Capacitors

Question 1

What is a capacitor

Answer 1

an electrical component which stores charge

Question 2

Define Capacitance , C

Answer 2

the amount of charge stored per unit pd by a capacitor

Question 3

State the equation for capacitance

Answer 3

C= Q/V

c- capacitance (F)

q - charge (C)

v - pd (V)

Question 4

What are capacitors made up of?

Answer 4

theyre made up of two conductors seperated from one another by a gap which often contains an insulating material ( dielectric)

Question 5

Describe the charging process of a capacitor with reference to the flow of electrons

Answer 5

1. As soon as you close the switch, electrons flow onto the plate connected to the negative terminal of the power source, so a negative charge builds up
2. build-up of negative charge causes electrons to be repelled off the plate connected to the positive terminal, making that plate positive (those electrons will attract towards the positive terminal)
3. An equal but opposite charge builds up on each plate causing a potential difference between the plates.
4. initially current through circuit is high but as charge builds up on the plates, electrostatic repulsion makes it harder for more electrons to be deposited
5. when p.d across plates​ = to source pd, the capacitor is fully charged and current stops flowing

Question 6

What is the equation for the total capacitance of two or more capacitors in parallel?

Answer 6

Ctotal = C1 + C2 +..

Question 7

What is the equation for the total capacitance of two or more capacitors in series?

Answer 7

1/Ctotal = 1/C1 + 1/C2 +..

Question 8

What does the area under the graph of pd against charge represent?

Answer 8

the energy stored by a capacitor

Question 9

State the 3 equations to find the energy stored by a capacitor

Answer 9

W= ½QV

W=½Q²/C

W=½V²C

Question 10

True of False : Capacitors can only store relatively small quantities of energy compared to batteries

Answer 10

true

Question 11

State and explain an everyday use of capacitors

Answer 11

flash photography - when you take a picture , the capacitor has to discharge really quickly to give a short pulse of high current to create a brief, bright flash

Question 12

What will you find between the two charged plates?

Answer 12

an electric field

Question 13

How is a uniform electric field created between the plates?

Answer 13

when a capacitor is connected to a power source, positive and negative charge builds up on opposite plates. The insulating material (which could be an air gap) stops charge moving between the two plates creating a potential difference which creates a uniform electric field

Question 14

Usually when charging or discharging a capacitor you will have a fixed resistor within the circuit , what is the effect of this resistor?

Answer 14

the resistor slows down the process of charging/discharging

Question 15

Describe an experiment to investigate capacitors in series and parallel using ammeters and voltmeters.

Answer 15

1. Connect some capacitors in series along with a variable resistor and a switch then close the switch
2. constantly adjust the variable resistor to keep the charging current constant for as long as you can
3. a data logger connected to a voltmeter can be used to record the p.d over time.Once fully charged open the switch.
4. ensure capacitors have discharged then connect them in parallel then close switch and repeat steps 2 and 3
5. You can then plot graphs of current against time and charge against pd

Question 16

What is the time constant?

Answer 16

the time taken for the charge, p.d or current in a discharging capacitor to fall to 37% of its initial value

its also: the time taken for the charge or p.d of a charging capacitor to rise to 63% of its maximum value

Question 17

What 2 factors does the time taken for a capacitor to charge or discharge depend on?

Answer 17

1. the capacitance of the capacitor
2. the resistance of the circuit

τ=CR

Question 18

What equations do we require for charging a capacitor?

Answer 18

Q = Q0(1 - e^-t/RC) & V = V0 (1 - e^-t/RC) where V0 is the battery PD and Q0=CV0

Question 19

Demonstrate what the Q-t , V-t and I-t graphs for a charging capacitor look like

Answer 19

Question 20

What shape does a Q-t , V-t or I-t graph for a discharging capacitor have?

Answer 20

exponential decay