Lab A

# Charge

On cold, clear days, rubbing almost any object seems to cause it to be attracted to or repelled from other objects. After being used, a plastic comb will pick up bits of paper, hair, and cork, and people wearing polyester clothing in the winter walk around cursing the phenomenon dubbed in TV advertisements as "static cling".

We are going to begin a study of electrical phenomena by exploring the nature of the forces between objects that have been rubbed or that have come into contact with objects that have been rubbed. These forces are attributed to a fundamental property of the constituents of atoms known as charge. The forces between particles that are not moving or that are moving relatively slowly are known as electrostatic forces

## Objectives

• To discover some of the basic properties of particles which carry electric charges.
• To develop an appreciation for careful technique. See Caveats.

## Materials

 Scotch tape strips Silk, polyester, and wool fabric Small aluminum balls on strings Glass rods Rabbit fur Pivot stand for rods Metal rod Plastic rods (PVC and perspex) Wimshurst machine Metal spheres on insulation stands

## Part I: The scotch tape trick...

### Procedure

1. You and your partner should each stick a 10 cm or so strip of scotch tape on the lab table with the end curled over to make a non-stick handle. Peel the tape off the table, and bring the tip of the non-sticky side of the tape slowly toward the tip of your partner's strip.
Examine how the distance between the strips affects the interaction between them. Describe the interaction between the two strips. Do they attract or repel one another? How does this interaction seem to depend on the distance between the strips?
2. Stick two strips of tape with "handles" on the table, and label them "B" for bottom. Press another strip of tape with a "handle" on top of each of the B pieces. Label these strips "T" for top. Pull each pair of strips off of the table with handle B and touch them until they have lost any electrostatic attraction to your hand. Then quickly pull the top and bottom strips apart using B & T handles. Examine the interaction between the two T strips when they are brought slowly toward one another.
Examine the interaction between the two B strips.
Examine the interaction between a T and B strip.
Examine the interaction between the strips and other objects, such as your hand.

## Part II: Triboelectricity

### Intro

Anyone who has ever felt the "zap" of an electric shock after walking across a carpet and then touching a metal door knob has experienced that two objects rubbing together can create electrostatic charges. Whenever two different materials rub against each other, it is likely that one will leave with more electrons than it started with…the other will leave with less. This is called Triboelectricity (tribo means friction). From the study of chemistry, we learn that different materials have different desire for electrons. (This is called electronegativity.) Some materials are very greedy and will always steal electrons from things they come in contact with, others are more willing to give upelectrons.

As the neutrally charged person walks across the wool carpet, his leather soled shoes have less deisre for electrons than the wool carpet. As a result, electrons get stolen from the shoe by the carpet. With every step the person becomes more and more positively charged. That charge distributes itself over thebody. When the positively charged person gets near the metal door, he will actually attract charges from the door which jump in the form of a spark. Notice how only the negative charges (electrons) are free to move.

It is important to point out that if he was wearing rubber soled shoes on a wool carpet, his shoes would steal electrons from the carpet. He would become more negatively charged with each step. When he gets near the door the electrons will jump from him to the door. From his point of view it would look and feel the same as it did in the first example. He can't tell whether the charges jumped to him or from him.

If we did a study of many materials and put them in order from those with the least desire for electrons to those with a very strong desire for electrons, we would have created a Triboelectric series. If two items from the list are rubbed together, then the item that is higher on the list will end up more positive and the lower one will end up more negatively charged. For example, if leather were rubbed with wool, the leather becomes positive and the wool negative. Yet if rubber is rubbed with wool, the rubber becomes negative and the wool positive. (It is important to note that this series is true only if the samples are clean and dry. The presense of moisture, dirt, or oils may cause some of the items to interact differently.)

The goal of Part II is to study many materials and put them in order from those with the least desire for electrons to those with a very strong desire for electrons.

### Procedure

1. Put a mark or piece of tape on one side of each rod, so that you can identify the hand-held side from the side which is rubbed by fabric.
2. Rub one of the fabrics vigorously back and forth around one end of the rod, then place the rod in the frictionless pivot holder.
3. Rub one of the fabrics vigorously back and forth around one end of a second rod and place this rod near and perpendicular to the first rod. Observe whether or not the force between the two rods is attractive or repulsive. Notice how strong the force is and whether or not the hand held side of either rod shows different behavior than the rubbed side of that rod.
4. Do all combinations of fabrics and rods, carefully tabulating your results, with the goal of placing all eight things (silk, polyester, rabbit fur, wool, PVC (white) plastic, perspex, glass, metal rod) in a table (below). The rod in the holder should be recharged if it loses its charge.
 More likely to lose electrons. Becomes more   More likely to steal electrons. Becomes more
1. Your table may be in reverse order since you cannot easily distinguish negative from positive charge. Reverse order is okay. If one fabric rubbed against one rod gives an extra large effect, place the fabric and rod at opposite ends of the table. If one rod is hard to charge, place it near the middle. Rods that repulse will be on the same side of the table and the fabrics that charged them will be on the other side of the table. Rods that have a strong attraction will be on opposite sides of the table. However, weak attraction can be due to induction. Induction only requires one of the rods to be charged. To see if a rod is actually charged, compare its rubbed side with its hand-held side. If no difference is observable, it is either uncharged or weakly charged. [Warning: keep hand away from freely pivoting rod since induction in the hand can induce an attractive force on the rod.]

## Part III: Measure how force depends upon distance

### Introduction

A large metal sphere will be charged up with a wimshurst machine, or some other comparable device. An aluminum ball hanging from a string will be attached to a transparent ruler on a support stand. Once the aluminum ball touches the metal sphere, it will take some of the charge from the sphere and be repulsed. The strength of the repulsive force, F, can be calculated from the angle, , at which the string slants away from the veritcal.

### Procedure

1. Measure the length of the string from the tape to the top of the aluminum ball. Use centimeters for all length measurements.
2. Charge up the metal sphere and touch the aluminum ball to it. Keep the metal sphere charged.
3. Your first measurement should have the aluminum ball close to the metal sphere. Look down on the transparent ruler to measure the horizontal displacement, x, of the aluminum ball from the vertical drop. Discharge the sphere to avoid getting shocked and measure the horizontal distance of the vertical drop from the edge of the sphere.
4. Uncharge the aluminum ball and then repeat steps 2 and 3 several more times choosing a larger distance, d, from the sphere each time. Your last measurement should havex at least a factor of 5 smaller than its value in the first measurement.
 x d

### Analysis for Part III.

1. Using the free force diagram for the ball, calculate how the electrostatic force, F, depends upon angle Θ. Note: The tension in the string also depends upon Θ, so your solution must eliminate T. Your final answer will be
F = constant • trig (Θ)
where trig (Θ) is some trigonometric function of Θ.
2. Calculate Θ and trig(Θ) for each data point.
3. Calculate r, the distance from the center of the aluminum ball to the center of the sphere for each data point.
r = x + d + radius of sphere.

4. Use Graphical Analysis to plot trig (Θ) vs. r and use a power function automatic fit, to find the r - dependence of the force. Print your plot and fit, and comment on your result. Do you know what the r - dependence should be?

### Part IV: Mr. Coulomb's trick

Coulomb devised a clever trick for determining how much force charged objects exert on each other without knowing the actual amount of charge on the objects. He transferred an unknown amount of charge, q, to a conductor, which meant it spread evenly all over the surface of the conductor. He then touched the newly charged conductor to an identical uncharged one. The conducting objects would quickly exchange charge until both had q/2 on each, since they were identically shaped. After observing the effects with q/2, Coulomb would discharge one of the conductors by touching a large piece of metal to it and then repeat the procedure to get q/4 on each conductor, and so on.

Using this procedure, measure the force between two pith balls or between a single ball and a larger charged object. Determine the dependence of the force on the charge present. Plot your results using Graphical Analysis.

## Caveats

Static electricity is notorious for producing inconsistent results. Make sure every result you write down can be repeated. Document the steps you took to acquire consistency for any result you report. Document the problems you encountered in gaining consistent data.