Which unit measures the resistance of an electric current?

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

Answer: (B)

Resistance is measured in ohms—the unit that expresses how strongly a material resists the flow of current. This is tied to Ohm’s law, which states V = I × R, linking voltage, current, and resistance. Knowing the voltage and resistance lets you find the current, or knowing the current and voltage lets you find the resistance. The ohm is defined as one volt per ampere, so a higher resistance means less current for a given voltage. The other units have different meanings: ampere measures the amount of current, volt measures electrical potential difference, and watt measures the rate of electrical power transfer. That's why the correct unit for resistance is the ohm.