5.E Charging a capacitor Flashcards
When do we know that a capacitor is fully charged?
When the current in the circuit is equal to zero. The current across the capacitor is equal to pd across the terminals of the power supply.
Why do we use a resistor in charging/discharging of a capacitor?
To slow down the rate of charging/discharging.
What is another reason we may use a resistor in a circuit?
To avoid an impulse current passing (max current as pd initially would be 0). This can damage the components of the circuit.
What happens to charge and pd during charging?
They both increase exponentially .
What happens to current during charging?
The current decreases exponentially.
What happens during the process of charging a capacitor?
1) When the switch is closed current starts to flow. The charge carriers (usually electrons) start to build up on the plate of the capacitor.
2) If the charge carriers are electrons then a negative charge builds up on the plate, this repels the electrons on the other plate leaving that one charged as well (only opposite charge)
3) Equal and opposite charge builds up on each plate causing a pd between the plates, since these are separated by an insulator and create an electric field.
4) Initially current is high, but as charge builds up, electrostatic repulsion makes it harder and harder for more charge carriers to be deposited (hence exponential graphs).
Equation for current when charging? What does each symbol in the equation represent?
It =(Io) e^-t/CR It = current after x time Io = initial current e = exponential function t = time from start of charging CR = time constant
Equation for pd and charge when charging? What does each symbol in the equation represent?
Qt = (Qo)e^-t/CR Vt = (Vo)E^-t/CR
How do we work out the time constant?
T = C x R
where T = time constant
C = capacitance
R = load resistance
What is the time constant?
The time the capacitor takes to discharge to 1/ of original or charge to 1-1/e of original.
