Electrostatics
The Atom
• All matter is
composed of atoms
that contain:
• Protons (positively
charged particles)
• Neutrons (neutrally
charge particles)
• Electrons (negatively
charged particles)
Every atom has a positively
charged nucleus surrounded
by negatively charged
electrons.
Electric charge, “q”, is measured in
Coulombs, C.
• All electrons are identical in mass and quantity of
charge :
q = - 1.6 x 10-19 C
• All protons are identical to other protons and
although they are more massive than electrons,
they have the same quantity of positive charge as
electrons have negative charge:
q = + 1.6 x 10-19 C
Compare the mass difference!
Electrical forces explain bonding in
molecules.
• All neutrons are identical to other neutrons,
have more mass than protons, and have neither
positive or negative charge.
• Atoms typically have the same number of
protons as electrons so the atom has a net
charge of zero.
• When an atom has lost or gained electrons, it is
charged and called an ION.
Charles Coulomb, mid
1700’s, studied and published
papers about the electrostatic
force between 2 charged
objects.
The fundamental rule of elctricity:
Like charges repel; opposites attract.
A neutral object will attract both positive
and negative charges
Ben Franklin was
the first to use the
terms “positive”
and “negative” to
describe electrical
charge. Mid 1700’s
Hmmm..
+++
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Coulomb’s Law calculates the electrical
force between charges:
where k is a constant, k = 9 x 109 N·m2/C2
Sample problem using the formula
• sample problem
Sample Problems using logic
• The electrostatic force between two charges located 2
meters apart is 0.10 N. What will the force between
these charges when they are located 1 meter apart?
Solution: The distance has halved. The force will become
stronger if the charges are closer.
• To get the factor, put ½ in the place of d (remember to
square it).
• 1/(1/2)2 = 1/(1/4) = 4; now multiply the
original force times the factor
• (.10)(4) = .40N
• A 2-C charge and a 4-C charge attract each
other with 10 N of force. How much will a 2-C
charge and an 8-N charge attract each other
when placed the same distance apart?
• Solution: one of the charges doubled. The
force will be stronger since force and charge
are directly related.
• The force will double: 10 N X 2 = 20 N