Question

If in a transformer the number of turns of primary coil and secondary coil are 5 and 4 respectively and 240 V is applied to primary coil, then the ratio of current in primary and secondary coil is
(A) 4:5
(B) 5:4
(C) 5:10
(D) 8:12

Answer 1

Hint In a transformer, the ratio of voltage in the secondary coil to that in the primary coil is equal to the ratio of the number of turns in the secondary coil to that of the primary coil.
Current and voltage are inversely proportional to each other.

Complete Step by step solution
The transformer is an electrical device that works on the principle of electromagnetic induction which induces alternating currents from one voltage to another voltage.
A step-up transformer induces the low voltage and high current of the primary side to high voltage and low current of the secondary side
A step-down transformer converts the high voltage and low current of the primary side to low voltage and high current of the secondary side.
We know that for a transformer
$\dfrac{{Number\;of\;turns\;in\;secondary\;coil}}{{Number\;of\;turns\;in\;primary\;coil}} = \dfrac{{Voltage\;of\;seconday\;side}}{{Voltage\;of\;primary\;side}}$
$\dfrac{{{N_S}}}{{{N_P}}} = \dfrac{{{V_S}}}{{{V_P}}}$
Since voltage inversely proportional to current we will have
$\dfrac{{Voltage\;of\;secondary\;side}}{{Voltage\;of\;primary\;side}} = \dfrac{{Current\;in\;primary\;coil}}{{Current\;in\;seconday\;coil}}$
Therefore we get
$\dfrac{{{N_S}}}{{{N_P}}} = \dfrac{4}{5} = \dfrac{{{V_S}}}{{{V_P}}}$
And
$\dfrac{{{V_s}}}{{{V_P}}} = \dfrac{4}{5} = \dfrac{{{I_P}}}{{{I_s}}}$ is the ratio of currents in the primary and secondary coils of the transformer.

Hence the correct option is A

Additional Information
The directions of the secondary (induced) current and the primary are opposite in the coils. Transformers are mainly helpful in the distribution and regulation of power across long distances.

Note We described from the above solution that by adjusting the turns ratio of the transformer we can get the desired ratio of voltages and currents