What is flowing
Metals are full of loose electrons. Each atom in a copper wire gives up one or two, and they wander freely among the fixed metal ions like a gas trapped in a sponge. With nothing pushing them they drift at random and nothing happens.
Connect a battery and they all feel a push in the same direction. The whole crowd begins to drift round the loop. That drift is an electric current, measured in amperes: one ampere is about six billion billion electrons passing a point each second.
The push and the obstacle
The battery’s push is its voltage. A higher voltage shoves each electron harder and the crowd moves faster. Everything the electrons have to squeeze through pushes back, and that is resistance. A thin wire resists more than a thick one, a long one more than a short one, and a hot one more than a cold one, because the ions are jiggling harder and get in the way.
Current is simply the push divided by the obstacle. Double the voltage and the current doubles; double the resistance and it halves. That rule is Ohm’s law, and it is the single most useful fact in all of electronics.
Try it: set the voltage to Car and slide the resistance from the bottom to the top. Watch the bulb and the speed of the crowd.
Why the light is instant
Electrons drift slowly, less than a millimetre a second in a household wire, so it seems the bulb should take hours to light. It does not, because the wire was already full of electrons before the switch was thrown. Closing the circuit sets the whole crowd moving at once, like water in a pipe that is already full: turn the tap and water comes out of the far end immediately, though no single drop has travelled far.
Where the energy goes
Electrons bumping through the bulb’s thin filament give up energy with every collision, and the filament heats until it glows. Everything a circuit does, lighting a lamp, turning a motor, charging a phone, is energy handed over by electrons pushing through something that resists them. The battery puts the energy in; the resistance decides where it comes out.
Two kinds of current
A battery pushes one way, giving direct current. The mains supply reverses its push fifty or sixty times a second, giving alternating current, in which the electrons shuffle back and forth rather than drifting anywhere at all. Alternating current is used for the grid because it is easy to change its voltage with transformers, and high voltage lets power cross the country through thinner wires with less loss.