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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…

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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…

Page 3

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A Semi-Conductor ­ A material with lower conduction electrons than a conductor, therefore a higher

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…

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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…

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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…

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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

Page 7

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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…

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greater at low temperature.

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

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…

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individual atoms. 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…

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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,…


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