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The Electrostatic Force
• Charge of objects
• And separation distance
– Influence the force between them.
As q increases, F increases.
As D increases, F decreases.
Distance
Force
Coulomb’s Law
• Notice that d is squared!!!!!! This is
another example of an inverse-square
law.
1 2
2
c
elec
k q q
F
d
=
...
Symmetries
• Recall Newton’s Law of Gravitation:
• And Coulomb’s Law:
• Notice the similarities??
1 2
2
c
elec
k q q
F
d
=...
Proportionality Constants
• Notice their relative size!!! 1020
difference!
1 2
2
c
elec
k q q
F
d
=
1 2
2grav
Gm m
F
d
=
G...
Using Coulomb’s Law
• Why do they use such small numbers for ordinary
objects? Wouldn’t it make more sense to call
them 1 ...
Using Coulomb’s Law
• So what sort of charges are we talking about?
q is 1-10 C
VERY LARGE
q is more like
10-6
or 10-9
C
The Elementary Charge
• http://webphysics.davidson.edu/applets/pqp_preview/contents/pqp_errata/c
d_errata_fixes/section4_5...
The Elementary Charge
• Robert Millikan -1909
– electric charge is quantized.
• Quantized - things can only have certain
v...
The Elementary Charge
• Millikan demonstrated that all the oil
droplets had charges which were multiples
of each other!
• ...
Using Coulomb’s Law
• Why is Coulomb’s Law called an “inverse-
square law?”
• Because of the d2
in the equation.
• Doublin...
The Inverse-Square Butter Gun
Bohr’s Model of Hydrogen
• You’ve seen the similarities between
Newon’s U.L.o.G. and Coulomb’s Law.
Niels Bohr used this s...
Columbs law pp
Columbs law pp
Columbs law pp
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Columbs law pp

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Columbs law pp

  1. 1. The Electrostatic Force • Charge of objects • And separation distance – Influence the force between them.
  2. 2. As q increases, F increases.
  3. 3. As D increases, F decreases. Distance Force
  4. 4. Coulomb’s Law • Notice that d is squared!!!!!! This is another example of an inverse-square law. 1 2 2 c elec k q q F d = q charge (Coulombs or C) Distance (meter of m) Electric Force (Newtons or N) kc (8.99 x 109 N-m2 /C2 )
  5. 5. Symmetries • Recall Newton’s Law of Gravitation: • And Coulomb’s Law: • Notice the similarities?? 1 2 2 c elec k q q F d = 1 2 2grav Gm m F d =
  6. 6. Proportionality Constants • Notice their relative size!!! 1020 difference! 1 2 2 c elec k q q F d = 1 2 2grav Gm m F d = G (6.67 x 10-11 N-m2 /kg2 ) was a proportionality constant Newton’s Law of Universal Gravitation kc (8.99 x 109 N-m2 /C2 ) is the proportionality constant Coulomb’s Law
  7. 7. Using Coulomb’s Law • Why do they use such small numbers for ordinary objects? Wouldn’t it make more sense to call them 1 C instead?? • The answer lies in the famous “Oil Drop Experiment.”
  8. 8. Using Coulomb’s Law • So what sort of charges are we talking about? q is 1-10 C VERY LARGE q is more like 10-6 or 10-9 C
  9. 9. The Elementary Charge • http://webphysics.davidson.edu/applets/pqp_preview/contents/pqp_errata/c d_errata_fixes/section4_5.html
  10. 10. The Elementary Charge • Robert Millikan -1909 – electric charge is quantized. • Quantized - things can only have certain values, not just any value. • Ex. Is it possible to own 2 ½ bicycles??? • Animation 1 (Quicktime) • Animation 2 (Java)
  11. 11. The Elementary Charge • Millikan demonstrated that all the oil droplets had charges which were multiples of each other! • All the droplets carried some multiple of what is called the Elementary Charge, 1.6 x 10-19 Coulombs. • This is the charge of a single proton or electron. • Ordinary objects have charges like 10-9 C, this is still about 10 billion excess charges!!!
  12. 12. Using Coulomb’s Law • Why is Coulomb’s Law called an “inverse- square law?” • Because of the d2 in the equation. • Doubling the distance between objects cuts the force by ¼ , just like Newton’s Universal Law of Gravitation. • Ex. Sound intensity, Spray paint, the “Butter Gun”
  13. 13. The Inverse-Square Butter Gun
  14. 14. Bohr’s Model of Hydrogen • You’ve seen the similarities between Newon’s U.L.o.G. and Coulomb’s Law. Niels Bohr used this similarity to create a model of the Hydrogen atom. • Just like the Earth orbiting the Sun, Bohr pictured the electron orbiting the proton. • Animation

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