Which of the following is a unit of electrical resistance?

• A. Ohm
• B. Volt
• C. Ampere
• D. Siemens

Answer: (A)

Resistance is the property that opposes current flow in a conductor. The unit that measures this property is the ohm, symbolized by the Greek letter omega (Ω). Ohm's Law ties voltage, current, and resistance together with V = I × R, so resistance can be expressed as R = V / I. By definition, 1 ohm equals 1 volt per ampere. The other terms describe different electrical quantities: voltage (potential difference), current (flow of charge), and siemens (conductance, the reciprocal of resistance). For example, doubling the voltage while keeping resistance the same doubles the current; changing resistance while keeping voltage constant changes the current inversely. In short, the unit of electrical resistance is the ohm.