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Physics > P2 - Electicity > Flashcards

P2 - Electicity Flashcards

To revise/learn topic p2

1
Q

What is a diode?

A

An electrical component which can be used to regulate the potential difference is circuits and to make logic gates.

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2
Q

What does LED stand for?

A

Light Emitting Diode

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3
Q

When do LEDs produce light?

A

When the current flows through them in a forward direction.

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4
Q

What are LEDs used for?

A

They are often used for indicator lights In electrical equipment, such as computers and television sets.

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5
Q

Why is the use of LEDs increasing?

A

They use a much smaller current than other types of lighting

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6
Q

Describe the resistance and direction of flow in a diode.

A

It has very high resistance in one direction, which means that current can only flow in the other direction

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7
Q

What are thermistors?

A

temperature-dependent resistors

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8
Q

What does LDR stand for?

A

Light dependant resistor

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9
Q

In the most common type of thermistor, what happens to the resistance when the temperature increases?

A

It decreases - the current can now flow through it.

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10
Q

What happens to the resistance of LDRs as the light intensity increases?

A

It decreases, allowing more current to flow through it.

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11
Q

Give some applications of thermistors

A

Fire alarms, car engine temperature sensors and electronic thermostats

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12
Q

Give two equations to find the charge flow (Q).

A

Charge flow = current x time (Q = I t)

Charge flow = energy transferred / voltage (Q = E/V)

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13
Q

Give the equation that connects current (I) , charge flow (Q) and time (t).

A

Q = I t

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14
Q

What is the equation that connects resistance, potential difference and current in an ohmic resistor.

A

V = I * R

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15
Q

Give the equation that connects power, energy transferred and time

A

E = P * t

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16
Q

Give the equation that connects potential difference, charge flow and energy transferred.

A

E = Q * V

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17
Q

Give three formulas to find the power of a component in watts

A 
P = E / t
P = VI
P = I^2 * R
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18
Q

What is the difference between a fuse and a circuit breaker?

A

A fuse has a wire that blows every time the current is too high. A circuit breaker is mechanical and when the current is too high, a magnet flips a switch that breaks the circuit.

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19
Q

What are the advantages and disadvantages of a fuse?

A

Advantages: It is cheap and stops the current quickly.

Disadvantages: once they blow you have to replace them.

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20
Q

What are the advantages and disadvantages of a circuit breaker.

A

Advantages: they can be reset and don’t have to be replaced.

Disadvantages: they are slower to stop the current and are mechanical so can be broken easier. They are also more expensive

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21
Q

What are two ways to increase the magnetism of a solenoid?

A

Increase the current and the number of coils.

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22
Q

Is magnetic charge stronger on flat areas or pointed ones?

A

Pointed ones.

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23
Q

what is current?

A

the flow of electrical charge

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24
Q

what is the unit of current?

A

ampere, A

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25
Q

what is potential difference?

A

the driving force that pushes the charge round

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26
Q

what is the unit of potential difference?

A

the volt, V

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27
Q

what is resistance?

A

anything that slows the flow down

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28
Q

what is the unit of resistance?

A

Ohm, Ω

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29
Q

what does total charge through a circuit depend on?

A

current and time

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30
Q

does the circuit symbol for a cell have one or two?

A

1

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31
Q

does the circuit symbol for a battery have one or two?

A

2

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32
Q

what do ohmic resistors have?

A

a constant resistance

33
Q

what is an I-V characteristic?

A

a graph which shows how the current (I) flowing through a component changes as the potential difference (V) across it increases

34
Q

what type of components have an I-V characteristic that’s a straight line?

A

linear components (e.g. a fixed resistor)

35
Q

what type of components have curved I-V characteristics?

A

non-linear components (e.g. a filament lamp or a diode)

36
Q

describe the I-V characteristic of a Ohmic resistor

A

a straight, diagonal line passing through the origin

37
Q

describe the I-V characteristic of a filament lamp

A

a curved line that forms a sort of S shape and passes through the origin

38
Q

describe the I-V characteristic of a diode

A

a line along the x-axis (V) until just after it passes through the y-axis, then it curves sharply up before straightening into a straight line

39
Q

explain the I-V characteristic of a filament lamp

A

as the current increases the temperature of the filament increases, so the resistance increases. This means less current can flow per unit pd, so the graph gets shallower, hence the curve

40
Q

explain the I-V characteristic of an ohmic resistor

A

at a constant temperature the current through an ohmic conductor is directly proportional to the potential difference so you get a straight line

41
Q

explain the I-V characteristic of a diode

A

Current will only flow through a diode in one direction. The diode has a very high resistance in the reverse direction

42
Q

how can you calculate the resistance at any point on the I-V characteristic?

A

using the formula R = V / I (V=IR)

43
Q

what are some applications of LDRs

A

automatic night lights, outdoor lighting and burglar detectors

44
Q

what can sensing circuits be used for?

A

to turn on or increase the power to components depending on the conditions that they are in

45
Q

the bigger a components resistance, the bigger/smaller its share of the potential difference.

A

bigger

46
Q

how is energy transferred in a kettle?

A

kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle

47
Q

how is energy transferred in a handheld fan?

A

Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor

48
Q

with a higher current is more or less energy transferred to the thermal energy stores of the components (and then the surroundings)?

A

more

49
Q

what does the total energy transferred by an appliance depend on?

A

how long the appliance is on for and its power

50
Q

what is the power of an appliance?

A

the energy that it transfers per second

51
Q

what is a power rating? what does it tell you?

A

appliances are often given a power rating - they’re labelled with the maximum safe power that an appliance can operate at. You can usually take this to be their maximum operating power. The power rating tells you the maximum amount of energy transferred between stores per second when the appliance is in use

52
Q

how does the power rating help customers choose between models?

A

the lower the power rating the less electricity an appliance uses in a given time and so the cheaper it is to run.
However, a higher power doesn’t necessarily mean that it transfers more energy usefully. An appliance may be more powerful than another, but less efficient, meaning that it may only transfer the same amount of energy (or even less) to useful stores.

53
Q

what is energy transferred per charge passed?

A

potential difference

54
Q

what happens when an electrical charge goes through a change in potential difference?

A

energy is transferred

55
Q

what is the national grid?

A

a giant system of cables and transformers that covers the UK and connects power stations to consumers

56
Q

what are some events that could cause the demand for electricity to increase?

A

demand increases when people get up in the morning, come home from school or work, and when it starts to get dark or cold outside. popular events like a sporting final being shown on TV could also cause a peak in demand.

57
Q

what are some measures power stations have in place to cope with an unexpectedly high demand?

A

power stations often run well below their maximum power output, so there’s spare capacity to cope with a high demand, even if there is an unexpected shut down of another station. Lots of smaller power stations that can start up quickly are also kept on standby, just in case

58
Q

does the national grid use a high or low potential difference? what about current?

A

it uses a high pd and a low current

59
Q

why does the national grid use a high potential difference and a low current?

A

to transmit the huge amount of power needed, you need either a high potential difference or a high current (P=VI). A higher current would mean losing a lot of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings, so it’s much cheaper to boost the potential difference up really high and keep the current as low as possible. This makes the national grid an efficient way of transferring energy.

60
Q

what is the potential difference in the wires of the national grid?

A

400,000 volts

61
Q

what is used in the national grid to get the voltage up to 400,000 volts?

A

step-up transformers (and big pylons with huge insulation)

62
Q

what do all transformers have?

A

two coils, a primary coil and a secondary coil, joined with an iron core

63
Q

how do step-up transformers work?

A

step-up transformers have more turns on the secondary coil than the primary coil. As the pd is increased by the transformer, the current is decreased

64
Q

do step down transformers have more turns on the primary or secondary coil?

A

primary

65
Q

what is the equation linking the pd and current of both the coils in a transformer? Why is this?

A

pd across primary coil (V) x current in primary coil (A) = pd across secondary coil (V) x current in secondary coil (A)
VpIp = VsIs
this is because transformers are nearly 100% efficient, so the power in primary coil = power in secondary coil

66
Q

how efficient are transformers?

A

nearly 100%

67
Q

is the power in the primary coil greater than, equal to or less than the power in the secondary coil?

A

equal to

68
Q

why does a moving charge transfer energy?

A

the charge does work against the resistance of the circuit (work done is the same as energy transferred)

69
Q

what are electrical appliances designed to do?

A

electrical appliances transfer energy between stores electrically - they are designed to transfer energy to components in the circuit when a current flows

70
Q

give the equation to work out the amount of energy transferred by electrical work

A 
energy transferred (J) = Power (W) x Time (s) 
(E = Pt)
71
Q

how does a power source work?

A

energy is supplied to the charge at the power source to ‘raise’ it through a potential. The charge then gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit. A battery with a bigger pd will supply more energy to the circuit which flows round it, because the charge is raised up ‘higher’ at the start

72
Q

what is the equation to work out energy transferred using charge flow and potential difference?

A 
Energy transferred (J) = charge flow (C) x potential difference (V)
(E = QV)
73
Q

give the formula to find power using potential difference and current. Include units

A

Power (W) = Potential difference (V) x Current (A)

P = VI

74
Q

give the equation to find power using current and resistance

A

power (W) = current (A)^2 x resistance (ohms)

P = I^2 x R

75
Q

how is electricity distributed around the UK?

A

through the national grid

76
Q

what does the national grid do?

A

the national grid transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the grid where it’s needed (the demand) - e.g. homes and industry

77
Q

what are step-down transformers used for?

A

to increase the current and decrease the potential difference of electricity transferred over the national grid in order to make it safe for the local consumer

78
Q

describe an experiment to find a component’s I-V characteristic

A
  1. set up a circuit with an ammeter, a variable resister, and the component you want to test in series, and a voltmeter connected in parallel around the component.
  2. Begin to vary the variable resister. This alters the current flowing through the circuit and the potential difference across the component
  3. take several pairs of readings from the ammeter and voltmeter to see how the potential difference across the component varies as the current changes. Repeat each reading twice more to get the average pd at each current
  4. swap over the wires connected to the cell, so the direction of the current is reversed, and repeat the experiment with a negative current
  5. plot a graph of the current against voltage for the component.
79
Q

how can you calculate the resistance at any point on an I-V characteristic?

A

since V = IR, you can calculate the resistance at any point on an I-V characteristic by calculating R = V/I

Physics (13 decks)

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