Charge and Discharging 8-9 Flashcards
When a capacitor is connected to a battery what direction do the electrons flow?
The electrons flow onto the plate connected to the negative terminal of the battery, so a negative charge builds up. (Electrons end up in opposite direction to conventional current.)
What is the distribution of charge between the plates of a fully charged capacitor?
An equal but opposite charge builds up on each plate, causing a potential difference between the plates.
Why does the current through the circuit decrease with time as the capacitor is charged?
As charge builds up on the plates, electrostatic repulsion makes it harder and harder for more electrons to be deposited.
How do you know the capacitor in a circuit is fully charged?
When the p.d. across the capacitor is equal to the p.d. across the battery, the current falls to zero. The capacitor is fully charged.
Draw the graph of the charge remaining on the plates of a capacitor while it is discharging.
What two factors does the time it takes to charge up or discharge a capacitor depend on?
1) The capacitance of the capacitor (C). This affects the amount of charge that can be transferred at a given voltage.
2) The resistance of the circuit (R). This affects the current in the circuit.
How can the rate of the discharge of a capacitor be expressed as?
dQ/dt = -Q/RC
What is the equation that gives he charge lft on the plates of a capacitor discharging from full?
Q = Q0e-t/RC
Draw the graph of V against t for a capacitor charging.
Draw the graph for V against T for a capacitor discharging.
The current-time graph for a charging and a discharging capacitor is just the same as the voltage-time graph for a discharging capacitor.
The time constant t is a measure of the time taken for what to occur in the capacitor?
The time constant is the time taken for the charge on a discharging capacitor (Q) to fall to 37 of Q0, or for the charge of a charging capacitor to rise 63% of Q0.
What effect does adding a larger resistance in series with the capacitor do?
The larger the resistance in series with the capacitor, the longer it takes to charge or discharge.
