Lab Report Day 8 - Electric Charge and Forces

1. Balloon

In this lab, Professor Mason uses an animal fur to rub the balloon.  We predict what would happen to the balloon when it is rubbed with the fur and placed on the glass. We predicted that the balloon will stick to the glass. From experiment, we find that our prediction is correct. On the balloon, it exerts gravity and frictional force on the y direction and electric force and normal force on the x direction. Each force cancels out and it is equilibrium. So the balloon will stick to the glass window. We then predict what would happen to the balloon if it is rubbed with silk. We predict that it would stick to the glass as well. From the experiment, we find that our prediction is true.
In both cases, the balloon get electric force by rubbing the animal fur or silk. It brings charge to the balloon.

We define charge as “stuff attracts other things electrically”.

Comparison of Mass and Charge

2. Interactions of Tape Strips

If we peel two tape off the table and bring the non-sticky side of it toward each other, the tape will repel each other. The closer they are, the larger distance they will repel.

Then, we place two strips on tape on the table sticky side down and label B for bottom and T for top.
When we put two top strips together, they will repel each other.

When we put two bottom strips together, they will repel each other.

When we put a top and a bottom strip together, they attract each other.

When the two strips stick together, it means they have the opposite charge; one is positive and one is negative. When the two strips repel to each other, it means they have same charges, either both positive or both negative.

Conclusion for Interactions of Tape Strips

Practice Problems:

We use trigs to solve for the angle.

From the practice, we find out that Y and x^-2 are proportional to each other. Then we need to prove our prediction.

 3. Electric Force Law LoggerPro

We predicted that the relationship between force and distance r is inversely proportional.

 By using LoggerPro, we are able to plot a graph and find the values. In this case, our theoretical value is 2, while our experimental value is 1.999. It is very close to it. The reason why it is not exact 2 is because we some times click the point not at the middle of the ball.

 If two charges have same charge, then they will repel. If they have opposite signs, they will attract to each other.

Conclusion for Electric Force Law Video Analysis Activity

 Van De Graaff Generator 

The Van De Graaff generator can charge other objects.

Professor Mason places a wig on the Van De Graaff generator. We noticed that the hair is sticking up. The reason why the hair goes up is because the generator is putting charges into the wig. Since all the charges are same, they want to repel each other.

When we touch it, the hair goes back down, and goes up again later. The reason why it acts like that is because when we touch the hair, the charges are transfer to our hand, and we give some position to neutral the electrons. However, since it is inputting large amount of negative charges, it will soon be filled with negative charges again, which causes the hair to repel each other.

(The video is rotated in the middle and rotates back in the end)

Then, Professor Mason put a Franklin motor on the generator. The Franklin motor immediately spins because the charge accumulated at the points. It is a conservation of momentum situation, the electrons streamed out of those points and moved the entire system.

Lastly, a streamer is put on the generator. When the it is turned on, the streamers goes up. It happens because the generator gives lots of negative charge which causes it to repel.

We then talk Van Der Graaff generator works. It rubs electrons by a motor using a belt that full of electrons and metal conducts the electrons outward through ground that generates a flow of electron.

We also play with the Storm Ball. 

Conclusion:

In today's class, we learn how to combine the electric force with things we learnt from 4A. We learn how to calculate the electric force and how the Van Der Graff generator works. We also learn how to using Coulomb's Law for calculation.