Lab#1 Static Electricity 02 03 2021 PDF

Preview

Summary

Download Lab#1 Static Electricity 02 03 2021 PDF

Description

Lab # 1

INTRODUCTORY PHYSICS HUNTER COLLEGE Static Electricity: Coulomb’s Law It is always a good idea to read up on a topic before you conduct a lab exercise. For this lab it is suggested that you review Giancoli Chapter 16 on Static Electricity. A site called “The Physics Classroom” is also a good place to brush up on any topic in Intro Physics that you want to. From early civilizations on, humanity has been aware of the phenomenon of static electricity. Who hasn’t walked on a carpet, touched a door knob and experienced a small electric shock? The first record of the study of static electricity was that of the pre-Socratic philosopher Thales of Miletus (around 600BCE). He found that rubbing amber (a fossilized tree resin) with something (he didn’t say what) caused it to attract bits of leaves and twigs. Twenty-eight hundred years later scientists have a pretty good idea of how rubbing things together generates forces of attraction and repulsion. Charles Coulomb back in 1785, building on the work done by others earlier, was able to formulate and verify a mathematical description of these forces. ( Both as scalar and vector arrangements)

Here q is the charge in coulombs, r is the distance between charges in meters and k is the electrostatic constant 9 x 109. C (N m2)/C2. The inverse square arrangement is strikingly similar to the form of Newton's Law of Universal Gravitation. In this lab exercise, you will create demonstrations of static electricity. In addition, through PhET simulations and using Coulomb's Law, you will be able to calculate the forces that arise from the interaction of such charges.

A. Demonstration of Static Electricity ( S.E.) First go to Youtube to watch a series of S.E. phenomena. In your browser go to

Static Electricity Demonstrations Part one Induction // Homemade Science with Bruce Yeany. 1

We will watch this together. You are welcome to view it later on your own time. You can also watch Part 2. However, you need not be restricted to just these demos. There are plenty more. Once you have done viewing S.E. demos, pick two that you will try to replicate. Obviously, you are not in a lab where lab materials can be presented to you. So this exercise entails directing you to actually create objects that can demonstrate how objects become charged and how forces due to S.E. can be created. Make sure that the demos you select can be done in your home with ordinary household material. DO NOT go out and buy expensive parts. Take pictures of what you have made and paste them into your report. Be sure to identify the demos you have created. After your project is complete answer the questions below. If for whatever reason, you are unable to actually build something, say so explicitly. Then describe what you would have done and respond to the questions below as if you had built it. I would have done the balloon experiment. Simple but effective in understanding. I would rub the balloon on top of hair, they would give off electrons while the balloon would accept them. The balloon would become negatively charged and the hair would be positively charged . If I put the balloon on the wall it would stick. The wall is an insulator and the process to which it sticks would be via induction.

Sketch how charges are distributed among the objects in your demo as they are rubbed. Scan and paste your sketches into the report. 2 Describe why an object becomes charged through friction. When two insulating materials rubbed against each other, it causes them to become electrically charged. This is due to the static electricity and this causes polarization. This is dependent on electron affinity. Objects that will gain electrons become more negatively charged and those that lose electrons becomes positively charged.

3. Explain why some objects become negatively charged while others become positively charged when rubbed. Electrons are transferred when two objects are rubbed against each other. Due to electron affinity, one object will gain more electrons, leading to it becoming more negatively charged while the other object which lost electrons will become more positively charged. 4. Rubber with Wool and Glass with Silk. They are rubbed together. Which is negative and which is positive. Explain why. Rubber when rubbed with wool will become more negatively charged as it has a higher electron affinity. The wool would be more positively charged due to losing electrons. When glass and silk are rubbed together, due to triboelectric effect, the glass will be more positively charged as it will lose electrons and the silk will gain more electrons, thus, being negatively charged. 5. Why can a charged object attract a neutral object? 2

Neutral objects are attracted to either a positive or negative charge and this is dependent on the concentration of charges in the charged object. This results in polarization via induction. 6. Can you support this statement: “An object becomes positively charged when protons migrate to it after being rubbed”? Why or why not? This statement is false due to the charged being the result of the transfer of electrons not protons. This is because in a molecule, the electron is outside of the nucleus and has weak nuclear forces compared to the protons which are close to the nucleus and have strong nuclear forces. Due to this force, they are more tightly bonded and are not likely to leave unlike the electrons mentioned earlier. 7. Who first labelled charges Positive and Negative. This person was also ambassador to France (and apparently was engaged in a number of “foreign affairs”.) This person incidentally also was the first to use the term electrical battery. Benjamin Franklin. 8. Why does the term “ground” come into play when dealing with electricity of any kind? Ground in the case of electricity is a safety measure or a point where electricity is flowed back into the ground in the case that there is any fault in the wiring. It can also can be any piece of metal. 9. How does a Wimshurst machine work and why is it useful? A Wimhurst machine is a device that will make a electrostatic potentital based on the plates and their diameters. It works on the basis of induction instead of friction. There were quadrants in which negative and positive charges are made across the induction plates while the plates rotate. 9. What did the people of a Dutch town called Lieden or Leyden do to store S.E?

B. Determining the Force Between Charges Copy and paste this PhET url into your address bar at the top. https://phet.colorado.edu/en/simulation/coulombs-law

Click the Macro Scale to get to this screen

3

Fool around with the controls to see how the simulation works. 2 As shown below, set the blue charge on the left at -2uC and the red one on the right at 4uC. Using the ruler, arrange the blue one at 0 and the red one at 10. Also be sure to check the Force Values on the right.

Record the Distance and Force values on the table below when the centers of the two spheres are 10 centimeters apart. Then move the center of the red sphere to 9 centimeters and record the force again. Continue this way for the other distance values.

Distance (cm)

Force (Newtons)

Distance (cm)

Force (Newtons)

10

7.190N

6

19.972N

9

8.877N

5

28.760N

8

11.234N

4

44.938N

7

14.674N

3

79.889N

RESPOND TO THIS QUESTION: Why is it that we record distances from the Center of the spheres and not from their edges? 4

We measure from the center as it allows for better accuracy. GRAPH (Nice and Neat) Now using a full sheet of graph paper, plot the data with F on the y axis and Distance on the x axis. Sketch a line of best fit for the data points. What kind of line of best fit should it be if it is not a straight line?

Distance in Centimeters versus the Force in Newtons For Coulomb's Law 90

80

70

Force (N)

60

50

40

30

20

10

0

2

3

4

5

6

7

8

9

10

11

Distance (cm)

Figure 1: Coulombs law demonstrated with distance measured in centimeters and force in Newtons. This is a parabola type graph

Now work out these problems and show your work! After finding the solution, verify your answer by manipulating the spheres in the simulation adjusting for position and charge. Be sure that you check both the Force and Scientific Notation boxes. How close were you?

Post Lab Questions 5

1.

The blue charge on the left ( -3 microcoulombs) is at the 3-centimeter mark. If the red charge on the right (6 microcoulombs) is located at the 9-centimeter mark, what is the magnitude of the attractive force between them?

F=k (q1*q2)/(r^2)  F = (9x10^9) * (3x10^-6) * (6x10^-6) / (6x10^-2)^2  45N

2.

By doing one quick mathematical step, predict what the attractive force between them would be if the red charge were moved to the 6-centimeter mark? Verify your answer in the simulation.

F=k (q1*q2)/(r^2)  F = (9x10^9) * (3x10^-6) * (6x10^-6) / (3x10^-2)^2  180N

3. How far apart would the two charges have to be if the attractive force between them was -20 newtons? Verify your answer in the simulation. 6

F=k (q1*q2)/(r^2)  20N = (9x10^9) * (3x10^-6) * (6x10^-6) / (r^2) 20N = 1.62x10^-1/(r^2)  1.62x10^-1/20 = r^2  r^2 = 8.1x10^-3  √8.1x10^-3  9x10^-2  9cm = r

These next problems require that you retrieve your vector analysis skills in order to solve them. They cannot be applied to the simulations

4. What would the net force be (remember signs) on a 2 microcoulomb charge if it were 2 centimeters to the left of the blue charge ( -3 microcoulombs)? The red charge (6 microcoulombs) continues to be 6 centimeters to the right of the blue one. Show your calculations. F = F1 + F2 F = (9x10^9)/(4x10^4) (-6 + 3/4) = -21/4 x (9/4 x10^13)  (-189/16 x 10^13N) = = 1.18 x 10^14

5. Two charges are 24 centimeters apart. On the left, the charge is +4 x 10-6 C and, on the right, the charge is +9 x 10-6C. A third charge between them is +3 x 10-6C. How far from the left charge must the middle charge be positioned, so that the net force acting on the middle charge is zero. Show how you would set up the equation, then use Google or a smartphone calculator to determine the actual value. F1 =F2

7

F = k x (q1 xq3)/(r^2) = k x q3 x q2 / ((0.24 – r)^2)  4x10^-16/(r^2) = 9x 10^-6 / (0.24 – r)^2  (2x10^3) / (r) = 3x10^3 /(0.24 – r)  2 /r = 3 /(0.24 -r) = 0.48 /5 = 5r/5  r = 9.6 cm

6.

Three charges are placed at the corners of a right triangle as shown in the figure below. The bottom leg is 4 meters wide and the vertical leg is 3 meters long.

2C 5 meters Hypotenuse 3 meters tall

-1C

4 meters

8C

Find the net force on the 2-coulomb charge at the top. Show how you set it up. Hint: Remember your vectors. You need to find the sum of the x and y forces. Use Google or your smartphone to avoid the drudge arithmetic. Since the net force is a vector, find both the magnitude and angle relative to the VERTICAL axis.

F1 = kq1q2 / (r^2) = 9x10^9 x 2 x1 / (3)^2 = 2 x10^9 N F2 = kq1q2/ (r^2) = 9x10^9 x 2 x 8 / (5)^2 = 5.76 x10^9 N |Fr| = 4.84x10^9N

8...