New Here (mech engineer of 8 years)

  • #1
swademc
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How did you find PF?
I was reading about resistive power loss: P = I^2R and came across a question about how to understand it more intuitively. The answers did not satisfy me.
Forget the mathematical derivations because the answer is wind (and you know it). It too has an exponential relationship with energy loss, and you can experience it yourself.

It's not too hard to pedal a bike at 20km/h, much less easy at 30km/h, and almost impossible at 40km/h with respect to the wind. The energy lost to wind resistance (ultimately heat) increases exponentially with wind speed. This is also why cars with double the power can't go double the speed.

In a similar way, a linear increase in current flowing through a circuit will result in an exponential increase in resistive losses (heat).

This is just another example where electrons behave somewhat like a fluid. They're bumping around and losing energy when they "hit" stuff. For some reason it's exponential. Don't question it unless you really need/want to.

Use a combination of experience, intuition and maths to solve your problems, and don't get caught up with the people who will inevitably say "err, that's technically not how that works, you should derive everything from quantum theory", because in most cases it doesn't matter how things work. It only matters that they do.

Now go and get some.

(sorry physicists, I'm an engineer)
 
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  • #2
> The energy lost to wind resistance (ultimately heat) increases exponentially

Is this precise? I thought "viscous friction" from the air is proportional (in force) to the square of the speed, thus energy loss also should be polynomial... intuitively, bullets won't fly if it is exponential, neither supersonic aircrafts...

As a sidenote, I remember good demonstration of this nonlinear dependency: we were on a honeymoon trip aboard some old river cruise ship. Among other entertainments there was scheduled an excursion to pilot's post and small lecture by captain. The ship had 3 engines with dedicated propeller each. While explaining controls captain have said see, now we are driving with only middle engine engaged full speed, it gives 18km/h. Now we turn on two other engines full-speed and you see... you see, it is 24km/h. Fluid friction, you know, so if we care about saving fuel, we shouldn't hurry.
 
  • #3
Please restrict discussions about physics to the technical forums. This forum is only meant as an opportunity to introduce yourselves.
 
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