Capacitors

Question 1

Define Capacitance

Answer 1

Charge stored per unit Volt [F]

Question 2

What do the gradient and area under this graph represent?

Answer 2

Gradient → Capacitance

Area → Work done (Energy Stored)

Question 3

What is wrong with this?

Answer 3

C = capacitance → not the charge!!!

Question 4

When building a capacitor how do you maximize the capacitance?

Answer 4

1. Increase the area of the plates
2. Decrease the plate separation
3. Place dielectric between plates

Question 5

What does it mean if the relative permittivity of a dielectric (ε_r) is 5.0?

Answer 5

The capacitor stores 5x more charge with the dielectric between the plates!

Question 6

How does adding a dielectric increase the capacitance of a capacitor?

Answer 6

1. Dielectric contains polarised molecules
2. They align with the field between the plates
3. Bigger negative charge attracts more electrons onto negative plate
4. Repels more electrons away from positive plate
5. V same but Q has increased

Question 7

What happens if the dielectric is removed?

(Capacitor still connected to battery)

Answer 7

1. Polarised molecules removed
2. Some electrons leave negative plate
3. Attracts more electrons to positive plate
4. Q has decreased but V same
5. C decreases (C=Q/V)

Question 8

What happens if the dielectric is removed?

(When the Capacitor is disconnected from battery)

Answer 8

1. Polarised molecules removed
2. But charge is trapped on plates
3. Same Q but with lower C
4. V increases (V=Q/C)

Question 9

How does this capacitor charge?

(When switch 1 is closed)

Answer 9

1. Electrons flow from the negative terminal of the battery
2. To the connected parallel plate (right plate)
3. Electrons are repelled from the opposite plate (left)
4. And attracted to the positive terminal of the battery
5. Charge across Parallel plates

Question 10

How does this capacitor discharge?

(When switch 2 is closed)

Answer 10

1. Electrons flow from the negative plate (right)
2. Through the resistor
3. To the other plate (left)
4. Decreasing charge difference across plates