Download Electric charge and current

Electric current
When you turn on an electric light an electric current flows in the wire. Do not think of it like water
coming from a tap – the electricity current does not flow out from the switch – it is already in the
wire – connecting the lamp to the power supply via the switch simply gives it the energy to flow.
This energy can come from a variety of sources – kinetic as in a dynamo, a chemical reaction in a
cell, light falling on a photoelectric cell, heating the junction of two metals in a thermocouple, sound
in a microphone or mechanical stress in a piezo-electric crystal.
When an electric current flows electrical energy is converted to other forms of energy such as
heat, light, chemical, magnetic and so on.
We will now look more closely at the nature of an electric current.
Consider a piece of metal wire - a very much enlarged view of which is shown in Figure 1 .
atom/ion
electron
Figure 1
A piece of wire is made of millions of atoms and each one of these has its own cloud of electrons.
However in a metal there is a large number of electrons that are not held around particular nuclei
but are free to move at high speed and in a random way through the metal. These are known are
free electrons and in a metal there are always large numbers of these. It is when these free
electrons are all made to move in a certain direction by the application of a voltage across the
metal that we have an electric current.
The difference between a metal (a large and constant number of free electrons), a semiconductor
(a few free electrons, the number of which varies with temperature) and an insulator (no free
electrons) is shown in Figure 2.
semiconductor
insulator
conductor
Figure 2
Each electron has only a very small amount of electric charge, and it is more convenient to use a
larger unit when measuring practical units of charge. This unit is the coulomb.
The charge on one electron is -1.6x10-19 C.
Usually written as e. You would need about 5x1018 electrons to have a charge of one coulomb!
The electrical charge passing any one point in a circuit in one second is called the electric current,
and it is measured in Amperes (A).
The Amp can be defined in the following way:
A current of 1A flows in a wire if a charge of 1C passes any point in the wire
each second.
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Current = Charge / Time
or
Charge = Current x Time
Example problems
1. Calculate the current in a wire if a charge of 240C passes a given point in 3 minutes.
Current = Q/t = 240/180 = 1.33A
(NB time in seconds)
2. A current of 25 mA flows for 10 ms.
(a) What charge has passed?
(b) How many electrons have flowed past that point in the circuit
(a) Charge = Current x time = 0.025 x 0.010 = 0.00025 C = 2.5x10-4 C
(b) Number of electrons = 2.5x10-4/1.6x10-19 = 1.56x1015 !
An alternative definition of the ampere (amp) based on fundamental quantities is:
A current of one amp is flowing in two parallel conductors placed one metre
apart in a vacuum when there is a force between them of 2x10-7 Nm-1.
You can find out more about this definition in the file headed: Forces between currents
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