physics//electromagnetism//electric circuit//voltage source

A voltage source is a device that keeps a nearly constant voltage between its terminals while current is drawn from it, by turning some other energy (chemical, mechanical, light) into electrical push, and it is what makes a circuit run for hours instead of an instant. A battery, a generator, a solar cell and the bench power supply are all voltage sources.

A voltage source is a device that keeps a nearly constant voltage between its terminals while current is drawn from it, by turning some other energy (chemical, mechanical, light) into electrical push, and it is what makes a circuit run for hours instead of an instant. A battery, a generator, a solar cell and the bench power supply are all voltage sources.

The contrast with a charged object makes the point. A charged capacitor also has a voltage, but every bit of charge that flows out lowers it, so a circuit fed from it runs down along a falling curve. A battery holds its voltage because a chemical reaction moves charge back to the terminal as fast as the circuit takes it, for as long as the reactants last; its voltage barely moves until the end, then falls quickly.

The water picture holds here, with a correction. A battery is less a full tank whose pressure drops as it empties and more a pump that keeps the pressure steady until its fuel runs out; that is the property that defines a source.

Real sources sag under load. Every source has some internal resistance, so drawing a large current costs a little voltage inside the source itself; a cold or old battery has more, which is why it fails first at high current.

Capacity is a separate number. A battery's voltage says how hard it pushes, its capacity (in ampere-hours or watt-hours) how long it can keep going: a phone battery holds roughly 15 watt-hours at about 3.7 V.

The cell has one too. A neuron's sodium-potassium pump spends ATP (the molecule cells use as a rechargeable energy currency) to keep the ion gradients, and with them the membrane's resting voltage, steady against every spike that discharges them (sodium-potassium pump in Lobeworks/17).