1. 6.1 Capacitance A capacitor is an electrical reservoir.
 two metal plates separated by an insulating material
 two wires connect plates to a circuit
Applications of capacitors
Circuits for
- smoothing unwanted voltage variations
- producing pulses or oscillations
- tuning radios
- filtering; to remove unwanted frequencies
- timing
2. Back up power supplies

7. CHARGING A CAPACITOR Tap closed: Engine to push balls
 balls can not move
 ball reservoir uncharged

8. CHARGING A CAPACITOR Tap Open: Reservoir does not store balls,
there are just
more balls
at the top
Tap Open:
 engine pushes balls onto top half of reservoir
 rubber membrane stretches
 balls are pushed out of lower reservoir

9. CHARGING A CAPACITOR Load/resistance applied:
 membrane pushes balls from the top of the
reservoir to the bottom
 powering the load -for a short while

10. e + _ e Current in a capacitor circuit
 battery pushes electrons on to lower plate
 electrons flow off the top plate at the same rate
 charge has been moved from one plate to another
( not stored )

11. Vb = Vr + Vc Vc and Vr Vb so no more current flows
From Kirchoff’s Law:
Vb = Vr + Vc
At t = 0 sec:
 capacitor is uncharged
and Vc = 0
 all battery voltage is
across the resistor so
Vb = Vr
As the capacitor charges:
Vc and Vr
until: Vb = Vc
and Vr = 0
so no more current flows Vb

12. Charging a capacitor at constant current
When the switch is closed
the variable resistor is
adjusted continually
to maintain a constant
charging current
A data logger or stop watch
records the the pd
at measured times.

13. Charging a capacitor at constant current
When the switch is closed
the variable resistor is
adjusted continually
to maintain a constant
charging current
A data logger or stop watch
records the the pd
at measured times.

14. Charging a capacitor at constant current
When the switch is closed
the variable resistor is
adjusted continually
to maintain a constant
charging current
A data logger or stop watch
records the the pd
at measured times.
Total charge Q transferred:
Q = I t
I = 15 ų A

15. Total charge Q transferred:
Q = I t
I = 15 ų A

16. Total charge Q transferred:
Q = I t
I = 15 ų A
Charge Q of a capacitor is directly proportional to the applied voltage V

17. Total charge Q transferred:
Q = I t
I = 15 ų A
Charge Q of a capacitor is directly proportional to the applied voltage V
Q = C V

18. Total charge Q transferred:
Q = I t
I = 15 ų A
Charge Q of a capacitor is directly proportional to the applied voltage V
Q = C V
Gradient:
Δ Q = C
Δ V

19. Total charge Q transferred:
Q = I t
I = 15 ų A
Charge Q of a capacitor is directly proportional to the applied voltage V
Q = C V
Gradient:
Δ Q = C
Δ V