Chapter 15'2 Electric Forces Coulombs Law

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Chapter 15.2Electric ForcesCoulombs Law

• Force between two point charges
• Superposition of forces from several point
  charges.

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Magnitude of charge

• Units of charge- Coulomb , C
• Charge on electron - e
• e 1.60x10-19 C

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Experimental results

Magnitude of F proportional to product of
charges q1q2 Inversely proportional to distance
squared Direction of F along line between
charges.
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Coulombs Lawforce between 2 point charges or 2
spherical distributions of charge
ke 8.99x109 Nm2/C2
9x109 Nm2/C2
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Compared to Gravitational Force
1kg 1kg r1m
FG 7x10-11 N
1C 1C r 1m
Fe 9x109 N about 1020 times more
than FG
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Magnitude of charge
How much charge is one mole of electrons?
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Suppose you had 2 charged spheres each with a
charge equivalent to 1mole of electrons,
separated by 1000 m what would be the force
between the two charges?
A large force!!!!
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Suppose we had two small 5 g spheres each
carrying charge q. They were suspended from a 1
m thread at an angle of 100 from the vertical
direction. Find q.
1m
q
q
r
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Electrical Forces
Interaction between electrons and nuclei in
atoms. Most terrestrial macroscopic electrical
forces involve relatively small amounts of charge
lt10-6 C because matter is mostly
neutral Atmospheric electrical forces-Thunderstorm
s - large amounts of charge separated by rising
air.
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Superposition PrincipleForce between several
point charges
F23
F
q1
q3
F13
q2
F F13 F23
Net force vector sum of forces
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Three charges are placed a the corners of a
square with the length of each side 2.0 cm.
Find the force on q3.
q1
q3-2x10-6 C
q1q21x10-6 C
q2
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Charge A and charge B are 3.00 m apart, and
charge A is 2.00 C and charge B is 3.00 C.
Charge C is located between them at a certain
point and the force on charge C is zero. How far
from charge A is charge C?
A
B
C
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Electric forces

• For two point charges the force on a charge can
  be calculated by Coulombs Law
• This can be extended to several charges. The net
  force is the vector sum of forces.
• But for large numbers of charges we use the idea
  of the electric field.