An electric current can be broadly defined as charged particles in motion - a flow of charge. The most obvious effect of a current would seem to be the transfer of charge from one place to another. So it is, when a charged body is discharged by touching it with an earthed wire, or when lightning strikes a tree. However, it is important to realise that most currents flow in closed loops or circuits with no net transfer of charge. When there is a steady current the distribution of charge around a circuit remains constant and every part of the circuit remains substantially neutral.
The effects of electric current
You
can't see anything move in an electric current but currents do have a number of
extremely important effects, which can be broadly classified as shown in the
first column of table 3.1. This list is by no means exhaustive. Some of the
topics are discussed in later chapters. In this chapter we are concerned in
general terms with the charge, mass and energy transferred by a current
The measurement of electric current
Electric
current is measured by means of a device which responds quantitatively to one
or other of the effects of the current. Because current is a flow of charge,
the device usually has to be connected so that the current goes through it.
Each
system has advantages and disadvantages. The choice is determined by the
exigencies of the situation. For example a clip-on ammeter does not require the
interruption of the current, but it is bulky and not very sensitive. By
contrast a digital ammeter responds quickly. Moving coil meters have the
advantage of simplicity and long-term stability.
Movement of charge
Any
movement of charge constitutes an electric current. The charge carriers could
be electrons in a vacuum, electrons in a metal, 'holes' in a semiconductor or
ions in a solution. The charge may move in free space, through a conductor or
on a conveyor belt. An obvious manifestation of charge movement is an electric
spark. At each spark some charge is transferred. Together a sequence of sparks
makes up an intermittent current in which each spark contributes a current
pulse. If the spark gap is narrowed so that the sparks become more frequent,
they tend to merge to make a continuous current.
Definition and unit of current
The
value of a current in a wire at any point is defined to be equal to the rate of
flow of net positive charge past that point. The direction of the current is
defined to be the direction of flow of positive charge.
If the charge carriers are actually
negative then the direction of the current is opposite to the flow direction of
the particles. Thus a current of one ampere to the right could be either
positive charge flowing to the right at 1 coulomb per second or negative charge
flowing to the left at 1 coulomb per second, or some combination of flows in
both directions such 0.5 C.s-1 each way.
The
SI unit of electric current is the ampere (symbol A), equal to one coulomb per
second. Because current is easier to measure than charge, the physical
standards have been established using current as the base quantity, so the
coulomb is defined as an ampere-second (A.s). The ampere is defined in terms of
magnetic effects
Current
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When
there is an electric current in a circuit, energy is generally being
transferred from a source to a load: energy is transferred from a battery to
a lamp or from a nerve cell to measuring electrodes. In such circuits, the
current is associated with the energy transfer.
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