Capacitors

Question 1

What happens to the p.d. across a capacitor as it discharges

Answer 1

decreases

Question 2

what happens to the resistance of an NCT thermistor when the temperature increases

Answer 2

decreases

Question 3

how to tell if a graph shows exponential decay

Answer 3

y decreases to 0.37 of its initial value after every time constant

Question 4

how to add capacitances of capacitors in series

Answer 4

1/C = 1/C1 + 1/C2 + …

Question 5

how to add capacitances of capacitors in parallel

Answer 5

C = C1 + C2 + …

Question 6

time constant

Answer 6

the time taken for the V, I or Q to decrease to 1/e of its initial value

Question 7

how both plates (X and Y) of a capacitor acquire equal but opposite charge

Answer 7

electrons are removed from X. X becomes positive. The size of charge is the same because an equal number of electrons are removed and deposited on the plates

Question 8

area under V-Q graph

Answer 8

work done

Question 9

what happens to current as capacitor charges

Answer 9

decreases

Question 10

what happens to p.d. of capacitor as capacitor charges

Answer 10

increases

Question 11

what happens to charge as it charges

Answer 11

increases

Question 12

how to charge a capacitor

Answer 12

connect the capacitor to a DC voltage source like a battery or a cell

Question 13

2 applications of a capacitor

Answer 13

camera flashes, converting alternating current into direct current

Question 14

where the initial energy stored in the capacitor is dissipated when it discharges

Answer 14

dissipated as heat in the resistor

Question 15

Explain in terms of movement of electrons how p.d across capacitor plates acquire an equal but opposite charge. (2)

Answer 15

• Electrons move from negative to positive terminals
• Equal number of electrons removed from one plate and deposited on the other

Question 16

Describe a lab experiment to determine the time constant of a capacitor-resistor circuit. (5)

Answer 16

• Capacitor connected to a power supply
• Power supply disconnected
• p.d. across capacitor is measured by a voltmeter
• time of discharge recorded with a stopwatch
v initial p.d. is time constant

Question 17

Explain in terms of movement of electrons how plates acquire an equal but opposite charge. (3)

Answer 17

• Electrons removed from one plate & deposited on other
• Equal number removed & deposited
• One plate becomes positively charged and the other negative

Question 18

Explain the effect on initial rate of discharge of a capacitor when two resistors are added to the circuit in parallel. (2)

Answer 18

• total resistance of circuit is halved
• therefore initial rate is doubled

Question 19

Explain how the plates of a capacitor become charged in terms of movement of charged particles. (2)

Answer 19

• Electrons flow from negative to positive terminals
• Electrons deposited on one plate and electrons removed from the other plate

Question 20

Describe and explain in terms of movement of electrons how p.d. across a capacitor changes as it discharges. (3)

Answer 20

• Electrons flow from positive to negative terminals.
• Charge on plates decreases
• p.d. decreases exponentially

Question 21

What happens if two or more capacitors are in parallel ?

Answer 21

p.d. is the same across each capacitor, total charged stored is equal to sum of individual charges, the total capacitance is equal to the sum of all the individual capacitances.

Question 22

For two or more capacitors in series what is the total p.d equal too ?

Answer 22

According to kirchhoffs second law it is equal to the sum of the individual p.d.s across the capacitors.

Question 23

what happens when a capacitor is connected to a cell?

Answer 23

electrons flow from the cell for a very short time. they cannot travel between the plates because of the insulation. the very brief current means electrons are removed from one plate and at the same time electrons are deposited onto the other plate. The plate on the positive side becomes deficient in electrons and acquires a positive charge.

Question 24

what happens when the pd across the plates is equal to the emf of the cell?

Answer 24

the current in the circuit falls to zero, the capacitor is fully charged and the net charge on the plates is zero.

Question 25

what is the time constant of a capacitor?

Answer 25

the product of capacitance and resistance, if is equal to the time taken for the pd to decrease to e^-1 of its initial value

Question 26

how do you investigate capacitors in series?

Answer 26

set the variable resistor to a high resistance and record it, close the switch and the capacitors will begin charging, record the initial value of the current in the circuit. then record the pd over time and constant adjust the variable resistor to keep the charging current constant for as long as you can, when the current drops to zero the capacitors are fully charged and then you can open the switch.

Question 27

how do you set up the circuit to investigate capacitors in parallel?

Answer 27

put 3 capacitors in parallel then a voltmeter across the third one. then put an ammeter, variable resistor, switch and battery in series with the voltmeter.

Question 28

how do you investigate charging and discharging capacitors?

Answer 28

close the switch so the capacitor begins to charge then record values on the ammeter and voltmeter. when the current on the ammeter is 0 the capacitor is fully charged then open the switch and take the same values across the time period to get the discharging values. when you plot a graph you should see a graph that increases then plateaus and then a graph that is exponential decay.