| 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 |
| Current
Electricity |
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| 12.6 |
What does
the current through an electrical circuit depend on? |
The size
of the current in a circuit depends on how hard the supply tries to push
charge through the circuit and how hard the circuit resists having charge
pushed through it. |
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Candidates
should use their skills, knowledge and understanding of how science works:
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• to interpret and draw circuit diagrams using standard symbols.
The following standard symbols should be known:
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• to apply the principles of basic electrical circuits to practical situations |
Simple feedback circuits - alarm systems - double switch circuits etc. |
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• Current-potential difference graphs are used to show how the current through a component varies with the potential difference across it. |
They are called 'characteristic curves' - you should know all of the ones below and how to explain their shape. |
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Their skills,
knowledge and understanding of how science works should be set in these
substantive
contexts: |
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The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor.
Potential difference, current and resistance are related by the equation:
potential difference (volt, V) = current (ampere, A) x resistance (ohm, ) |
V = I R |
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• The resistance of a filament lamp increases as the temperature of the filament increases. |
Is used for illumination - gets brighter as the current through it increases. |
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 • The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction. |
Can be used as a 'one-way' device in a circuit - only allows current to flow easily in one direction. |
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• The resistance of a component can be found by measuring the current through,
and potential difference across, the component.
• The current through a component depends on its resistance. The greater
the resistance the smaller the current for a given potential difference
across the component.
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• The resistance of a light-dependant resistor (LDR) decreases as light intensity increases. |
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• The resistance of a thermistor decreases as the temperature
increases (ie knowledge of negative temperature coefficient thermistor only
is required). |
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• The potential difference provided by cells connected in series is
the sum of the potential difference of each cell (depending on the direction
in which they are connected).
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• For components connected in series:
– the total resistance is the sum of the resistance of each component
– there is the same current through each component
– the total potential difference of the supply is shared between the
components. |
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• For components connected in parallel:
– the potential difference across each component is the same
– the total current through the whole circuit is the sum of the currents
through the separate components. |
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