Capacitors 6-7

Question 1

What are capacitors?

Answer 1

Capacitors are things that store electrical charge.

Question 2

What is capacitance a measure of?

Answer 2

Capacitance is a measure of how much chage a capacitor can hold.

Capacitance is in farads(F) - 1 Farad = 1 CV^-1

Question 3

Explain the use of capacitors in flash photography.

Answer 3

When you take a picture, charge stored in a capacitor flows through a tube of xenon gas which emits a bright light. In order to give a brief flash of bright light, the capacitor has to discharge really quickly to give a short pulse of high current.

Question 4

What is the use of capacitors in defibrillators?

Answer 4

During a heart attack, a person’s heart stops beating properly. A proper rythm can be restarted by giving the person an electric shock using a defibrillator. The circuit in a defibrillator canbe programmed to vary how much charge is stored depending on the size of the paitent. The charge is stored on a capacitor and then released in a short, controlled burst.

Question 5

What is the use of capacitors in back-up power supplies?

Answer 5

Computers are often connected to back up power supplies to make sure that you don’t lose any data if there’s a power cut. These often use large capacitors that store charge while the power on then release that charge slowly if the power goes off. The capacitors are designed to discharge over a number of hours, maintaining a steady flow of charge.

Question 6

How do you find the capacitence of a capacitor in a circuit.

Answer 6

1) Set up a test circuit to measure the current and potential difference.
2) Constantly adjust the variable resistor to keep the charging current constant for as long as you can (it’s impossible when the capcitor is nearly fully charged.)
3) Record the p.d. at regular intervals until it equals the battery p.d.
4) Plot Q against V and the gradient from the graph will be the capacitance. (Obtain Q from Q=It)

Question 7

What equation allows you to calculate the energy stored in a capacitor?

Answer 7

E = 1/2QV

You find the energy stored by the capacitor from the area under a graph of p.d. against charge stored in the capacitor.

Question 8

How do you derive the second equation to calculate the energy stored in a capacitor?

Answer 8

You know: 1/2QV

C=Q/V so Q=CV

So E=1/2CV x V

so:

E = 1/2CV^2