Capacitors

Question 1

what is a capacitor

Answer 1

• electrical component that stores charge between 2 plates
• insulator called dielectric placed between to prevent charge from travelling across gap

Question 2

capacitance

Answer 2

charge stored per unit potential difference across 2 plates

Question 3

relative permittivity

Answer 3

ratio of charge stored with the dielectric between the plates to the charge stored when the dielectric is not present

the greater the relative permittivity the greater the capacitance of the capacitor

er = Q/Q0

Question 4

area under the graph of charge against pd represent

Answer 4

energy stored by the capacitor

Question 5

describe the Q against t graph for the discharging of a capacitor through a resistor

Answer 5

exponential decay curve

Question 6

describe the v against t graph for discharging of a capacitor through a resistor

Answer 6

exponential decay curve

Question 7

describe the I against t graph for the discharging of a capacitor through a resistor

Answer 7

exponential decay curve

Question 8

describe the Q against t graph for the charging of a capacitor through a fixed resistor

Answer 8

curves up and over

Question 9

describe the v against t graph for the charging of a capacitor through a resistor

Answer 9

curves up and over

Question 10

time constant

Answer 10

time it takes for the charge in a capacitor to fall to 37% of the initial value

given by RC

a capacitor is considered fully charged after 5 time constants

Question 11

how was 37% derived when using the time constant

Answer 11

• start with formula Q=Q0E^-t/RC
• when t = rc the formula becomes Q=Q0E^-1
• E^-1 = 0.37 which is where 37% came from

Question 12

half time of a capacitor

Answer 12

T1/2 = 0.69RC

Question 13

what equations do we require for charging a capacitor

Answer 13

Q = Q0(1-E^-t/rc)

Question 14

how does a capacitor charge

Answer 14

• electrons move from negative to positive around the circuit
• 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 15

describe and explain in terms of the movement of electrons how the pd across a capacitor changes when it discharges across a resistor

Answer 15

• electrons move in opposite direction than when the capacitor was charging up
• charge on one plate A decreases as it loses electrons, and plate B gains electrons, neutralising them
• pd decreases exponentially across the plates

Question 16

use of capacitors

Answer 16

plash photograph, nuclear fusion, backup power supplies

Question 17

expressions fro energy stored by a capacitor

Answer 17

E = 1/2(Q^2/c) = 1/2(QV) = 1/2(CV^2)

Question 18

what factors affect the time taken for a capacitor to charge or discharge

Answer 18

• the capacitance of the capacitor. this affects the amount of charge that can be stored by the capacitors at any given potential difference across it
• the resistance of the circuit. this affects the current in the circuit and how quickly it flows hence how quickly the capacitor charges or discharges