Ohm’s Law Explained Simply (Voltage, Current, Resistance and Power)
A beginner-friendly explanation of Ohm’s law and electrical power with worked examples for students and hobbyists.
The three quantities
Voltage (V, in volts) is the electrical “push”. Current (I, in amperes) is how much charge flows. Resistance (R, in ohms, Ω) is how strongly a component resists that flow.
Ohm’s law
V = I × R. Rearranged, I = V ÷ R and R = V ÷ I. Example: a 12 V supply across a 100 Ω resistor drives a current of 12 ÷ 100 = 0.12 A (120 mA).
Adding power
Power (P, in watts) is P = V × I, which also gives P = I² × R and P = V² ÷ R. In the example above the resistor turns 12 × 0.12 = 1.44 W into heat, so a small ¼ W resistor would overheat — you would need one rated for at least 2 W.
The Ohm’s law calculator finds the other two values from any two you know.
Reading real resistors
Through-hole resistors show their value with coloured bands. Use the resistor colour code tool to turn the bands into ohms and tolerance before you build your circuit.
Always work safely: mains electricity is dangerous and should only be handled by qualified people.