Inside the battery
Kids tend to ask these questions after Big Idea 4. Each animation answers one.
Why can ions cross the liquid, but electrons can’t?
The liquid inside a battery, the electrolyte, has lithium salt dissolved in it. A lithium ion is a whole atom with an electric charge, so it can drift through the liquid like a swimmer crossing a pool.
Electrons can only move through materials that have loose electrons to pass them along, like the copper in a wire. The liquid has none, so for an electron it’s a wall. The motor sits on the wire, so every electron has to go through it and spin it on the way.
- Follow one atom slows down to a single lithium atom and narrates each step.
- Cut the wire stops everything, even the ions. Ions and electrons have to move together, one for one.
- Electron tries the shortcut shows an electron bouncing off the liquid.
Swipe sideways to see the whole drawing.
Two paths: blue electrons take the metal wire through the motor, orange ions swim through the liquid. Every ion that crosses is matched by one electron.
Why does lithium give up its electron in the first place?
A lithium atom has 3 protons and 3 electrons. Two electrons sit close to the center, held tight. The third sits alone, farther out, and is only loosely held.
The other side of the battery, the cathode, pulls on electrons much harder. Moving there is a downhill trip that releases energy, like a ball rolling down a hill. The height of that hill is the battery’s voltage, about 3.7 volts per cell.
Swipe sideways to see the whole drawing.
Lithium has 3 protons and 3 electrons. Two electrons hug the nucleus in a tight inner layer. The third orbits alone, far out. Try pulling each one.
The animation uses cobalt oxide, which many phone batteries use. Tesla hasn’t said exactly which materials are in the Roadster’s battery; many batteries use nickel or iron instead, and the idea is the same.
Are electrons always negative?
Yes. Every electron has exactly the same negative charge. What changes is the atom. Lose an electron and it becomes a positive ion, like Li⁺. Gain one and it becomes a negative ion, like the chloride in table salt.
The names are an accident of history. Around 1750, Benjamin Franklin picked which kind of charge to call positive, about 150 years before anyone discovered the electron. That’s why circuit diagrams draw current flowing from + to −, while the electrons actually travel from − to +. You can see it in the first animation: electrons leave the anode, the negative side.
An independent lesson. Not affiliated with, sponsored by or endorsed by Tesla, Inc. or SpaceX.