OCR A Physics A-level Topic 6.1: Capacitors

Flashcards covering the key concepts of capacitors from OCR A Physics A-level Topic 6.1

What is a capacitor?

An electrical component that stores charge on two separated metallic plates.

What is the purpose of the insulator (dielectric) in a capacitor?

To prevent charge from travelling across the gap and to increase the capacitance.

Define capacitance (C).

The charge stored (Q) per unit potential difference (V) across the two plates, C = Q/V.

What is the unit of capacitance?

Farad (F), equivalent to CV⁻¹.

What happens when a capacitor is connected to a DC power supply?

A brief current flows as electrons are drawn from one plate and deposited on the other until the potential difference equals the power supply's e.m.f.

What does the dielectric do to increase capacitance?

It polarizes in the electric field, effectively increasing the charge stored on the plates.

What is electrical permittivity?

A property of dielectrics that describes its ability to polarize and strengthen charge storage capability.

State Kirchhoff's voltage law.

The sum of the e.m.f.s in any closed loop in a circuit is equal to the sum of the potential differences in the same loop.

How do you calculate total capacitance in series?

1/CT = 1/C1 + 1/C2 + 1/C3 + … + 1/CN

State Kirchhoff's current law.

The total current flowing into a node in a circuit must be equal to the total current flowing out of that node.

How do you calculate total capacitance in parallel?

CT = C1 + C2 + C3 + … + CN

Why is work required to charge a capacitor?

Like charges repel, so work must be done to deposit electrons on the negative plate. Work is done to remove electrons from the positive plate.

What does the area under the charge vs. potential difference graph represent?

The work done in charging up the capacitor, which is the energy stored in the capacitor.

What is the formula for energy stored in a capacitor?

W = (1/2)QV = (1/2)V²C = Q²/2C

Name three applications of capacitors.

Short pulses of energy (camera flashes, touch screens), uninterrupted power supplies (UPSs), and converting AC to DC.

How do capacitors help in converting AC to DC?

They store energy as the p.d. rises and discharge as it falls in a smoothing circuit, maintaining a more constant current.

How is a capacitor discharged?

By disconnecting the power supply and connecting another electrical component, often a resistor.

What happens to the electrons during discharge?

The electrons repel one another and flow round the circuit, dissipating electric energy as heat in the resistor, until the potential difference is zero.

What two factors does the time constant for discharging depend on?

Capacitance and the magnitude of the resistance in the discharging circuit.

What is the initial potential difference across the plates before discharge?

V0 = Q0/C, where Q0 is the initial charge stored on the plates.

What is the relationship between current and time in a discharging capacitor?

I = −dQ/dt (the negative sign indicates conventional current is opposite to electron flow).

What is the formula for charge as a function of time in a discharging capacitor?

Q = Q0e^(-t/RC)

What is the formula for voltage as a function of time in a discharging capacitor?

V = V0e^(-t/RC)

What is the formula for the charging capacitor?

VC = V0(1 − e^(-t/RC))

Define the time constant (τ) for a capacitor-resistor circuit.

τ = RC; when t = τ, the charge on the capacitor will have decreased to approximately 37% of its original value during discharge.