capacitors

Question 1

what do capacitors do

Answer 1

store up electrical charge

Question 2

what is a capacitor made of

Answer 2

two conducting plates operated by a gap or dielectric

Question 3

what happens when a capacitor is connected to a power source

Answer 3

positive and negative charge build up on opposite plates

Question 4

what happens when positive and negative charge builds up on the plates of a capacitor

Answer 4

a uniform electric field is created between them

Question 5

what is capacitance

Answer 5

the amount of charge per unit potential difference

Question 6

capacitance equation

Answer 6

C = Q/V
Q is charge in coloumbs
v is potential difference in volts
capacitance in farads

Question 7

what units will you see farads expressed in

Answer 7

uF — microfarads (× 10–6)
nF — nanofarads (× 10–9)
pF — picofarads (× 10–12

Question 8

what is permittivity

Answer 8

Permittivity is a measure of how difficult it is to generate an electric field in a certain material.

Question 9

what is the relative permittivity

Answer 9

The relative permittivity is the ratio of the permittivity of a material to the permittivity of free space:

Question 10

what is relative permittivity known as

Answer 10

dielectric constant

Question 11

what happens when no charge is applied to a capacitor in terms of molecules

Answer 11

Imagine a dielectric is made up of lots of polar molecules — they have a positive end and a negative end.
When no charge is stored by the capacitor, there is no electric field — so these molecules point in a bunch of random directions.

Question 12

what happens when charge is applied to a capacitor in terms of molecules

Answer 12

When a charge is applied to a capacitor, an electric field is generated. The negative ends of the molecules are attracted to the positively charged plate and vice versa. This causes all of the molecules to rotate and align themselves with the electric field. The molecules each have their own electric field, which in this alignment now opposes the
applied electric field of the capacitor. The larger the permittivity, the larger this opposing field is. This reduces the overall electric field, which reduces the potential difference needed to charge the capacitor — so the capacitance increases.

Question 13

how do you work out the capacitance of a capacitor using the dimensions
of the capacitor and the permittivity of the dielectric

Answer 13

C=Aε₀εr/d

Question 14

what happens as charge builds up on the plates of a capacitor

Answer 14

electrical energy is stored by the capacitor

Question 15

on a charge against potential difference graph for a capacitor what is the area under it

Answer 15

energy stored

Question 16

how do you know that the charge against potential difference will be a straight line through orgin

Answer 16

because potential difference across a capacitor is directly proportional to the charge stored on it

Question 17

if there’s a higher capacitance what is true about the energy stored

Answer 17

the energy stored will be higher

Question 18

energy equation for capacitor

Answer 18

E=1/2 QV

Question 19

explain how a capacitor begins charging

Answer 19

As soon as the switch closes, current starts to flow. The electrons flow onto the plate connected to the negative terminal of the dc power supply, so a negative charge builds up.
This build-up of negative charge repels electrons off the plate connected to the positive terminal of the power supply, making that plate positive. These electrons are attracted to the positive terminal of the power supply. An equal but opposite charge builds up on each plate, causing a potential difference between the plates. Remember that no charge can flow between the plates because they’re separated by an insulator (a vacuum, gap or dielectric). As charge builds up on the plates, electrostatic repulsion makes it harder and harder for more electrons to be deposited. When the p.d. across the capacitor is equal to the p.d. across the power supply, the current falls to zero.

Question 20

how to discharge capacitor

Answer 20

Remove the power supply from the test circuit on and close the switch to complete the circuit. Let the capacitor discharge whilst the data logger records potential difference and current over time. When the current through the ammeter and the potential difference across the plates are zero, the capacitor is fully discharged.

Question 21

the time it takes to discharge and charge depends on

Answer 21

1) The capacitance of the capacitor (C). This affects the amount of charge that can be transferred at a given potential difference.
2) The resistance of the circuit (R). This affects the current in the circuit.

Question 22

how would you describe the the growth rate of charge or potential difference

Answer 22

exponential

Question 23

how do you calculate the charge on the plates at a given time after a capacitor begins charging

Answer 23

Q=Q₀(1- e ^-t/RC)

Question 24

how do you calculate the potential difference on the plates at a given time after a capacitor begins charging

Answer 24

V=V₀(1- e ^-t/RC)

Question 25

how do you calculate the current at a given time as a capacitor is charging

Answer 25

I=I₀e ^-t/RC

Question 26

why does it always take same length of time for the charge to halve no matter how much charge you start with

Answer 26

because the charge left on the plates falls exponentially with time as a capacitor discharges

Question 27

how to work out the charge left on the plates as a capacitor discharges

Answer 27

Q=Q₀e ^-t/RC

Question 28

how to work out the potential difference across the plates as a capacitor discharges

Answer 28

V=V₀e ^-t/RC

Question 29

how to work out the current as a capacitor discharges

Answer 29

I=I₀e ^-t/RC

Question 30

what is the time constant

Answer 30

the time constant, is the time taken for the charge, to fall to 37% of its value when fully charged.
or
It’s also the time taken for the charge or potential difference of a charging capacitor to rise to 63% of its value when fully charged.

Question 31

what is true about the time when there is a larger resistance in series with the capacitor

Answer 31

the longer the it takes to discharge or discharge

Question 32

what happens when you take natural log of of both sides of Q=Q₀e ^-t/RC

Answer 32

ln(Q) = (-1/RC)t + ln(Q₀)

Question 33

what is the time to halve

Answer 33

time taken for the charge current or potential difference of a discharging capacitor to reach half of the value it was when it was fully charged

Question 34

time to halve equation

Answer 34

T(1/2)=0.69RC