21, Capacitance Flashcards
What is the circuit symbol for a capacitor?
Two parallel lines.
What does C = Q/V mean?
Capacitance = Charge stored per unit P.D. across it.
What is the unit of capacitance?
The Farad (CV^-1).
How do you store a charge in a capacitor?
Supply a direct current across the capacitor forcing electrons onto one plate and off the other, the current in the circuit stops once the P.D. across the plates = the E.M.F. of the power supply.
What is the total capacitance of two capacitors in parallel?
The sum of the capacitance (area of plates has increased).
What is the total capacitance of two capacitors in series?
The inverse of the sum of the inverses of the capacitance (gap between plates increases).
Where does the energy on a capacitor come from?
The electrons on the plate repel each other so if there is no E.M.F. to keep them on the plates there will be a current in the opposite direction to the current used to provide the charge to the plates.
What quantity is the area under a P.D. charge graph?
Work done.
What does the equation W = (1/2)QV mean and where does it come from?
The work done on a capacitor = 1/2 * charge on the capacitor * P.D. It is derived from the area under a P.D. charge graph.
A circuit has a fully charged capacitor in parallel with a resistor in parallel with a power supply with a closed switch between the power supply and the other components, describe what happens as the switch is opened.
The capacitor starts by placing a P.D. across the resistor equal to the E.M.F. of the battery, since the charge in the capacitor is decreasing and Q = VC, the P.D. decreases causing the current to drop (I=V/R), this cycle means that P.D. and charge and current all follow an exponential decay over time.
What does the equation x = x_0 e^(-t/CR) mean and where does it come from?
x can have I, V or Q substituted into it giving the current I, V, or Q of a discharging capacitor-resistor circuit over time.
How do capacitors store energy?
When charged one plate has positive charge and another negative charge creating an electric field between the plates which stores electric potential energy since electrons have had work done to move them onto the plates.
Why are capacitors useful when providing energy?
They have very little internal resistance so can deliver very high currents.
What is the time constant of a capacitor-resistor circuit?
Denoted by tau it is equal to CR and is the time taken for I/V/Q to decrease to e^-1 * original value.
What does the equation x = x_0 (1-e^(-t/CR)) come from?
It is the discharging equation for P.D. and charge (since the charge and P.D. stored in the capacitor increase over time).
