MOTION MOUNTAIN

22 1 electricity and fields
F I G U R E 8 A simple set-up to
confirm electric charge
conservation: if rubbed fur is
moved from the first pot to the
second, the charge taken away
from the first pot is transferred to
the second, as shown by the two
electrometers (© Wolfgang
Rueckner).
In summary, electric fields start at bodies, provided they are charged. Charging can
be achieved by rubbing and other processes.There are two types of charge, negative and
positive. Charge can flow: it is then called an electric current. The worst conductors of
current are polymers; they are calledinsulators ordielectrics. A charge put on an insulator
remains at the place where it was put. In contrast, metals are good conductors; a charge
placed on a conductor spreads all over its surface. The best conductors are silver and
copper.This is the reason that at present, after two hundred years of use of electricity, the
highest concentration of copper in the world is below the surface of Manhattan. Charges
can flow through air if the electric field is strong enough; this is a spark or, when the
spark is large, a lightning bolt.
Electric charge
Because all experiments with electric charge can be explained by calling the two charges
positive and negative, we deduce that some bodies have more, and some less charge than
an uncharged,neutral body. Electric charges thus only flow when two differently charged
bodies are brought into contact. Now, if charge can flow and accumulate, we must be able
to somehow measure its amount. Obviously, the amount of electric charge on a body,
usually abbreviatedq, must be defined via the influence the body, say a piece of sawdust,
feels when subjected to a field. Charge is thus defined by comparing it to a standard
reference charge. For a charged body of massmaccelerated in a field, its chargeqis
determined by the relation
q
= dp/dt
q ref dp ref /dt , (1)
i.e., by comparing its momentum change with the momentum change of the reference
charge. Charge thus determines the motion of bodies in electric fields in the same way
that mass determines the motion of bodies in gravitational fields. Charge is therefore the
second intrinsic property of bodies that we discover in our walk.
In practice, electric charge is measured with electrometers. A few such devices are
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net