Ohm's law
Summary
Ohm's law states that the potential difference across a conductor equals the current through it times its resistance, V = IR. It holds for ohmic materials, where R stays constant, but not for every device. The MCAT tests how current changes when V or R changes.
Detail
Meaning: Ohm's law relates the potential difference across a conductor to the current through it. For an ohmic material at constant temperature, current is proportional to voltage, so the ratio V/I, the resistance, stays constant. A graph of V against I is then a straight line through the origin with slope R. Non-ohmic devices, such as diodes, give curved graphs.
Equation: V = IR, where V is potential difference in volts (V), I is current in amperes (A), and R is resistance in ohms (Ω).
On the MCAT: Passages give circuits with series and parallel resistors. Find the equivalent resistance, then the battery current, then each voltage drop. Typical predictions: at fixed V, doubling R halves I; adding a parallel branch lowers equivalent resistance and raises total current. Power follows from P = IV = I²R.
Common confusion: Students read V = IR as saying that more voltage changes the resistance. For an ohmic resistor, R depends on material, length, cross-sectional area and temperature, so changing V changes I, not R. Also, use the voltage across the component in question, not the battery voltage, unless it is the only element in the loop.
Sources
- Physics (Giancoli)
- Fundamentals of Physics (Halliday, Resnick, Walker)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.