21. capacitance

Question 1

what is a capacitor?

Answer 1

a capacitor is an electrical component that stores electrical charge in a circuit

Question 2

what is capacitance

Answer 2

the capacitance of an object is a measure of the amount of charge stored in the object per potential difference used to store it

Question 3

what equation is used for the definition of a capacitor

Answer 3

C=Q/V

Question 4

why are capacitors useful?

Answer 4

• can store energy when connected to charging circuit
• can dissipate stored energy when disconnected from circuit

TEMPORARY BATTERY!

Question 5

how are capacitors commonly used in electrical devices

Answer 5

it maintains power supply while batteries are being changed

this prevents loss of information in volatile memory

Question 6

what’s the difference between batteries and capacitors (ENERGY STORES)

Answer 6

• capacitor stores potential energy in the electric field
• battery stores potential energy in the chemical store

Question 7

what’s the difference between batteries and capacitors (SIZE)

Answer 7

• capacitors can store relatively little charge
• batteries can store relatively large amounts of charge

Question 8

what’s the difference between batteries and capacitors (SPEED)

Answer 8

• capacitors charge and discharge quickly
• batteries charge and discharge slowly

Question 9

give some uses of a capacitor

Answer 9

• defibrillator: quick discharge in electric shock
• camera flash: quick discharge causing light
• ignition switch: quick discharge leading to starter circuit of motor

Question 10

what is the Farad (F) equal to

Answer 10

CV^-1

Question 11

what is the capacitance of 1 farad?

Answer 11

when 1 couloomb of charge stored due to a potential difference of 1V being applied across a capacitor

Question 12

what are 2 very important consideration of C=Q/V

Answer 12

• V (potential difference) is the pd across the capacitor, not the circuit
• charge cannot be created or destroyed, so charge will also be the charge of the circuit as the capacitor is charging or discharging

Question 13

what does a capacitor consist of?

Answer 13

two conducting parallel plates, seperated by a gap. these parallel plates will produce a uniform electrical field between them.

Question 14

what happens when a capacitor is connected to a direct current source

Answer 14

1. charge builds up at the plates
2. the plate connected to the -ve terminal of the power supply gains electrons MAKING IT NEGATIVELY CHARGED
3. the plate connected to +ve terminal loses electrons MAKING IT POSITIVELY CHARGED.
4. the movement of electrons happens because they are REPELLED by the negative plate, and ATTRACTED to the positive terminal
5. this will only happen if the current which flows through the capacitor is direct

Question 15

what do the charges due to the potential difference in the plates

Answer 15

charged object would have to do work to move between the plates

Question 16

how does the potential difference between plates form

Answer 16

electrons cannot travel between the plates as the air gap is a poor conductor, this allows for a potential difference to form.

Question 17

what happens if you improve the insulator between the plates?

Answer 17

a greater charge imbalance can form between the two plates

this would increase the capacitance or charge stored

because there is less chance of electrons travelling between the plates

Question 18

how to increase the potential difference across plates?

Answer 18

use a material less conductive than air. this material is called a dialectric.

Question 19

what force will any charged particle in the dialectric experience

Answer 19

electrostatic.

it is a charged object in an electrical field, and therefore becomes an electrical potential store.

Question 20

how is the energy from a capacitor released

Answer 20

moving electrons around the circuit

Question 21

what happens when a capacitor is discharged?

Answer 21

• current in the circuit increases (as there are more electrons flowing with greater energy)
• potential difference across the capacitor decreases (as there is less electric potential energy stored in the dialectric)

Question 22

what does the charge imbalance between the plates do?

Answer 22

• forms a UNIFORM electric field between them
• makes the two plates equally but oppositely charged

Question 23

what is the capacitance between two plates if the insulator is air/vacuum?

like a symbol

Answer 23

C₀

C is the symbol for capacitance

Question 24

what are common examples of dialectric materials

Answer 24

wax paper
polythene

Question 25

what is permittivity

Answer 25

the measure of how difficult it is to generate a uniform electric field in a certain material

Question 26

what is the permittivity of free space

Answer 26

how difficult it is for a vacuum to generate a uniform electric field. this is a physical constant ε₀=8.85x10^-12 Fm^-1

Question 27

what is relative permittivity

Answer 27

how well a material is to generate an electric field **COMPARED TO** how well free space generates an electric field εr = Q/Q₀ εr = C/C₀ εr = ε/ε₀ IT HAS NO UNITS

Question 28

what is another name for relative permittivity

Answer 28

dialectric constant

Question 29

what value is directly proportional to the capacitance of the plate

Answer 29

Area of the plate (m^2)

Question 30

what value is inversely proportional to the capacitance of the plate

Answer 30

distance between the plates (m)

Question 31

when a capacitor is charged, which way will polarised molecules face? what other interesting feature do they have?

Answer 31

the positive of the molecule will face towards the negatively charged plate (and vice versa) they will produce their own electrical field, as they are charged particles. THIS IS IN THE OPPOSITE DIRECTION OF TO THE CAPACITOR

Question 32

what is a practical investigation you could do to observe how a capacitor charges and discharges?

Answer 32

1. connect capacitor, power pack and resistor in series circuit 2. make sure that longer leg of capacitor is connected to the positive terminal of the power supply 3. find the resistance of the resistor by the banding on its side 4. place multimeter in parallel with capacitor (set to 20V) 5. record the potential difference across the capacitor every 5s for 120s, this is charging the capacitor. start at t=0 6. after 120s, turn off power pack. 7. record pd across capacitor every 5s for 120s. this is discharging the capacitor

Question 33

what does a graph of (v or q) against t look like/ show

Answer 33

**looks like:** arc through origin, high gradient which then goes to flat line **shows:** potential difference increases exponentially when the capacitor is being charged. this is an example of exponential decay, as the rate of change decreases over time.

Question 34

what does the graph of I against t look like/ show?

Answer 34

**looks like:** L shape but more curved **shows:** as more charge is stored on the capacitor, there is less current in the circuit. **IMPORTANT NOTE:** this is current measured in the circuit, NOT THE CAPACITOR

Question 35

how to know when capacitor is fully charged

Answer 35

ammeter reads zero max num of electrons on the plates, no more charge can move onto the plates, making the current drop to zero

Question 36

what is the equation for charging a capacitor (for v and q)

Answer 36

V= V0 (1 - e^(t/CR) ) Q= Q0 (1 - e^(t/CR) )

Question 37

equation used for the discharge of the capacitor for V, Q, and I

Answer 37

V= V0 e^(t/CR) Q= Q0 e^(t/CR) I = I0 e^(t/CR)

Question 38

what is time constant

Answer 38

time taken for a value to fall by 63% Time Constant = C x R

Question 39

what equation can be found from a V against Q graph

Answer 39

E=1/2 QV E= area under the graph