AQA P2 2.4 Using mains electricity safely and the power of electrical appliances

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  • Created by: Rchilds
  • Created on: 06-06-17 13:34
What is direct current (d.c.) and what can supply it?
Current that always flows in the same direction. Cells and batteries supply d.c.
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What is alternating current (a.c.) and what can supply it?
Current that is constantly changing direction. It is supplied by the mains electricity.
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What does a.c. and d.c. look like on a potential difference against time graph?
a.c. look like a wave, d.c. is a horizontal line
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What is the voltage and frequency of mains electricity?
230V and 50Hz
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What does frequency mean?
How many oscillations happen every second (so 50Hz means the current changes direction 50 times every second)
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How do you find frequency from a p.d. against time graph for a.c. current?
Look in the question for the scale of the graph. frequency = 1/time period in seconds
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What is the difference between a two core and three core cable?
3 core has 3 wires in (line, neutral and earth wires), 2 core only has the live and neutral wires. 2 core is often used for appliances with plastic cases as they are double insulated so don't need an earth wire.
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What colour are the live, neutral and earth wires and where do they go in a plug?
Live = brown and is connect to the lower right pin. Neutral = blue and is connected to the lower left pin. Earth = green and yellow and is connected to the top pin.
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What is a fuse?
A thin piece of wire found in a plug, connected to the live wire. It is a safety feature that helps prevent electrocution.
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What are wires made from and why?
Copper wires because copper is a very good conductor and is ductile (can be stretched into wires easily without snapping). Plastic coating to insulate them.
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Why is brass used for the pins on a plug?
It is a good conductor (although not as good a copper) but is much stronger as it is an alloy and therefore is more durable
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How does a fuse work to help protect the user of an appliance?
If an electrical fault occurs and the current is too great, the fuse will melt, disconnecting the circuit. The fuse will melt when the current exceeds the rating on it.
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How does a fuse work with an earth wire to protect a user? (2 cards)
If the live wire becomes connected to the metal casing of a device and a user touches it, current will flow through them to the ground and electrocute them. The Earth wire is therefore connected to the casing to give the current an alternative path..
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...fuses continued
...that has a MUCH LOWER RESISTANCE. This causes a HIGH CURRENT to flow, which melts the fuse and breaks the circuit.
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How do you choose an appropriate fuse for a circuit?
The fuse rating must be just above the normal current the appliance takes. Note: they usually get you to calculate the current in a appliance using the electrical power equation (P=IV) then state the fuse value.
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Why do difference fuses have different ratings?
The wires in them have different thicknesses. Thicker wires allow more current to flow through them before they melt so have a higher rating.
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What is an RCCB?
Residual Current Circuit Breakers. They are another way of disconnecting a circuit if there is an electrical fault.
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How does an RCCB work?
RCCBs operate by detecting a difference in the current between the live and neutral wires. If the neutral wire has a lower current, current must be flowing elsewhere (potentially through a person).
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What are the advantages and disadvantages of using an RCCB rather than a fuse?
RCCBs operate much quicker than a fuse. Fuses can only be melted once then must be replaced but RCCBs can just be reset (switch). RCCBs are more expensive to install, fuses are cheap.
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What is double insulation and what symbol does a double insulated appliance take?
An appliance that has a plastic (insulator) case. The symbol is a small square inside a bigger square.
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Why do double insulated appliances have a plastic top pin on their plug?
Because they don't have/need an earth wire. The earth wire is only needed to conduct current away from the casing, but the casing is an insulator so it doesn't conduct anyway.
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What is power? What is its unit?
Power is the rate of energy transfer (how quickly energy is transferred) it is measured in Watts (W)
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What are the advantages and disadvantages of buying more efficient devices? e.g. Compact Fluorescent Lamps (CFLs)
They waste less energy as heat so bills are less/less impact on environment as less demand from power station etc.
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If charge is passed through a potential difference, energy is transferred E =VxQ. What do these symbols mean?
E = energy transferred (joules), V = potential difference (Volts), Q = charge (coulombs). (note: this is just a rearrangement of the defintion of voltage from P2.3, voltage is work done per unit charge V = E/Q)
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Other cards in this set

Card 2

Front

What is alternating current (a.c.) and what can supply it?

Back

Current that is constantly changing direction. It is supplied by the mains electricity.

Card 3

Front

What does a.c. and d.c. look like on a potential difference against time graph?

Back

Preview of the front of card 3

Card 4

Front

What is the voltage and frequency of mains electricity?

Back

Preview of the front of card 4

Card 5

Front

What does frequency mean?

Back

Preview of the front of card 5
View more cards

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