# Why is there an induced EMF in a plastic ring?

If we were to pull a plastic ring across the boundary of a magnetic field, why does an emf form? If this was a metal ring I would fully understand because the electrons are free to move in a metal thus allowing it to collect together and form charged sides. However, plastic is an insulator so how can it have it's charges moving? It's even mentioned in the answers that due to the insulating nature of plastic no current will flow, but then how do the charges separate in the first place?

If it helps, I typically think of the Loretnz force acting on charges for electromagnetic induction.

Also with this induced emf, would there be a difference in magnitude of the emf between the metal and plastic ring?

An electromotive force doesn't require a conductor --- it doesn't even require matter. The electromagnetic field is a local property of the vacuum, governed by Maxwell's equations. The relevant one in this case is

$$\vec\nabla \times \vec E = -\frac\partial{\partial t}\vec B$$

That is, at any point in space, a changing magnitude or direction for the magnetic field is inextricably associated with an electric field with nonzero curl. Because electromagnetism obeys all the symmetries of special relativity, it doesn't matter whether the point of interest is stationary and the field there is changing, or if the point of interest is moving through a region of static but nonuniform magnetic fields.

Now, a "conductor" is some material with the property of "always" having $$\vec E=0$$ inside. Since the changing magnetic field is associated with $$\vec E \neq 0$$, then the charges in the conductor where $$\partial\vec B/\partial t \neq 0$$ must move to produce $$\vec E=0$$ by superposition. You can think of that as motion due to the Lorentz force if you like. The Lorentz force,

$$\vec F = q\left(\vec v\times\vec B + \vec E \right),$$

implies that a charge must be moving to experience a force from a static magnetic field. However a changing magnetic field produces a nonzero $$\vec E$$, and can therefore exert force on stationary charges.

• This is the correct answer. The question confuses emf and current, I think. – Jerry Schirmer Dec 30 '18 at 15:14
• As a layman, I would read that as "just as there would be an emf induced in no plastic ring at all"? – rackandboneman Dec 31 '18 at 2:00
• @rackandboneman Yes. The emf is a property of the changing magnetic field, whether the space of interest is occupied by a plastic ring, a metal ring, a calico cat, or nothing at all. – rob Dec 31 '18 at 4:15
• @rob I'm still a bit confused. I can accept the fact that a changing magnetic field intrinsically creates a changing electric field with it. What I don't get is how can this changing E field create an emf or voltage if there is no separation of charge? EDIT: Upon thinking more closely I can actually understand why there is an EMF (by summing the loop of electric field) but now I can't seem to understand why there should be a circular electric field produced. At the risk of being blasted for asking metaphysics, could you explain this? – John Hon Dec 31 '18 at 6:48
• @JohnHon The summing-of-fields-over-loop that you like is related to the differential equation for $\vec\nabla\times\vec E$ by Stokes' Theorem. – rob Dec 31 '18 at 13:16

The electron shells of atoms get distorted ie move a small amount relative to the nuclei, so that electric dipoles are induced.