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:
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.

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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. This makes sense because electrons are negatively charged and are therefore repelled by the negative
terminal and attracted to the +ve terminal. This is the actual direction that current flows in a circuit.
Worked Example:
The current through resistor X would be 3.0 A.…read more

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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 resistance.
Drift Velocity = Current/ Number Density x Area x Electronic Charge.
A good conductor has a huge number of conduction electrons that are free to move. Metals such as
copper are good conductors, because they have large numbers of these conduction electrons.
Insulators have very few or no conduction electrons.…read more

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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.f of 3V, but the right cell has an e.m.f of 1.5V. The negative terminals of each cell are
connected, therefore we must subtract the e.m.f's. The overall e.m.f would be 1.5V.…read more

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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.
This is because some of the voltage may be used in heating an extension lead/wire. In addition, some of
the voltage may be lost due to internal resistance of a battery.…read more

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This is because V = IR. At a constant temperature, R is constant, so V is directly proportional to I. Note on a V/ I graph,
if V is directly proportional to I then a straight line passing through the origin is produced. This is true for all variables
that are directly proportional to each other.
Diodes, Bulb, thermistors and light dependent resistors don't follow Ohm's Law.
Only resistors follow Ohm's Law.…read more

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I-V for an LED:
I-V for an LED:
Diodes are simple devices which allow an electric current to pass in only one direction. The resistance in the
opposite direction we say is infinitely high (the current in this direction is zero).
Diodes only operate above a threshold voltage. Below this threshold voltage, the resistance is infinite and
the current is zero. As the voltage increases, the current rises exponentially.
Uses and Benefits of L.E.…read more

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Light Dependent Resistors: Resistance decreases as light intensity decreases. Sensitivity is greater in low
light.
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.…read more

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The conduction electrons must progress through a more turbulent mass of atoms. This
increases the resistance, thus also the resistivity.
Variation of Resistance of a Semiconductor with Temperature:
An increase in temperature means a lower resistance.
As the temperature increases, more electrons can break free of their atoms to become conduction electrons.
At the same time there are more collisions, but this number is small in comparison.
This increases the current, which decreases the resistance, and thus the resistivity.…read more

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The kilowatt-hour is a unit of energy, not power. 1 kWh=3.6x106J
It is easier for companies to measure in kilowatt-hours as typical energy consumption per month in a home
in about 100 kWh, which is easier to deal with than 3.6x108J because to the average person, standard form
means nothing, and large numbers scare people.…read more

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