12 - Capacitance Flashcards
What is a capacitor?
An electrical component which separates charge.
To parallel metal plates separated by an insulator.
What is a dielectric?
The insulator between the metal plates of a capacitor.
Define capacitance.
The charge stored on a capacitor per the unit potential difference across it.
C = Q / V
What is the unit of capacitance?
Farad
How does a capacitor gain charge?
Electrons flow to one side of the capacitor, giving it a negative charge. This causes an equal amount electrons to be repelled from the other side of the capacitor, making the current flow.
How does a capacitor reach maximum charge?
When the potential difference across the capacitor is equal to the e.m.f of the cell.
How do you add capacitors in PARALLEL?
Add the capacities.
Kirchoff’s second law, p.d. across each loop is the same
How do you add capacitors in SERIES?
Add the reciprocal of the capacities.
Kirchoff’s second law, p.d. of each component in loop is equal to e.m.f of cell
How does a capacitor discharge?
Electrons flow from the negatively charged side of the capacitor to the positively charged side of the capacitor, through a circuit that connects the two sides, until the P.D across the capacitor is 0.
What is the energy stored by a capacitor represented by on a graph of p.d. against charge?
The area under the graph
What happens to the p.d. across a capacitor as the charge stored by it decreases?
It decreases as well
Describe the graph of a capacitor discharging over time.
Exponential decay
How does the current through a resistor connected to a discharging capacitor change over time?
It exponentially decays
How does the charge stored by a capacitor change over time as the capacitor is discharging?
It exponentially decays
What is the time constant of a capacitor-resistor circuit?
The product of the capacitance and the resistance.
What is the time constant for a discharging capacitor?
The time taken for the capacitor to discharge to 37% of its original value.
How does the current through a charging capacitor circuit change over time while the capacitor is charging?
It decreases exponentially
How does the p.d. across a resistor connected to a charging capacitor vary with respect to time?
Why?
It decreases exponentially
Kirchoff’s 2nd Law: Sum of p.d.s in a closed loop must equal e.m.f of cell
How does the charge stored by a capacitor change over time as the capacitor is charged?
It increases exponentially
How does the p.d. across a capacitor change over time as the capacitor is charged?
It increases exponentially
What equation related current, p.d. across and resistance of the resistor in the a charging capacitor circuit?
V = IR
What equation links the charge, capacitance and potential difference across the capacitor when it is charging?
Q = VC
How are diodes and capacitors used to smooth an AC current?
The capacitor charges while the AC current is positive and then discharges while the current is negative.
Only positive current is allowed to flow due to a diode.
What is a “ripple” in a current smoothed by a diode and a capacitor?
The difference between the m
How can we reduce the ripple in an output voltage smoothed by a diode and a capacitor?
Use a capacitor with a time constant much higher than the period of the circuit.
What happens to the charge when the distance between the plates is doubled?
No change
What happens to the capacitance when the distance between the plates is doubled?
Capacitance halves.
C = ε0A/d
