Flashcards
Capacitors
Cards in this deck (12)
- 1What is the purpose of a capacitor?To store electrical energy in a circuit, by holding charge on its plates.
- 2How is capacitance defined, and what is its unit?C=Q/V (unit: farad)
- 3What does it mean to say that the charge on a capacitor is Q?One plate has charge +Q, and the other charge -Q.
- 4How do you add capacitors in series and in parallel.Capacitors add in series like resistors add in parallel, and in parallel like resistors add in series. Try proving this.
[Hint: For capacitors in series, Q is equal; for capacitors in parallel, V is equal. Can you explain why?] - 5What is the initial charging current, and explain qualitatively how does changes over time?Initial charging current is simply determined by the resistance of the rest of the circuit. However, as charge builds up on the plates, it repels the charges moving through the wires, so the current drops as the charge on the plates increases. Charging stops when the p.d. between the capacitor plates is equal to the e.m.f. of the cell.
- 6What is the time constant and how is it related to the half-time of charging the capacitor?T=RC
Half-time=T*ln(2) - 7A capacitor can be discharged by connecting a resistor between its two plates. Give an expression for the discharge current at any time.discharge current = (p.d. between plates)/(resistance of the resistor)
- 8Derive expressions relating the charge on a capacitor during its discharge to the time t since the discharge began, and the initial charge on the capacitor. Do the same for current and voltage..
- 9What is the convention for taking a capacitor to be fully charged/discharged?When t >= 5RC
- 10What is the equation for the energy stored in a capacitor? Prove it.E = 1/2 QV = 1/2 CV^2 = 1/2 Q^2/C
- 11How much of the energy developed by a cell is dissipated as heat in fully charging up a capacitor? Why is it important to always charge up a capacitor via a resistor?1/2
This is the portion of the energy developed by the cell that is dissipated in the rest of the circuit, regardless of what is in the circuit. If there is no resistor, the wires may overheat. - 12What is the effect of inserting a dielectric between the capacitor plates? Why does this happen?It increases the capacitance of the capacitor. The dielectric contains polar molecules which orient themselves due to the electric field in the capacitor in a direction opposed to the field. This means the effective field in the capacitor is decreased. Hence the potential difference across the capacitor is decreased (for fixed d), and thus the capacitance must be increased (for fixed Q).
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- A level Physics (A level) (Paper 2) Flashcards on Capacitors, created by muon neutrino on 31/10/2017.
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- muon neutrino
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