1. CAPACITORS

2. IF A CAPACITORS JOB IS TO STORE ELECTRICAL CHARGE, WHERE WOULD THEY BE USEFUL?

3. USES Found in radios/tv's/computers Control timings in car and burglar alarms Camera flashes Tumble drier - to get the drum to rotate All devices where a sudden burst of energy is required Lasers used in nuclear fusion

4. CAPACITORS A capacitor is an electrical device which stores charge. Capacitance,C is defined as the amount of charge stored per unit p.d. C = Q / V Capacitance is measured in the unit of FARAD, F. 1F = 1C per V One Farad is large so capacitance is normally quoted in micro farads, nano farads or pico farads.

5. CIRCUIT SYMBOLS

6. HOW DO THEY WORK? A capacitor consists of two conducting plates. One connected to positive and the other to negative. These plates become 'charged' The amount of energy stored by the capacitor depends on how much charge is moved to the plates. The plates are separated by an insulating material called the DIELECTRIC. The plates need to have a large area and to achieve this capacitors consist of two long stripes of metal, separated by the dielectric and rolled up like a Swiss roll.

7. TO CHARGE To charge,connect a capacitor to a battery - electrons flow from the negative terminal to the negative plate. At the same time electrons are repelled away from the positive plate to the positive terminal of the battery. A negative charge builds up on the the negative plate and a positive charge on the positive plate - this increases the p.d. across the plates. Eventually the build up of charge prevents any more charge arriving and the movement of the electrons stops.

8. TO DISCHARGE To discharge, remove the battery and reconnect the capacitor to the circuit - the electrons on the negative plate are driven around the circuit to neutralise the charge on the opposite plate. The charge on both the plates falls. Therefore the p.d. across them decreases. The capacitor is 'discharged' when there is no more charge left on the plates (no current).

9. EXPONENTIAL DISCHARGE The time taken to charge/discharge depends on the capacitance of the capacitor, C and the resistance, R of the circuit (which affects the current) When a capacitor discharges, the amount of charge left on the plate falls exponentially with time. As C=Q/V, voltage and current also change exponentially with time.

10. COMBINED CAPACITANCE Two or more capacitors in parallel have the same p.d. across each one (as each can store the same charge) In parallel Ct = C1 + C2 +.. Two or more capacitor in series share the p.d across them (each capacitor stores some of the charge) In series 1/Ct = 1/C1 + 1/C2 +..

11. ENERGY STORED BY A CAPACITOR In a capacitor, electrical energy provided by the battery is stored by the capacitor Work is done removing charge from one plate and depositing on the other plate. The energy stored by the capacitor is equal to the work done by the battery. From the area under the graph: W =1/2 QV

12. OTHER ENERGY EQUATIONS Using W = 1/2 QV and C = Q/V, come up with other energy stored/ work done equations.

13. TIME CONSTANT The time constant of a capacitor tells us how long it takes for a capacitor to discharge The time constant (tau) is the time it takes for the p.d. to fall to 1/e of it's original value (or 37%) Time constant = capacitance x resistance Why is the unit of time constant, seconds?

14. EXPONENTIAL EQUATIONS As stated earlier - charge, voltage and current on a capacitor all decay exponentially with time. Equations can be used to calculate the charge, voltage or current left on a capacitor after time t.