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Physics P2 > 5 Mains electricity > Flashcards

5 Mains electricity Flashcards

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1
Q

What is direct current?

A

current in one direction only

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2
Q

What is alternating current?

A

current which repeatedly reverses its direction

flows one way then the opposite way in successive cycles

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3
Q

What are examples of dc and ac?

A

direct: battery in a torch
alternating: mains electricity

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4
Q

What is the peak voltage of an alternating potential difference?

A

the maximum voltage measured from zero volts

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5
Q

What does every mains circuit have?

A

a live wire and a neutral wire

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6
Q

What is a neutral wire?

A

the wire of a mains circuit that is earthed at the local sub-station so its potential is zero

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7
Q

What does ‘earthed’ mean?

A

connected to the ground by means of a conducting lead or wire

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8
Q

Why does the current through a mains appliance alternate?

A

the mains supply provides an alternating potential difference between the 2 wires

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9
Q

What is the potential difference between the live wire and the ‘earth’ known as?

A

(the neutral wire is ‘earthed’ at a local sub-station)

known as the potential or voltage of the live wire

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10
Q

Why is the live wire dangerous (due to its voltage)?

A

its voltage repeatedly changes from + to - and back every cycle (reaching over 300V in each direction)

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11
Q

What is an oscilloscope?

A

a device used to display the shape of an electrical wave

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12
Q

What can we use an oscilloscope to measure?

A

the peak potential difference and the frequency of a low voltage ac supply

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13
Q

What is Y-gain control?

A

an oscilloscope control used to adjust the height of the waveform

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14
Q

What is time base control?

A

an oscilloscope control used to space the waveform out horizontally

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15
Q

How do we measure the frequency of an ac supply?

A

we measure the time period of the waves
formula:
frequency = 1
——————————
time taken for one cycle

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16
Q

What is a cable?

A

2 or 3 insulated wires surrounded by an outer layer of rubber or flexible plastic

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17
Q

What is a plug?

A

a plug has an insulated case and is used to connect the cable from an appliance to a socket

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18
Q

What is a socket?

A

a mains socket is used to connect the mains plug of a mains appliance to the mains circuit

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19
Q

What is a three-pin plug?

A

a 3-pin plug has a live pin, a neutral pin and an earth pin-

the earth pin is used to earth the metal case of an appliance so the case cannot become live

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20
Q

What are socket and plug cases made out of?

A

stiff plastic materials which enclose the electrical connections so you are not electrocuted by the live wires

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21
Q

Why is plastic used?

A

it is a good hard-wearing electrical insulator

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22
Q

What does a mains cable consist of?

A

2 or 3 insulated copper wires surrounded by an outer layer of flexible plastic material

23
Q

How do sockets make the mains circuit safe?

A
  • the earth wire of the circuit is connected o the ground at your home
  • the longest pin of a 3-pin plug is designed to make contact with the earth wire of a wall socket- so when you plug an appliance with a metal case to a wall socket, the case is automatically earthed
24
Q

How are plugs made safe/effective?

A
  • the pins are made of brass because its a good insulator that does not rust or oxidise (copper is not as hard as brass even though it conducts better)
  • the case material is an electrical insulator- the inside is shaped so the wires and pins cannot touch each other when the plug is sealed
  • the plug contains a fuse between the live pin and the live wire so if too much current passes through the wire in the fuse then it will melt and cut the live wire off
25
Q

In a three-pin plug (or three core cable) what are the different colours of the wires?

A
  • live wire: brown
  • neutral wire: blue
  • earth wire: green and yellow
    (a 2 core cable does not have an earth wire)
26
Q

How are cables made safe/effective?

A
  • copper is used for the wires because it is a good electrical conductor and bends easily
  • plastic is a good electrical insulator and therefore prevents anyone from touching the cable from receiving an electric shock
  • two-core cables are used for appliances which have plastic cases (e.g. hairdryers, radios)
  • cables of different thicknesses are used for different purposes e.g. cables joining wall sockets in a house must be thicker than those joining light fittings (this is because more current passes along the wall socket cables so the wires in them must be much thicker which helps stop the heating effect of the current making the wires too hot)
27
Q

What is a fuse?

A

it contains a thin wire that heats up and melts if too much current passes through it - when this happens we say the fuse ‘blows’

28
Q

What is the rating of a fuse?

A

the maximum current that can pass through it without melting the fuse wire

29
Q

Where should the fuse always be placed?

A

in series with the live wire between the live wire and the appliance- if the fuse blows, the appliance is then cut off from the live wire

30
Q

Why should a fuse always have the correct current rating for the appliance?

A

this is because if the current rating is too large, the fuse will not blow when it should and the heating effect of the current could set the appliance/connecting cable on fire -
provided the correct fuse is fitted, the connecting cable must be thick enough to make the heating effect of the current in the cable insignificant

31
Q

Why is earthing important?

A

this is too ensure you don’t get an electric shock as without it, the metal frame would become live if the live wire touched it

32
Q

What is a circuit breaker?

A

an electromagnet switch that opens (switches off or ‘trips’) when there is a fault- this stops the current in the live wire flowing

33
Q

How does a circuit breaker work?

A

the electromagnet is connected in series with the live wire and is the current is too large then the magnetic field of the electromagnet is strong enough to pull the switch contacts apart - once the switch is open it stays open but it can be reset once the fault that made it trip is put right

34
Q

Why are circuit breakers used instead of fuses?

A
  • work faster

- can be reset more easily and quickly

35
Q

How is a Residual Current Circuit Breaker (RCCB) different to an ordinary circuit breaker and how is it better?

A
  • cuts off the current in a wire when it is different to the current in the neutral wire
  • works even faster, can be used when there is no earth connection, more sensitive
36
Q

What is the power supplied to a device?

A

the energy transferred to it each second

37
Q

How can we calculate the power of an appliance? (include units)

A

power (in watts,W) = energy transferred (in joules,J)
———————————————-
time (in seconds,s)
P = E
t

38
Q

For any electrical appliance, what is the current, potential difference and power supplied?

A
  • the current through it is the charge that flows per second
  • the potential difference across it is the energy transferred to the appliance by each coulomb of charge that passes through it
  • the power supplied to it is the energy transferred to it each second which is the electrical energy it transfers every second
39
Q

How can we calculate the power supplied? (include units)

A 
power supplied (in watts,W) = current (in amperes,A) x potential difference (in volts,V)
P = IV
40
Q

How can we work out the correct rating in amperes for a fuse?

A

= electrical power (in watts,W)
——————————————-
potential difference (in volts,V)

41
Q

What is an electric current?

A

the flow of charge

42
Q

How can we calculate charge? (include units)

A

charge (in coulombs,C) = current (in amperes,A) x time (in seconds,s)
Q = It

43
Q

What happens when an electrical charge flows through a resistor?

A

energy transferred to the resistor makes it hot

44
Q

Why does a resistor get hotter when electrical charge flows through it?

A

when a resistor is connected with a battery, electrons are made to pass through the resistor by the battery - each electron repeatedly collides with the vibrating metal ions of the resistor which transfers energy to them so they therefore gain kinetic energy and vibrate even more so the resistor becomes hotter

45
Q

What does the energy transferred in a certain time in a resistor depend on?

A
  • the amount of charge that passes through it

- the potential difference across the resistor

46
Q

How can we calculate energy transferred? (include units)

A 
energy transferred (in joules,J) = potential difference (in volts,V) x charge (in coulombs,C)
E = VQ
47
Q

When charge flows around the circuit for a certain time, how much electrical energy is supplied by the battery compared to the components?

A

the electrical energy supplied by the battery is EQUAL to the electrical energy transferred to all the components in the circuit

48
Q

Why are electrical faults dangerous?

A

they can cause electric shocks and fires

49
Q

What should you never touch in a mains appliance?

A
  • a plug or socket with wet hands (lowers your resistance to shocks
  • a bare wire
  • a terminal at a potential of more than 30V
50
Q

What does an electrician selecting a cable for an appliance need to use?

A
  • a two-core cable is the appliance is double insulated and no earth wire is needed
  • a three-core cable if an earth wire is needed because the application has a metal case
  • a cable with conductors of suitable thickness so the heating effect of the current in the cable is insignificant
51
Q

What should be considered when choosing a new electrical appliance?

A

the power and efficiency rating

52
Q

What appliances should be checked regularly to check that they are safe/effective for use?

A
  • cables, plugs and sockets for damage e.g. cracks
  • smoke alarms and infrared sensors
  • fuses/circuit breakers (if blown or tripped should not be used until someone has repaired them)
  • if there is a burning smell from an overheating appliance then it should be checked immediately
53
Q

Compare different types of bulb and which ones are more efficient:

A

Filament bulb: very inefficient- energy from bulb gradually makes the plastic parts of the bulb socket brittle so they crack
(also halogen bulbs which are slightly more efficient than filament bulbs but much less efficient than low energy bulbs)

Low energy bulbs: much more efficient so they don’t become hot like filament bulbs…there are different types)

  • compact fluorescent bulbs - now used for room lighting
  • light-emitting diodes - used for spot lights and known as high-power LEDs which operate at low voltage and power, much more efficient

Physics P2 (7 decks)

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