The GATE Grind

GATE 2015 EC – Question 23

Analog Circuits · Diode Circuits · 1 mark · Multiple choice

In the circuit shown, the switch SW is thrown from position A to position B at time $t = 0$. The energy (in $\mu$J) taken from the 3 V source to charge the 0.1 $\mu$F capacitor from 0 V to 3 V is

A 3 V source in series with a $120\ \Omega$ resistor and a switch SW that connects the 0.1 μF capacitor to the source when in position B.
  1. 0.3
  2. 0.45
  3. 0.9
  4. 3

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Show answer and explanation

Correct answer: (C) 0.9

Explanation

Charging a capacitor from a constant source through a resistor draws the charge $Q = CV$ from the source at voltage $V$. The energy from the source is $QV = CV^2 = 0.1\ \mu\text{F} \times 9 = 0.9\ \mu$J. Half of this is stored in the capacitor and half is lost in the resistor.