Voltage and current
Resistance and current
Georg Ohm's quantitative law |
In this unit you will investigate in a quantitative way, how voltage, current and resistance relate to each other in a circuit.
Voltage and current
The higher the voltage of a battery, the more potential energy per coulomb
of charge it can supply. Voltage is sometimes referred to as potential
difference (or PD), which indicates the energy difference between the
terminals of the battery or power supply. Mains electricity, 240 volt
has much more potential energy per coulomb than a 1.5 volt cell. This
is what makes mains electricity so dangerous, with enough energy to seriously
injure or kill.
All other things being equal, an increase in voltage will produce
a corresponding increase in current, or flow of electrons. Voltage and
current are related in a positive way, as one increases the other increases
and vice versa. The graph to the right illustrates the positive correlation
between current and voltage.
Note: In real circuits, resistors, like light globes, heat up and increase
their resistance as the current flows though the circuit. As resistance
increases for a set voltage, the current no longer increases proportionally.
A more realistic graph for a real circuit is shown here.
Resistance and current
There is another variable called resistance. Resistance can slow you down,
for example, the greater the resistance when you are trying to move through
a crowd, the slower you can move. Your progress is impeded - you might
find the word 'impedance' used in a specialised way to describe a form
of resistance in some more advanced electronics texts.
Assuming that the voltage remains the same, what would happen to
the current if the resistance within a circuit is increased? This is like
plugging more globes into a circuit, as the resistance increases, the
current decreases. In this case, there is a inverse relationship between
the two variables. As the resistance increases, the current decreases,
provided all other factors are kept constant.
Materials with low resistance, metals for example, are called electrical conductors and allow electricity to flow easily. Those materials with high resistance, like plastics, are called electrical insulators.
Georg Ohm put these observations into an equation to allow measurement of each variable. This law is named after him and is called Ohm's Law. See the topic Ohm's law.
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