How Cars Work
Energy Back
CHAPTER ELEVEN
Six chapters ago, in Chapter 5, we asked a question. A 3,000-pound car going 60 miles per hour has an enormous amount of kinetic energy — one-half mass times speed squared, you might recall. To stop the car, you have to get rid of that energy. Where does it go?
In a gasoline car, the answer was: it gets turned into heat in the brake pads, and then that heat dissipates into the air. The energy is gone. The car burned gasoline to gain that energy in the first place. Then, every time it stopped, it deliberately threw the energy away.
There is a Try It from that chapter where you rubbed your palms together hard for fifteen seconds and felt them get warm. That is what brakes do. The chapter ended with a line: there is nowhere else for the car’s energy to go.
It turns out, in an electric car, there is.
You can put the energy back in the battery.
The system that does this is called regenerative braking. It is one of the cleverest tricks in any modern car, and it follows from a single piece of physics we have already met.
A motor and a generator are the same device
Back in Chapter 3, we used this rule of physics:
A collapsing magnetic field through a coil of wire creates a voltage in that coil.
At the time, we used it to explain how the ignition coil makes 20,000 volts to fire a spark plug. But the rule is more general than that. Any time a magnetic field changes through a coil — getting stronger, getting weaker, moving past, doesn’t matter — a voltage appears in the wire.
The motor we built in the last chapter uses the OTHER direction of this rule. We send a current through a coil, the coil becomes a magnet, and that magnet pushes against another magnet to make rotation.
What if you ran the motor backwards?
Imagine you have an electric motor — coils inside a stator, magnets on a rotor — but instead of feeding current to the coils, you take the shaft of the rotor and spin it yourself. The permanent magnets on the rotor sweep past the coils on the stator. From the coils’ point of view, magnetic fields are constantly changing — getting stronger as a magnet approaches, weaker as it leaves, then the next magnet does the same.
By Chapter 3’s rule, that means voltages appear in the coils.
If you connect those coils to a battery (or any other electrical thing), current flows out of them. The motor, when it is being spun, is making electricity.
This is not a different device. It is the same device. A motor and a generator are the same device. The difference is which direction you run it: feed current in, get rotation out; feed rotation in, get current out. It is one of the most elegant symmetries in physics, and it is the entire foundation of regenerative braking.