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physics spec > electricity > Flashcards

electricity Flashcards

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1
Q

condition for electrical charge to flow

A

for electrical charge to flow through a closed circuit, the circuit must include a source of potential difference

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2
Q

define the size of the electric current

A

the rate of flow of electrical charge

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3
Q

does current have different values at different points in a single closed loop?

A

no, a current has the same value at any point in a single closed loop

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4
Q

what does the current through a component depend on and how

A

the current through a component depends on both the resistance of the component and the potential difference across the component. the greater the resistance of the component, the smaller the current for a given potential difference across the component

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5
Q

function of a thermistor

A

a component whose resistance is related to temperature

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6
Q

function of an LDR

A

a component whose resistance is related to to the amount of light falling in it

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7
Q

function of a diode

A

they only allow current to flow through them in one direction

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8
Q

function of an LED

A

LEDs emit light when they conduct electricity; current can only flow through them in one direction.

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9
Q

function of an on/off switch

A

an on-off switch allows current to flow when closed

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10
Q

function of a cell

A

a cell supplies electrical energy

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11
Q

function of a battery

A

batteries supply electrical energy and are made up of multiple cells

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12
Q

function of a resistor

A

it restricts the flow of current in a circuit

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13
Q

function of a variable resistor

A

a component used to vary and control the current in a circuit

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14
Q

function of a lamp

A

a component that transforms electrical energy into light

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15
Q

function of a fuse

A

a safety device that will break the circuit and stop current flowing

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16
Q

function of a voltmeter

A

used to measure potential difference between two points in a circuit. connected in parallel

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17
Q

function of an ammeter

A

used to measure current in a circuit. connected in series

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18
Q

relationship between resistance and ohmic conductors; what would this look like on a graph?

A

the current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. this means that the resistance remains constant as the current changes

on a graph plot potential difference against current; the resistance is a straight line through the origin (extends on both negative and positive values)

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19
Q

relationship between resistance and components; what would this look like on a graph?

A

the resistance of components, such as lamps, diodes, thermistors and LDRs is not constant; it changes with the current through the component. the resistance of a filament lamp increases as the temperature of the filament increases

on a graph plot potential difference against current; the resistance is a curved line from the negative values that goes through the origin and curves in the positive values

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20
Q

relationship between resistance and diodes; what would this look like on a graph?

A

the current through a dioxide flows in one direction only; the diode has a very high resistance in the reverse direction

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21
Q

how does resistance change in
a) a thermistor
b) an LDR

A

a) the resistance of a thermistor decreases as the temperature increases
b) the resistance of an LDR decreases as light intensity increases

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22
Q

rules for components connected in series

A
  • there is the same current through each component
  • the total p.d. of the power supply is shared between the components
  • the total resistance of two components is the sum of the resistance of each component e.g. Rₜₒₜₐₗ = R₁ + R₂
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23
Q

rules for components connected in parallel

A
  • the p.d. across each component is the same
  • the total current through the whole circuit is the sum of the currents through the separate components
  • the total resistance of two resistors is less than the resistance of the smallest individual resistor
  • total resistance is the reciprocal of all of the resistances added together
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24
Q

why does adding resistors in parallel decrease total resistance

A

in a parallel circuit, the net resistance decreases as more components are added, because there are more paths for the current to pass through

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25
Q

why does adding resistors in series increase total resistance

A

in a series circuit, the net resistance increases as more components are added; the current is the same in every single component in the circuit, so the more resistors we add, the harder it is for current to flow

26
Q

why are parallel circuits more advantageous than series circuits

A

if a wire were to be faulty in the series circuit, the whole circuit would be broken, whereas if a wire were to be faulty in a parallel circuit, only the junction(s) affected would be broken, and the other lamps would still function

27
Q

what is the mains electricity

A

an ac supply, which in the UK, the domestic electricity supply has a frequency of 50Hz and is about 230V

28
Q

difference between direct and alternating current

A

direct current is the movement of charge in only one direction, whereas alternating current is the backward and forward movement of charge

29
Q

describe alternating current

A

the p.d. constantly changes, reaching a peak of 230V in one direction, reverting to 0, then reaching a peak of 230V in the opposite direction and repeating. there are 50 of these cycles per second, so the frequency is 50Hz

30
Q

function and colour of the live wire

A

brown; the live wire carries the alternating potential difference from the supply. this wire has a p.d. of 230V

31
Q

function and colour of the neutral wire

A

blue; the neutral wire completes the circuit. this wire has a p.d. of 0V

32
Q

function and colour of the earth wire

A

green and yellow stripes; the earth wire is a safety wire to stop the appliance becoming live. this wire would be at 0V

33
Q

why is the insulation covering each wire colour coded

A

for easy identification

34
Q

how does a fuse work

A

fuses have different capacities for how much current can pass through; if it surpasses the amount, then the fuse overheats and melts which breaks the circuit, to ensure that other components which could be dangerous with excessive amounts of current do not receive it

35
Q

what happens when charge flows

A

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

36
Q

what do all domestic appliances do

A

transfer electrical energy into other forms of energy, typically kinetic and thermal energy; hence we can divide appliances into two main categories: those that transfer electrical energy to the kinetic energy of motors, or those that transfer electrical energy to the thermal energy of heating devices

37
Q

energy transfer to the mains in a blender

A

they are powered by electrical energy, which is then transferred to the kinetic energy of electric motors to allow for movement (some energy will be dissipated as thermal energy due to friction)

38
Q

energy transfer to the mains in a kettle

A

they are powered by electrical energy, which is transferred to thermal energy in the heating element

39
Q

examples of appliances that fit under transferring electrical energy into both kinetic and thermal energy categories

A

hair dryers, washing machines

40
Q

what are everyday appliances designed to do

A

bring about energy transfers

41
Q

define power rating

A

the maximum power that appliances can safely operate at / the maximum rate of energy transfer

42
Q

describe the difference in power rating between appliances designed to transfer electrical energy to thermal energy vs kinetic energy

A

usually, appliances designed to transfer electrical energy to thermal energy have a much higher power rating than those designed to transfer to kinetic energy, as it takes much more energy per second to heat than to move

43
Q

misconception about power

A

a higher power does not necessarily mean that an appliance transfers more energy usefully i.e. appliance 1 might have a very high power but a very low efficiency

44
Q

define the national grid

A

a system of cables and transformers linking power stations to consumers

45
Q

what is transferred using the national grid

A

electrical power

46
Q

what is the issue with getting electricity to homes? how can this issue be reduced; what is the problem with this resolution?

A

energy is lost in power cables due to the resistance of the wires; we can reduce energy loss by building power stations near to homes, but this isn’t feasible, as people don’t want to live near power stations

47
Q

how are unwanted energy transfers reduced in the national grid

A

by using step-up and step-down transformers; step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use

48
Q

why is electrical power transferred at a very high p.d. across step-up transformers

A

in our P=IV equation, if we increase the potential difference keep the power (rate of energy transfer) the same, then we have a lower current. a lower current results in less energy being lost

49
Q

define insulators; give examples

A

insulators are materials that do not conduct electricity, as electrons cannot move through them e.g. cloths and plastic rods

50
Q

why are metals good conductors of electricity

A

electrons can easily flow through metals

51
Q

what happens when you rub certain insulating materials together

A

when they are rubbed against each other (e.g. a cloth and a plastic rod), they become electrically charged. negatively charged electrons are rubbed off one material and on to the other. the material that gains electrons becomes negatively charged and the material that loses electrons becomes positively charged. these positive and negative charges are equal and opposite

52
Q

why is static electricity called that

A

because the two insulators now become electrostatically charged

53
Q

what happens when two electrically charged objects are brought close together

A

they exert a force on eachother; two objects that carry the same type of charge repel, two objects that carry different types of charge attract. attraction and repulsion between two charged objects are examples of non-contact force

54
Q

how can static electricity cause a spark under certain conditions

A

when we rub two insulators and electrons are transferred from insulator A to insulator B, insulator A becomes positively charged and B becomes negatively charged. the electrons from insulator B are therefore strongly attracted to insulator A’s positive charge, so sometimes, these electrons can jump across the gap when they are brought close to each other, causing a spark

55
Q

how do you prevent sparks leading to fires e.g. on aircraft fuelling

A

the refuelling truck and the aeroplane are earthed to prevent any charged building up and leading to a spark

56
Q

how do sparks commonly take place

A

between an object with a static charge and an earthed object

57
Q

describe how sparks take place between static charged objects and earthed objects - what assumptions do you make

A

as electric charge builds up on an object, the p.d. between the charged object and the earthed object increases. with a high enough p.d., the electrons can jump across the gap and be transferred from the charged object to the earthed object. you must assume that the uncharged object is earthed, not positive

58
Q

when is an electric field created

A

a charged object creates an electric field around itself

59
Q

determine the strength of an electric field

A

the electric field is strongest close to the charged object; the further away from the charged object, the weaker the field

60
Q

what happens when another charged object is placed in the electric field

A

it experiences a non-contact force; the force gets stronger as the distance between the objects decreases

61
Q

how can we draw electric field lines

A

draw lines perpendicular to the surface with arrows that point in the direction of a positive particle that is placed within the field i.e. field lines for a positive object will go away from the object (if you place a positive object in the field it will be repelled) and field lines for a negative object go towards the object (if you place a negative object in the field it will be attracted)

physics spec (10 decks)

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