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IGCSE Physics > 09 Circuits And Mains Electricity > Flashcards

09 Circuits And Mains Electricity Flashcards

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1
Q

Why would an appliance NOT need an earth wire

A
  • its double insulated
  • has plastic casing or the electrical components are incased in plastic
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2
Q

Power equation

A

Power = current x voltage

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3
Q

Power

A

The rate of energy transfer
OR
the amount of energy transferred per second

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4
Q

Purpose of a fuse - simple

A
  • designed to cut off the flow of electricity to an plainte if current becomes too large due to a fault/surge
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5
Q

Fuse circuit symbol

A

Rectangle with line through it

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6
Q

Purpose of a fuse

A
  • if the current in a wire becomes too large (exceeds stated value)
  • fuse heats up and melts
  • breaks the circuit and stops the current
  • makes sure more current doesn’t flow through the circuit and cause a fire/spark/explosion
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7
Q

Equation to find right fuse for appliance

A

Current = power/voltage

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8
Q

Why should a fuse always be slightly higher than current needed by appliance

A
  • always choose next size
  • if the fuse current rating is too low, it will break the circuit even when an acceptable current is flowing
  • if the fuse current rating is too high, it will not break the circuit in enough time before damage
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9
Q

Energy transferred equation

A

Energy transferred = current x voltage x time

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10
Q

Common electrical safety hazards

A

Damaged insulation - is someone touches an exposed piece of wire they can experience lethal shock
Overheating of cables - when too much current is in a small wire it can melt the insulation and expose live wires leading to a fire
Damp conditions - if someone comes in contact with live wires it can cause electrocution or fire

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11
Q

Insulation

A
  • electrical wires are covered with an insulating material e.g. rubber
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12
Q

What are wires usually made of

A

Copper

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13
Q

Double insulation

A
  • insulation around wires
  • non-metallic case that acts as a second layer of insulation
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14
Q

Risk of no earth wire

A
  • many electrical appliances have metal cases -> potential electrical safety hazard
  • if live wire comes into contact with case, the case would be electrified and can be a risk of electrocution to people who touch it
  • earth wire reduces this risk
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15
Q

Earth wire

A
  • provides a low resistance path to earth
  • causes a surge of current in the earth wire and hence also in the live wire
  • the high current through the fuse causes it to melt and break
  • this cuts off the supply of electricity to the appliance making it safe
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16
Q

Circuit breaker and its advantage

A

Consists of an automatic electromagnet switch that breaks the circuit if current exceeds a certain value
Adv - doesn’t melt/break so it can be reset and used again, works faster

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17
Q

Current passing through a resistor(or wire) results in

A

Electrical transfer of energy

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18
Q

Current

A

Rate of flow of charge

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19
Q

Current in a resistor results in.. and examples of when its useful

A

Current passing through results in electrical transfer of energy
Temperature of resistor increases due to collisions of the free electrons within the wire
Some of the energy is dissipated into surroundings by heat

Used for electric heaters/ovens/hobs, toasters and kettles

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20
Q

Mains electricity can be supplied by

A

Alternating current or direct current from a cell/battery

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21
Q

Direct current

A

A steady current, constantly flowing in the same direction in a circuit from positive -> negative
- potential difference across a cell travels in one direction only
- has fixed positive terminal and a fixed negative terminal
- produced by cells and batteries

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22
Q

Examples of direct current

A

Electric cells or batteries

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23
Q

Things in a dc circuit

A

Cell/battery, bulb, resistor and voltmeter in parallel

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24
Q

Alternating current

A

A current that continuously changes direction, going back and forth
- travels from positive terminal to negative terminal (two identical terminals)
- produced by electrical generators e.g. mains electricity

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25
Q

Mains electricity frequency

A

50 Hz

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26
Q

Mains electricity potential difference

A

230V

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27
Q

Plastic/polythene rod being charged by rubbing with a cloth

A
  • both the rod and cloth are insulating material
  • polythene rod may be given a charge by rubbing it witha cloth
  • the occurs because negatively charged electrons are transferred from one material to another
  • the rod gain electron
  • since electrons are negatively charged the rod becomes negatively charged
  • as result the cloth loses electrons and is left with an equal positive charge
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28
Q

In components in series circuits have

A

Same current

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29
Q

Current is the same at

A

Each point in a series circuit

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30
Q

Decreasing voltage of a power source will

A

Reduce current

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31
Q

Increasing the number of components in a series circuit increases

A

The total resistance
- hence less current flows through the circuit

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32
Q

In a parallel circuit what happens to current

A
  • current splits at a junction
  • electrons have different paths to take
  • sum of current on individual branches equals total current
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33
Q

Current is ___ at a junction

A

Conserved

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34
Q

In a series circuit the total voltage of a power supply is

A

Shared between the components

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35
Q

For two identical components with equal resistance, the voltage across them will be

A

The same
- equal to half the total voltage of the power supply

36
Q

For two non-identical components (with different values of resistance) the voltage will be

A

Higher across the component with higher resistance
Lower across the component with lower resistance

37
Q

Advantages of series circuit

A
  • all of the components are controlled by a single switch
  • fewer wires required
38
Q

Disadvantages of series circuit

A
  • components can’t be controlled separately
  • if one component breaks, all the others stop working
39
Q

Advantages of parallel circuit

A
  • the components can be individually controlled using their own switches
  • if one component breaks the others will continue to function
40
Q

Disadvantages of a parallel circuit

A
  • many more wires can be complicated to set up
  • all branches has same voltage as supply making it more difficult to control the voltage across individual components
41
Q

Why do bulbs dimmer in a series circuit when another one is added

A
  • voltage shared between bulbs is in series
  • less voltage across each bulb therefore bulb is dimmer
  • adding a second bulb doubles resistance of circuit which halves the current
42
Q

Voltage

A

Energy per unit charge
- or energy transferred per unit charge passes

43
Q

What is voltage across a cell or power supply a measure of

A

The energy transferred from chemical store electrically to wires per coulomb

44
Q

What happens to the total resistance and current in a circuit as more bulbs are added in parallel

A

Adding bulbs in parallel decreases the total resistance -> v=i/r
As resistance decreases, current increase for the same voltage

45
Q

Fixed resistor - how would a line change for a resistor with half the resistance

A
  • steeper
  • double the gradient
  • double the current
46
Q

Total resistance equation

A

R = R1 + R2

47
Q

Components connected in series summary

A
  • current is the same at a;; point and in each components
  • voltage of the power supply is shared between components
  • total resistance is the sum of the resistances of each components
48
Q

Components connected in parallel summary

A
  • current from supply splits at branches
  • voltage across each branch is the same
  • total resistance is less than that of each component
49
Q

Components with linear IV graphs

A

Fixed resistors, wires

50
Q

Components with linear IV graphs

A

Fixed resistors, wires

51
Q

Components with non-linear IV graphs

A

Filament lamps, diodes, LDR, thermistors

52
Q

Components with non-linear IV graphs

A

Filament lamps, diodes, LDR, thermistors

53
Q

Describe IV graph for wire/resistor

A
  • relationship is linear/directly proportional
  • resistance is constant
  • current is directionally proportional as graph is a straight line through origin
54
Q

Describe IV graph for filament bulb

A
  • relationship is non-linear and not directionally proportional
  • not straight line, so voltage and current do not increase for decrease by same amount
  • resistance changes as slope is not constant
  • as voltage increase the current increases at a proportionally slower rate
55
Q

Describe why the IV graph for filament lamp is non linear

A

As current through a filament lamp increases, the resistance increases because
- higher current causes the temperature of the filament to increase
- higher temperature causes atoms in a metal lattice to vibrate more
-causes increase in resistance -> more difficult for free electrons (current) to pass through
- since resistance opposes current, it causes it to increase at a slower rate

56
Q

Describe IV graph for a diode

A

Forward bias - shown by sharp increase in voltage and current on right of graph
Reverse bias - shown by 0 reading of current and voltage on left of graph
- shows resistance is very high

57
Q

What do lamps and LEDs indicate

A

Presence of current in a circuit

58
Q

Voltage equation

A

Voltage = current x resistance

59
Q

Charge equation

A

Q = I x T
Charge = current x time

60
Q

Electric current in a solid metallic conductor

A

Flow of negatively charged electrons

61
Q

Voltage across 2 components connected in parallel is

A

The same

62
Q

Volt

A

Joule per coulomb

63
Q

Energy transferred equation

A

En ergo transferred = charge x voltage

64
Q

Conductor

A

A material that allows charge (electrons) to flow through it easily

65
Q

Examples of conductors

A

Copper, steel, aluminum

66
Q

Metals conduct electricity well because

A

Current is the rate of flow of electrons
So the more easily electrons are able to flow the better the conductor

67
Q

Insulator

A

Has no free charges so it doesn’t allow the flow of charge through easily

68
Q

Examples of insulators

A

Rubber, plastic, wood, glass

69
Q

Examples of insulators

A

Rubber, plastic, wood, glass

70
Q

Investigating how insulating materials can be charged by friction

A
  • rub both ends of a polythene rod with a cloth
  • rub both ends of an acrylic rod
  • bring each end of the rods together
  • record observation of the motions of the rod
  • repeat 3x
71
Q

What happens to electrons when two insulating materials are rubbed together

A

Electrons will transfer from one insulator to the other insulator

72
Q

Polythene rod is given a negative charge by rubbing it with a cloth because

A

Electrons are transferred to the polythene rod from cloth
Electrons are negatively charged so the rod becomes negatively charged

73
Q

Acetate rod is given a positive charge when rubbed with a cloth because

A

Electrons are transferred away from acetate to the cloth
Electrons are negatively charged so when the acetate loses negative charge, it becomes positively charged

74
Q

If the material is repelled by the polythene rod

A

The materials have the same charge

75
Q

If the material is attracted to the polythene rod

A

They have opposite charges

76
Q

Neutral atom

A

Number of electrons equals number of protons
- equal charges cancel out, overall charge is 0

77
Q

Opposite charges

A

Attract

78
Q

Same charges

A

Repel

79
Q

Charging by friction

A

When certain insulating materials are rubbed against each other they become electrically charged

80
Q

The object the electrons are transferred TO becomes

A

Negatively charged

81
Q

The object the electrons transfer FROM becomes

A

Positively charged

82
Q

Use of electrostatic charges in photocopiers

A
  • image of document is projected onto a positively charged copying plate
  • plate loses its charge in light areas and keeps the positive charge in dark areas
  • a negatively charged ink is applied to the plate and sticks to where there’s positive charge as positive and negative charges attract
  • ink is transferred to white paper
  • paper is heated so the powder sticks
83
Q

Dangers of electrostatic charge

A
  • risk of electrocution
  • risk of fire and explosion due to a spark
84
Q

Earthing

A

Used to prevent the dangerous build-up of charge
- this is done by connecting the vehicles to the earth with a conductor

85
Q

Fuelling vehicles

A
  • build up of static charge is a potential danger when refuelling aeroplanes
  • fuel runs through pipes at a fast rate and fuel is flammable
  • friction between fuel and pipe causes the fuel to gain charge
  • is the charge causes a spark the fuel could ignite and cause explosion
86
Q

How to prevent spark when fuelling plane

A

Fuel tank is connected to earth with a copper wire called the bonding line
Conductor earths the plane by carrying the charge through to the earth which removes risk of any sparks

IGCSE Physics (15 decks)

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