Electric Charge
When a rubber rod is rubbed against fur, electrons
are removed from the fur and deposited on the rod.
Electrons negative
move from - -
positive
fur to the - -
++++
rubber rod.
The rod is said to be negatively charged because of an
excess of electrons. The fur is said to be positively
charged because of a deficiency of electrons.
Glass and Silk
When a glass rod is rubbed against silk, electrons are
removed from the glass and deposited on the silk.
glass Electrons positive
move from + +
glass to the negative + +
silk silk cloth. - - - -
The glass is said to be positively charged because of a
deficiency of electrons. The silk is said to be
negatively charged because of a excess of electrons.
The Electroscope
Laboratory devices used to study the
existence of two kinds of electric charge.
Pith-ball Gold-leaf
Electroscope Electroscope
The First Law of Electrostatics
Like charges repel; unlike charges attract.
Neg
Pos Neg
Pos Neg Pos
The Quantity of Charge
The quantity of charge (q) can be defined in
terms of the number of electrons, but the
Coulomb (C) is a better unit for later work. A
temporary definition might be as given below:
The Coulomb: 1 C = 6.25 x 1018 electrons
Which means that the charge on a single electron is:
1 electron: e- = -1.6 x 10-19 C
Units of Charge
The coulomb (selected for use with electric
currents) is actually a very large unit for static
electricity. Thus, we often encounter a need to
use the metric prefixes.
1 mC = 1 x 10-6 C 1 nC = 1 x 10-9 C
1 pC = 1 x 10-12 C
Coulomb’s Law
The force of attraction or repulsion between two
point charges is directly proportional to the product
of the two charges and inversely proportional to the
square of the distance between them.
F
- q q’ +
qq '
r F 2
F F r
q q’
- -
Calculating Electric Force
The proportionality constant k for Coulomb’s
law depends on the choice of units for charge.
kqq ' Fr 2
F 2 where k
r qq '
When the charge q is in coulombs, the distance r is
in meters and the force F is in newtons, we have:
9 Nm
2 2
Fr
k 9 x 10
qq ' C2
CONCLUSION: Chapter 18
Electric Force