How Speedometers Works

[Carl Fredrik Ahl]

I wonder why the speed-cup in this case will follow the magnet: 

Since they're electric currents, and they're moving in an electrical conductor inside a magnetic field, another law of electromagnetism says they will create motion. How? The currents actually make the speed-cup rotate in such a way that it tries to catch up with the spinning magnet. 

Source: https://www.explainthatstuff.com/how-speedometer-works.html

Edited January 15, 2019 by Strange
Changed link to be ... well, a link

[swansont]

You could analyze the dynamics, or consider the alternative: What other option is there? If the cup rotated the other way, it would see an ever-larger changing field, which would result in a larger force, and spin faster. Repeat. That would never reach steady-state and would require infinite energy. So the opposite case is unphysical.

[mathematic]

This belongs in classical physics, not quantum theory.

[Strange]

20 hours ago, mathematic said:

This belongs in classical physics, not quantum theory.

!

Moderator Note

Consider it done.

 

[Sensei]

Quote

How Speedometers Works

Take a look at how my bike speedometer works: wheel has attached e.g. little magnet, laser or LED, when magnet is moving close to wire (or photoactive element) on the body of bike, there is created electric signal. Electronics is counting the all wheel spins. Wheel radius and circumference are constants. So distance travel by device is [math]2 \pi r * spins [/math]. Electronics is counting also time. Divide distance by time, and you will get average speed to show to driver.

 

Edited January 15, 2019 by Sensei