Lesson 3: Atoms and Charge
So far we've mostly been talking about 'charges'.
This was because we were trying to tell a story about energy. This story is much easier to tell if we try and forget about what's 'really happening' and just pretend that positive charges carry energy around a circuit.
But in this lesson we'll put our energy story to one side for a bit try and conjure up some images of what you might see if you had a special microscope that meant you could look inside the wires of a circuit. Even then we won't pretend we know what's really going on but we'll try and describe some little sketches that will help us think about some of the issues.
At the end of the lesson we'll introduce the idea of conventional current. This will mean we can tell the same story about energy for all moving charges, whether they're electrons in a wire, ions in an electrolyte or electrons in a spark.
The scale of atoms
So let's start off by imagining what might be happening in the wires of an electric circuit. The first thing to remember is that the wire is made from atoms just like you, me, gibbons, mountains, tangerines, air, water and every other substance you can think of except light.
Atoms are absolutely tiny. If you wanted to build a model of a pea big enough that you could just see its atoms, the model of the pea would have to be the size of the Earth.
But atoms themselves are made up of even smaller parts. At the centre of an atom is the tiny nucleus. The nucleus is so small that if you made a model of an atom as big as a sports stadium, the nucleus would be about the size of an orange.
The rest of the atom is made up of electrons and empty space. Lots of empty space. The reason why things feel solid is that there are big electric fields acting between the electrons and the nucleus. These electric fields mean that atoms can't pass through each other.
No experiment has ever measured the size of an electron and it may be they don't have any size at all. Perhaps they're just pure points.
Electrons in conductors and insulators
In most materials like paper, glass, stone or plastic each atom keeps its electrons to itself. These materials are called electrical insulators because they don't have electrons that are free to move around.
But with metals each atom has a very loose hold on one or two of its outer electrons and they're free to whizz around between the atoms. These electrons are said to be , which is another way of saying they're homeless. They have no idea which atom they came from. So metals are electrical conductors.
When you think of something being a 'conductor of electricity' you need to be a bit careful. There isn't this extra stuff called 'electricity' that runs through wires. All being a conductor means is that the electrons that are already there because they're part of the atoms that make up the material are free to move around. The more electrons that are free to move and the less their paths are blocked, the better a material is at conducting electricity.
Now if you remove an electron from an atom you're leaving it with a slight positive charge. So we can think of a metal wire being made up of slightly positive atoms, which are called 'ions', locked together in a regular grid structure. Amongst this scaffolding of positive ions there is a gas of free electrons zooming aimlessly in all directions.
Wires don't start empty
The key point to note here is that the wire is already full of free electrons. Electric circuits are all about the movement of electrons that are already there. The wires don't start off empty and then electrons from the battery rush down them like water through an empty pipe.
Movement of electrons in the wires of an electric circuit
So what happens to the electrons in a wire when it is part of an electric circuit? The simple answer is that they start moving away from the negative end of the battery and towards the positive.
The most important point to remember is that the electrons are already there in the wires and all the electrons everywhere in the circuit start moving at almost exactly the same time. The other important point is that the progress of the electrons along the wire is extremely slow, perhaps half a metre an hour.
It's worth thinking about this in a little more detail.
Imagine a piece of wire that isn't part of a circuit. It's full of free electrons. We can think of these electrons whizzing around in all directions at about 1% of the speed of light. When the circuit is set up each electron gets a little nudge from the one next to it. These nudges are passed down the wire at around the speed of light, which is why the electrons start moving at almost exactly the same time. The movement along the wire because of the push from the battery is very small compared with the random movement of the electrons. Imagine a bee buzzing around the inside of a glass as you slide it very slowly along a table. The bee is still moving along the table but it spends a lot of its time going in the opposite direction.
How electrons interact with atoms in a metal
Up to now we've been talking about free electrons. But the vast majority of a wire isn't made up of free electrons but of atoms, or at least atoms that are missing an electron, called positive ions.
We can think of electrons colliding with ions and bouncing off. This is a reasonable mental picture but if you try and make it explain too many things it breaks down.
We can use it to explain why electric currents cause heating: electrons bash into ions, making them vibrate more - more vibration is identical to higher temperature.
And we can kind of convince ourselves that it explains why the higher the temperature the higher the resistance: ions vibrate more, electrons find it harder to get past.
A fuller explanation involves the idea of 'phonons'. Because the lattice of positive ions that make up a metal is very regular, the vibrations tend not to be random but have wave-like properties. These waves are explained by quantum theory and are called phonons. Electrons tend to be scattered off phonons rather than individual atoms. The higher the temperature the more phonons there are, the more scattering of the electrons and so the harder it is for them to progress along the wire - hence higher resistance.
This explanation is one I'm by no means fully comfortable with and the main lesson to draw is not to get too tied to any simple model of electrons and ions because you'll quickly find things that it doesn't really explain.
We've seen that in wires the positive things (ions) stay in the same place and the negative things (electrons) move about. If it's the electrons moving then why do we show positive charges in our animations?
The answer is to do with the history of electricity. All the laws about electricity, magnetism and energy were framed with the idea that there were positive charges moving in electric circuits. It was only in the early 20th century that scientists that they could unify lots of ideas in physics and chemistry if they imagined the existence of electrons. Electrons are negative.
This was a bit of a blow but is quite easy to get round. All we need to say is that negative charges moving from left to right are identical to positive charges moving in the opposite direction. We just pretend the negatives no longer exist.
It's actually easier to make this simple adjustment than to rewrite all the physical laws. Find out more about why Furry Elephant uses conventional current.