Which formula expresses current I when voltage E and resistance R are known?

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

Answer: (D)

The current is found using Ohm’s Law: the relationship between voltage, current, and resistance. If you know the voltage across a resistor and its resistance, current is obtained by dividing voltage by resistance. So I equals E divided by R.

This makes sense because volts per ohm equals amperes, so the units line up correctly.

Other forms don’t directly express current when voltage and resistance are known: E = IR would be a way to relate the same quantities but doesn’t isolate current; V = IR expresses voltage in terms of current and resistance and would require rearranging to solve for current; and I = R / E is the reciprocal of the correct form, which isn’t correct for current.