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Coulomb’s Law Problems

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Presentation on theme: "Coulomb’s Law Problems"— Presentation transcript:

1 Coulomb’s Law Problems
1. Find the force between charges of +1.0 C and -2.0 C located 0.50 m apart in air. k q1 q2 F = r2 ( 9 x 109 Nm2/C2 )( +1.0 C )( -2.0 C ) = ( 0.50 m)2 F = x 1010 N negative sign indicates attraction

2 2. q1 = μC F = N r = m (a) What is the sign of the second charge? A positive force indicates repulsion, and since q1 is negative, q2 must also be negative

3 2. q1 = μC F = N r = m (b) What is the magnitude of the second charge? q2 = ? k q1 q2 Solve for q2 r2 F = r2 r2 r2 F = k q1 q2 k q1 k q1 r2 F ( 0.30 m )2 ( N ) q2 = = k q1 ( 9 x 109 Nm2/C2 )( x 10-6 C ) q2 = C = x 10-6 C = μC must change μC to C

4 3. F = N q1 = q2 = x 10-6 C r = ? k q1 q2 Solve for r r2 F = r2 r2 r2 F = k q1 q2 k q1 q2 r2 = F F F k q1 q2 r = F (9 x 109 Nm2/C2)(5 x 10-6 C)(5 x 10-6 C) = 0.25 N r = m (don’t forget to take square root)

5 4. Two spheres, with charges +20.0 μC and -10.0 μC,
are 10.0 cm apart. Find force of attraction. q1 = x 10-6 C q2 = x 10-6 C k q1 q2 F = r2 (9 x 109 Nm2/C2)(+20.0 x 10-6C)(-10.0 x 10-6 C) = ( 0.10 m)2 F = N

6 5. The spheres are touched and separated again. What is the force?
When the spheres are touched, the charge re-distributes +10 μC -10μC If each sphere had the same charge, but of opposite sign, the net charge would be zero (the sum of +10 and -10 is zero) +10 μC -10μC Both spheres neutral N N

7 5. The spheres are touched and separated again. What is the force?
With one sphere +20 μC and the other -10 μC, the overall charge is (+20) + (-10) = +10 μC +20 μC -10μC +20 μC -10μC When the spheres are touched, the +10 μC distributes so that each sphere has +5 μC +5.0 μC +5.0 μC +5.0 μC +5.0 μC

8 Two spheres, each with charge +5.0 μC, are 10.0 cm
apart. Find force of attraction. q1 = +5.0 x 10-6 C q2 = +5.0 x 10-6 C k q1 q2 F = r2 (9 x 109 Nm2/C2)(+5.0 x 10-6C)2 = ( 0.10 m)2 F = N

9 6. Find the distance between two spheres, one -3.0 x 10-5 C
and the other -2.5 x 10-4 C, if the electric force is 1.0 N. k q1 q2 Solve for r r2 F = r2 r2 r2 F = k q1 q2 k q1 q2 r2 = F F F k q1 q2 r = F (9 x 109 Nm2/C2)(-3 x 10-5 C)(-2.5 x 10-4 C) = 1.0 N r = m (don’t forget to take square root)

10 7. A sphere of charge +3.5 μC experiences a force of
-3.0 N when separated by a second sphere by 0.25 m. Find the charge of the second sphere. k q1 q2 Solve for q2 r2 F = r2 r2 r2 F = k q1 q2 k q1 k q1 r2 F ( 0.25 m )2 ( N ) q2 = = k q1 ( 9 x 109 Nm2/C2 )( x 10-6 C ) q2 = C = x 10-6 C = μC must change μC to C


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