Electric Force

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Electric Force

• PHYS 2326-2

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Coulombs Law 3-d

• Forces work in 2 3 dimensions also
• Use vector forces
• Keep track of units, directions and signs
• Total force is vector sum of forces
• Exploit symmetry when possible

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Example 1

• q1 1E-6C, q2 2E-6C, q3 -3E-6C, q4 4E-6C
• r2 0.1m, r3 0.1m, m 2kg, k 9.00E9
  Nm2/C2
• Notice q3 and q4 are on an x axis while q2 and q4
  are on a y axis.
• What is the initial acceleration on q4 if all q
  start at rest?

a
ay
F
ax
m
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Equations

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Example 1 Answers

• r1 0.1414 m
• ? 45
• F1 1.8N
• F2 7.2N
• F3 -10.8N
• Fx -9.53N, ax -4.76m/s2
• Fy 8.47N, ay 4.24m/s2
• a 6.37m/s2, 41.7 2nd quadrant

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Example 2

• Ring of charge Q radius a in the y,z plane
  centered around the x axis. A point charge q at
  distance x along the x axis

dQ ds
r
dFx
a
?
dFy
x
dF
Q
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Example 2

• Electric force at x?
• What is F at the center of the ring?
• What is F if x is much greater than a?
• What is F if Q10uC, q -5uC, a0.1m and x 0.2m?

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Equations
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Answers
(b) F0 at x 0.
(c)
(a)
(d) F-8.05N î