Capacitors Electricity Higher Physics (copy)

Capacitor

A device designed to store charge

Capacitance

Capacitance is the ratio of the charge stored on the capacitor to the voltage applied

What is meant by a capacitor of 1 farad

A capacitor of 1 farad will store 1 coulomb of charge when the potential difference across it is 1 volt

Capacitance equation

C = Q/V

Method: investigating charging current

1. Start the timer when you connect the switch to position A. Record values of current (I) on the ammeter every 15 seconds (timed with stop clock)

2. Stop taking readings when the current has reduced to 0A

3. Plot a graph with time on x-axis and current on y-axis

Method: investigating charging voltage

1. Start the timer when you connect the switch to position A. Record values of voltage (V) on the voltmeter every 15 seconds (timed with stop clock)

2. Stop taking reading when the voltage has reached a maximum value

3. Plot a graph with time on the x-axis and voltage on y-axis

Charging capacitor

• Initially all the potential difference is across the resistor and the current is high

• As current flows and electrons arrive at the capacitor plate

• Electrons repel each other, so the more there are on the plate, the greater the resistance, so the lower the current and the greater the potential between the plates

• Eventually the repulsion force between the electrons equals the force from the supply and the current is zero

Discharging capacitor

• When connected in a circuit without a power supply giving an opposing force, the electrons on the plate repel and a current flows

• The lamp converts that electrical energy into heat and light

• The potential across the capacitor decreases to zero

Factors affecting capacitor time to charge

There are 2 factors that affect the time for a capacitor to charge - the resistance of the resistor and the capacitance of the capacitor

Resistance affecting charging time

• The greater the resistance the lower the current so the longer it takes to charge the capacitor

• The lower the resistance the higher the current so it takes less time to charge the capacitor

Capacitance of capacitor affecting charging time

• The greater the capacitance of the capacitor, the more charge it can store so it takes longer to charge

• The smaller the capacitance, the less charge it can store, so it fully charges faster