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Electric Force

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... build up in the atmosphere until it discharges in a spectacular lightning bolt. It can build up on you by walking across the carpet and zap you when you reach ... – PowerPoint PPT presentation

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Title: Electric Force


1
Electric Force
  • PHYS 2326-1

2
Electric Charge
  • Sometimes called Static Electricity
  • It can make your hair stand on end
  • It can build up in the atmosphere until it
    discharges in a spectacular lightning bolt
  • It can build up on you by walking across the
    carpet and zap you when you reach for the door
    knob

3
Types of Charge
  • Experiments showed two types
  • Ben Franklin named them positive and negative
  • Oppositely charged objects attract
  • Similarly charged objects repel
  • Millikan discovered charge came in multiples of a
    fundamental amount

4
Matter
  • Consists of atoms that form molecules
  • Atoms made from common building blocks -
    Particles or subatomic particles
  • These particles include
  • Protons
  • Neutrons
  • Electrons

5
Atomic Structure
  • Experiments led to the structure of the atom
  • Similar to a solar system
  • Heavy dense core nucleus consisting of
  • Protons
  • Neutrons
  • Electrons apparently orbiting the nucleus
  • Atoms are electrically neutral

6
Electric Charge
  • Protons are positive
  • Electrons are negative
  • Neutrons are neutral

7
Electron
  • Has negative charge
  • Charge -1.602176 x 10-19Coulombs
  • Mass 9.10938 x 10-31 kg

8
Proton
  • Has positive charge
  • Charge 1.602176 x 10-19C
  • Mass 1.672621 x 10-27 Kg

9
Neutron
  • No charge
  • Mass 1.674927 x 10 -27 Kg

10
Electric Charge
  • Negative more electrons than protons
  • Positive fewer electrons protons

11
Coulombs Law
  • F k q1 q2 / r2
  • K Coulombs Constant
  • q1 first charge C
  • q2 second charge C
  • r distance between the two charges m
  • K 1 / (4 p eo ) 8.9876 x 10 9 Nm2/C2
  • eo permittivity of free space
  • 8.8542 x 10-12 C2/Nm2

12
Example 1a
  • A hydrogen atom consists of a proton and an
    electron. What is the gravitational force
    between them if they are separated by the Bohr
    radius of 0.5292 x 10-10 m?
  • F G m1 m2 / r2
  • G 6.67 x 10-11 N m2/kg2
  • m1 1.67 x 10-27 kg mass of proton
  • m2 9.11 x 10-31 kg mass of electron
  • F (6.67 x 10-11 ) (1.67 x 10-27 ) (9.11 x 10-31
    ) / (0.5292 x 10-10 )2
  • F 3.623 x 10-47 N

13
Example 1b
  • Proton and electron separated by 0.5292 x 10-10
    m, electron charge 1.60 x 10 -19 C
  • F k q1 q2 / r2
  • F(9.00 x 109)(1.60 x 10 -19 )(-1.60 x 10 -19
    )/(0.5292 x 10-10 )
  • F -8.227 x 10-10 N
  • Attractive or repulsive force?

14
Example 1c
  • How strong is the electric force compared to the
    gravitational force?
  • Take a ratio R Fe/Fg
  • R 8.227 x 10-10 / 3.623 x 10-47
  • 2.27 x 1039

15
Example 2a
  • Three charges are located on the x-axis, the
    first at x0, the second at x 5cm and the third
    at location x. This third charge is located
    where there is no electrical force on it.
  • q1 1.00 x 10 -6 C, q2 2.00 x 10 -6 C, q3
    3.00 x 10 -6 C
  • What is x?
  • F31 F32 0 , Fkqq/r2
  • F12 7.2 N

16
Example 2a
  • F13 k q1 q3 / x2
  • F23 k q2 q3 / (x0.05)2
  • Condition forces on q3 sum to 0
  • F13 -F23
  • k q1 q3/x2 -k q2 q3 / (x0.05)2
  • q1/x2 -q2 / (x0.05)2
  • q1(x0.05)2 -q2(x2)
  • 1(x0.05)2 2(x2)
  • x2 0.10x 0.0025 2 x2
  • -x2 0.10x 0.0025 0
  • x 0.1207m 12.07cm

17
Example 2b,c
  • force on q1
  • F12 7.2N
  • F13 (9E9)(1E-6)(3E-6)/(0.1207)2
  • F131.853N
  • F23 (9E9)(-2E-6)(3E-6)/(0.12070.05)2
  • F23 1.853 N
  • F1 1.8537.2 F2 1.853 7.2
  • F1 9.05 N
  • F2 -9.05 N
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