Physics Electric Current Revision Notes

Notes that I created on Electric Current for AS Physics

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  • Created by: George
  • Created on: 14-05-11 18:54
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Electric Current
Page 109
Chapter 9
Making a current
The conventional current travels from the positive terminal of the cell to the negative terminal of
the cell (conventional current)
A wire is made of metal. Inside the metal there are negatively charged electron which are free to
move (delocalised or conduction electrons) they allow a metal to conduct an electric current.
The atoms of a metal bind tightly together they usually form a regular array.
In a typical metal (such as copper or silver) one electron from each atom breaks free to become
a conduction electron (the atoms remain positive). However since there are equal numbers of
free electrons (negative) and ions (positive) so the metal has no overall change ­ it is neutral.
The cell exerts an electrical force on the conduction electrons that makes them travel along the
length of the wire. Since the electrons are negative, they flow away from the negative terminal.
This is the opposite of the conventional current.
There is current at all points in the circuit as soon as the circuit is completed. This means we do
not have to wait for the charge to travel around from the cell. This is because charged electrons
are already present throughout the metal.
A current can also be a flow of positive charges an example of this is a beam of protons.
A solution which conducts is called an electrolyte as it contains both positive and negative ions
(they move in opposite directions when connected to a cell).
Particles which contribute to an electric current are known as charge carriers (these can be
electrons, protons or ions).

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Kirchhoff's first law
Current may divide up where a circuit splits into two spate branches. For example, a current of
5.0A may split at a junction or a point in a circuit into two separate current of 2.0A and 3.0A.
Current will not disappear, or with the same respect gain extra! This is the basis of Kirchhoff's
first law.
The sum of the currents entering any point in a circuit is equal to the sum of the currents
leaving the same point.…read more

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If the current increases, the drift velocity must increase. Therefore current and drift velocity are
directly proportional. However if the wire is thinner, the electrons move more quickly so mean
drift velocity is inversely proportional to current.
In a material with a lower density of electrons the mean drift velocity must be greater so mean
drift velocity is inversely proportional to number density.
For a high rate of flow, the water moves fast, this corresponds to a greater current.…read more

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