Moving charge produces a magnetic field and a moving magnetic field produces current.

First of all, if you have charge moving through a wire, then there’ll be a magnetic field that develops around the wire. You’ve probably seen experiments with iron shavings and how they form lines on a piece of paper when a magnet is placed under the paper. The lines go from one end of the magnet to the other in 3 dimensions. Even though the paper only shows 2 dimensions, the magnetic lines still go all around the magnet from one end to the other. The iron pieces line up with the magnetic field and arrange themselves in lines.

What kind of lines do you think would appear around a wire if you were to do the same experiment? Here’s where it’s a little different. You’re not going to see the same kinds of lines because the magnetic field doesn’t form along the wire. It forms around the wire.

The way this effect is normally used is by coiling the wires in lots of loops all in the same direction. This causes the magnetic field to bend around so that it’s going one direction inside the loops and the other direction outside the loops. It also concentrates the magnetic field and you can get a noticeable magnetic effect with current flowing through the looped wire.

This is called an electromagnet because it produces a magnetic field only when electricity is flowing. Listen to the full episode or read the full transcript below to learn about how this effect can be used by computers as well as to change AC voltages.

Transcript

The most important part of that description is moving. If you only remember one thing from this episode, let that be it. There’s some more details that I’ll explain as well as how this effect can be used.

First of all, if you have charge moving through a wire, then there’ll be a magnetic field that develops around the wire. You’ve probably seen experiments with iron shavings and how they form lines on a piece of paper when a magnet is placed under the paper. The lines go from one end of the magnet to the other in 3 dimensions. Even though the paper only shows 2 dimensions, the magnetic lines still go all around the magnet from one end to the other. The iron pieces line up with the magnetic field and arrange themselves in lines.

What kind of lines do you think would appear around a wire if you were to do the same experiment? Here’s where it’s a little different. You’re not going to see the same kinds of lines because the magnetic field doesn’t form along the wire. It forms around the wire.

If you were to poke a hole in the paper and stick the wire through the paper, then you might be able to see some magnetic lines appearing in circles around the wire. I haven’t tried this experiment so don’t know how much current is needed before you can see anything.

The way this effect is normally used is by coiling the wires in lots of loops all in the same direction. This causes the magnetic field to bend around so that it’s going one direction inside the loops and the other direction outside the loops. It also concentrates the magnetic field and you can get a noticeable magnetic effect with current flowing through the looped wire.

This is called an electromagnet because it produces a magnetic field only when electricity is flowing. If you then put a piece of iron or steel which has iron in it inside the coils, then it can pull the metal and hold it inside the coils. Or if the metal is fixed in place but the coil is allowed to move, then the coil can be held in place around the metal. And by adjusting the amount of current, you can control the strength of the magnetic field.

This is how speakers work inside your computer. The coil is attached to a springy surface such as paper and as the current is adjusted it causes the coil to move back and forth. This also moves the paper which pushes on the air. As the paper pushes on the air, it causes sound waves.

This is also how most electric motors work. A motor will have several coils that each interact with permanent magnets and push and pull on the magnets. Either the coils will move and spin inside the permanent magnets or the coils will be fixed in place and the permanent magnets will spin outside the coils. By changing the direction of the current, the electromagnets can switch their polarity completely and change from pushing on a magnet for a while and then start pulling instead.

Computers use motors too in order to drive fans that keep the computer cool. And robots definitely use motors to move around.

An interesting thing about a motor is that they can sometimes be used to generate electricity. The same principle that makes a motor turn when a current is applied will also cause it to generate current when turned manually. This creates a generator.

This is because of the other effect that I mentioned at the beginning of the episode when I said that a moving magnetic field will cause current to flow through a wire. By turning a motor manually, you’re causing the coils to move past permanent magnets. Because the wire is coiled in the same direction many times, then there’s a long length of wire passing through the magnetic field and this causes the charge to want to flow along the entire length of the wire. A simple spin with your fingers is enough to light a small lamp. The important part is movement. A generator that’s sitting still will not generate anything.

I promised last week to also explain transformers and how they can be used to change voltage levels but only for alternating current. It works like this.

You start out with some coils that you then run some current through. This creates a magnetic field like I just described. But instead of putting permanent magnets next to the coils like in a motor or putting some other metal next to the coils that can move around, you put some more coils. These secondary coils are completely separate. In other words, you have two long wires each coiled up next to each other.

If the first coil or the primary coil as it’s usually called is energized with a constant current, then it’ll create a constant magnetic field. This constant field will have no effect on the secondary coil. There will be a brief moment when current first starts flowing through the primary coil and the magnetic field is expanding to its full strength when the secondary coil will react. And another brief moment when current stops flowing through the primary coil and the magnetic field is collapsing. But other than that, once the magnetic field is in place is won’t be moving anymore and the secondary coil will not be affected at all.

This is what happens when the primary coil is powered by direct current.

But if we put alternating current into the primary coil, then the current will increase, stop increasing, then start decreasing until it reaches zero. Then it goes the other way increasing, stopping, and decreasing. Because the current is always changing, then the magnetic field in the primary coil is constantly expanding and collapsing. That’s movement. And that causes a similar current to be generated in the secondary coil.

That explains how AC is able to bridge the gap between the two coils. And by the way, when I say gap, there’s usually still some iron between the coils which just helps the magnetic field pass from one coil to the next. But that doesn’t explain how the voltage can be increased or decreased.

For that, all we need to do is adjust how many loops of wire are in the primary coil vs. the secondary. The more loops there are, the higher the voltage. If both coils have the same number of loops, then the transformer will not change the voltage at all. But if there are more loops in the secondary, then the output voltage will be higher than the input voltage. And if there are more loops in the primary, then the output voltage will be lower.