What electricity is
What is already in a wire
Everything on this bench is made of atoms, and every atom carries charge: a positive core and negative electrons around it. In a copper wire the outermost electrons are barely held. They wander from atom to atom all the time, in every direction, going nowhere in particular. That is a wire with nothing happening in it.
the wire is already full; closing the loop moves all of it at once.
Current, and how much of it
Current is what you get when those electrons drift one way on purpose. We measure it in amperes, and one amp is a lot of electrons: about six billion billion passing a point every second. The kit works in thousandths of an amp, milliamps, written mA. A red LED wants about 20 mA. The pushbutton passes that easily; your finger does not.Try it on the bench: Light the LED →Try it on the bench: Load and sag →Try it on the bench: Squeeze it →
current is a count past a point, and in one loop it is the same at every point.
Voltage, the push
Voltage is the push. A battery uses a chemical reaction to pile up extra electrons on one terminal and leave the other short of them, and the difference between the two is the voltage, measured in volts. Nine volts means each bit of charge that makes the trip from one terminal to the other gives up nine joules per coulomb of energy along the way, in the resistor as heat and in the LED as light.Try it on the bench: Lightning →Try it on the bench: Sun to light →
volts are the push, not the stuff being pushed.
The picture every shop teacher draws
The water analogy is the one every shop teacher reaches for, and it is good: voltage is pressure, current is the flow in litres per second, and a thin pipe is resistance. A pump is the battery. It gets you through the first half of this course.
water leaves a cut pipe; charge does not leave a cut wire — it stops.
What really happens
Here is where it breaks. Cut a pipe and water pours out. Cut a wire and nothing pours out, because charge cannot leave the metal; the electrons just stop drifting. And a battery does not squirt electrons down an empty hose. The wire is already full of them, end to end, so when you close the switch the whole column starts moving at once. That is why the LED lights the instant you press the button, even though any one electron crawls along at less than a millimetre a second.
the push arrives at once; the charge itself moves slower than a walk.
One habit to pick up
One more habit to pick up now. We draw current flowing from + to −, the way it was guessed before anyone knew about electrons. The electrons actually go the other way. Both descriptions give the same answers, so the shop uses the old arrow and does not lose sleep over it.