Quantitative relationship between current, voltage and resistance
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.

Click for larger image 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.

Click for larger image
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.

Click for larger image 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's quantitative law
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.

Copyright owned by the State of Victoria (Department of Education and Early Childhood Development). Used with Permission.

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Qualitative Quantitative Voltage Electric current