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edexcel igcse physics > unit two: electricity > Flashcards

unit two: electricity Flashcards

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1
Q

current unit

A

amperes (A)

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2
Q

charge unit

A

coulomb (C)

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3
Q

resistance unit

A

ohm

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4
Q

voltage unit

A

volts (V)

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5
Q

power unit

A

watts (W)

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6
Q

purpose of the earth wire

A

to protect you if an appliance develops a fault by. providing a path fo current to escape through the user

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7
Q

voltage supplied to homes in the UK

A

230 V

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8
Q

what is a fuse

A

a cylinder or cartridge whcih obtains a thin piece of wire made from a metal with a low melting point

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9
Q

what is the fuse there for

A

if there is too large a current then the fuse wire becomes very hot and melts. the circuit is now incomplete and reduces the chance of getting a shock or electrical fire

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10
Q

size of fuses

A

3A
5A
13A

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11
Q

why do many appliances have a metal casing

A

if the live wire becomes damaged or breaks and comes into contact with the casing the earth wire provides a low resistance path for the current, which would probably be big enough to blow the fuse and turn the circuit off.

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12
Q

where should a switch always be placed and why

A

in the live wire so when the switch is o pen no energy in the form of electricity can reach an appliance. if the switch is placed in the neutral wire, energy can still enter a faulty appliance and possibly cause an electric shock

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13
Q

if we want wires to become warm (heating elements) what do they have

A

high resistance

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14
Q

if we dont want wires to become warm what do they have

A

low resistance

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15
Q

why will a 70J bulb be brighter than a 50@ oe

A

it transfers 70J of electrical energy every second as opposed to 50J

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16
Q

what is power measured in

A

joules per second or watts

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17
Q

1kW is

A

1000W

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18
Q

power (W) =

P =

A

current (A) x voltage (V)

I x V

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19
Q

energy (J) =

E =

A

power (W) x time (s)

P x t

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20
Q

direct current

A

current/voltage that isi always in the same direction and have the same value. on a graph it would be a straight line

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21
Q

alternating current

A

an electric current that reverses its direction many times a second at regular intervals, typically used in power supplies.

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22
Q

what is an electric current

A

a flow of charge

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23
Q

electrons flow through all metals , so metals are

A

conductors of electricity

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24
Q

voltage (V) =

V =

A

current (A) x resistance (ohms)

I x R

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25
Q

what usually happens in a conductor

A

the electrons are free to move between the atoms. the number of electrons flowing in any one direction is equal to the number of electrons flowing in any other direction.

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26
Q

what happens in a conductor when a cell or battery is connected across a conductor

A

the electrons start flowing away from the negative terminal and to the positive terminal. now there is a net flow of charges or electric current.

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27
Q

how do you measure current

A

using an ammeter

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28
Q

the size of an electric current indicates…

A

the rate at which the charge flows

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29
Q

if there is a current of 1A then it means

A

1C of charge is passing along the wire each second

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30
Q

charge (C) =

Q =

A

current (A) x time (s)

I x t

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31
Q

what is a volt

A

a joule per coulomb

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32
Q

energy (j) =

A

charge (c) x voltage (v)

Q x V

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33
Q

how do you measure volts

A

using a voltmeter

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34
Q

series circuit

A

only one path the current can take

35
Q

parallel circuit

A

multiple paths and junctions that the current can take

36
Q

if one bulb in a series breaks,

A

it causes a gap int he circuit and all of the other bulbs will stop working

37
Q

the energy in a series circuit is shared between all the bulbs, so the more bulbs you add,

A

the dimmer they all become

38
Q

if one bulb in parallel breaks,

A

only bulbs in the same branch of the circuit break so not all of the bulbs will break

39
Q

in a series circuit how is the current distributed

A

the current is the same in all parts and not used up

40
Q

in a parallel circuit how is the current distributed

A

the current will be different in different parts of the circuit,

41
Q

voltage (V) =

V =

A

current (A) x resistance (ohms)

I x R

42
Q

practical: how to investigate how current varies with voltage with different components

A

have a circuit with a switch, variable resistor , voltmeter, ammeter and piece of wire. turn up the variable resistor to maximum and close the switch to take readings from the. ammeter and voltmeter. alter value of variable resistor or and take a new reading. repeat at least six times

43
Q

on a current vs voltage graph what does a flat slope indicate

A

a high resistance

44
Q

what does a fixed resistor do

A

to control the sizes of currents and voltages. without it the voltage across the bulb may cause too large a current and the bulb will break

45
Q

how is a variable resistor different to a fixed resistor

A

you can vary the resistor, allowing the bulb to shine more brightly or less brightly.

46
Q

what is a thermistor

A

a resistor whose resistance changes quite.a lot even with small changes in temperature

47
Q

what is a light dependent resistor

A

it has a resistance that changes when light is shone on it. in the dark its resistance is high

48
Q

where are thermistors used

A

temp sensitive circuits like fire alarms

49
Q

where are LDRs used

A

automatic lighting controls and burglar alarms

50
Q

what are diodes

A

special resistors that allow charges to follow through them in one direction.

51
Q

when diodes are used in rectifier circuits what do they do

A

they convert alternating current into direct current

52
Q

PRACTICAL: investigate how insulating materials can be charged by friction

A
  1. Before beginning the experiment, ensure the plate of the electroscope is uncharged (you
    can do this by touching it with your finger). The leaf should hang straight down next to the
    stem.
  2. Charge up either a perspex or polythene rod by rubbing it with a cloth.
    ○ The polythene rod should become negatively charged as the cloth deposits
    electrons onto its surface.
    ○ The perspex rod should become positively charged as the cloth removes electrons
    from its surface.
  3. Hold the rod near the plate of the electroscope and the leaf should be repelled from the
    stem, showing that the rod is charged.
53
Q

conclusion of the practical: investigate how insulating materials can be charged by friction

A

The charge from the rod is transferred to the metal plate and travels down the stem and leaf of the
electroscope. The stem and leaf therefore carry the same charge and repel each other.

54
Q

PRACTICAL: sticking a balloon to a wall

A
  1. Take the inflated balloon and hold it against the wall for a few seconds and then let go of it.
    If it is uncharged it should fall to the ground.
  2. Using the same balloon, rub it vigorously with a cloth or against your hair
  3. Once again hold it against the wall and release it. If it has been successfully charged, it will
    stick to the wall.
  4. Electrons have rubbed on or off the balloon, giving it an overall charge. This overall charge
    repels the same charges on the wall but attracts the opposite charges. It is this attraction of
    opposite charges that causes the balloon to stick.
55
Q

PRACTICAL: deflecting a stream of water

A
  1. Rub the polythene rod with the cloth so that it becomes charged.
  2. Run the tap so that there is a strong stream of water and then hold the charged rod
    alongside the stream.
  3. The water should deflect. This is because the charged molecules in water are attracted or
    repelled according to the charge of the rod.
56
Q

is current the same in all parts of a series circuit

A

yes there is only one route for the electrons to flow so the current must be the same in all parts of a series circuit.

57
Q

is voltage the same in all parts of a series circuit

A

no, the energy supplied by cell is ‘shared’ between the components so the voltage across each component will vary. However, the voltages across each component must sum to the voltage of the cell

58
Q

is current the same in all parts of a parallel circuit

A

no, there is several routes for the electrons to flow so the current will vary in the different branches, although the currents in the branches must sum to the current in the main circuit.

59
Q

is voltage the same in all parts of a parallel circuit

A

yes each component has its own connections to the cell so does not have to ‘share’ the energy. The voltage across each branch in the circuit will be the same.

60
Q

when does the resistance of a thermistor increase

A

when temperature increases

61
Q

what would happen if the current flowing through a diode in a circuit was reversed

A

the current would not flow anymore

62
Q

Explain the meaning of insulated wire

A

covered with a non conductor

63
Q

Explain the meaning of a 5A fuse

A

a thin wire that melts when theres a current in excess of 5A to break the circuit

64
Q

Explain the meaning of earthed

A

outside case made of metal. Direct connection to ground

65
Q

Explain the meaning of double insulated

A

outside case made of an insulator so cannot get a shock from it

66
Q

why type of energy does coal use to generate electricity

A

thermal energy

67
Q

electrons are __________ charged

A

negatively

68
Q

protons are ___________ charged

A

positively

69
Q

if an atom is neutral/no overall charge what must it have

A

equal numbers of protons and electrons

70
Q

what are electrical insulators

A

substances that dont allow electricity to flow through them

71
Q

what are electrical conductors

A

materials that allow electricity to flow through them

72
Q

e.g. of electrical insulators

A

plastic, rubber, glass and wood

73
Q

e.g. of electrical conductors

A

copper, graphite, tin and gold

74
Q

if two charged objects have similar charges

A

the objects repel eachother

75
Q

if two charged objects are oppositely charged

A

they attract eachother

76
Q

explain the force between charged and uncharged objects (like a balloon)

A

after the balloon has been charged with static electricity, (before being close to the wall), the charge is evenly distributed. the wall is uncharged the balloon is negative. this means that some of the balloons electrons will be repelled from the wall’s surface. this gives the surface of the wall a slight positive charge that attracts the negatively charged baloon.

77
Q

how does static electricity help electrostatic paint spraying

A

as the drops of paint emerge from the spray gun, they are charged. as the drops all carry the same charge they repel and spread out as a thin spray. the metal object has a wire attached to it with an electrical supply that attracts the paint drops

78
Q

how does static electricity help inkjet printers

A

each spot of ink is given a charge. when it falls between a pair of deflecting plates, electrostatic forces direct it to the right position. the charges on the plates change hundreds of times a second, so that each drop falls in a different position, forming pictures and words on the paper.

79
Q

how does static electricity help photocopiers

A

positive charges are sprayed onto a turning drum whose surface is covered with selenium (metal). a bright light is shone on to the sheet of paper to be copied. the white parts of the paper reflect the light, causing the selenium to lose its charge but dark parts do not. a negatively charged carbon powder called toner is blown across the drum, and sticks to just those parts of the drum that are charged. a sheet of paper is now pressed against the drym and picks up the pattern of the carbon powder. the powder is then fixed in place by a heater.

80
Q

how does static electricity help electrostatic precipitators

A

mant heavy industrial plants produce a lot of smoke. electrostatic preciptators can reduce the risks of these to humans and the environmenta and to buildings. as the smoke intially rises, it passes through highly charged wires. (-50,000 V). as they pass through, the ash and dust particles become negatively charged. higher up the chimmney, these charged particles are attracted by and stick to, large metal earthed plates. the cleaner smoke si then released. when the earthed plates are completely covered in dust and ash they are tapped hard, and all the dust and ash falls into a collection box to be emptied.

81
Q

how many tonnes of dust and ash can be removed from the smoke each hour in large coal fired power stations

A

50-60 tonnes

82
Q

problems with static electricity

A

friction causes aircrafts to become charged with static electricity. after landing, there is the possibility of charges escaping to earth as a spark of flash of electricity. if this takes place during refuelling it could cause an explosion.

83
Q

solution to the gain of static electricity by aircraft/fuel tankers

A

earth the plane with a conductor as soon as it lands, and before refuelling, allowing the charge that has built up to flow to earth.

edexcel igcse physics (13 decks)

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