Physics Laboratory 5

Static Electricity

Objective:

In this lab students will try to separate electric charges and give macroscopic objects a net charge. Students will then investigate the interaction between the charged objects.

Static electricity is a well-known phenomenon. It affects many industries in diverse environments. Static charge buildup can result in potentially dangerous electrical shocks, which can cause fires, explosions and severe damage to sensitive electronic components. Static charge buildup can be caused by friction between two surfaces. This is called triboelectrification. Electrons migrate from the surface of one material to the surface of the other. Upon separation of the two surfaces, one surface loses electrons and becomes positively charged. The other surface gains electrons and becomes negatively charged.

As the pressure and the speed of contact and separation increase, the amount of the static charge buildup increases. Rapidly moving materials can quickly develop charges, which produce a potential of more than 25,000 volts.

In this lab students will separate electric charges. They will consult the triboelectric sequence to find out which object acquires a positive, and which object acquires a negative charge. They will then investigate the interaction between the charged objects.

Equipment needed:

	Sticky tape (Scotch tape or similar tape)
	Rags and rods
	Styrofoam cup and string

Procedure:

Before starting your experiment, open a word document. Then address points (a) - (i) below. After each step, write down what you have observed. (Address the points highlighted in red.)

(a) Obtain a piece of sticky tape, about 15 - 20 cm in length. For ease in handling, make "handles" by folding each end of tape to form portions that are not sticky. Press the tape firmly onto a smooth, unpainted surface, for example, onto your textbook. Then quickly peel the tape off the surface and hang it from a support. Describe the behavior of the tape as you bring objects, such as a finger or a pen, towards it.

(b) Make another piece of tape as described above. Bring the second tape toward the first. Describe your observations. It is important, that during this experiment you keep your hands and other objects away from the tapes. Explain why this precaution is necessary. Describe how the distance between the tapes affects the interaction between them?

(c) Press two pieces of tape onto the surface and write a B (for bottom) on them. Then press another tape on top of each B tape and label it T (for top). Pull each pair of tapes off the surface as a unit. After they are off the surface, separate the T and B tapes. Hang one of the T tapes and one of the B tapes from a support. Describe the interaction between the following pairs of tape when they are brought near one another.

	Two T tapes
	Two B tapes
	One T and one B tape

(d) Among your belongings find a rag and a rod-like object. The rag and the rod should be made from materials near the opposite ends of the triboelectric sequence. Consult the table of triboelectric materials below.

The items on top are less attractive to electrons and become positively charged, while the items on the bottom are more attractive to electrons and become negatively charged.

	Human Hands (usually too moist though) (very positive)
	Rabbit Fur
	Glass
	Human Hair
	Nylon
	Wool
	Fur
	Lead
	Silk
	Aluminum
	Paper
	Cotton
	Steel (neutral)
	Wood
	Amber
	Hard Rubber
	Nickel, Copper
	Brass, Silver
	Gold, Platinum
	Polyester
	Styrene (Styrofoam)
	Saran Wrap
	Polyurethane
	Polyethylene (like scotch tape)
	Polypropylene
	Vinyl (PVC)
	Silicon
	Teflon (very negative)

Choose a rod and a rag, for example a piece of PVC pipe and fur. Rub the rod vigorously with the rag and then hold the rod near newly-made T and B tapes hanging from a support. Compare the interactions of the rod with the tapes to the previously observed interactions between the tapes. Describe any similarities or differences.

(e) The rod and the tapes interact, because they are electrically charged. Answer the following questions based on the observations you have made thus far.

	How many different types of charge do there appear to be? Explain.
	Which tape, T or B, has a positive charge? Explain.
	How do two objects that are positively charged interact? Explain how you can tell.

(f) Now remove all tape from the support. Cut a Styrofoam cup into small pieces, for example squares 1 to 2 cm on each side. Attach a small piece of Styrofoam to approximately 30cm of insulating string and hang it from a support. Touch the Styrofoam piece to a charged rod and observe the behavior of the piece after it touches the rod. Is the piece charged? If so, does the piece have net charge with the same sign the rod? Explain how you can tell.

(g) Hold a charged rod as shown. The top view shows a plane through the center of the rod.

Explore the region near the rod with a test piece that has a charge of the same sign as the rod. Based on your observations, sketch vectors to represent the net electric force on the test piece at each of the points marked by an Ä.

Compare the magnitude of the force F on the piece of Styrofoam at different points. If the amount of charge q on the Styrofoam piece were doubled, would the electric force exerted on the Styrofoam piece at each location change? If so, how? If not, explain why not.

Would the ratio F/q change? If so, how? If not, explain why not.

The quantity F/q evaluated at any point is called the electric field E at that point. Compare the magnitude of the electric field at different points. Would the magnitude or the direction of the electric field change if:

	the charge on the rod were increased? Explain.
	the magnitude of the test charge were increased? Explain.
	the sign of the test charge were changed? Explain.

(h) The electric field can be represented with vectors or alternatively with field lines. The diagram below shows a two-dimensional top view of the electric field lines representing the electric field for a positively charged rod.

Identify the feature of the electric field lines which gives information about the magnitude of the field?

(i) Hold a piece of paper flat in front of you. The paper can be thought of as a part of a larger plane surface. Identify a single line which can be used to specify the orientation of the plane of the paper, so that someone else can hold the paper in the same or in a parallel plane?

	The area of a flat surface can be represented by a single vector, called the area vector A. What does the direction of the vector represent? What does the magnitude of the vector to represent? Explain.
	Describe the direction and magnitude of the area vector A for the sheet of paper shown below.
	Describe the direction and magnitude of the area vector A for each of the individual squares that make up the sheet.
	Consider the paper folded into a hollow triangular tube as shown below. Can the entire sheet be represented by a single vector with the characteristics you defined above? If not, what is the minimum number of area vectors required? Explain.
	If the graph paper is formed into a tube as shown below, can the orientation of each of the individual squares that make up the sheet of graph paper still be represented by dA vectors? Explain.

Open Microsoft Word and prepare a report using the template shown below.

Name:
E-mail address:

Laboratory 5 Report

List your comments, descriptions, explanations, and answers pertaining to points (a)-(j). Make sure that you have addressed the points highlighted in red.