# Question about charged circular dish-an old topic

#### neila9876

A circular dish is charged with an iron rod in the center. When the dish rotates, someone on the ground considers that there is current and generates magnetism and the iron rod becomes a magnet; while someone else standing on the dish considers that there is no current and no magnetism generated and the iron does not become a magnet...
Is the iron rod a magnet or not?
Thank you.

#### oz93666

You are suggesting rotation is dependent on whether you are standing on the disc or not .. that someone on the disc would think he was not rotating . But he would experience rotation evident by centrifugal forces he feels ..

So rotation is not relative , and the iron rod must become a magnet .

Imagine you were standing on a perfectly spherical and homogeneous planet ... If the planet were not spinning gravity would be exactly the same everywhere on the surface ... You measure it and find it is exactly the same , so the planet has no spin . So what is the planet's non rotation relative to??? The total mass of the Universe???

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1 person

#### neila9876

...

Is gravity the same force as electromagnetic?

#### HallsofIvy

Rotation is an acceleration, not velocity, so is NOT "relative"!

No, "gravitational force" is NOT a type of "electro-magnetic" force. There are four fundamentally different "fundamental forces":
gravitational force
electro-magnetic force
strong nuclear force
weak nuclear force

1 person

#### neila9876

Is the iron rod a magnet or not? Thank you.

#### topsquark

Forum Staff
Is the iron rod a magnet or not? Thank you.
HallsofIvy and oz93666 are saying is that there is no inertial reference frame where the spinning disc is not spinning because the spin creates an angular speed. So the charged disc spins in any inertial reference frame. The charge rotating on the disc creates a magnetic field inside the iron so it acts as an electromagnet.

-Dan

#### neila9876

furhter step

Device 1: A plastic disc fixed on a plastic axis. Brush the disc with a piece of cloth or leather which one is appropriate. (charge by friction) Put the cloth or leather on the table nearby. Rotate the device. Use a compass to detect.
Device 2: Two plastic disces fixed on a plastic axis. Render the disces carry the same amount of counter charge. (I don't know how to realize this idea) Rotate the device. Use a compass to detect.
Guys must do it with good insulation and make sure they do it with enough accuracy.
In case 1, the charge on the disc moves relative to the charge on the cloth (or leather) while in case 2, charge on the disces without relative movement.
Logically, they are different affairs and there might be different out come.
Watch from the angle of electric potential energy, you can feel something different too.

#### Woody

At first thought it seems counter-intuitive, but a reference frame under constant rotation is actually an accelerating reference frame!
Or rather (more precisely) if you consider a reference frame following any point on the rotating disk, then that reference frame is changing direction and is therefore accelerating.

Because it is accelerating, rather than inertial, the symmetries you are expecting don't hold.

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#### neila9876

Intuition vs ration

Maybe situation of acceleration is a bit complicated. Next, simplified the resarch.
An electron moves to you in constant velocity, you will feel that magnetism occurs, while the electron is fixed over the ground and you move to it in constant velocity, you will feel no magnetism occurs.
According to the relative movement principle, these two cases are equivalent. Why difference occurs in our sence?
Take the remaining positive charge into consideraton, the answer appears.
In the former case, the electron might move relative to the remaining positive charge, while in the latter case, the electron might not move relative to the remaining positive charge. They might be different cases logically.
Intuition of mankind might not be always wrong.
funny?

#### neila9876

Oz39666 seems to look deep into the cosmos, combination of vectors,,,the moon, the sun, the galaxy...the total mass of cosmos, isn't it??? That's gravity.
But can you ignore the other part of the charge pair in case of electromagneitc?