Which statement expresses Ohm's law and identifies the unit of resistance?

• A. V = IR; resistance unit is the ohm (Ω).
• B. I = VR; resistance unit is the ohm (Ω).
• C. V = IR^2; resistance unit is the volt (V).
• D. R = V/I; resistance unit is the ampere (A).

Answer: (A)

Ohm’s law shows how voltage, current, and resistance are connected through a simple proportional relationship. The voltage across a resistor equals the current through it times its resistance: V = IR. This form directly expresses resistance as the constant of proportionality between voltage and current, meaning the same R works no matter the level of applied voltage (within the ohmic range).

The unit of resistance is the ohm, defined as 1 volt per ampere (Ω = V/A). So using V = IR with V in volts and I in amperes naturally gives R in ohms. The other forms either mix up the dependence (I = VR would imply current scales with voltage times resistance) or misstate the unit (R = V/I is a valid rearrangement, but its unit is ohms, not amperes), or square the resistance (V = IR^2), which isn’t correct for Ohm’s law.