# Discharging a Capacitor

Discussion in 'MCAT Study Question Q&A' started by collegelife101, May 13, 2014.

1. ### collegelife101

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May 2, 2014

A resistor, a fully charged capacitor, and a switch are all connected in series, with the switch initially open. The switch is then closed, allowing the capacitor to discharge. During this discharging process, which of the following quantities DECREASES?

I.

The current through the resistor

II.

The energy stored in the capacitor

III.

The voltage of the battery

As the capacitor discharges, the voltage across its plates decreases according to V = Q/C. Because the voltage drop across the resistor must equal the capacitorâ€™s voltage, VC â€“ VR = 0, the voltage across the resistor correspondingly decreases. Therefore, by V = IR, so does the current across the resistor, meaning I is true. The power dissipated by a resistor is given by P = I2R, so as the current decreases, so does the power dissipated across the resistor, meaning III is true. Moreover, as the charge of the capacitor decreases, so does the potential energy stored, according to PE = Q2/2C. Thus II is true.

I'm a little confused on where the battery is in this situation. If the power dissipated by a resistor decreases, how does that tell us that the voltage of the battery is decreasing?

Thanks!