why can’t electric cars be charged by their own movement?
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Some of them are, it’s called regenerative braking. You usually still lose more energy than you gain, though
I think some of them are to an extent. They do use the motion of the car to recharge. However, due to friction, no machine is perpetual. If the engine expels 100 kw of power, then perhaps only 95 kw actual transfers to moving the car forward. That 95 km could then (at best) charge to battery, but again power is lost so now the battery as 90 kw. So now the car engines has 90 kw of power, but only 85 kw goes to motion. That 85 kw of motion charges to battery to 80 kw. This will continue to decrease until the car runs dry.
Inefficiency.
Energy conversions are never 100% efficient. Taking X amount of electrical energy stored in a battery and converting it to kinetic energy in a motor leaves you with X-Y. Taking some of that energy and converting it again to electrical energy would leave you with Y-Z, and the cycle would continue.
Because in order to turn the kinetic energy of the car into electrical energy, you need to take away that kinetic energy – slowing the car down. And with everything else in physics, nothing is 100% efficient much less > 100% efficient.
Regenerative braking DOES provide some electrical energy to electric cars and hybrids, but it’s just a small supplement.
Why don’t they just have a generator connected to a gear that’ll spin the generator?
Most electric cars do take advantage of converting their movement into energy, but that can only be done while braking (or going down hill – which is still effectively “braking”). Instead of using regular brakes to convert the movement into heat (and thus slowing down) they use the movement to spin the motor and use it to generate a little bit of electricity. It is known as “Regenerative Braking”.
It is not very efficient – but it does add some extra range to the vehicle that it would not otherwise have.
Energy in may never be equal to energy out.
It’s a law of thermodynamics.
Converting electric charge to mechanical rotation isn’t perfect. There are always losses.
Reversing it also has losses.
100% power in battery. Only 70-ish% ends up being rotation.
If you tried to then turn rotational energy back into a charge, the car would have to stop moving. Because your rotation would go to 0, as charge comes back.
You start with either charge, or rotation. You MUST end with
either charge OR rotation, or half and half, or 70/30… something.
You may not have both. If you started with one, and ended with both, you’d need to have made more energy for no cost.
Add in those losses we mentioned earlier.
And 100% battery charge becomes 70% rotation 0% battery as the other 30% energy turns to heat, which you can’t get back.
Then that 70% goes to 49% battery, 0% rotation, as 30% goes ro heat again. You always lose.
Gas powered cars do this by consuming chemical fuel.
Conservation of energy. You could do that, but you’d still lose energy due to air resistance and friction with the road
Let’s say they do charge while they drive. Perpetual energy machines are literally physically impossible (Literally violates the laws of physics). You will never get more energy out of something than you put in. There’s a reason why patent offices will basically reject perpetual motion machine patents outright.
So that means at best you will get back the energy you put in. Which means at best you can stop and go again with the energy, but you would never be able to go faster than you started. Which is also impossible because it’s a physical impossibility to 100% efficiently convert the energy in a car (Literally violates the laws of physics). Which means doing this will always result in you slowing to a stop unless you’re on a hill.
Now electric cars do have regenerative breaking. This lets them charge their batteries when they slow down/stop. This works because the electric motor also works as a generator. In fact, nearly all non-solar power plants (and even some solar plants) essentially use the equivalent of giant electric motors to produce power.
They can charge through deceleration but the kilowatts produced from that are minimal. They could charge through mini wind turbines, but the drag they would create would offset the energy output. They could charge through solar panels but the surface area on a car isn’t enough to reliably keep a vehicle working.
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