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

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

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

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

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 )

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

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.

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.

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

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

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

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

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

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