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Note: Items in red are measurements that must be made as you do the experiment.

Introduction

Two charged particles exert a force upon each other according to the relation: Eq. 1 In this lab you will measure this force with the torsion balance depicted in Figure 1, first by varying distance, and second by varying charge. When spheres A and B interact, A will be pushed away from B. To bring sphere A back to its original position, you will twist the wire through an angle q (theta). Given the rotation spring constant of the wire, you will be able to calculate the force required to keep sphere A in place and the repulsive force acting between the two spheres. Helpful hint: The spheres lose their charge by transferring electrons to or from the air or other objects. Be careful not to touch the spheres, and try to work quickly, especially if the air is humid.

Calibration

You will need to establish the zero-force position on the torsion balance from which to measure the twist angle q (theta).

1. Remove the charge on both spheres by touching them with your finger.
2. Rotate the twist angle pointer adjustment until sphere A is at the zero position.
3. Rotate the protractor scale so that 0º is in line with the twist-angle pointer.

Distance

Measure how the electrostatic force is affected when the charge separation distance is varied.

1.       Bring the two charges into contact with one another and charge them by rubbing one of the rods and then sliding the rod along the surface of one of the spheres.

2.       Move sphere B to the 15cm mark on the meter stick and measure the force between the two spheres by measuring the angle q (theta) on the twist angle pointer required to keep sphere A in place. The force is given by Eq. 2, where s = 2.36x10^–5 N/degree.  Then move sphere B to the 25 cm mark and repeat the measurement.  Record your results.

Eq. 2

3.       Using your F₁₅ force measurement, calculate the amount of charge on sphere A (assume that the spheres are equally charged).

4.       From your force measurements, determine the ratio F₁₅/F₂₅ and compare this to the ratio you would predict based on the 1/r² dependence of Coulomb’s Law.

Measured F₁₅:       N  (Examples: 1.23 , 1.32e-12. Do not use commas or spaces.)
Measured F₂₅:       N
Calculated q_A:       C
Measured F₁₅/F₂₅:
Predicted F₁₅/F₂₅: 

Charge

Measure how the electrostatic force is affected when the charge magnitude is varied.

1. Recharge spheres A and B as you did previously.
2. Move sphere B to the 15cm mark on the meter stick and measure the electrostatic force.
3. Touch a similar sphere C to ground and then to sphere B to remove half of its charge.
4. Remove sphere C and measure the force between A and B again.

Before touching C, F₁₅:     N
After touching C, F₁₅:        N
Measured F_before/F_after:   
Predicted F_before/F_after: