two lab assignments

­­­Online Electric Interaction Lab

Charged Objects Interactions and Coulomb’s Law

• To discover some of the interactions of particles that carry electric charges.

• To understand how Coulomb’s law describes the forces between charges.

After rubbing some items together there seems to be some new force acting upon them. Consider when you comb your hair with a plastic comb, your hair will start to move toward the comb. Unroll a thing of saran wrap to cover some leftovers and the saran wrap will try to stick to everything, even through it is inherently not sticky to the touch. Walking across carpet in socks allows you to annoy your friends and family with a shock. These and other phenomena of this nature are caused by the electric force between objects that have been rubbed or pulled apart. This force is due to the fundamental property of the constituents of atoms known as charge.

We begin the study of the electric force by determining when the force is repulsive versus attractive, which should lead to an understanding of how many different types are charge exist in the natural world. Then by measuring this force we can qualitatively determine which physical properties determine the magnitude of the force between charged objects by Coulomb’s law, the mathematical relationship that describes the vector force between two small-spherical objects. Finally, we determine a new quantity called the electric field which will be used to explain how small test charges behave in the presence of other charges at different locations in space.

Coulomb’s Law:

Exploring the Nature of Electrical Interactions

1. Open the simulation Triboelectricity.

Question 1: Looking at the balloon, cat, and wall are any of them positively charged, negatively charged, or neutral? Explain.

They are all neutral. They have an equal amount of positive and negative charges.

1. Grab the balloon and bring it to the wall. Observe its interaction with the wall. Next take the balloon above the cat (but do not touch the cat yet) and drop it. Observe its interaction with the cat.

Question 2: Does the balloon seem to have an attraction or repulsion with the wall or cat?

It has an attraction with both

1. Rub the balloon on the cat, don’t worry the cat is okay with this.

Question 3: What do you observe now as the charged state of the balloon and the cat?

The charged state of the balloon is negative and of the cat is positive.

1. Bring the balloon next to the wall, observe its interaction. Bring the balloon next to the cat the same way you did in step 2. Observe its interaction.

Question 4: Does the balloon seem to have an attraction or repulsion with the wall or cat?

It seems to have an attraction with both

Question 5: Did the wall ever gain or lose charge? Why then did it experience the interaction described in Question 4? Use your textbook to explain this phenomenon.

Exploring the Force between charged particles.

1. Open the simulation Coulombs Law Lab. Press Begin.

2. Click Show Grid and Show Angle.

3. Click on Charge One and Charge Two. You should see the mass and charge of each tiny sphere. You may change the charge of the spheres to a value of your choosing, but must be charged. Record your values in Table 1. You may need to convert the units.

Table 1

1. Reviewing Newton’s Second Law we can determine that there are 3 forces acting on the charge on the right. Using the figure and Newton’s Second Law write the sum of the forces in the horizontal direction and in the vertical direction.

1. Since the Tension force is unknown solve for it using the vertical direction equation and plug that into the horizontal equation. Remember that Coulomb’s Law is .

2. Use algebraic steps to make the formula look like .

Question 6: Either upload a picture of your work or type your work showing steps 4-6 to answer this question.

1. Using the simulation move the charge on the left back and forth, measure the distance between the two hanging charged spheres as r and record the value in Table 2. You will then measure the angle of the hanging charge, record this value in Table 2.

2. Repeat Step 7 until you have a total of 10 points.

3. Calculate the value of 1/r2 for each r value and record in Table 2. Calculate for each angle value and record in Table 2.

4. Make a plot of vs 1/r2 with on the y-axis and 1/r2 on the x-axis This should now be a linear plot of the form .

Table 2

Question 7: Upload a picture of your graph with displayed Trendline equation to answer this question.

Question 8: What do the slope and y-intercept values represent?

1. Using the slope value from the trendline and your equation in step 6 solve for the electrostatic constant , use the percent error formula to compare your calculated result to the known value .

Question 9: Either upload a picture of your work or type your work showing step 11 to answer this question.

Question 6: How close do the values compare and what would be some causes of error?

1. You have two charged objects. How would you determine if they have like or unlike charges? What would you need to determine if they are charged positively or negatively?

2. Two like charges are separated by some distance. Describe quantitatively what will happen to the force exerted by one charge on the other if

   1. The distance between the charges is doubled

   2. The distance between the charges is halved

   3. One of the charges is replaced by a charge of the same magnitude but opposite sign

3. A charged sphere of mass 0.20 g is hanging from a long string. A charged probe of charge C is brought near the hanging mass. The distance between the probe and hanging mass is now 5.0 cm and the mass makes an angle of 15 degrees with respect to the vertical. What is the charge on the hanging mass?

4. Consider a system of charges. One charge of 2.0 nC is located at (-20.0 cm, 0), another charge of -3.0 nC is located at (0, 30.0 cm), and the last charge of 2.0 nC at (20.0 cm,0). What is the magnitude of the force on the -3.0 C charge from the other two charges?