Capacitors

Question 1

What is capacitance?

Answer 1

The amount of charge an object is able to store per unit potential difference across it

Question 2

What is the unit for capacitance?

Answer 2

Farads

Question 3

What are capacitors made up of?

Answer 3

Two electrical conducting plates separated by an electric insulator(dielectric)

Question 4

Describe the current-time graph for a capacitor

Answer 4

• Current in microamps
• As time increases, current stays the same
• Area under the graph is charge stored on plates, Q = It

Question 5

Describe the charge-p.d graph for a capacitor

Answer 5

• Charge in microcoulombs
• Charge calculated by Q = It
• Q and p.d are directly proportional
• Gradient of the graph = Capacitance

Question 6

State 2 applications of a capacitor and describe how they works

Answer 6

• Camera flash: camera battery charges the capacitor for a few seconds and all the charge is put into the flash almost instantly
• Power supplies: The capacitor smooths out the variations in the current supplied

Question 7

What happens to the plates of a capacitor while it charges?

Answer 7

One becomes positively charged while the other becomes negatively charged

Question 8

Derive the energy stored equation for a capacitor

Answer 8

• charge-p.d graph is directly proportional
• Area under graph = Energy stored by capacitor
• Area of a triangle = 1/2 x base x height
• Energy stored by capacitor = 1/2QV

Question 9

What is permittivity?

Answer 9

A measure of how difficult it is to generate an electric field in a medium

Question 10

What are polar molecules?

Answer 10

Molecules with one end being positively charged and the other end being negatively charged

Question 11

Describe the alignment of molecules in a dielectric without an electric field

Answer 11

The molecules are aligned randomly throughout the dielectric

Question 12

Describe the alignment of molecules in a dielectric with an electric field

Answer 12

• The negative ends of the molecules are attracted to the positively charged plate
• The positive ends of the molecules are attracted to the negatively charged plate
• Each molecule has its own electric field, which opposed the applied electric field of the capacitor

Question 13

Describe the flow of electrons as a capacitor charges

Answer 13

• Electrons flow from the negative terminal of the supply onto the plate connected, building a negative charge on the plate
• This repels the electrons on the other plate, moving them to the positive terminal of the supply, making that plate positive

Question 14

Describe the current-time graph for a charging capacitor through a fixed resistor

Answer 14

• y intercept = Initial current
• As time increases, current decreases
• L shaped

Question 15

Describe the p.d-time graph for a charging capacitor through a fixed resistor

Answer 15

• As time increases, voltage increases
• Exponential for max voltage
• r shaped

Question 16

Describe the charge-time graph for a charging capacitor through a fixed resistor

Answer 16

• As time increases, charge increases
• Exponential for max charge
• r shaped

Question 17

Describe the discharging current-time graph for a discharging capacitor through a fixed resistor

Answer 17

• y intercept = initial charge
• As time increases, current decreases
• L shaped

Question 18

Describe the p.d-time graph for a discharging capacitor through a fixed resistor

Answer 18

• y intercept = initial p.d
• As time increases, p.d decreases
• L shaped

Question 19

Describe the charge-time graph for a discharging capacitor through a fixed resistor

Answer 19

• y intercept = initial charge
• As time increases, charge decreases
• L shaped

Question 20

State the log-linear equation for charge of a discharging capacitor

Answer 20

ln(Q) = (-1/RC)t + ln(Qo)
- y axis = ln(Q)
- gradient = (-1/RC)
- x axis = t
- y intercept = ln(Qo)

Question 21

What is the time constant?

Answer 21

• When t = RC
• Also the time taken for the charge of a discharging capacitor to fall to about 37% of initial charge

Question 22

State the time to halve equation

Answer 22

T(1/2) = ln(2)RC = 0.69RC

Question 23

Derive the equation for the time to halve

Answer 23

• Q = 1/2Qo = Qoe^t/RC
• 1/2 = e^-t/RC
• ln(1/2) -> ln(2^-1) -> -ln(2) = -t/RC
• t = ln(2)RC