A battery produces direct current, meaning the current flows steadily in one fixed direction the entire time the circuit is closed. The electricity delivered through a wall socket is fundamentally different: it's alternating current, meaning the direction of current flow itself reverses on a regular, repeating cycle, fifty times per second in most of the world's power grids and sixty times per second in North America and a handful of other countries. This isn't an incidental technical detail — it's the specific property that made large-scale electrical power distribution practical in the first place.
The voltage doesn't just reverse, it rises and falls smoothly
In an alternating current circuit, the voltage doesn't just flip abruptly between two fixed values; it varies smoothly over time, typically following a sine wave, rising from zero to a peak, back through zero to an equal and opposite peak in the other direction, and back to zero again, with one full cycle of this pattern being what's referred to as one full alternation, repeating fifty or sixty times every second depending on the grid. Because the voltage and current are constantly changing rather than holding steady, the effective heating or working power an alternating current circuit delivers is usually described using a root-mean-square value rather than the peak value the voltage briefly reaches, since the peak alone would overstate the circuit's actual average delivered power.
Alternating current made long-distance power transmission practical
The main reason alternating current became the global standard for power distribution, over the direct current systems that were also being developed around the same time, is that alternating current voltage can be efficiently stepped up and down using transformers, a device that only works with a changing, alternating voltage and doesn't function on a constant direct-current voltage in the same simple way. Stepping voltage up to very high levels for the trip along long-distance transmission lines and then back down to safer levels for use in homes and businesses dramatically reduces the energy lost to resistance along the way, making it practical to generate power at large centralised power stations and distribute it efficiently across a wide surrounding region — a capability that direct current transmission systems of the same era genuinely lacked, and that was central to the historical rivalry between competing early power distribution systems.
What we're still unsure about
The basic physics of alternating current, its standard frequencies around the world, and the transformer-based reasoning behind why it won out for large-scale power distribution are all well-established, thoroughly documented electrical engineering, not subject to any real scientific dispute. What's more a matter of engineering and economic judgement than settled fact is how quickly high-voltage direct current transmission — a more modern technology that addresses some of alternating current's own limitations over very long distances or undersea routes — will continue expanding its role within grids still built predominantly around alternating current standards, a genuinely live, ongoing area of infrastructure planning and investment rather than a question with one fixed, universally agreed answer.
This sits inside Alternating Current Circuits, one of eight topics in Electromagnetism, one of five domains in Physics, one of seventeen subjects the app can quiz you on.