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GCSE Physics > P5 Electricity In The Home > Flashcards

P5 Electricity In The Home Flashcards

1
Q

What are the two types of electricity supply?

A
  • Alternating current (AC)
  • Direct current (DC)
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2
Q

What is the current in ac supplies?

A
  • Constantly changing direction
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3
Q

How are alternating currents produced?

A
  • Alternating voltages in which the positive & negative ends keep alternating
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4
Q

What type of supply is the UK mains supply (a.k.a the electricity in your home)?

A
  • An ac supply
  • At around 230V
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5
Q

What is the frequency of the ac mains supply?

A
  • 50 cycles per second (50 Hertz (Hz))
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6
Q

What type of supply are cells & batteries?

A
  • Direct current (dc)
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7
Q

What is direct current (dc)?

A
  • A current that is always flowing in the same direction
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8
Q

What is direct current created by?

A
  • Direct voltage
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9
Q

How are most electrical appliances connected to the mains supply & what does it mean?

A
  • Most electrical appliances are connected to the mains supply by three-core cables
  • This means that they have three wires inside them, each w/ a core of copper and a coloured plastic coating
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10
Q

What shows the purpose of a cable?

A
  • The colour of the insulation on each cable shows its purpose
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11
Q

Why are the colours the same for every appliance & why?

A
  • The colours are always the same for every appliance
  • This is so that it is easy to tell the different wires apart
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12
Q

What is the colour of the neutral wire?

A

Blue

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13
Q

What is the neutral wire used for?

A
  • Completes the circuit
  • When the appliance is operating normally, current flows through the live and neutral wires
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14
Q

What is the potential difference (voltage) of a neutral wire?

A

0V

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15
Q

What is the colour of the live wire?

A

Brown

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16
Q

What is the live wire used for?

A
  • Provides the alternating potential difference from the mains supply
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17
Q

What is the potential difference (voltage) of the live wire?

A

230V

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18
Q

What colour is the earth wire?

A

Green & yellow

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19
Q

What is the earth wire used for?

A
  • To protect the wiring & for safety
  • Stops the appliance casing from becoming live
  • Doesn’t carry a current unless there is a fault
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20
Q

What is potential difference (voltage) of an earth wire?

A

0V

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21
Q

Why can a live wire give you an electric shock?

A
  • Your body (just like the earth) is at 0V
  • This means that if you touch the live wire, a large potential difference is produced across your body & a current flows through you
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22
Q

Why are electric shocks dangerous to humans?

A
  • Could injure/kill you
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23
Q

Why is there still a risk of electric shock when the plug socket/light switch is turned off (i.e. the switch is open)?

A
  • A current isn’t flowing but there’s still a potential difference in the live wire
  • If you made contact with the live wire, your body would provide a link between the supply & the earth, so a current would flow through you
24
Q

Why are any connections between live & earth wires dangerous?

A
  • If the link creates a low resistance path to earth, a huge current will flow, which could result in a fire
25
Q

Why does a moving charge transfer energy?

A
  • Because the charge does work against the resistance of the circuit (work done is the same as energy transferred)
26
Q

What are electrical appliances designed to do?

A
  • Transfer energy to components in the circuit when a current flows
27
Q

How do kettles transfer electric energy?

A
  • Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside the kettle
28
Q

How is electrical energy transferred in a fan?

A
  • Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor
29
Q

Why can’t appliances transfer all energy usefully?

A
  • The higher the current, the more energy is transferred to the thermal energy stores of the components (and then the surroundings)
30
Q

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

A
  • How long the appliance is on for & its power
31
Q

What is the power of an appliance?

A
  • The energy it transfers per second
  • So, the more energy it tranfers in a given time, the higher its power
32
Q

What is the formula for energy transferred by electrical work?

A
  • Energy transferred (J) = Power (W) x time (s)
  • E = Pt
33
Q

What is a power rating?

A
  • Appliances are often given a power rating
  • I.e. they’re labelled w/ the maximum safe power that they can operate at
  • You can usually take this to be their maximum operating power
34
Q

Why are power ratings useful?

A
  • It helps customers to choose between models - the lower the power rating, the less electricity uses in a given time and so the cheaper it is to run
35
Q

Why doesn’t higher power mean usefulness?

A
  • An appliance may be more powerful but less efficient, meaning that it might still only transfer the same amount of energy (or even less) to equal stores
36
Q

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

A
  • Energy transferred
37
Q

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

A
  • Energy is supplied to the charge at the power source to ‘raise’ it through a potential
  • The charge gives up this energy when it ‘falls’ through any potential drop in components elsewhere in the circuit
  • That means that a battery with a bigger pd will supply more energy to the circuit for every coulomb of charge which flows around it, because the charge is raised up “higher” at the start
38
Q

What is the formula for energy transferred?

A

Energy transferred (J, E in formula) = Charge flow (C, Q in formula) x Potential difference (V, V in formula)

39
Q

What is the formula for power of an appliance?

A
  • Power (W) = Potential difference (V) x Current (A)
  • P = VI
  • or P = I^2R
40
Q

What is the national grid?

A
  • A giant system of cables and transformers that covers the UK & connects power stations to consumes (anyone who is using electricity)
41
Q

What power does the national grid transfer?

A
  • The national grid transfers electrical power from power stations anywhere on the grid (the supply) to anywhere else on the gird where it’s needed (the demend) - e.g. homes & industry
42
Q

Why do power stations have to meet demand?

A
  • Throughout the day, electricity usage (the demand) changes
  • Power stations have to produce enough electricity for everyone to have it when they need it
43
Q

What are power stations able to predict?

A
  • When the most electricity will be used through
44
Q

When causes demand in electricity to increase?

A
  • When people get up in the morning
  • Come home from school/work
  • When it starts to get dark/cold outside
  • Poopular events like a sporting final
45
Q

Why do power stations run at well below their maximum power output?

A
  • So there’s spare capacity to cope w/ a high demand, even if there’s an unexpected shut-down of another station
  • Therefore lots of smaller power stations that can start up quickly are also kept in standby just in case
46
Q

How is the national grid’s power transmitted?

A
  • To transmit the huge amount of power needed, you need either a high potential difference or a high current
47
Q

What is the problem w/ high current?

A
  • You lose loads of energy as the wires heat up & energy is transferred to the thermal energy store of the surroundings
  • Therefore it’s much cheaper to boost the pd up really high (400000 V) & keep the current as low as possible
48
Q

What makes the national grid efficient?

A
  • For a given power, increasing the pd decreases the current, which decreases the energy lost by heating the wires & the surroundings
  • This makes the national grid an efficient way of transferring energy
49
Q

How do we get the potential difference of a power station to 400000V?

A
  • To get the potential difference to 400000V for efficient transmission we use transformers (and big pylons with huge insulators)
50
Q

How many coils does a transformer have?

A
  • Two coils
  • A primary coil & a secondary coil
51
Q

What are coils joined with?

A
  • Iron core
52
Q
  1. How is potential difference increased?
A
  • Step-up transformer
  • They have more turns on the secondary coil than the primary coil
  • As the pd is increased by the transformer, the current is decreased
53
Q
  1. How is potential difference decreased?
A
  • The pd is then reduced again at the local consumer end using a step-down transformer (the current is therefore increased by this transformer)
  • They have more turns on the primary coil than the secondary coil
54
Q

What is the formula for the power of a primary coil?

A

power = pd x current

55
Q

Why does the power in the primary coil equal the power in the secondary coil?

A
  • Because transformers are nearly 100% efficient
  • Therefore, pd across primary coil (V) x current in secondary coil (A) = pd across secondary coil (V) x current in secondary coil (A)
  • V(P)I(P) = V(S)I(S)

GCSE Physics (7 decks)

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