Capacitors

capacitor

an electronic component that stores electrical energy in an electric field, similar to a temporary battery

direction of conventional current

positive to negative

direction of electron flow

negative to positive

graph of charge against time when charging up

graph of voltage against time when charging up

graph of current against time when charging up

graph of charge against time when discharging

graph of voltage against time when discharging

graph of current against time when discharging

equation for capacitance

measured in farads charge/voltage q/v

how to get capicitance from a voltage over charge graph

gradient = 1/capicitance

equation for energy stored in capacitor

½ x Q x V area under graph of voltage against charge

its ½ because energy is used in transferring electrons from 1 plate to another

use of capacitor

old cameras

fast discharge

large current over short time

made bulb bright

things that affect rate of discharge i.e shape of discharge graph

• resistance

• capacitance - lots of charge takes longer to discharge than small amounts of charge

equation of graph of charge against time discharging

equation of graph of voltage against time discharging

equation of graph of current against time discharging

equation of graph of voltage against time charging

equation of graph of current against time charging

equation of graph of charge against time charging

time constant

time it takes for the voltage or charge to decay to 36.8% of its initial value when discharging, or to rise to 63.2% of its final value when charging

measuring the charge on a capacitor

1. Set up the apparatus like the circuit above, making sure the switch is not connected to X or Y (no current should be flowing through)

2. Set the battery pack to a potential difference of 10 V and use a 10 kΩ resistor. The capacitor should initially be fully discharged

3. Charge the capacitor fully by placing the switch at point X. The voltmeter reading should read the same voltage as the battery (10 V)

4. Move the switch to point Y

5. Record the voltage reading every 10 s down to a value of 0 V. A total of 8-10 readings should be taken
   

how to draw a graph from equation of voltage against time discharging

using logs