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Practice A- Coulomb’s Law

1. A balloon rubbed against denim gains a charge of −8.0 μC. What is the electric force between the balloon and the denim when the two are separated by a distance of 5.0 cm? (Assume that the charges are located at a point.)

2. Two identical conducting spheres are placed with their centers 0.30 m apart. One is given a charge of +12 × 10⁻⁹ C and the other is given a charge of −18 × 10⁻⁹ C.

a. Find the electric force exerted on one sphere by the other.

b. The spheres are connected by a conducting wire. After equilibrium has occurred, find the

electric force between the two spheres.

3. Two electrostatic point charges of +60.0 μC and +50.0 μC exert a repulsive force on each other of 175 N. What is the distance between the two charges?

Practice B- The Superposition Principle

1. Three point charges, q1, q2, and q3, lie along the x-axis at x = 0, x = 3.0 cm, and x = 5.0 cm, respectively. Calculate the magnitude and direction of the electric force on each of the three point charges when q1 = + 6.0 μC, q2 = +1.5 μC, and q3 = −2.0 μC.

2. Four charged particles are placed so that each particle is at the corner of a square. The sides of the square are 15 cm. The charge at the upper left corner is +3.0 μC, the charge at the upper right corner is −6.0 μC, the charge at the lower left corner is −2.4 μC, and the charge at the lower right corner is −9.0 μC.

          a. What is the net electric force on the +3.0 μC charge?

          b. What is the net electric force on the −6.0 μC charge?

          c. What is the net electric force on the −9.0 μC charge?

Practice C- Equilibrium

1. A charge of +2.00 × 10⁻⁹ C is placed at the origin, and another charge of +4.00 × 10⁻⁹ C is placed at x = 1.5 m. Find the point between these two charges where a charge of +3.00 × 10⁻⁹ C should be placed so that the net electric force on it is zero.

2. A charge q1 of −5.00 × 10⁻⁹ C and a charge q2 of −2.00 × 10⁻⁹ C are separated by a distance of 40.0 cm. Find the equilibrium position for a third charge of +15.0 × 10⁻⁹ C.

3. An electron is released above the Earth’s surface. A second electron directly below it exerts just enough of an electric force on the first electron to cancel the gravitational force on it. Find the distance between the two electrons.