Capacitors

Circuit symbol

+-

Experiment: To find how the charge on a capacitor varies with potential difference across it.

Potential difference across capacitor (V)

Charge, Q (C)

Small capacitance

Large capacitance

Charge is directly proportional to potential difference.

Q = constant x V

Q = CV where C is the capacitance of the capacitor

C = Q ÷ V or the gradient of the graph

C = Q

V

1 farad (F) equals 1 coulomb (C) per volt (V)

Careful as C is for coulombs and capacitance

1 Farad is a very large unit so smaller units are often used.

µF micro farad x10 -6

nF nano farad x10 -9

pF Pico farad x10 -12

Potential difference across capacitor (V)

Charge (C)

Energy stored in the capacitor equals the are under the QV graph = ½QV

As E = ½ QV and Q = CV show that:-

E = ½CV2 and E = ½ Q2

CTutorial Questions p67

Qu 7 to 14

How a capacitor charges and discharges click here

V

t

Large capacitance

V

t

Large resistance

Max I

Current / A

60 Time / s

Large capacitance

Max I

Current / A

60 Time / s

Max I Large resistance

Max V is supply voltage

Max I = V ÷ R

Charging and discharging a capacitor

A capacitor – Current and and A.C.

frequency

Current

Current is directly proportional to frequency

A resistor – Current and and A.C.

frequency

Current

Current is not dependant on frequency

Uses of a capacitor

This capacitor passes AC signals but blocks DC signals. Used between the stages of an audio system to pass on the audio signal (AC) without any steady voltage (DC) which may be present.

1.

2.This capacitor blocks AC and lets DC through.

Used in audio systems to remove high frequency hiss.

3. Camera flash

4. To smooth a voltage which rectifies (changes) AC to DC

Tutorial Questions 15 to 23 and SAQs to page28

Neon bulb