Which of the following expresses Ohm's law to calculate current from voltage and resistance?

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

Answer: (D)

Current in a circuit depends on how hard the voltage pushes charges and how much the circuit resists that flow. Ohm's law gives a direct way to calculate it: current equals voltage divided by resistance. In other words, as voltage goes up with the same resistance, more current flows; as resistance goes up with the same voltage, less current flows. This relationship also means the unit works out correctly: volts divided by ohms gives amperes.

The other forms don’t match this calculation. Reversing the division to current equals resistance divided by voltage would imply more current with more resistance, which isn’t right. Multiplying voltage and resistance would give a value with incorrect units for current. Expressing resistance as voltage divided by current doesn’t give current directly from voltage and resistance, even though it’s a rearranged form of the same law.