2. Capaitors
2 conductors separated by an insulator
Capacitance iss the ability of a conductor to
store charge. Measured in Farads F.
C = Q Q
p.d V V
Q = CV
V = Q/C
3. Parrallel plate capacitor with an area A and
separated by distance d
C = ε0 εr A
d
Capacitors in parrallel
C = C1 + C2
Capacitors in series
1/C = 1/C1 + 1/C2
4. Eg
A parallel plate capacitor is charged to 200V
in a vacuum. It has an area of 100mm2 and
separated by 20mm space filled with mesh
of εr = 5. What is the capacitance?
5. C = ε0 εr A
d
= 8.85 x 10-12 x 5 x 100 x 10-3
20 x 10-3
= 2.21 x 10-10 F.
6. Energy stored in a charged capacitor
= average potential difference x charge
½ QV
V = Q/C
W = ½ QQ/C = ½ Q2/C
Can still be expressed as
½ CV2
substituting Q = CV
7. Eg
Consider a circuit with 3 capacitors of 4 μF
each in parallel with a potential of 50V
across the circuit. What is the capacitance?
What is the energy stored?
8. C = C1 + C2 + C3
= 12 μF
Energy stored
= ½ CV2 = ½ x 12 x 10-6 x 502
= 15 x 10-3 J
9. Uses of capacitors
Tuning in radio circuits
Prevention of sparking in switches
Charge storage
Smoothening rectified current from dc power
supplies
Blocking noise in ac amplifiers.
