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physics > P3 - electricity > Flashcards

P3 - electricity Flashcards

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1
Q

what is charge?

A
  • a physical quantity, which can be either positive or negative
  • it causes a particle to experience a force when placed in an electric field
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2
Q

what is a conventional current?

A

the rate of flow of charge from positive to negative

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3
Q

what is a coulomb?

A
  • the unit of charge
  • equivalent to the charge on 6.25x10^18 electrons
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4
Q

what is a current?

A

the rate of flow of electric charge

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5
Q

what is an electric field?

A

a region in which another charged object will experience a force

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6
Q

what is an electrostatic force?

A

the name given to the force that occurs between charged particles

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7
Q

what is induced charging?

A
  • to cause a neutral object to gain a localised charge by placing a charged object near to it
  • caused by electrons in the neutral object being attracted or repelled from its surface by the charged object
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8
Q

what are ions?

A

charged particles formed when atoms lose or gain electrons

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9
Q

what is Ohm’s law?

A

the potential difference that occurs across a conductor is directly proportional to the current flowing through it, if the temperature of the conductor is constant

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10
Q

what is a parallel circuit?

A

a circuit with more than one loop; each loop works independently of the rest

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11
Q

what is potential difference?

A

the amount of energy transferred (work done) per unit charge

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12
Q

what is resistance?

A

the property of a material to reduce the flow of electrical current due to collisions of free electrons with metal ions

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13
Q

what is a series circuit?

A

a circuit with only one loop

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14
Q

what is a build up of static caused by?

A

transferring electrons

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15
Q

what happens when two materials are rubbed together?

A
  • electrons are transferred from one to the other
  • if materials are conductors, the electrons will flow back into or out of them, so they stay neutral
  • if the materials are insulators, electrons can’t flow so a positive static charge is left on the object that lost electrons and a negative static charge is left on the object that gained electrons
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16
Q

what does the direction of electron transfer depend on?

A

the materials

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17
Q

what happens when enough charge builds up?

A

it can suddenly move, causing sparks or shocks

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18
Q

what do like charges do?

A

repel

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19
Q

what do opposite charges do?

A

attract

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20
Q

what do electrically charged objects do to each other?

A

exert a force on one another

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21
Q

what happens to the forces that electrically charged objects exert on each other, the further apart the objects are?

A

these forces get weaker the further apart the two objects are

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22
Q

what is the force between two charged objects known as?

A
  • electrostatic attraction (if they attract each other)
  • electrostatic repulsion (if they repel each other)
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23
Q

what type of forces are electrostatic attraction/repulsion?

A

non-contact force

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24
Q

what do electrically charged objects do to small neutral objects placed near them?

A

attract

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25
Q

e.g. if you hold a charged rod above some small scraps of paper the scraps will ‘jump’ towards it

why does this happen?

A

the charged rod induces a charge in the paper
- if the rod is positively charged, it attracts the electrons in the paper towards it
- if it’s negatively charged it repels the electrons; this gives the surface of the paper near the rod an opposite charge to the rod, so the rod and the paper are attracted together

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26
Q

what can you use to test for charge?

A

a gold leaf electroscope

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27
Q

how can a gold leaf electroscope be used to test for charge?

A
  • if a negatively charged insulator touches the zinc plate, some of its charge is transferred to the electroscope, and conducted down to the metal stem and gold leaf
  • this negatively charges both the stem and the gold leaf, which repel each other, this makes the gold leaf rise
  • if you touch the plate with a positively charged insulator, electrons will flow into it from the plate, stem and leaf
  • again, the stem and leaf will have the same charge and the leaf will rise
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28
Q

what do electric charges have?

A

an electric field

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29
Q

where do electric field lines go from?

A

positive to negative

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30
Q

what do closer electric field lines mean?

A

the closer together the field lines are, the stronger the field, and the stronger the force a charged object in the field experiences

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31
Q

what do electric fields cause?

A

electrostatic forces

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32
Q

what happens when the electric fields around two charged objects interact?

A

a force is produced

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33
Q

what happens if the field lines between the charged objects point in the same direction?

A

the field lines ‘join up’ and the objects are attracted to each other

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34
Q

what happens if the field lines between the charged objects point in opposite directions?

A

the field lines ‘push against’ each other and the objects repel each other

35
Q

in a single closed loop, what will the current be like at any point?

A

the current will have the same value

36
Q

what is an ammeter?

A
  • the ammeter measures the current flowing through the component
  • must be placed in series with the component
  • can be put anywhere in series in the main circuit, but never in parallel like the voltmeter
37
Q

what is a voltmeter?

A
  • the voltmeter measures the potential difference across the component
  • must be placed in parallel with the component under test, not the variable resistor or battery
38
Q

what usually happens to resistance with an increase in temperature?

A

it increases

39
Q

what happens to an electron when it flows through a resistor?

A

some of its energy is transferred to the thermal energy store of the resistor (heating it up)

40
Q

what happens to the particles as the resistor heats up?

A

they start to vibrate more

41
Q

why does the particles vibrating more increase resistance?

A

with the particles joggling around it’s more difficult for the charge-carrying electrons to get through the resistor; the current can’t flow as easily and the resistance increases

42
Q

most resistors have a [ ] to the the current that can flow

A

for most resistors, there’s a limit to the current that can flow

43
Q

what does more current lead to?

A

more current means an increase in temperature, which means an increase in resistance, which means the current decreases again

44
Q

what happens to the graph for the filament lamp at high currents?

A

it levels off

45
Q

what is a diode?

A
  • a special device made from semiconductor material like silicon which lets current flow freely through it in one direction, but not in the other
  • diodes can also be used to get direct current from an alternating supply
46
Q

how many directions does current flow in through a diode?

A

current only flows in one direction through a diode

47
Q

what are diodes useful in?

A

in various electronic circuits e.g. in radio receivers

48
Q

what is an LDR?

A

a light dependent resistor; a resistor that’s dependent on the intensity of light

49
Q

when is the resistance of an LDR highest?

A

in darkness, the resistance is highest
- as light levels increase the resistance falls so the current through the LDR increases

50
Q

what are some applications of LDRs?

A
  • automatic night lights
  • outdoor lighting
  • burglar detectors
51
Q

what is a thermistor?

A

a temperature-dependent resistor

52
Q

how does the resistance of a thermistor change with temperature?

A

in hot conditions, the resistance of a thermistor drops; in cool conditions, the resistance goes up

53
Q

what are thermistors used for?

A

they’re used as temperature detectors e.g. thermostats, irons and car engines

54
Q

what can you use LDRs and thermistors in?

A

sensing circuits

55
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

56
Q

how are components connected in series circuits?

A
  • the different components are connected in a line, end to end, between the positive and negative terminals of the power supply
  • current has to flow through all of the components to get round the circuit, so if you remove one of them it can have a big affect on the others
57
Q

how are components connected in parallel circuits?

A
  • each component is separately connected to the positive and negative terminals of the supply
  • this means if you remove or disconnect one of them, it will hardly affect the others at all
58
Q

which type of circuit is usually best?

A

parallel circuits are usually more sensible to connect things e.g. in cars and household electrics, where you have to be able to switch everything on and off separately

59
Q

how would you add more cells to a circuit?

A

connect them in series not parallel since connecting several cells in series, all the same way (+ to -), gives a bigger total p.d.

60
Q

what is potential difference like in series circuits?

A

potential difference is shared between the various components
- so the p.d.s round a series circuit always add up to equal the p.d. across the power supply

61
Q

why is potential difference shared in series circuits?

A

because the Toal energy transferred to the charges in the circuit by the power supply equals the total energy transferred from the charges to the components

62
Q

what is current like in series circuits?

A

current is the same everywhere

63
Q

what is the size of the current in a series circuit determined by?

A

the total p.d. of the power supply and the total resistance of the circuit (I=V/R)

64
Q

what is resistance like in series circuits?

A

resistance adds up
- the total resistance is the sum of the individual resistances

65
Q

why is the resistance of two or more resistors in series bigger than the resistance of just one of the resistors on its own?

A

because the battery has to push each charge through all of them

66
Q

why is it that the bigger the resistance of each component, the bigger its share of the total p.d.?

A

because more energy is transferred from the charge when moving through a large resistance than a small one

67
Q

what will happen to the potential difference across all the components if the resistance of one component changes?

A

the potential difference across all the components will change too

68
Q

what is potential difference like in parallel circuits?

A

potential difference is the same across all branches
- since each charge can only pass down one branch of the circuit, so it must transfer all the energy supplied to it by the source p.d. to whatever’s on that branch

69
Q

what is current like in parallel circuits?

A

current is shared between branches
- the total current flowing round the circuit equals the total of all the currents through the separate branches

70
Q

in a parallel circuit, what is there where the current either splits or rejoins?

A

junctions

71
Q

what must the total current going into a junction equal?

A

the total current leaving

72
Q

how is the total resistance of a parallel circuit calculated?

A

difficult to work out, but it’s always less than that of the branch with the smallest resistance

73
Q

what can you use to investigate series and parallel circuits?

A

bulbs
(see booklet/revision guide)

74
Q

what is anything that’s supplying electricity also supplying?

A

energy

75
Q

where does an electrical current transfer energy from?

A

the power supply

76
Q

what awesome examples of power supplies?

A
  • cells
  • batteries
  • generators
77
Q

what does an electrical current transfer energy to?

A

the components of the circuit

78
Q

why is energy supplied to the charge at the power source?

A

to ‘raise’ it through potential

79
Q

what does the charge give up when it ‘falls’ through a potential drop in any components elsewhere in the circuit?

A

the charge gives up this energy

80
Q

why will a battery with a bigger p.d. supply more energy to the circuit for every coulomb of charge which flows around it?

A

because the charge is raised up ‘higher’ at the start and more energy will be transferred in the circuit too

81
Q

why does a greater resistance of a component lead to a bigger drop in p.d.?

A

the greater the resistance of a component, the more energy the charge has to transfer to it to pass through, so the bigger the drop in p.d.

82
Q

what is power?

A

the rate of energy transfer

83
Q

what does the power of a component tell you?

A

how much energy it transfers per second

84
Q

what are some units used for power?

A

J or kWh

physics (11 decks)

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