Ohm's Law
The relationship between voltage, current and resistance - the most-used calculation in electrical work.
Why this matters
Where it occurs
Every time an electrician tests a circuit, sizes a resistor, checks a coil or verifies a sensor loop, Ohm's Law is the tool that predicts what the meter should read.
Why calculate it
It converts a circuit diagram into expected numbers, so a real measurement can be compared against a calculated truth.
What decision it supports
Is the reading normal, shorted or open? Is the conductor sized correctly? Which fuse or breaker belongs in this circuit?
What happens if it is wrong
A wrong resistance calculation can mean a coil that never pulls in, a circuit that overheats, or a protection device that fails to protect.
The concept
Ohm's Law states that the current through a circuit is directly proportional to the voltage applied and inversely proportional to the resistance: more push (voltage) means more flow (current); more obstruction (resistance) means less flow.
It is written V = I x R, and rearranges three ways: I = V / R, R = V / I. In trade work, choose whichever form gives you the unknown you need.
Two habits separate professionals from guessers: always convert milliamps to amps before calculating, and always compare the calculated value with the measured one - a mismatch is the fault-finding signal.
Formula
Ohm's Law
V = I x R
- V = Voltage (volts (V))
- I = Current (amperes (A))
- R = Resistance (ohms (Ω))
Worked example
Problem: A 24 V control circuit powers a relay coil of 12 Ω. What current should the coil draw?
- Identify the unknown: current I.
- Rearrange the formula: I = V / R.
- Substitute: I = 24 / 12.
- Calculate: I = 2 A.
2 A. If the meter reads far more, suspect a short; far less, an open or high-resistance connection.
Quick solve: practice it
A 24 V circuit drives a relay coil of 8 Ω. What current flows?
Quick-solve habit: calculate, then ask yourself - does this answer make sense in real units?
Games that use this mathematics
Apply the calculation inside a workplace simulation.