Phy MC Questions

14. Statement 1 is wrong. The higher the power, the lower is the resistance of the bulb.

Power = voltage^2/resistance
i.e. power is inversely proportional to resistance. Hence, the resistance of the 40w bulb is half that of the 20w bulb.

Statement 3 is right.
Let R be the resistance of the 40w bulb, thus the resistance of the 20w bulb is 2R
hence, 40 = V^2/R where R = voltage applied

When the bulbs are connected in series, total resistance = (R + 2R) = 3R
Power = V^2/(3R) = (1/3).(V^2/R) = (1/3) x 40 w = 13.3 w < 20 w

16. Power delivered by the source at primary coil
= 12 x 2 w = 24 w
Hence, power trnasferred to the secondary coil = 24 x 0.8 w = 19.2 w

Since the turn-ratio of the transformer is 100/50 = 2:1
voltage at secondary = 12 x (1/2) v = 6 v
Resistance R = 6^2/19.2 ohms = 1.875 ohms

Therefore, option B is the correct answer.
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