A generator is essentially just an electric motor working in reverse. 

An electric motor is essentially just a tight coil of copper wire wrapped around an iron core that’s free to rotate at high speed inside a powerful, permanent magnet. When you feed electricity into the copper coil, it becomes a temporary, electrically powered magnet, in other words, an electromagnet and generates a magnetic field all around it.

This temporary magnetic field pushes against the magnetic field that the permanent magnet creates and forces the coil to rotate. The coil can be made to rotate continuously in the same direction, spinning round and round and powering anything device that requires an electric current. 

So how is a generator different? Suppose you have an electric drill with a rechargeable battery inside. Instead of letting the battery power the motor that pushes the drill, what if you did the opposite? What if you turned the drill back and forth repeatedly? What you’d be doing would be manually turning the electric motor’s axle around.

That would make the copper coil inside the motor turn around repeatedly inside its permanent magnet. If you move an electric wire inside a magnetic field, you make electricity flow through the wire, in effect, you generate electricity. So keep turning the drill long enough and, in theory, you would generate enough electricity to recharge its battery. That, in effect, is how a generator works.

In practice, you need to put in a huge amount of physical effort to generate even small amounts of electricity. You’ll know this if you have a bicycle with dynamo lights powered from the wheels: you have to pedal somewhat harder to make the lights glow—and that’s just to produce the tiny amount of electricity you need to power a couple of torch bulbs. A dynamo is simply a very small electricity generator.

At the opposite extreme, in real power plants, gigantic electricity generators are powered by steam turbines. These are a bit like spinning propellers or windmills driven using steam. The steam is made by boiling water using energy released from burning coal, oil, or some other fuel. (Note how the conservation of energy applies here too. The energy that powers the generator comes from the turbine. The energy that powers the turbine comes from the fuel. And the fuel, if it’s coal or oil originally came from plants powered by the Sun’s energy. The point is simple: energy always has to come from somewhere.)

Most of the time we    take electricity for granted. We switch on lights, TVs, or clothes washers without stopping to think that the electrical energy we’re using has to come from somewhere.

But what if you’re working outdoors, in the middle of nowhere, and there’s no electricity supply you can use to power your chainsaw or your electric drill?  One possibility is to use cordless tools with rechargeable batteries. Another option is go for pneumatic tools, such as jackhammers. These are entirely mechanical and powered by compressed air instead of electricity.

A third option is to use a portable electricity generator. It’s simply a small petrol engine, similar to the compact engine you get on a motorcycle, with an electricity generator attached. As the engine chugs away, burning up gasoline, it pushes a piston back and forth, turning a generator and producing a steady electric current as its output. With the help of a transformer, you can use a generator like this to produce pretty much any voltage you need, anywhere you need it.

As long as you have enough gasoline, you can make your own electricity supply indefinitely. But remember the conservation of energy: run out of gas and you run out of electricity as well!

eddie@afrowebs.com