TOM: I am with Robert Llewellyn, from Scrapheap Challenge and Red Dwarf and many other things
besides, but more importantly: electric car enthusiast.
What are we in?
ROBERT: We're in a Tesla Model S.
TOM: I am not good with strong acceleration.
If you’re in a fossil-fuel powered car, then it gets lighter as you burn off fuel.
Racing cars have to manage that really carefully.
Too much fuel, they're too heavy, they're slowed down.
Too little fuel, they run out on the track.
But I had a thought the other day: does that apply to electric cars as well?
Because energy is equivalent to mass, according to the physics I learned.
ROBERT: Yeah.
TOM: So as you lose energy, or as you give up that energy into kinetic...
do you lose mass, do you technically become lighter?
I thought I'd be able to researching that really easily.
I wasn’t.
The internet has many many conflicting amateur physicists' opinions about this.
So I asked professional physicists.
I asked more than half a dozen professional physicists, including Professor Philip Moriarty,
from Sixty Symbols.
And the overwhelming consensus is that when the battery is full, it is heavier.
Bloody---- [choking noise]
God, that's fast!
We're fi--- we're already at -- 60!
70!
80!
Now, electrons aren’t being moved out of the battery right now.
It’s more subtle than that, in ways I don’t understand and that it would take several
hours, or maybe several years, of physics lectures to me to understand.
120!
But we can work out exactly what the maximum difference is, using the world’s most famous
equation.
Energy -- 135!
Energy is equal to mass times the speed of light squared.
Now, this battery in this car holds about 85 kilowatt-hours, which is about 306 million
joules.
Divide both sides of that equation by the speed of light squared, and the result is
about 3.4 micrograms, or about -- oh, that's strong braking!
-- wow! -- or about the weight -- all through that, my voice was up here, I was straining.
Or about the weight of a small grain of sand.
It's impossible to measure, we lost more than that off the tyres during that brake run.
ROBERT: [laughter]
TOM: And I could spit out the window and we would get a hundred thousand times more weight
lost.
Literally.
I did the maths.
But the bottom line is: that acceleration, that use of battery power has meant that we
are lighter and so as we accelerate again...
...we are going ever so slightly faster.
Robert Llewellyn’s new book is called News from the Clouds, it's out now.
Robert, thank you very much for being a wonderful test driver!