voltmeters

Meaning: A voltmeter reads the potential difference (voltage) across a circuit element. It is wired in parallel with that element, so both have the same voltage across them. To avoid disturbing the circuit, it must divert almost no current, so an ideal voltmeter has infinite resistance and a real one has a very high resistance. An ammeter is the opposite: it goes in series and has very low resistance.

Equation: V = IR, where V is the potential difference across the element (volts, V), I is the current through it (amperes, A), and R is its resistance (ohms, Ω). The voltmeter reading across a resistor equals I times that resistor's R.

On the MCAT: A passage shows a circuit diagram with meters, and you pick the correct placement or predict a reading. You may use Kirchhoff's loop rule or a voltage divider to find the voltage across one resistor. A common question asks what happens if a voltmeter is placed in series. Its huge resistance cuts the current to nearly zero, so it reads almost the full source voltage. Another question: a real voltmeter in parallel slightly lowers the equivalent resistance of that branch, so its reading is slightly lower than the true value.

Common confusion: Students mix up voltmeter and ammeter placement. A voltmeter goes in parallel with high resistance, so it adds a negligible current path. An ammeter goes in series with low resistance, so it adds a negligible voltage drop.