Coulomb’s Law

Coulomb’s Law

Name: ____________________________

Open the simulation: on the “Macro Scale” box. It will open the simulation.

Familiarize yourself with the simulation. Play around with the settings, change the magnitude of the charges, the distance between the charges, and observe how the magnitude and direction of the force change. When you are finished testing all the settings, click on the orange reset button.

Step 1: Set change 1 to +4µC and charge 2 to +8µC. Set the distance “r” between the charges to 2 cm (by dragging one or both charges along the scale, e.g. if charge 1 is at 4 cm mark and charge 2 at 6 cm mark, the distance between them r = 2 cm = 0.02 m, remember 1 m = 100 cm). Record the magnitude of charges (in coulombs, remember that 1µC = 10-6C), the distance between them (in meters), and the magnitude of the force in the table on the next page.

Notice that the force of q1 on q2 is the same as that of q2 on q1. Also notice that the direction of the force in the simulation shows that the two charges repel each other (as you would expect since both the charges are positive).

Step 2: Keeping the charges fixed q1 = 4µC and q2 = 8 µC, change the distance between them to 3 cm or 0.03 m, record the force again in the table, along with the distance and magnitude of charges.

Step 3: Repeat step 2 for distances 4 cm, 5 cm, 6 cm, 7 cm, 8 cm, 9 cm and 10 cm. Record all the data in the table.

Charge q1

(C) Charge q2

(C) r

(m) Force on q1 by q2


4 x 10-6 8 x 10-6 0.02 Step 4: Go to: the top you will see plot, and below it you will see a data table. Click on the “Clear” link to the right of the data table to clear the data.

Step 5: Use your mouse to select only the data in the last two columns of your table above (starting from 0.02, not including the heading row that has r and Force on q1 by q2), copy this data. Paste this data into the data table by clicking in the table on your browser and pressing ctrl+v on the keyboard.

It will ask you: Is your data arranged by rows or by columns? Select “By Row” and click “Next”.

Then it will ask you: Which columns contains the X-axis data? Click on the left column and click “Next”.

Finally, it will ask you: Is this correct (with all the data displayed”? Click “Apply”.

Now you should see the graph created from your data.

Take a picture of your graph (or a screenshot) and paste it in the space below.

Step 6: Click on the link “Fit Method” under the graph, from the drop down menu click on “Nonlinear” and then on “Power: y = axb”.

Now click anywhere outside the “Fit Method” menu. You will notice the fit parameters under your graph. The computer has fit your data to the function and : y = axb providing you all the fit paramters, like R2:1, aR2:1 etc.

At this point we are only interested in the fit function that is given on the third line under the graph. It should be something like y = 0.1876x^-1.56. Write down this function in the space below:

y =

What is the coefficient (experiment): a =

What is the power (experiment):b =

In 1785, French physicist Charles-Augustin de Coulomb discovered that the force between two point like charged objects with charges q1 and q2 separated by a distance r is given by:

F=kq1q2r2We call it Coulomb’s law.

In our fit above to the data that we collected; x represents r the distance. So, if you compare the fit function above to the Coulomb’s law, you should expect that:

The coefficient: a = keq1q2

And the power (theory):b = -2.0Use the experimental value from your fit above and the actual value from theory to calculate the percent error. Show all your work below.

% error= btheory-b(experiment)b(theory) x 100

% error= ______________

Now since: a = kq1q2 and you know the value of coefficient a from the fit above, and the charges are fixed throughout the experiment, i.e. q1 = 4 x 10-6 C, and q2 = 8 x 10-6 C, use all these values to find the value of the Coulomb’s constant ke, using the equation below. Show all your work:

a = kq1q2k (experiment) =

Actual value of k is: k (actual) = 9 x 109

Use the experimental value from your fit above and the actual value from theory to calculate the percent error. Show all your work below.

% error= kactual-k(experiment)k(atual) x 100

% error= ______________