EMF, Internal Resistance & Cells — A/L Physics

Electromotive Force (EMF, ε) The total energy supplied per unit charge by a source (cell or generator) when no current is drawn. EMF is the maximum potential difference a source can provide across its terminals when the circuit is open (no current flows). It is measured in volts (V). For example, a 1.5 V battery has an EMF of 1.5 V, meaning it gives 1.5 J of energy to each coulomb of charge passing through it.

Internal Resistance (r) The opposition to current flow within the source itself, due to the material and chemical composition of the cell. Every real cell has some internal resistance, typically a few ohms or less. When current flows, some energy is lost as heat inside the cell (I²r loss). This causes the terminal voltage (V) to drop below the EMF. For example, a car battery has very low internal resistance (~0.01 Ω) to deliver high starting current, while a small button cell may have higher internal resistance.