AS Physics Electricity OCR Notes

AS Physics Electricity OCR Notes

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Module 1: Electric Current
Spread 1: Electric Current and Charge:
Charge = Current x Time. Q=IT.
One coulomb is the total charge supplied by a current of one amp, in a time of one second. 1C = 1A
X 1s.
The charge on a single electron is 1.6x10-19.
Electric Current in a Metal:
When copper is examined in detail, it is found to be crystalline. This means that the atoms occupy
fixed positions, arranged in a regular lattice. Each copper atom has one electron that is free to
move. These delocalised electrons are known as conduction electrons. These conduction electrons
enable electrical conductivity of a metal.
When there is no current, the general drift of an electron is random.
When there is a current, the electrons generally drift in a certain direction.
Spread 2: Kirchoff's first law:
Key Definitions:

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Conventional Current: A model that describes movement of charge in a wire. Conventional
current travels from the positive terminal of a battery to the negative terminal.
Kirchoff's First Law: The sum of the current entering a junction is the same as the current leaving
the junction. Charge is conserved.
Electron flow is in the opposite direction to conventional current, moving towards the positive
terminal of the battery.…read more

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Electron Drift Velocity: The average velocity of an electron as it travels through a wire, due to a
Conductor ­ Material with a high number density of conduction electrons. Therefore, conductors
must have low resistance, to allow electrons to flow.
A Semi-Conductor ­ A material with lower conduction electrons than a conductor, therefore a
higher resistance.
Insulator ­ A material with a small number of conduction electrons, therefore a very high
Drift Velocity = Current/ Number Density x Area x Electronic Charge.…read more

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Key Definitions:
Electromotive Force: E.m.f is the energy transferred per unit charge, when one type of energy is
converted into electrical energy. It is the energy per charge that is added to the circuit.
Electromotive Force = Electrical Energy Transferred/Charge.
Each cell above is 1.5V. Since the negative terminal of the left cell is connected to the positive
terminal of the right cell, we can add the individual e.m.f. `s. Therefore, the overall e.m.f would be 3V.
The left cell has an e.m.…read more

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Potential Difference: p.d. is the electrical energy transferred per unit charge, when electrical
energy is converted into another form of energy. The potential difference is also the difference in
energy per unit charge that enters a component, and the energy per unit charge that leaves it.
The reading on a voltmeter across a component differs from the e.m.f of a battery.
Across the supply, we read 230V but across the lawn mower it reads 216V. This means that 14 JC-1
are lost.…read more

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Ohm's Law: The current through a conductor is proportional to the potential difference across it,
provided physical conditions, such as temperature, remain constant.
Resistance: Resistance is basically opposition to flow of charge. R = V/ I. The definition of resistance is the
potential difference across a component / current flowing through it. Resistance is measured in Ohms.
This is because V = IR. At a constant temperature, R is constant, so V is directly proportional to I.…read more

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The graph is a curve for a filament lamp. Doubling the voltage does not however, produce double
the current. The value of V/I rises as I increases. As the current increases, the filament gets hotter;
this increases the resistance. The electrons in the filament of the lamp vibrate more and thus,
collide with the metal ions more often. The ions also vibrate, making it difficult for electrons to pass
through.…read more

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They are being used increasingly more for light sources as a substitute to filament bulbs.
One can be used as the `on' indicator on a TV; 100 can be used in a traffic light.
They emit strong sources of light.
They switch on instantly.
Very robust and versatile.
Operate on low p.ds.
Have a long working life.
They must be connected the correct way round as they only allow current to flow in one direction.…read more

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Spread 5&6: Resistivity:
Key Definitions:
Resistivity = Resistance x Cross-Sectional Area/ Length of a Wire. Unit is ohm metres.
Factors that Affect Resistance of a Wire:
Length: A longer wire has more resistance, as the path for electrons is increased, thus collisions
occur more often.
Area: A thin wire has more resistance than a thick wire, as there is less space for electrons to flow.
The space is more compact, so collisions are more likely.…read more

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Key Definitions:
Power: Power is the rate at which energy is transferred, measured in watts.
Fuse: These are devices introduced into circuits to protect the wiring from excessive currents.
These high currents cause wires to get hot, damaging them, and could result in fires. When a fuse
is subjected to higher currents than what they are designed to permit, it will get too hot and melt,
breaking the circuit and preventing hazardous currents.
Kilowatt-hour: Unit of energy. Used by electricity companies when charging for electricity.…read more


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