If a circuit with E = 12 V and R = 4 ohms has a current I, what is I?

• A. 1 A
• B. 4 A
• C. 3 A
• D. 12 A

Answer: (C)

Current is determined by Ohm's law: I = V / R. With a 12-volt source across a 4-ohm resistor, the current is 12 divided by 4, which equals 3 amperes. If you sanity-check the other options: 1 amp would need only 4 volts, 4 amperes would need 16 volts, and 12 amperes would need 48 volts across the same resistance. So 3 A fits the given voltage and resistance.