# Worksheet: Calculating the Energy Stored in a Capacitor

In this worksheet, we will practice relating the charge stored in and the voltage applied across a capacitor to the energy stored in it.

Q1:

A capacitor has a charge of 2.5 µC when connected to a battery of 6.0 V. How much energy is stored in this capacitor?

Q2:

A 165 µF capacitor is used in conjunction with a dc motor. How much energy is stored in the capacitor when 119 V is applied?

Q3:

Electronic flash units for cameras contain a capacitor for storing the energy used to produce the flash. In one such unit, the flash lasts for s, with an average light power output of 270 kW.

If the conversion of electrical energy to light is efficient (because the rest of the energy goes to thermal energy), how much energy must be stored in the capacitor for one flash?

What is the capacitance of the capacitor if the potential difference across its plates is 125 V when it stores enough energy for one flash?

Q4:

Suppose that the capacitance of a variable capacitor can be manually changed from 100 pF to 800 pF by turning a dial, connected to one set of plates by a shaft, from to . With the dial set at (corresponding to ), the capacitor is connected to a 500 V source. After charging, the capacitor is disconnected from the source, and the dial is turned to . If friction is negligible, how much work is required to turn the dial from to ?

Q5:

A prankster applies 300 V to a 65.0 µF capacitor and then tosses it to an unsuspecting victim. The victim’s finger is burned by the discharge of the capacitor through 0.130 g of flesh. What is the temperature increase of the flesh? Take the specific heat of tissues to be 3,500 J/kg⋅K.

Q6:

On a particular day, it takes J of electrical energy to start a truck’s engine. What is the capacitance of a capacitor that could store that amount of energy at 8.0 V?

Q7:

How much energy is stored in a 12 µF capacitor whose plates are at a potential difference of 3.0 V?

Q8:

In open-heart surgeries, a very small amount of energy is used to defibrillate the heart. A 5.00 µF capacitor is used in a heart defibrillator, which stores 30.0 J of energy.

What is the potential difference across the capacitor in the defibrillator?

How much charge is stored by the capacitor in the defibrillator?