WEBVTT Kind: captions Language: en-GB 1 00:00:00.140 --> 00:00:04.300 TOM: I am with Robert Llewellyn, from Scrapheap Challenge and Red Dwarf and many other things 2 00:00:04.300 --> 00:00:07.209 besides, but more importantly: electric car enthusiast. 3 00:00:07.209 --> 00:00:08.290 What are we in? 4 00:00:08.290 --> 00:00:09.460 ROBERT: We're in a Tesla Model S. 5 00:00:09.460 --> 00:00:11.810 TOM: I am not good with strong acceleration. 6 00:00:11.810 --> 00:00:17.250 If you’re in a fossil-fuel powered car, then it gets lighter as you burn off fuel. 7 00:00:17.250 --> 00:00:19.800 Racing cars have to manage that really carefully. 8 00:00:19.800 --> 00:00:21.740 Too much fuel, they're too heavy, they're slowed down. 9 00:00:21.740 --> 00:00:24.570 Too little fuel, they run out on the track. 10 00:00:24.570 --> 00:00:28.410 But I had a thought the other day: does that apply to electric cars as well? 11 00:00:28.410 --> 00:00:32.079 Because energy is equivalent to mass, according to the physics I learned. 12 00:00:32.079 --> 00:00:33.079 ROBERT: Yeah. 13 00:00:33.079 --> 00:00:38.309 TOM: So as you lose energy, or as you give up that energy into kinetic... 14 00:00:38.309 --> 00:00:40.129 do you lose mass, do you technically become lighter? 15 00:00:40.129 --> 00:00:43.110 I thought I'd be able to researching that really easily. 16 00:00:43.110 --> 00:00:44.110 I wasn’t. 17 00:00:44.110 --> 00:00:48.510 The internet has many many conflicting amateur physicists' opinions about this. 18 00:00:48.510 --> 00:00:50.399 So I asked professional physicists. 19 00:00:50.399 --> 00:00:54.420 I asked more than half a dozen professional physicists, including Professor Philip Moriarty, 20 00:00:54.420 --> 00:00:55.539 from Sixty Symbols. 21 00:00:55.539 --> 00:01:01.969 And the overwhelming consensus is that when the battery is full, it is heavier. 22 00:01:01.969 --> 00:01:03.679 Bloody---- [choking noise] 23 00:01:03.679 --> 00:01:05.689 God, that's fast! 24 00:01:05.689 --> 00:01:09.700 We're fi--- we're already at -- 60! 25 00:01:09.700 --> 00:01:10.700 70! 26 00:01:10.700 --> 00:01:11.700 80! 27 00:01:11.700 --> 00:01:14.000 Now, electrons aren’t being moved out of the battery right now. 28 00:01:14.000 --> 00:01:17.079 It’s more subtle than that, in ways I don’t understand and that it would take several 29 00:01:17.079 --> 00:01:22.200 hours, or maybe several years, of physics lectures to me to understand. 30 00:01:22.200 --> 00:01:23.200 120! 31 00:01:23.200 --> 00:01:27.460 But we can work out exactly what the maximum difference is, using the world’s most famous 32 00:01:27.460 --> 00:01:28.590 equation. 33 00:01:28.590 --> 00:01:30.659 Energy -- 135! 34 00:01:30.659 --> 00:01:32.820 Energy is equal to mass times the speed of light squared. 35 00:01:32.820 --> 00:01:39.610 Now, this battery in this car holds about 85 kilowatt-hours, which is about 306 million 36 00:01:39.610 --> 00:01:40.610 joules. 37 00:01:40.610 --> 00:01:44.380 Divide both sides of that equation by the speed of light squared, and the result is 38 00:01:44.380 --> 00:01:49.090 about 3.4 micrograms, or about -- oh, that's strong braking! 39 00:01:49.090 --> 00:01:56.620 -- wow! -- or about the weight -- all through that, my voice was up here, I was straining. 40 00:01:56.620 --> 00:01:59.170 Or about the weight of a small grain of sand. 41 00:01:59.170 --> 00:02:03.759 It's impossible to measure, we lost more than that off the tyres during that brake run. 42 00:02:03.759 --> 00:02:04.759 ROBERT: [laughter] 43 00:02:04.759 --> 00:02:10.629 TOM: And I could spit out the window and we would get a hundred thousand times more weight 44 00:02:10.629 --> 00:02:11.629 lost. 45 00:02:11.629 --> 00:02:12.629 Literally. 46 00:02:12.629 --> 00:02:13.629 I did the maths. 47 00:02:13.629 --> 00:02:18.011 But the bottom line is: that acceleration, that use of battery power has meant that we 48 00:02:18.011 --> 00:02:20.370 are lighter and so as we accelerate again... 49 00:02:20.370 --> 00:02:25.440 ...we are going ever so slightly faster. 50 00:02:25.440 --> 00:02:29.849 Robert Llewellyn’s new book is called News from the Clouds, it's out now. 51 00:02:29.849 --> 00:02:33.870 Robert, thank you very much for being a wonderful test driver!