Electric Charge and Coulombs Law

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Electric Charge and Coulombs Law
TSW define and apply electric charge and
Coulombs Law in order to solve electrostatic
equilibrium problems
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Fundamental Charge The charge on one electron.
e 1.6 x 10 -19 C
Unit of charge is a Coulomb (C)
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Two types of charge
Positive Charge A shortage of electrons.
Negative Charge An excess of electrons.
Conservation of charge The net charge of a
closed system remains constant.
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Nucleus
Negative Atom Number of electrons gt Number of
protons -2e -3.2 x 10-19C
Neutral Atom Number of electrons Number of
protons
Positive Atom Number of electrons lt Number of
protons 2e 3.2 x 10-19C
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Electric Forces
Like Charges - Repel
Unlike Charges - Attract
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Coulombs Law Gives the electric force between
two point charges.
Inverse Square Law
k Coulombs Constant 9.0x109 Nm2/C2 q1
charge on mass 1 (C) q2 charge on mass 2 (C) r
the distance between the two charges (m)
The electric force is much stronger than the
gravitational force.
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Example 1
Two charges are separated by a distance r and
have a force F on each other.
If r is doubled then F is
¼ of F
2F
If q1 is doubled then F is
16F
If q1 and q2 are doubled and r is halved then F
is
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Example 2 Two 40 gram masses each with a charge
of 3µC are placed 50cm apart. Compare the
gravitational force between the two masses to the
electric force between the two masses. (Ignore
the force of the earth on the two masses)
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The electric force is much greater than the
gravitational force
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Example 3
Three charged objects are placed as shown. Find
the net force on the object with the charge of
-4µC.
F1 and F2 must be added together as vectors.
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2.3cos451.6
?
29º
2.3sin451.6
F1 lt - 4.5 , 0.0 gt
F2 lt 1.6 , - 1.6 gt
Fnet lt - 2.9 , - 1.6 gt
3.31N at 209º
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Example 4 Two 8 gram, equally charged balls are
suspended on earth as shown in the diagram below.
Find the charge on each ball.
r
r 2(30sin10º)10.4cm
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Draw a force diagram for one charge and treat as
an equilibrium problem.
Tsin80º
80º
Tcos80º