You will not be asked to state it. You will be asked to use it under pressure.
The theory questions are not hard. They are fast, and they are where people bank the time they need for the calculations.
On this page
The relationship
Voltage equals current times resistance. Everything else on the theory portion is that sentence rearranged, or that sentence combined with power.
Power is voltage times current. Substituting one into the other gives you the versions with squares in them, which you never need to memorize separately because you can derive them in about four seconds.
Memorizing twelve formula variants is how people run out of room. Learn two and rearrange.
The rearrangements you actually use
- Given voltage and resistance, current is voltage over resistance.
- Given power and voltage, current is power over voltage.
- Given current and resistance, power is current squared times resistance.
- Given voltage and resistance, power is voltage squared over resistance.
The last two are the ones people reach for a chart to find. They fall out of substitution and are worth deriving once so you trust them.
Where volt-amperes and watts diverge
On a purely resistive load they are the same number, which is why the distinction feels academic until it is not.
On anything with a motor or a transformer in it they are different, and the code works in volt-amperes for load calculations precisely because it does not want to assume a power factor.
A question that gives you a power factor is telling you it wants the distinction. A question that does not is usually resistive.
Three-phase, and the only extra piece
Three-phase power brings in the square root of three. That is the entire difference in the arithmetic.
It appears when you are relating line values to total power, and it does not appear when you are working within a single phase.
Getting this wrong produces an answer off by about 1.73 or about 3, which is often close enough to a distractor to be chosen.
What to practice
Speed, not understanding. You already understand this.
The theory questions are the cheapest points on the paper and the fastest to bank. Every minute saved here is a minute available for a load calculation, which is where the paper actually gets hard.