)

$160$

A transformer takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit. A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
If we change the number of turns in the coils we change the induced emf. This allows us to change (transform) the voltage from the primary to the secondary coil.
The Turns Rule is:

$\frac{Ns}{Np}=\frac{Vs}{Vp}$

Where,
$Ns=$ number of turns on the secondary coil
$Np=$ number of turns on the primary coil
$Vs=$ voltage across the secondary coil
$Vp=$ voltage across the primary coil
So if number of turns on the secondary coil is lesser than on the primary coil, the output voltage will be lesser than the input voltage. This is called a step down transformer.
Hence, The number of turns of the secondary coil as per the given question can be obtained as follows:
Let the input voltage be $240$ V and the output voltage be $8$ V. Let $4800$ be the number of turns of primary coil and x be the number of turns of secondary coil.

$\frac{Ns}{Np}=\frac{Vs}{Vp}$;

That is, $\frac{x}{4800}=\frac{8}{240}$$\Rightarrow x=4800\times \frac{8}{240}=160$ turns.

Therefore, the number of turns of secondary coil is $160$ turns.