6.1 Capicitance

Question 1

What is a Capacitor?

Answer 1

Two metal plates of opposing charges, separated by a dielectric sandwiched and rolled up

Question 2

How does a Capacitor Work?

Answer 2

Electrons flow towards the positive flow of the power supply, the plates gain an equal and opposite charge as they lose and gain an equal amount of electrons

Question 3

What is Capacitance?

Answer 3

The charge stored per unit potential difference

Question 4

What is a Farad?

Answer 4

Charge per potential difference/volt
The unit of capacitance

Question 5

What Happens if too much Charge is Added to a Ceramic Capacitor?

Answer 5

The dielectric breaks down and allows a current to conduct through it so the capacitor is destroyed

Question 6

Why must the Voltage supplied to a Capacitor be Polarised?

Answer 6

One specified terminal must always have positive potential with respect to the other

Question 7

What is the Time Constant of a Circuit?

Answer 7

The time constant (τ) is the time taken for the charge of a capacitor to fall to 1/e of its initial value and is the product of Capacitance and Resistance

Question 8

What relationship does discharging a Capacitor have?

Answer 8

Exponential decay

Question 9

What does the Capacitance of a Parallel Plate depend on?

Answer 9

The separation of the plates (d)
The area of overlap (A)
The dielectric between them

Question 10

What is Electric Potential Energy (E)?

Answer 10

The external energy required (work done) to bring a charge from infinity to that point in an electric field

Question 11

What is Electric Potential (V)?

Answer 11

The work done by an external force per unit positive charge to bring a charge from infinity to a point in an electric field

Question 12

What are Uses of Capacitors?

Answer 12

Camera Flash
Particle Accelerators
Backup supply for computers and emergency lighting when power briefly cuts

Question 13

What’s a Smoothing Capacitor?

Answer 13

Changes alternating currently to direct current
• Rectifier Circuit changes alternating to smooth direct circuit as the diode allowed current in one direction but the capacitor means that instead of just positive cycles it smooths the current to a near constant voltage

Question 14

How do you find the Permittivity of a Dielectric other than Free Space?

Answer 14

ε=ε(r)ε(0)
Where:
ε = Permittivity of Dielectric
ε(r) = Relativity Permittivity of Dielectric
ε(0) = Permittivity of Free Space

ε(r) can be found by finding the charge stored by the dielectric divided by the charge stored by a dielectric of free space

Question 15

What is the Equation for the Total Capacitance Stored in Parallel?

Answer 15

C(Total) = C(1) + C(2) + …

Question 16

What’s the Equation for Total Capacitance in Series?

Answer 16

1/C(Total) = 1/C(1) + 1/C(2) + …

Question 17

What does the Area under a Graph of Charge against PD equal?

Answer 17

Energy stored by the capacitor

Question 18

What is the Process of Charging a Capacitor?

Answer 18

1. Electrons move from negative to positive around the circuit
2. The electrons are deposited on plate A, making it negatively charged
3. Electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge
4. Electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a potential difference across the plates
5. When the p.d across plates = source p.d., the capacitor is fully
   charged and current stops flowing