Transcript Capacitors - Kelso High School

Capacitors
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+
Circuit symbol
Experiment: To find how the charge on a capacitor varies
with potential difference across it.
Large capacitance
Charge, Q
(C)
Small capacitance
Potential
difference across
capacitor (V)
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
1 farad (F) equals 1 coulomb (C) per volt (V)
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
Charge (C)
Potential
difference across
capacitor (V)
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
C
Tutorial Questions p67
Qu 7 to 14
How a capacitor charges and discharges click here
V
V
Large
capacitance
Max V
is
supply
voltage
Large
resistance
t
t
Current
/A
Max
I
Current
/A
Max I
Large capacitance
Max I
60
Time / s
Max I = V ÷ R
Large
resistance
60
Time /
Charging and discharging a capacitor
A capacitor – Current and and A.C.
Current
frequency
Current is directly proportional to frequency
A resistor – Current and and A.C.
Current
frequency
Current is not dependant on frequency
Uses of a capacitor
1.
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.
2.
This capacitor blocks AC and
lets DC through.
Used in audio systems to
remove high frequency hiss.
3. Camera flash
Neon bulb
4. To smooth a voltage which rectifies (changes) AC to DC
Tutorial Questions 15 to 23 and SAQs to page28