PHY 112 Lab 1

Lab 1: Electrostatic Forces 1. Testable Question: How is the electrostatic force (F e ) deflection of a pith ball related to the distance between the charged rod and the charged ball (r)? 2. Hypothesis: As the distance between the charged rod and the charged pith ball increases, the electrostatic force (F e ) decreases since the mutual attraction between the charged objects decreases. 3. Variables: Control: length of the string (L), mass of the pithball (m), charge on pith ball(Q P ), charge on rod (Q R ) Independent: distance between pith ball and rod (r), distance between the pith ball and vertical reference (Δx) Dependent: electrostatic force (F e ) 4. Experimental Design: Control(s): m= 0.100 g ; L= 25.0 cm i r (cm) Δx (cm) F e (N) 1-8 5. Materials: ● Pith ball with a string ● Meter stick ● Fur ● Ruler ● Scale ● Table Clamp ● Rod clamps ● Charging Rods ● White Board ● Balance ● Cell phone ● Ring stand

6. Procedure: 1. Record the mass of the pith ball and length of string. 2. Attach the pith ball to the rod clamp. Measure the length of the string to the center of the pith ball using a ruler. Place the whiteboard behind the experiment set up to make taking measurements easier. 3. Rub the piece of fur along the PVC pipe to create a charge. 4. Place the charged PVC pipe near the pithball, but do not touch it. Slowly move the rod from 50 cm in the direction of 100cm at a constant speed. Record the movement of the rod and pith ball with your cell phone from directly on 50 cm as possible. 5. Play the video in slow motion to determine the distance between the pith ball and the rod, as well as the vertical reference over 8 frames. 6. Record the distance between the pith ball and the rod and the distance between the pith ball and the vertical reference for 8 frames in the data table. 7. Calculate tan ( Θ ) using the equation: Δ x √ L 2 − ( x 2 ) 8. Calculate the electrostatic force using the equation: F g × tan ( Θ ) 9. Plot F e v. r, using Microsoft Excel. Find the equation of the line and the R 2 value. 10. The equation being investigated is Coulumb’s Law: F e = ( k Q R Q P ) ∙r 2 7. Data Table: Control(s): m= 0.100 g ; L= 25.0 cm i r (cm) Δx (cm) F e (10 -3 N) 1 1.90 12.9 0.381 2 1.80 12.8 0.385 3 1.80 12.7 0.391 4 1.50 12.5 0.404 5 1.40 12.3 0.416 6 1.30 12.0 0.434 7 1.30 11.8 0.446 8 1.20 11.6 0.457

Graph 3: F e v. 1/r 2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.34 0.36 0.38 0.4 0.42 0.44 0.46 0.48 Fe v. 1/r2 1/r2 (1/cm2) Fe (10-3 N) MS: 0.381 TS: 2.00 % Error: | MS − TS | TS × 100% = | 0.381 − 2.00 | 2.00 × 100% = 80.9% 9. Conclusion: Based on the graphs, electrostatic force (F e ) is inversely linear to the distance between the pith ball and rod (r) based on the equation: F e = 0.483r -0.381 . 10. Evaluation: The hypothesis was supported in this experiment. As the distance between the charged rod and the charged pith ball (r) increased, the electrostatic force decreased because the mutual attraction between the charged objects decreased. This is depicted by the inversely linear relationship of electrostatic force (F e ) to distance between the two charged objects (r). The accuracy of the measurements were a failure at 80.9%. The theoretical slope was greater than the measured slope, due to the systematic errors of humidity and not taking the video directly on the 50 cm mark. This would have caused inaccurate results in data when measuring the distance between the two charged objects (r) and the vertical reference (Δx). The level of precision, the R 2 value, of F e was considered to be terrible. With a 0.9431 R 2 value the random errors of not moving the PVC pipe with a constant speed and the vibrations of the pith ball are possibilities for the lack of precision. These random errors would have created spontaneity in the measurements of the distance between the pith ball and the charged rod (r).