Charge polarization
Also called Induced charge separation, Polarization of charge
Charge polarization is the separation of positive and negative charge inside an object caused by a nearby charged object or an external electric field. The object's net charge does not change, which is how a neutral object can still be attracted.
The AP CEDs call the process induced charge separation and use polarization for its result. EK 10.2.A.1.ii in AP Physics 2, matched by EK 8.2.A.1.ii in C: E&M, says induced charge separation occurs when the electrostatic force between two systems alters the distribution of charges within the systems, resulting in the polarization of one or both. EK 10.2.A.1.iii adds that it can occur in neutral systems.
Conductors and insulators polarize differently.
- In a conductor, free electrons migrate across the whole object. C: E&M EK 10.1.A.6 frames this as a consequence of the conductor staying an equipotential surface in the external field.
- In a dielectric, EK 10.4.A.1 says the charges are not as free to move as in a conductor, so the material itself becomes polarized, and EK 10.4.A.3 says the field created by the polarized dielectric opposes the external one.
Why a neutral scrap of paper jumps. Polarization puts the opposite charge nearer the rod and the like charge farther away. Coulomb attraction between the near pair beats the repulsion between the far pair, so the net force is attractive whichever sign the rod carries.
Nothing has been charged. Take the rod away and an ordinary conductor relaxes back to neutral everywhere. Making the separation permanent needs a ground connection broken at the right moment, which is charging by induction.
Unrelated word alert: polarization in Unit 14 means a transverse wave confined to one plane.