It’s just possible to start a fire with a 1.5 AA battery by passing the current through a narrow piece of foil-covered gum wrapper. I’ve never actually succeeded but I’ve come close.
Oh, a fresh AA has more than enough power to start a fire.
My NiMh AA’s are rated at 2.3 AH, which is around 17W for ten minutes. They can also dump enough current into a dead short to make a piece of wire red-hot.
Let’s run the numbers:
Open circuit voltage = 5 V
Short circuit current = 300 mA
V[sub]th[/sub] = 5 V
R[sub]th[/sub] = 5 V / 300 mA = 16.67 Ω
So the first-order Thévenin equivalent of their lemon battery would be a 5 V voltage source in series with a 16.67 Ω resistor. (This assumes V[sub]th[/sub] was still 5 V when it was shorted, which is probably not the case. That’s why I am calling this a first-order analysis.)
You can get maximum power from their battery - and to the load - when the load (motor or whatever) has a resistance of 16.67 Ω.
With a load resistance of 16.67 Ω, the current will be 150 mA and the power to the load will be 0.375 watts, or 0.0005 HP.
Let’s assume the car needs to have a 500 HP engine. This means there’s six orders of magnitude between the power of the lemon battery and the power of the 500 HP engine.
Go to the :27 sec mark in the video and see the lemon battery. They put all the zinc and copper strips in alternately and wired the strips of each kind together, not to the other kind. That’s not how they built it for the car, but that shows an alternate arrangement from the OP’s video. It uses a single lemon to make a lot of surface area for the electrodes.
For the car, they water jet cut strips of copper and zinc to make a large array that they then wired appropriately and jammed lemons on to bridge the strips at regular intervals.
They only got 5 V and 300 mA, so they rigged up a rube Goldberg that used the lemon battery to raise a bag of lemons to drop and spin a juicer to make lemonade that. Why? Because that’s what they do - crazy builds.
Since the lemon battery didn’t work, he next rigged a zipline in his backyard with a regenerative braking device that charged a drill battery. When that was full, he still didn’t have orders of magnitude enough juice, so he rigged a solar array on his roof and enlisted kids to design a way to keep the dust off them, so they built a sprinkler system that he now uses. Solar power charged the battery for the car.
See this video made in response to the OP’s video.
Late response to the OP: with the right kind of metal, you won’t even need a lemon; anything moist will do. Try sodium metal (or potassium if you want it spectacular)!
You know, I’ve seen that said before by amateurs, obviously by me because I quoted it unthinkingly as a transcriber.
What are they thinking–I mean, if you are a HS science teacher, you might want to figure out what is slipping them up, and what it is they think is happening, given a slippery knowledge of EE?
<snark>
You can start a fire with a lemon as long as the lemon is already on fire …
</snark>
No, its a massive short circuit inside the lemon, and one properly connected externally.