Question: 18 A 12 \mathrm{~V}, 60 \mathrm{~W} lamp is connected to the secondary of a step down transformer, whose primary is connected to ac mains of 220 V . Assuming the transformer to be ideal, what is the current in the primary winding?
(1) 3.7 A
(2) 0.37 A
(3) 0.27 A
(4) 2.7 A
Answer: Option (3)
Explanation:
For an ideal transformer, there is no power loss, so the power in the primary is equal to the power in the secondary.
The power of the lamp connected to the secondary is
P = 60 \, \mathrm{W}The voltage of the primary is
V_p = 220 \, \mathrm{V}Power in the primary circuit is given by
P = V_p I_pSubstituting the known values,
60 = 220 \times I_p I_p = \frac{60}{220} I_p = 0.27 \, \mathrm{A}Hence, the current in the primary winding is 0.27 \, \mathrm{A}.