| Physics
4451 (Unit 2) - exam to be taken in January of Y11 |
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Written
paper 25% |
45 minutes - 45
marks
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PHY2H |
| Mains
electricity |
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| 12.7 |
What is
mains electricity and how can it be used safely? |
Mains electricity is useful but can be very dangerous. It is important to
know how to use it safely. |
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Candidates
should use their
skills, knowledge and
understanding of how
science works: |
• to recognise errors in the wiring of a three-pin plug
• to recognise dangerous practice in the use of mains electricity
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• to compare potential differences of d.c. supplies and the peak potential
differences of a.c. supplies from diagrams of oscilloscope traces
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• to determine the period and hence the frequency of a supply from diagrams
of oscilloscope traces. |
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Their skills,
knowledge and understanding of how science works should be set in these
substantive contexts: |
• Cells and batteries supply current which always passes in the same
direction. This is called direct current (d.c.).
• An alternating current (a.c.) is one which is constantly changing
direction. |
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• UK mains supply is about 230 volts and mains electricity is an a.c. supply.
In the UK it has a frequency of 50 cycles per second (50 hertz).
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• Most electrical appliances are connected to the mains using cable and a
three-pin plug.
• The structure of electrical cable.
• The structure of a three-pin plug.
• Correct wiring of a three-pin plug.
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• If an electrical fault causes too great a current the circuit should be
switched off by a fuse or a circuit breaker.
• When the current in a fuse wire exceeds the rating of the fuse it
will melt, breaking the circuit.
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• Appliances with metal cases are usually earthed.
• The earth wire and fuse together protect the appliance and the user.
• The live terminal of the mains supply alternates between positive
and negative potential with respect to the neutral terminal.
• The neutral terminal stays at a potential close to zero with respect
to earth. |
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| 12.8 |
Why do
we need to know the power of electrical appliances? |
Electrical appliances transform energy. The power of an electrical appliance
is the rate at which it transforms energy. Most appliances have their power
and the potential difference of the supply they need printed on them. From
this we calculate their current and the fuse they need. |
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Candidates
should use their skills, knowledge and understanding of how science works: |
• to calculate the current through an appliance from its power and the potential
difference of the supply and from this determine the size of fuse needed. |
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Their skills,
knowledge and understanding of how science works should be set in these
substantive contexts: |
• Electric current is the rate of flow of charge.
• When an electrical charge flows through a resistor, electrical energy
is transformed into heat energy.
• The rate at which energy is transformed in a device is called the
power.
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• Power, potential difference and current are related by the equation:
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• Energy transformed, potential difference and charge are related by
the equation:
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• The amount of electrical charge that flows is related to current and time
by the equation:
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