Ohm's law & power
The relationship between voltage, current and resistance — and how it gives you power.
The relationship
Ohm's law ties together three quantities: voltage (the push), current (the flow), and resistance (the opposition). For a given resistance, more voltage drives more current; more resistance allows less current.
Rearranged, it lets you find any one of the three from the other two — the everyday tool for working out what a reading should be, or what's wrong when it isn't.
Power
Power — the rate of using energy, in watts — comes from voltage and current together. A heater's element, a motor's load, and a cable's heating all come back to power. It's why a high-resistance loose joint carrying load current gets hot: power is being dissipated where it shouldn't be.
On site
Ohm's law underpins fault-finding: an open (infinite resistance) means no current; a short (near-zero resistance) means huge current. Expected readings on a healthy circuit come straight from these relationships.
Related faults
Voltage drop on a long cable run
Equipment at the end of a long run misbehaves — dim lights, a contactor that won't hold, a motor struggling — because volt-drop along the cable leaves too little voltage at the load.
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.
Heater bank drawing uneven current across phases
A multi-element heater bank pulls noticeably different current on each phase. Heating is uneven, output is low, or a phase reads much lower than the others.
Related definitions
Series vs parallel circuits
In series, current is shared and voltage divides; in parallel, voltage is shared and current divides.
Voltage drop
Volts lost along a cable's resistance under load — why the far end of a long run can misbehave.
Heating element
A resistive conductor that turns electrical energy into heat — the business end of most electric heating.