A circuit carries a current of 2 A through a 5-ohm resistor. What is the emf (voltage) E?

• A. 10 V
• B. 2 V
• C. 5 V
• D. 7 V

Answer: (A)

Ohm's law links current, resistance, and voltage: the voltage supplied in a simple circuit is the product of the current through the element and its resistance. With a current of 2 A through 5 Ω, the voltage is V = I R = 2 × 5 = 10 V. In a basic source–resistor loop, the emf equals this voltage, so E is 10 V. The units work out since A·Ω = V.