Lab – Coulomb’s Law – Phet Simulation

This activity consists of two Parts

Part one: Eclectic force versus distance. Part two: Electric forces versus charge.

This lab uses the following PHeT simulation:

Coulomb’s Law

( https://phet.colorado.edu/sims/html/coulombs-law/latest/coulombs-law_en.html )

Objectives:

1- Satisfy Coulomb’s law experimentally

2- Study the parameters that affect the electric force

3- Determine the electric constant k

Theoretical Background:

Coulomb’s Law: “The magnitude of the electric force that a particle exerts on another is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.” Mathematically, the magnitude of this electrostatic force FE acting on two charged particles (q1, q2) is expressed as:

Where r is the separation distance between the charged objects and k is a constant of proportionality, called the Coulomb constant, k = 9.0 × 109 Nm2/C2.

Part one:

In this part of the experiment we will explore the inverse square relationship between distance and force. The charges will be held constant throughout.

1. Open the simulator for the macro scale and enter the following values:

,Move the charges so that is at the 0 cm mark and is at 8.0 cm Record these values and the force value into the third row of Table 1.

Calculate r2 and 1/r2 so that the entire third row is completed.

4

Table 1

q1=……… ….	q2=…… …….
r (cm)	r2 (m2)	1/r2 (1/m2)	FE (N)
10
9
8
7
6
5
4
3

2. Move to charges into different positions so that the rest of Table 1 can be completed.

3. Examine the values you obtained for FE. How did the force change as r increased from 3 cm to 9 cm?

4. How did the force change when the value of r was reduced by one half?

5. Using only the values from the data table, determine what the force would read if the distance were increased to 24 cm. Show your work in the space below. Do not use the formula for Coulomb’s Law to solve the problem.

Part two:

In this part of the experiment you will explore the linear relationship between force and charge, and you will use this relationship to experimentally determine Coulomb’s constant k.

 

6. Change the charge on q1 to 5c and adjust the distance between the two objects to 6 cm, record them in table 2. Set the charge on q2 to 10 c and record the corresponding value for the force between the two charges into the top row of table 2.

Table 2

q1 = 5 C	r=6 cm
q2 (C)	FE (N)
10
9
8
7
6
5
4
3

7. Adjust the values for q2 according to Table 2 and fill out the column for FE. Do not change the values for q1 or r.

8. Compare the values for FE when q2 is 4 c and when q2 is 8 c. Does the data support a linear relationship between charge and force? Explain.

Data Analysis

9. Graph the data from Table 2 by placing FE on the y-axis and q2 measured in coulomb s (not C) on the x-axis. Draw a line of best fit. The line should pass through the origin, so do not use a broken scale for either axis.

10. The slope of the best fit line can be used to calculate the electric constant k. Since, the equation is linear, we can compare Coulomb’s Law with the slope intercept equation:

but since b = 0 and FE and q2 are variables graphed on the y and x axes respectively:

where is the slope of your line of best fit.

From Coulomb’s Law, . Since we have two equations for FE, set the right hand sides equal to each other: . Use your slope value and solve for k. You must use standard units. Note that q2 cancels on both sides. Show your work below:Applications

11. In the figure below, 𝒒𝟏, 𝒒𝟐 and 𝒒𝟑 are point charges with 𝐪𝟏=𝟑𝟎 𝐧𝐂 , 𝒒𝟐 = -20.0 nC, and 𝐪𝟑=𝟑𝟎.𝟎 nC. The mass of each charge is 0.020 kg.

a) Determine the magnitude (positive answer) of the electromagnetic force for q1 and q2:

b) Determine the magnitude (positive answer) of the electromagnetic force for q3 and q2:

c) Calculate the total electromagnetic force on q2 due to the presence of both q1 and q3. Determine the magnitude (positive answer) of the total force.

d) Calculate the direction angle of the total force. Express the angle in positive degrees measured counter-clockwise from the positive x-axis. Assume that q2 is at the origin.

 

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