Applying Ohm’s law

Resistor combinations and applications

Resistors in series

When two resistors, R1 and R2, are connected one after the other in a circuit (series connection), the total resistance is simply the sum of the two:

R_total = R1 + R2

Example: R1 = 10 ohms and R2 = 15 ohms in series give R_total = 25 ohms.

Resistors in parallel

When two resistors are connected in parallel, the equivalent resistance is always less than the smaller of the two resistors. The formula (which pupils should be familiar with at secondary school level in a qualitative sense) is:

1 / R_total = 1 / R1 + 1 / R2

Why Ohm’s Law is useful

This law allows you, for example, to:

Choose the correct resistor to place across an LED (light-emitting diode) to prevent it from burning out.
Calculate the current that will flow through a device before plugging it in, to check that it does not exceed the limit withstood by a fuse.
Understand why an electrical wire heats up: the greater its resistance and the higher the current, the more energy is dissipated as heat.

Guided example

We want to protect an LED with a resistor. The power supply delivers U = 9 V, the LED requires a voltage of 2 V across its terminals, and we want a current of 0.02 A in the circuit.

The voltage across the protective resistor is 9 – 2 = 7 V. We therefore calculate:

R = U / I = 7 / 0.02 = 350 ohms

A resistor of approximately 350 ohms is therefore required.

Common pitfall