Download Chapter 18 Electric Forces and Electric Fields

Chapter 18 Electric Forces and
Electric Fields
The Origin of Electricity
• The electrical nature of matter is inherent in
atomic structure. An atom consists of small,
relatively massive nucleus particles called
protons and neutrons.
• 2 Kinds of Electric Charge:
• 1. Positive – proton
• 2. Negative - electron
A proton has a mass of
1.673 x 10-27 kg
• Neutron has 1.675 x 10-27 kg.
• Surrounding the nucleus is a diffuse cloud of
orbiting particles called electrons.
• An electron has a mass of 9.11 x 10-31kg.
• The magnitude of the charge on the proton is
exactly equal the magnitude of the charge on
the electron;
• The proton carries a charge + e, and the
electron carries a charge of – e.
• SI unit: coulomb (C )
• e = 1.60 x 10 -19C
• The symbol e represents only the magnitude
of the charge on a proton or an electron and
does not include the algebraic sign that
indicates whether the charge is + or -.
When an atom or any object carrying
no net charge, the
• object is said to be electrically neutral. The
neutrons in the nucleus are electrically neutral
particles (has no electric charge).
• The charge on an electron or a proton is the
smallest amount of free charge that has been
discovered. Charges of larger magnitude are built
up on an object by adding or removing electrons.
Thus, any charge of magnitude q is an integer
multiple of e, that is, q = Ne, where N is an
integer.
Because any electric charge q occurs in
integer multiples of
• elementary, indivisible charges of magnitude
e, electric charge is said to be quantized.
• Example: How many electrons are there in
one coulomb of negative charge?
• N = q/e = 1/1.6 ×10-19 = 6.26 × 10+18 electrons
• Law of Conservation of Electric Charge
• during any process, the net electric charge of
an isolated system remains constant (is
conserved).
• It is easy to demonstrate that 2 electrically
charged objects exert a force on one another.
• Like charges, repel and unlike charges attract
each other.
Note: the force has magnitude and direction
Direction of E out of the +ve charge
Direction of E toward the –ve charge
Conductors and Insulators
• Substances that readily conduct electric
charge are called electrical conductor.
• Metals such as copper, aluminum, silver, and
gold are excellent electrical conductors and
therefore are used in electrical wiring.
• Materials that conduct electrical charge poorly
are known as electrical insulators. Examples of
electrical insulators are rubber, many plastics
and wood.
Example: A Test Charge
• The positive test charge (pls see p 547) is qo =
+ 3.0 x 10-8 C and experiences a force F 6.0 x
10-8 N in the direction shown in the drawing.
• A) Find the force per coulomb that the test
charge experiences
• B) Using the result in (a), predict the force that
a charge of + 12 x 10-8 C would experience if it
replaced qo.