Capacitors

Question 1

What is a capacitor?

Answer 1

An object that can store electric charge

Question 2

Unit of capacitance

Answer 2

Farads (F)

Question 3

Capacitance (for a parallel plate capacitor)

Answer 3

Charge stored on one plate per potential difference between the plates

Question 4

Why does current decrease with time when charging a capacitor?

Answer 4

• It becomes more difficult to pull electrons from a positive plate and push them onto a negative plate
• So the rate of flow of electrons decreases

Question 5

When does a capacitor stop charging?

Answer 5

When the pd across it is equal to the pd across the battery

Question 6

Dielectric

Answer 6

• An electrical insulator
• Permittivity effects capacitance

Question 7

Relative permittivity

Answer 7

Ratio of materials permittivity to the permittivity of free space
(Always bigger than 1)

Question 8

Why do polar molecules increase a materials relative permitivity?

Answer 8

• Rotate to align with the electric field
• Charge of polar molecules near the plate opposes the charge on the plate
• Allows more charge to be stored on the plates

Question 9

Why is energy stored by a capacitor only half that supplied by the battery?

Answer 9

The other half is transferred to heat energy by resistance in the circuit

Question 10

What do V₀ , I₀ and Q₀ represent?

Answer 10

Maximum pd, current and charge

Question 11

Why does current decrease with time when discharging a capacitor?

Answer 11

• As Q decreases, there is less repulsion between the plates
• So electrons flow less quickly as time goes on

Question 12

Time constant

Answer 12

RC
Time for Q / V / I to decrease to 1/e of its original value in a discharging circuit