Introduction to SAT II Physics

Much like gravitational potential energy, there is no absolute, objective point of reference
from which to measure electric potential energy. Fortunately, we are generally not
interested in an absolute measure, but rather in the electric potential, or potential
difference, V, between two points. For instance, the voltage reading on a battery tells us
the difference in potential energy between the positive end and the negative end of the
battery, which in turn tells us the amount of energy that can be generated by allowing
electrons to flow from the negative end to the positive end. We’ll look at batteries in more
detail in the chapter on circuits.
Potential difference is a measure of work per unit charge, and is measured in units of
joules per coulomb, or volts (V). One volt is equal to one joule per coulomb.

Potential difference plays an important role in electric circuits, and we will look at it more
closely in the next chapter.

Conductors and Insulators

Idealized point charges and constant electric fields may be exciting, but, you may ask,
what about the real world? Well, in some materials, such as copper, platinum, and most
other metals, the electrons are only loosely bound to the nucleus and are quite free to
flow, while in others, such as wood and rubber, the electrons are quite tightly bound to
the nucleus and cannot flow. We call the first sort of materials conductors and the
second insulators. The behavior of materials in between these extremes, called
semiconductors, is more complicated. Such materials, like silicon and germanium, are
the basis of all computer chips.
In a conductor, vast numbers of electrons can flow freely. If a number of electrons are
transmitted to a conductor, they will quickly distribute themselves across the conductor
so that the forces between them cancel each other out. As a result, the electric field within
a conductor will be zero. For instance, in the case of a metal sphere, electrons will
distribute themselves evenly so that there is a charge on the surface of the sphere, not
within the sphere.

Key Formulas

Coulomb’s

Law

The Law of

Superpositio

n