Capacitors Circuit symbol + -
Jan 03, 2016
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