Electromotive Force of a Cell

Every electrical device — from a torch battery to a solar panel to a fuel cell — needs something to keep pushing charge around a circuit. That "something" is the electromotive force (emf). Without it, current would die out the instant resistance drained the charges' energy. Understanding emf is the foundation for analysing real circuits, cell combinations, and internal resistance.

Electromotive Force (emf) is the work done by a cell (or any energy source) in driving a unit positive charge around the complete circuit, including through the cell itself.

ε = \[\frac {dW}{dq}\]

SI Unit: volt (V), where 1 V = 1 J C⁻¹
Dimensional Formula: [ML²T⁻³A⁻¹]

• A cell converts chemical energy → electrical energy through reactions at its electrodes.
• Inside the cell, positive charge moves from the low-potential (negative) terminal → high-potential (positive) terminal — i.e., against the electrostatic field, driven by chemical action.
• Outside the cell (in the external circuit), conventional current flows from the positive terminal → the negative terminal, driven by the potential difference.

Analogy: Think of a cell like a water pump in a closed pipe loop. The pump does not create water; it lifts water from a low tank to a high tank, maintaining a pressure difference (like emf) that keeps water (charge) flowing through the pipe (circuit), even though gravity (resistance) constantly pulls it back down.

When external resistance R → 0 (cell terminals directly connected):