Capacitance Flashcards
Capacitance, combined capacitance, energy stored in capacitor, functions of capacitor.
How does a capacitor work?
To move charge onto the plates of a capacitor, it must be connected to a voltage supply.
The negative terminal of the supply pushes electrons onto one plate, making it negatively charged.
Electrons are repelled from the other plate, making it positively charged.
There is a flow of electrons all the way round the circuit. (Note: Conventional current flows in the opposite direction to the electrons.)
The current stops when the p.d. across the capacitor is equal to the e.m.f. of the supply. We then say that the capacitor is ‘fully charged’.
A capacitor does not store charge. Why?
One plate has charge of +Q and the other plate has charge –Q. Since the two plates store equal and opposite charges, the total charge on the capacitor is zero. (i.e. the capacitor does not store charge.)
Why does a capacitor store energy?
Work is done by the supply to separate the positive and negative charges. As a result, the capacitor stores energy as electric potential energy.
Define capacitance.
Ratio of charge to potential.
Define the capacitance of a parallel plate capacitor.
Ratio of charge on one plate to the potential difference between the plates.
Formula for capacitance.
C = Q/V
Define the farad.
One farad is the capacitance of a capacitor that stores a charge of 1 C when the potential difference is 1 V.
State the functions of capacitors.
(i) store energy
(ii) used in a smoothing circuit to reduce the ripple on rectified voltage
(iii) produce electrical oscillations
(iv) used in a tuning circuit
Derive the formula for combined capacitance for capacitors in series.
Two capacitors in series store equal charge.
The capacitors have different p.d.s across them.
See notes.
Derive the formula for combined capacitance for capacitors in parallel.
Two capacitors in parallel have the same p.d.
See notes.
Sketch a graph of V against Q for a capacitor.
Graph is a straight line passing through origin.
How do you find the potential energy stored in a capacitor from the graph of V against Q?
Potential energy stored in capacitor = area under graph.
Using the definition of potential difference, V = W/Q. So, work done by battery to charge the capacitor = QV. But the energy stored in the capacitor is only ½ QV. What happened to the rest of the energy supplied by the battery?
Only 50% of the energy supplied is actually stored in the capacitor. The other 50% is wasted due to the heating effect of the charging current. The resistances in the circuit, such as the resistance of the connecting wires and internal resistance of the battery, cause a heating effect when current passes through them.
Formulae for energy stored in capacitor
E = 1/2 QV or E = or E = (check notes)
A capacitor C1 is charged and has charge Q. It is then connected to an uncharged capacitor C2.
How do you find the final charge on C1 and C2?
• The capacitors are in parallel and have the same p.d. across them
• Their combined capacitance,
Total C = C1 + C2
• The charge Q is shared between the two capacitors but the total amount of charge stored must remain the same since charge is conserved.
