Capacitors

Question 1

Define capacitance

Answer 1

the charge stored per unit p.d. across the plates

Question 2

What is the time constant?

Answer 2

the time taken for the charge to fall to 0.37 (1/e) of its initial value

Question 3

What does the area underneath a V vs Q graph represent?

Answer 3

Energy stored by the capacitor

Question 4

What happens to the other half of energy supplied by a cell (the energy that isn’t stored in the capacitor)?

Answer 4

lost in the circuit resistance (internal resistance, R of wires,…) as heat energy

Question 5

Define a dielectric

Answer 5

an insulator composed of polar molecules

Question 6

What is a polar molecule?

Answer 6

a molecule with a positive and a negative end

Question 7

What happens when an electric field is applied across a dialectric?

Answer 7

the molecules rotate and align themselves anti-parallel to the electric field generated by the plates

Question 8

What does anti-parallel mean?

Answer 8

aligned but pointing in opposite directions

Question 9

What happens to the resultant electric field when a dielectric is placed in a capacitor?

Answer 9

• the resultant electric field is reduced

- the electric field from the dielectric opposes that from the capacitor

Question 10

Why does a dielectric increase the capacitance of the plates?

Answer 10

the reduced electric field allows electrons to leave the positive plate (and accumulate on the negative plate) more easily

Question 11

A dielectric is between parallel plates and a p.d. is applied across the plates. The capacitor is then disconnected from the power supply, what happens to the charge stored on the plates and the p.d. across the plates?

Answer 11

• nothing happens

- charge and p.d. stays the same

Question 12

A dielectric is between parallel plates and a p.d. is applied across the plates. The capacitor is then disconnected from the power supply. The dielectric is removed from between the plates by sliding it. What happens to the charge stored on the plates and the p.d. across the plates (show this mathematically)?

Answer 12

• charge stays the same, as it has no route to move
• C=(A x εo x εr)/d, εr decreases, so C decreased
• C = Q/V, if C decreases and Q is constant, therefore V increases
• E = 0.5QV, Q is constant, V increases, so E increases

Question 13

Why does the energy stored between the plates increase as the dielectric is slid out?

Answer 13

• there is work done against traction when you remove the dielectric

Question 14

What is the equation for relative permittivity (εr)?

Answer 14

• εr = εm / εo
• εm = permittivity of material
• εo = permittivity of free space

Question 15

What is the unit for capacitance?

Answer 15

F