Which expression gives current in terms of voltage and resistance?

• A. I = V * R
• B. I = V / R
• C. I = R / V
• D. I = V - R

Answer: (B)

Current is set by Ohm's law: the current through a conductor equals the voltage driving the current divided by the resistance the current faces. In symbols, I = V / R. This makes sense because increasing the voltage pushes more charge through for the same opposition, while increasing resistance reduces the current for a given voltage. The units line up: volts divided by ohms gives amperes.

The other forms don’t fit. I = V * R would combine volts and ohms to give a quantity with the wrong units for current. I = R / V would yield ohms per volt, which does not equal current. I = V - R mixes quantities that aren’t directly subtractable to produce current.