Actually eli6 because its not like I know nothing about electricity at all: I understand that 1 amp = 6.24\*10\^18 electrons per second and voltage is the pressure behind it. I understand that P=IV and that volts = energy per coulomb.
What I don’t understand is: (A) why power is the current multiplied by voltage and not current alone. In terms of the water analogy, the current is how much water is flowing through a pipe per second. If I’m filling a bucket with water then regardless of how much pressure and resistance there is, the only thing that matters for the bucket is how much water flows through per second, aka power which would be watts. But based on P=IV not being P=I, I know I’m wrong but I don’t know why. Why does current have to be multiplied by voltage to get the power? Is the water analogy just flawed here?
(B) How an ebike battery and system works. My ebike has 30-42v depending on SOC, but what does that mean? Does it mean that there will always be 30-42v in the motor and the wires and everything else, with the only changing thing being amps? What about amps, do they change based on voltage or in a different way? What happens to both voltage and amps when using different power settings in the display?
(C) Does ohm’s law apply to heat loss in ebikes? If yes, my current understanding is the following:
Power(heat loss) = VI = (IR)I = I²R
Power(useful kinetic energy) = VI
Wait, that would mean since both are VI, that would mean there will always be 50% useful energy and the other 50% is loss. I know this is wrong but I don’t know why. Where am I mistaken?
(D) in conclusion, is it true that an electric motor running at 2x power will have 4x the heat loss over the same duration of time? If so, what are the formulas behind it?
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In: Physics
(Re: A only)
If you’re filling a bucket, then the accumulated result is some litres/gallons of water. The analogous quantity in electricity is some coulombs of charge. And it’s important to understand that the amount of water/amount of charge, is not the same as measuring an amount of work done.
A barrel of water at the top of a big hill, can do a lot more work flowing down to a reservoir 3 miles below, than the same quantity can do if it’s already sitting in the lower reservoir and has nowhere lower to flow to.
(eta: the same reasoning applies to water flowing between reservoirs which are at different pressures for reasons other than gravity. Elevation just happens to provide a convenient, uniform pressure gradient in this analogy.)
And a coulomb of charge can do a lot more work flowing from a +48V source to 0V, than from +12 to 0.
Work, in kinematics, is an amount of force applied, multiplied by the distance over which the force moves. So the work that can be done by an amount of falling water is a certain number of newtons *times* a certain number of meters.
Analogously, the amount of work that a given amount of charge can do by “falling” from a high-voltage reservoir to a low-voltage one, is the amount of charge *times* the ‘distance’ it falls (measured in volts).
And, in both cases, power is work per unit time.
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