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?
Best Answer
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.