SAT Physics Subject Test

Chapter 8 Electric Forces and Fields


Materials can be broadly classified based on their ability to permit the flow of charge. If electrons were placed on a metal sphere, they would quickly spread out and cover the outside of the sphere uniformly. These electrons would be free to flow through the metal and redistribute themselves, moving to get as far away from one another as they could. Materials that permit the flow of excess charge are called conductors; they conduct electricity. Metals are excellent conductors. Metals conduct electricity because the structure of a typical metal consists of a lattice of nuclei and electrons, with about one electron per atom not bound to its nucleus. Electrons are free to move about the lattice, creating a sort of sea of mobile electrons. This freedom allows excess charge to flow freely.

Insulators, however, closely guard their electrons, and even extra ones that might be added. Electrons are not free to roam throughout the atomic lattice. Examples of insulators are glass, wood, rubber, and plastic. If excess charge is placed on an insulator, it stays put.

Midway between conductors and insulators is a class of materials known as semiconductors. As the name indicates, they’re less conducting than most metals, but more conducting than most insulators. Examples of semiconducting materials are silicon and germanium.

An extreme example of a conductor is the superconductor. This is a material that offers absolutely no resistance to the flow of charge; it is a perfect conductor of electric charge. Many metals and ceramics become superconducting when they are brought to extremely low temperatures.

  8. A solid sphere of copper is given a negative charge. Discuss the electric field inside and outside the sphere.

Here’s How to Crack It

The excess electrons that are deposited on the sphere move quickly to the outer surface (copper is a great conductor). Any excess charge on a conductor resides entirely on the outer surface.

Once these excess electrons establish a uniform distribution on the outer surface of the sphere, there will be no net electric field within the sphere. Why not? Since there is no additional excess charge inside the conductor, there are no excess charges to serve as a source or sink of an electric field line cutting down into the sphere because field lines begin or end on excess charges.

There can be no electric field within the body of a conductor.

In fact, you can shield yourself from electric fields simply by surrounding yourself with metal. Charges may move around on the outer surface of your cage, but within the cage, the electric field will be zero. Also, the electric field is always perpendicular to the surface, no matter what shape the surface may be.

You can see an example of this when you’re listening to the car radio and you pass through a tunnel. In a tunnel, you’re surrounded by metal, so the radio signals (which are electromagnetic waves composed of time-varying electric and magnetic fields) can’t penetrate inside, and you won’t get any reception.

  9. If an electron with mass Me makes a circular orbit of radius r around a proton with mass Mp, what is the speed v of the electron?

Here’s How to Crack It

Since the gravitational attraction between the particles is negligible compared to the electrostatic attraction, we can say that  for the electron to move in a circular path.  Therefore,