Electromotive force

Any device that maintains a current must do work on charge, pushing it from low potential to high potential inside the source so it can flow through the external circuit. Electromotive force is that work per unit charge. Batteries supply it through chemical reactions, generators through electromagnetic induction, solar cells through the photovoltaic effect. The unit is the volt, which is one joule per coulomb.

Real sources have internal resistance, so the voltage you measure across the terminals is less than the emf whenever current flows. The relationship is terminal voltage = ε − Ir, where ε is the emf, I is the current, and r is the internal resistance. A battery with an emf of 12 V and 0.5 Ω of internal resistance delivering 4 A shows a terminal voltage of 12 − (4)(0.5) = 10 V. Only when no current flows does the terminal voltage equal the emf.

In electrochemistry, the emf of a galvanic cell is the difference between the reduction potentials of its two half-reactions — the standard cell potential E°cell equals E°cathode minus E°anode. A positive cell potential indicates a spontaneous reaction, and it relates to free energy by ΔG° = −nFE°. When concentrations depart from standard conditions, the Nernst equation gives the actual cell potential. Emf also arises without any battery at all: Faraday's law of induction states that a changing magnetic flux through a loop induces an emf equal to the negative rate of change of that flux.

The MCAT tests all three faces of emf within the chemical and physical foundations section — circuit behavior and terminal voltage, galvanic cell potentials and spontaneity, and induced emf from changing magnetic fields. Being comfortable moving between the circuit definition and the electrochemical one is what most MCAT emf questions actually require.