Ohm's Law & Resistor Colour Codes

Revision: component symbols

Before tackling Ohm's Law, make sure you can recognise these circuit basics:

• Cells in series add their voltages together (two 1.5 V cells in series give 3 V); cells in parallel keep the same voltage but can supply current for longer.
• Lamps in series share the circuit's voltage and all go out if one fails (one path for current); lamps in parallel each get the full voltage and keep working if one fails (multiple paths for current).
• Switches in series behave like AND logic — every switch must be closed for current to flow. Switches in parallel behave like OR logic — current flows if any one switch is closed.
• Conventional current is defined as flowing from the positive terminal of a cell, through the circuit, to the negative terminal.

Ohm's Law — the relationship

Ohm's Law describes how voltage, current and resistance relate in a circuit: if the resistance stays constant, increasing the voltage across a component causes the current through it to increase too. Voltage is the "push"; resistance is the "opposition"; current is the resulting flow.

The three forms of the equation

Ohm's Law can be rearranged to solve for whichever quantity is unknown:

Resistor colour codes

Resistors control how much current flows in a circuit. Their resistance value must be marked somehow:

• Low-value resistors often simply have their resistance value printed on them in numbers.
• Higher-value resistors are marked with coloured bands instead, since printed text would be too small to read reliably.

On a standard four-band resistor: the first three bands combine to give the resistance value in ohms (the first two are significant digits, the third is a multiplier), and the fourth band is a tolerance rating — how accurate the resistor is, given as a percentage (e.g. gold = ±5%, silver = ±10%).