The charge stored per unit potential difference across a capacitor, C = Q/V.

What does Q refer to in C = Q/V?

The magnitude of the charge on one plate.

How does C change if the plate separation doubles?

It halves, as C is proportional to 1/d.

What is relative permittivity?

The factor by which a dielectric multiplies the capacitance compared with a vacuum; it has no unit.

What is a polar molecule?

A molecule with a positive end and a negative end, such as water.

How does a polar dielectric increase capacitance?

Its molecules rotate to align with the field, producing an opposing field, which reduces V for the same Q so C = Q/V is larger.

An isolated charged capacitor has a dielectric inserted. What happens to Q and V?

Q stays the same; V falls by a factor of εᵣ.

Capacitance and the parallel plate capacitorAQA A-Level Physics: Flashcards

Card 1 of 120 of 12 known

Question

Define capacitance.

Tap or press Space to reveal

Tap card or press Space to flip

See all 12 cards

Define capacitance.: The charge stored per unit potential difference across a capacitor, C = Q/V.
What is the unit of capacitance?: The farad, F = C V⁻¹.
What does Q refer to in C = Q/V?: The magnitude of the charge on one plate.
State the formula for the capacitance of a parallel plate capacitor with a vacuum between the plates.: C = Aε₀/d.
How does C change if the plate separation doubles?: It halves, as C is proportional to 1/d.
State the formula for a parallel plate capacitor with a dielectric.: C = Aε₀εᵣ/d.
What is relative permittivity?: The factor by which a dielectric multiplies the capacitance compared with a vacuum; it has no unit.
What is another name for relative permittivity?: The dielectric constant.
What is a polar molecule?: A molecule with a positive end and a negative end, such as water.
How does a polar dielectric increase capacitance?: Its molecules rotate to align with the field, producing an opposing field, which reduces V for the same Q so C = Q/V is larger.
An isolated charged capacitor has a dielectric inserted. What happens to Q and V?: Q stays the same; V falls by a factor of εᵣ.
What value of ε₀ is used?: 8.85 × 10⁻¹² F m⁻¹.

Exam questions on Capacitance and the parallel plate capacitor

A 470 µF capacitor is connected across a 12 V d.c. supply until it is fully charged.
Define the capacitance of a capacitor and state what the charge Q in the definition refers to.2 marks
A parallel plate capacitor has two identical square plates, each of area 0.020 m², separated by an air gap of 0.50 mm. Treat the relative permittivity of air as 1.0. ε₀ = 8.85 × 10⁻¹² F m⁻¹.
A sheet of dielectric of relative permittivity 4.0 is inserted so that it completely fills the gap. Calculate the new capacitance.2 marks
An air-filled parallel plate capacitor has a capacitance of 2.4 nF. It is charged to 160 V and then disconnected from the supply. A sheet of dielectric made of polar molecules is then slid between the plates so that it fills the gap, and the capacitance rises to 9.6 nF.
Explain how the polar molecules of the dielectric increase the capacitance of the capacitor.3 marks

See the full worksheet

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).