Electric charge

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
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
• 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
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
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

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