Capacitors

Question 1

What does a capacitor do?

Answer 1

Stores charge on its two plates when a p.d. is applied across it.

Question 2

How can you keep the charging current constant?

Answer 2

Using a variable resistor and constantly changing its resistance.

Question 3

What will the p.d. of the capacitor be when fully charged?

Answer 3

The same as the power supply (the variable resistor would’ve taken up the remaining p.d. before this point).

Question 4

Define the time constant.

Answer 4

The time taken for the capacitor to discharge to 37% of its initial voltage, or charge to 63% of its final voltage.

Question 5

Why does a dielectric increase permittivity (and therefore capacitance)?

Answer 5

The polar molecules in the dielectric line up along the electric field lines, creating an opposing electric field, reducing the p.d. across the plates, so same amount of charge can be stored with less V.

Question 6

What happens to Q, V and E if the dielectric is removed while the capacitor is connected?

Answer 6

• V is constant (battery maintains p.d.)
• Q decreases
• E decreases

Question 7

What happens to Q, V and E if the dielectric is removed while the capacitor is disconnected?

Answer 7

• Q is constant
• V increases (presence of dielectric decreased p.d. between plates due to charge collecting on its surface)
• E increases
• Work done removing dielectric is added to the energy stored.

Question 8

Define permittivity (use equation).

Answer 8

The charge per unit area on oppositely charged parallel plates when the electric field strength between the plates is one volt per metre.