If you connect a lamp to a battery, a current in the lamp causes it to glow. But what determines the size of the current? This depends on two factors:
the potential difference or voltage V across the lamp – the greater the potential difference, the greater the current for a given lamp
the resistance R of the lamp – the greater the resistance, the smaller the current for a given potential difference.
Now we need to think about the meaning of electrical resistance. The resistance of any component is defined as the ratio of the potential difference to the current.
This is written as:

$resistan ce = \frac{{potential\,difference}}{{current}} = R = \frac{V}{I}$

where R is the resistance of the component, V is the potential difference across the component and I is the current in the component.

You can rearrange the equation to give:

$I = \frac{V}{R}$ or $V = IR$

Table 8.2 summarises these quantities and their units.

Defining the ohm

The unit of resistance, the ohm ($\Omega $), can be determined from the equation that defines resistance:

$resistan ce = \frac{{potential\,difference}}{{current}}$

The ohm is equivalent to 1 volt per ampere:

Table 8.2: Basic electrical quantities, their symbols and SI units. Take care to understand the difference between V (in italics) meaning the quantity voltage and V meaning the unit volt.

Quantity	Symbol for quantity	Unit	Symbol for unit
current	I	ampere (amp)	A
voltage (p.d., e.m.f.)	V	volt	V
resistance	R	ohm	$\Omega $