What is the formula for current when voltage and resistance are known?

• A. I = V / R
• B. I = V × R
• C. I = R / V
• D. I = V - R

Answer: (A)

Current is the rate at which electric charge flows, and Ohm's law relates current, voltage, and resistance with V = I R. Solving for current gives I = V / R. This means current increases as voltage increases and decreases as resistance increases, assuming the other quantity stays the same. For example, with 9 V across 3 Ω, the current is 3 A; if the resistance doubles to 6 Ω, the current drops to 1.5 A. The other expressions don’t yield current in amperes because they mix units or produce quantities with the wrong dimensions (for instance, V × R has units of volt-ohms, not amperes; we’d get volt-per-volt if we did R / V, which isn’t current; and V − R mixes incompatible units).