Lab Report Day 11 - Gauss' Law, Microwave, Active Physics

1. Gauss' Law in Flatland

Here is the graph to show the electric flux of different charges, +1,+2 and -2 charges. We count the electric flux by counting the difference between number of lines into the charge region and number of lines out.  

This is a definition of the flux. It only depends on the number of lines in and out, and the surface area of it. 

2. Gauss’s Law in Three Dimensions

There is no electric charge in the surface since charges accumulate on the surface of objects.

3. What is the Excess Charge in a Metal (In class notes, not the lab notebook)

We make few assumptions on which way is the safest while there is lightening.

We guess that the safest way is to stay in the car. Here are some explanations. 

1.) Tree is tall (distance is short, easier to discharge)

2.) Is  better then 1, but still near the car (electrons travel outside car)

3.) Umbrella is a conductor. Distance is short

4.)  still have chance to get hit by lightening

5.) The electron will travel outside the car, since car is made by metal, which is a conductor, and the inner part of the car is an isolator. (Best choice)

The answer is that stay inside the car is safe. Our assumption is correct. 

4. Microwave Demos

We are going to heat several things in a microwave. The frequency for microwave is about 2.5 G Hz. At this range, microwave will be absorbed by water. The vibration of water molecules will heat up things. At this frequency, microwave will not be absorbed by plastic, glass or ceramics.

1.) We start with putting metal fork into microwave

We heat the metal fork up. We guess that there will be sparks since we are told that we should never put metal into microwave.

However, what we see is that——nothing visible happens. However, when we touch it,  it is hot. It has a larger mass. So it takes longer to heat up. 

2.) We then put CD into microwave. We first predict that there will be sparks and it will burn.

We can see that the CD is burnt, and there are bubbles on it. The charges are on the surface of the CD. Since CD has small mass of metal particles. So the charge could have larger affect. 

3.) We put a match which is on fire into microwave. We make a prediction that the flame will be larger. We guess that the flame becomes larger because microwave provide energy to the water vapor in air and ignite the gas.

We can see that the fire gets bigger. Also, the flame moves at a frequency of 2.5G Hz, which is the frequency of microwave.

When we light a match, the exhaust gases that contains unburnt fuel and water vapor goes above the flame. It adds lot of energy to its mixture, but water is what vibrates well. So the flame will vibrate at  a frequency of 2.5G Hz.

4.) We then heat steel wool.

We see lots of sparks while heating it up. Its mass is small, so when adding lots of charges, it discharges from steel wool which causes lots of sparks.

We then heat soap in microwave.

We do not see anything. We guess that it will bubbles like the CD. However, we do not see anything. Soap is expected to expand as there are air sac with water vapors in the soap. The experiment fail to show the effect. It might be because the water vapor has been dried out for a long period of time.

Theoretically, the soap will expand, since it discharges in the inside. 

Then, we heat up the Christmas Decoration Ball, which contains metal. 

It actually has two different results. When we place the sharp side down, which the whole ball is considered as sphere shape, nothing happens, because the charge on sphere are uniform. When we place the sharp side up, the ball is on fire. It contains coding over it. But the surface is very small. So at each area it contains lots of charge, which makes the ball on fire. 

We then heat up a light-ball. 

The bulb is filled with Ar gas. The light bulb will light up with different color. 

We then heat up a grape.

There is no effect on the grape since the grape is in round shape, but the grape will be burned if it is cut into half as the cut surface is no more round in shape.

Finally, we heat up a juice bag.

It has blue spark, and it expands a little bit. The water inside it becomes vapor so the volume increases which cause the bag expand. 

Active phys 1-5

Answers for Active Physics 1-5

Summary:

In today’s class, we learn what is flux and Gauss’s Law. We know that flux is determined by the number of electric field lines (which is related to the strength of electric field ), and the surface area. We know how microwave works. We learn what kind of material is able to absorb microwave and what cannot. 

5% Extra Credit on Celebration One

Lab Report Day 10 - Electric Dipole and Flux

Van De Graff and Cylinder Cage

We first charge put a cylinder cage with aluminum foil inside. We predict which direction the aluminum foil will go. We predict that both aluminum will move.

What we observe is that only the outer foil will move. This happens because the flux of inside is zero. The number of electric field coming in  and electric out is the same, which makes no electrons flow in the inner.

The set-up

ActivePhysics

We do active physics 1-4, 5-9 in class (which we finish after class.)

Answers for Active Physics Electric Flux

Needle Seat

As long as the weight is separately uniformly, it will not hurt.

The needle stands for electric field line. The number of needles represents the strength of the flux.

We get a conclusion that the shape does not affect the flux. If we increase the number of charges, the flux will also be bigger.

Conclusion

In today's class, we learn what is the electric flux. The bigger the surface area, the larger the flux.

Lab Report Day 9 - Electric Fields

Active Physics

This is the active physics we do in the beginning of class. (We finished it after class.)

Electric Field

Those are some features of an electric field. An electric field is caused by some charge particles. The magnitude of an electric field depends on the charge. The magnitude of an electric field also depends on the distance. Not until a charge is placed in an electric field does it experiences an electric force.

3D Models of Electric Field

A 3-D negative charge

Professor Mason use the polystyrene plate to make two hemispheres which represent positive and negative charges. A positive electric field points out, and the negative electric field points in. When we put the two opposite charge together, the positive electric field will go to the negative charged electric field. From this model, It helps visualize a positive charge and a negative charge.

We derive the formula for electric field

Doing some exercises.

Plotting the electric field vs. distance graph. We can see that it is a hyperbola shape. 

Some exercises.

Superposition of Electric Field Vectors

Using Excel to calculate the net electric field (using equation E=Kq/r^2)

We change the distances from 0.5 to10.0 cm with increment of 0.5. Using the information calculated on the spreadsheet, we get the net electric field. 

Extended Charge Distribution

 

Some Calculations. From Excel, we find that the electric field is 5.97*10^4. In here, we make some calculations and find that our value is 5.99*10^4, which is almost exactly what we get from the excel.

Electric Field Hockey

The electric hockey game is interesting. Some how I find that we can pause and remove the charge that we have no use, and the total charge will be deducted.

Conclusion

In today's class, we learn what a positive and negative charge look like. We also learn that since electric field is a vector, we can add the electric field together.

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.