Physics · Ch 1 — Electrostatics
Induced Electric field inside the dielectric
Induced Electric field inside the dielectric
When an external electric field Eo is applied to a dielectric, whether by inducing new dipoles in a non-polar dielectric or by aligning the pre-existing dipoles of a polar one, the resulting polarised material -- now filled with a huge number of tiny, roughly-aligned dipoles -- produces its own internal electric field, called the induced field. Crucially, the induced field inside the dielectric always points opposite to the applied external field, because each aligned dipole's own field (recall from section 1.4.2 that a dipole's field on its axial line points along p, but a chain of aligned dipoles produces a net internal field pointing from the induced positive surface charge toward the induced negative surface charge, which is opposite to the external field that caused the alignment in the first place) works to partially cancel the applied field within the material. The net field inside the dielectric is therefore the vector sum of the external field and this internal induced field, E_net = Eo - E_induced, always smaller in magnitude than Eo alone but, since a dielectric (unlike a conductor) has no free charges available to fully …
What this figure shows. A slab of dielectric material is drawn between two field lines representing an external applied field Eo pointing, say, to the right; inside the slab, a shorter set of field-line arrows labelled E_induced (or E_p) is drawn pointing to the left, opposite to Eo, representing the internal field created by the now-aligned induced or permanent dipoles within the polarised material. The net internal field, drawn as a still-shorter rightward arrow, is the vector sum Eo - E_induced, always weaker than the applied field Eo alone but, unlike inside a conductor, never reduced all the w …
What this figure shows. A charged balloon, having been rubbed and given a net charge, is shown pressed against (and sticking to) a neutral wall; close to the balloon's surface, the wall's own molecules are shown polarised, with the charge of opposite sign to the balloon's own charge drawn to the near surface of the wall, closer to the balloon than the like charge that is pushed slightly farther away on the far side. Because the attractive force from the nearer, opposite induced charge is slightly stronger than the repulsive force from the farther, like induced charge, the net force on the wall is attractive, which is exactly why a charged balloon can stick to an ordinary …