Great builders do not stand on pictures they will have to tear down. They learn each thing as it really is, however slow or strange it turns out to be.
⟨ transmission ends ⟩In First Light, the LED lit the moment the black lead touched its leg. Yet the LED sits a long way from the battery, about 15 cm of lead on a real bench. It seems the electricity must rush out of the battery and reach the LED almost at once. The bench below lets you check.
Below, the leads are joined and the LED is lit. The electricity in the leads is drawn as small green dots, and here they move at their real speed. One bit of it, at the battery’s + end, is marked with an amber ring. The clock beside the bench lets the hours pass as fast as you turn its hand. How long do you think the marked bit takes to reach the resistor? Does it get there almost at once, or does it take longer?
The bench is a 9 V battery, a 470 Ω resistor and a red LED, carrying 15 mA. In this model each battery lead is 15 cm of copper, 0.13 mm² thick. The electricity in it creeps at about 0.0085 mm each second, which is about 3 cm an hour. The dots are drawn at that real speed, so without the clock they never seem to move. In First Light they were sped up so that you could see them go round. In copper the electricity is carried by tiny particles called electrons, which drift the other way, from − round to +. Engineers draw it going from + to −, and so does this course. The Understand Electronics course explains why, in The Flow.
It took almost five hours. The electricity in the lead creeps along at about 3 cm an hour. That is slower than the tip of a wall clock’s hour hand. Expecting it to rush from the battery to the light is sensible, since the light comes on so fast. But the LED was lit all that time, so it never waited for electricity from the battery. That leaves a harder puzzle. If the electricity creeps, what lit the LED at once?
Start before the loop is closed. The black lead’s tip hangs just short of the LED’s short leg, so the LED is dark. Is there any electricity in the leads yet, or is it all still in the battery? The magnifier below shows what no real glass can, the electricity inside the metal.
No real magnifier can see this. The glass is drawn to show the electricity inside the metal, each dot standing for a vast number of electrons. They rush about in every direction, far faster than drawn here. With the loop open, as many go one way as the other, so they go nowhere.
Every lead was full before anything was joined. The copper inside each lead is packed with electricity from end to end, and so are the parts’ bare legs. It moves about in place, but it goes nowhere. The plastic sleeve and the table hold none that can move. Every metal is like this. It is full of electricity that is free to move. So the battery’s job is to push on the electricity that is already in the wires.
What happens to all that electricity when the loop closes? Below, three windows watch the leads. The first is where the red lead leaves the + end. The second is at the LED, and the third is where the black lead comes back to the − end. Each magnifies its lead 80 times, so the creep can just be seen at its real pace. When you join the black lead, which window do you think starts moving first?
The push that starts the electricity goes round a loop this size in a few billionths of a second. That is far too fast to see, so the three windows seem to start at the same moment.
None of them went first. All three windows started together, and when you pulled the lead away, all three stopped together. Electricity moving round a circuit is called current. The loop starts and stops as one because each bit of electricity pushes on the next. That push spreads round the loop at close to the speed of light. In a loop this size it takes a few billionths of a second. The electricity already inside the LED starts to move at once. That is why the light comes on the instant the loop closes.
A bicycle chain behaves in a similar way. Every link is already in place, all the way round. Turn the pedal, and every link moves at once, the ones at the back wheel too. Break one link, and nothing turns. The chain picture is wrong in one way. A chain moves as fast as your feet push it, while the electricity in a wire barely moves at all. What races round the wire is only the push.
First Light said that the electricity is not used up in the light. Now you can check it. A counter on the red lead counts the electricity going out of the battery each second. A second counter can be clipped on the black lead, where the electricity comes back. Do you think less electricity comes back along the black lead than goes out along the red lead?
The counters are drawn for this lesson. On count, each counts the dots crossing the white line in its window, and every dot stands for the same amount of electricity. On milliamps, the same count is shown in the unit engineers use. We will meet a real tool that measures current in The Three Numbers.
Just as much came back as went out. The current is the same all the way round the loop, in both leads and through both parts. None of the electricity is used up. What the resistor and the LED take from it is energy. The LED gives its share off as light, and the resistor gives its share off as a little heat.
Every dot on the counters stood for the same amount of electricity, so the counters were really counting amounts. An amount of electricity is measured in coulombs, written C. One coulomb is the electricity carried by a specific number of electrons.
The 10¹⁸ means a 1 followed by eighteen zeros, so one coulomb is a vast number of electrons.
How much electricity passes a point each second is the size of a current. Its unit is the ampere, or amp, written A. One amp is one coulomb passing a point every second.
On a bench, currents are usually small, so they are counted in milliamps. A milliamp (mA) is a thousandth of an amp. This LED runs on 15 mA, which is 0.015 A, and a small LED like this one can carry at most about 25 mA.
This changes how you read a real circuit. When a circuit stays dark, nothing is still on its way, because a current starts everywhere at once. If nothing lights, something is broken. The joints are the first place to look. A torch works the same way. It lights the moment its button closes, though the electricity in its wires creeps too slowly to see.
Four questions, if you want to check yourself.
You join the same battery and LED with leads ten times longer. When does the LED light?
The electricity in a lead creeps a few centimetres an hour. Why does the LED light the moment the loop closes?
A battery, a resistor and an LED are joined in one loop. Is the current bigger just before the resistor or just after it?
A resistor’s leg snaps in the middle of a lit loop. What happens to the current in the red lead, far from the break?
The two counters agreed, but they could not say what decides how big the current is. Would a second battery make it bigger? Would a different resistor make it smaller? The first half of the answer is the push behind every current, and that is the next lesson, The Voltage.
Lesson 2 done.
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