Capacitance (Unit 4)

Question 1

Series?

Answer 1

-capacitance - 1/c
- voltage- size
-charge - equal

Question 2

Parallel?

Answer 2

-capacitance- c1+c2
- voltage - equal
-charge- add

Question 3

What is a capacitor?

Answer 3

-a pair of conducting plates separated by an insulator

Question 4

How does a capacitor work (store energy)?

Answer 4

-When no p.d. across the capacitor the plates are neutral
and they have an equal number of electrons and positive ions
-When a p.d. is applied to the plates, electrons are forced to move and the plates acquire equal and opposite charge (net charge is zero).This stores energy until the electrons can flow back

Question 5

Definition of capacitance?

Answer 5

C - charge on plate/ pd between plates
C - Q/V

Question 6

Units of capacitance?

Answer 6

• F - CV^-1

Question 7

How to increase capacitance of a vacuum spaced capacitor?

Answer 7

• using a dielectric

Question 8

What is a dielectric?

Answer 8

• an insulator between the plates of the capacitor

Question 9

Electric field strength units?

Answer 9

Vm^-1

Question 10

Field lines in a capacictor?

Answer 10

• equal and parallel
• uniform in between plates

Question 11

How is a capacitor charged?

Answer 11

• electrons move from negative to positive around the circuit
  -the electrons are deposited on plate A, making it negatively charged
  -electrons travel from plate B to the positive terminal of the battery, giving the plate a positive charge
  -electrons build up on plate A and an equal amount of electrons are removed from plate B, creating a PD across the plates
  -when the PD across plates= source PD, the capacitor is fully charged and current stops flowing

Question 12

How does PD across a capacitor change, when it discharges across a resistor?

Answer 12

-electrons move in opposite direction than when the capacitor was charging up
-charge on plate A decreases as it loses electrons and plate B gains electron, so A and B become neutral
-PD decreases exponentially across the plates

Question 13

Equation for charging a capacitor?

Answer 13

Q=Qo(1−e^−t/RC)

Question 14

Equation for discharging a capacitor?

Answer 14

Q=Qoe^-t/RC

Question 15

What is the time constant?

Answer 15

• RC
• time for charge to decrease to 37% of it’s original value

Question 16

Investigation of the charging and discharging of a capacitor to determine the time constant practical ?

Answer 16

• set up prac
• start stopwatch
• record initial voltage
• record voltage every 5 seconds until voltage reaches 0v
• plot a graph voltage over time
• find what the time when the voltage is 37 percent of the initial
  -this gives the time constant

Question 17

What does the investigation of the charging and discharging of a capacitor to determine the time constant practical look like?

Answer 17

• Battery connected to parallel with a capacitor first then a resistor connected to a voltmeter
  -when charging capacitor switch makes first loop closed
• when discharging first loop is open

Question 18

Investigation of the Energy Stored in a Capacitor practical?

Answer 18

-Charge the capacitor by closing the switch and then record the
reading on the voltmeter.
- Repeat this for different capacitors so that the effect of
capacitance on energy stored can be evaluated.
- The energy stored by the capacitor (E) is found by E=½ CV 2
where C is capacitance and V is the voltage across the
capacitor.
- Work out the energy stored by each of the capacitors

Question 19

Investigation of the Energy Stored in a Capacitor what does it look like?

Answer 19

• battery connected to capacitor and voltmeter in parallel with switch in series for first loop

Question 20

V over t graph for discharging of a capacitor through a fixed resistor?

Answer 20

• downwards slide hitting x axis and levelling off

Question 21

I over t graph for discharging of a capacitor through a fixed resistor?

Answer 21

• downwards slide hitting x axis and levelling off

Question 22

Q over t graph for discharging of a capacitor through a fixed resistor?

Answer 22

• log graph