Electricity: what is actually moving, and what is not
Electrons drift through a wire slower than you walk. The signal travels near light speed.
Electricity is confusing mainly because three distinct quantities share one everyday word. Separate them and circuits become arithmetic.
The three quantities
- Current (amperes) - how much charge flows past a point each second. The rate.
- Voltage (volts) - energy given to each unit of charge. The push.
- Resistance (ohms) - how much the material opposes the flow.
A water analogy helps if you do not lean on it too hard: current is the flow rate, voltage is the pressure, resistance is the narrowness of the pipe. The analogy breaks down eventually, but it gets the relationships right.
They are tied by Ohm's law: V = I R. Voltage equals current times resistance. Nearly all of school circuit work is that equation applied in different arrangements.
The thing almost everyone gets wrong
Electrons in a wire move remarkably slowly - the drift velocity in a typical household cable is under a millimetre per second. Slower than you walk. Slower than a snail.
Yet the light comes on instantly when you flick the switch. Both are true, and the resolution is that the wire is already full of electrons. Pushing at one end makes the whole line shift almost immediately, in the same way a full pipe delivers water from the far end the instant you push at the near end - you are not waiting for the first molecule to travel the distance.
What travels near the speed of light is the electric field, not the electrons. The electrons barely move at all.
Parallel resistance dropping below the smallest branch surprises people. Adding another path gives charge somewhere else to go, so the overall opposition falls. This is why house wiring is parallel: every appliance gets the full 230 volts, and switching one off leaves the rest running.
Power, and why it is on your bill
P = V I - power equals voltage times current, in watts. Combine with Ohm's law and you also get P = I squared R, which is the more revealing form.
Because power lost as heat goes as current squared, doubling current quadruples the heat lost in a wire. This is the entire reason electricity is transmitted at hundreds of thousands of volts: the same power delivered at high voltage needs far less current, so the losses in the cables collapse. Transformers then step it back down before it reaches a house.
AC and DC
- DC - direct current, flowing one way. Batteries, and everything electronic internally.
- AC - alternating current, reversing 50 or 60 times a second. Mains supply.
Mains is AC because transformers only work with a changing current, and transformers are what make high-voltage transmission possible. This was argued over bitterly in the 1880s - Edison backed DC, Tesla and Westinghouse backed AC - and AC won on exactly this point.
Why it is dangerous. It is current through the body that harms, not voltage on its own. A static shock can be thousands of volts and is harmless because the current is minute and brief. Mains at 230 volts can push a lethal current through you because your body completes the circuit to earth. Dry skin resists strongly; wet skin far less, which is why water and electricity are a genuinely serious combination rather than a slogan.
What a circuit actually needs
A complete loop, a source of voltage, and something to use the energy. Break the loop anywhere and current stops everywhere in that loop - which is what a switch does, and what a fuse does deliberately when the current exceeds a safe value.