Resistance is defined as

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

NOTE: This is NOT the gradient of a V-I curve. Instead, just read off values and do the calculation.

Metal wire at constant temperature behaves such that its resistance is constant for different values of current. This means current is directly proportional to potential difference.

Materials and components behaving like this are called Ohmic materials and components.

The most obvious Ohmic electronic component is the fixed resistor.

If a bulb has a current of 0.25 A and runs at 240V, what is its resistance?

$$ R = \frac{240}{0.25} $$

$$ R = 960 \; \Omega $$

A resistor of 2.5 \(k\Omega \) has a potential difference of 3 V across it. What is the current flowing through it?

$$ I = \frac{3}{2.5 \times 10^3} $$

$$ I = 1.2 mA $$