PASCO ES-9070 User Manual

Includes

Teacher's Notes

and

Typical

Experiment Results

COULOMB BALANCE

Instruction Manual and Experiment Guide for the PASCO scientific Model ES-9070

012-03760E

05/99

012-03760E Coulomb Balance

T able of Contents

Section Page

Introduction .....................................................................................................1

Theory ............................................................................................................. 2

Equipment........................................................................................................3

Additional Equipment Recommended: .....................................................3

Tips for Accurate Results ................................................................................ 4

Setup .............................................................................................................5

Experiments:

Part A Force Versus Distance...................................................................7

Part B Force Versus Charge ......................................................................8

Part C The Coulomb Constant...................................................................9

Replacing the Torsion Wire........................................................................... 13

Teacher’s Guide............................................................................................. 15

Technical Support.................................................................Inside Back Cover

Coulomb Balance 012-03760E

Please—Feel free to duplicate this manual subject to the copyright restrictions below.

The PASCO scientific 012-03760E Coulomb Balance manual is copyrighted and all rights reserved. However, permission is granted to nonprofit educational institutions for reproduction of any part of the manual providing the reproductions are used only for their laboratories and are not sold for profit. Reproduction under any other circumstances, without the written consent of PASCO scientific, is prohibited.

Limited Warranty

PASCO scientific warrants the product to be free from defects in materials and workmanship for a period of one year from the date of shipment to the customer. PASCO will repair or replace at its option any part of the product which is deemed to be defective in material or workmanship. The warranty does not cover damage to the product caused by abuse or improper use. Determination of whether a product failure is the result of a manufacturing defect or improper use by the customer shall be made solely by PASCO scientific. Responsibility for the return of equipment for warranty repair belongs to the customer. Equipment must be properly packed to prevent damage and shipped postage or freight prepaid. (Damage caused by improper packing of the equipment for return shipment will not be covered by the warranty.) Shipping costs for returning the equipment after repair will be paid by PASCO scientific.

Equipment Return

Should the product have to be returned to PASCO scientific for any reason, notify PASCO scientific by letter, phone, or fax BEFORE returning the product. Upon notification, the return authorization and shipping instructions will be promptly issued.

NOTE: NO EQUIPMENT WILL BE

ACCEPTED FOR RETURN WITHOUT AN AUTHORIZATION FROM PASCO.

When returning equipment for repair, the units must be packed properly. Carriers will not accept responsibility for damage caused by improper packing. To be certain the unit will not be damaged in shipment, observe the following rules:

➀ The packing carton must be strong enough for the

item shipped.

➁ Make certain there are at least two inches of

packing material between any point on the apparatus and the inside walls of the carton.

➂ Make certain that the packing material cannot shift

in the box or become compressed, allowing the instrument come in contact with the packing carton.

Address: PASCO scientific

10101 Foothills Blvd. Roseville, CA 95747-7100

Credits

Author: Bruce Lee Editor: Dave Griffith

Phone: (916) 786-3800 FAX: (916) 786-3292 email: techsupp@pasco.com web: www.pasco.com

012-03760E Coulomb Balance

Introduction

The PASCO Model ES-9070 Coulomb Balance (Figure 1) is a delicate torsion balance that can be used to investigate the force between charged objects. A conductive sphere is mounted on a rod, counterbalanced, and suspended from a thin torsion wire. An identical sphere is mounted on a slide assembly so it can be positioned at various distances from the suspended sphere.

To perform the experiment, both spheres are charged, and the sphere on the slide assembly is placed at fixed

charged spheres

torsion wire

Figure 1. Experimenting with the Coulomb Balance

slide assembly

distances from the equilibrium position of the suspended sphere. The electrostatic force between the spheres causes the torsion wire to twist. The experimenter then twists the torsion wire to bring the balance back to its equilibrium position. The angle through which the torsion wire must be twisted to reestablish equilibrium is directly proportional to the electrostatic force between the spheres.

All the variables of the Coulomb relationship (F = kq

/R2) can be varied and measured using the

1q2

Coulomb Balance. You can verify the inverse square relationship and the charge dependence using the balance and any electrostatic charging source. However, for best results, we recommend you charge the spheres with a stable kilovolt power supply to ensure a reproducible charge throughout the experiment. To determine the Coulomb constant with reasonable accuracy, we recommend you use an electrometer and a Faraday ice pail to accurately measure the charge on the spheres. For more information about accuracy, read the section Tips for Accurate Results.

Coulomb Balance 012-03760E

Theory

Take one gram of protons and place them one meter away from one gram of electrons. The resulting force is equal to

1.5 x 1023 newtonsroughly the force it would take to lift an object from the surface of the Earth that had a mass about 1/5 that of the moonnot a small force.

So, if such small amounts of charge produce such enormous forces, why does it take a very delicate torsion balance to measure the force between charged objects in the laboratory? In a way, the very magnitude of the forces is half the problem. The other half is that the carriers of the electrical forcethe tiny proton and the even tinier electronare so small, and the electrons are so mobile. Once you separate them, how do you keep them separated? The negatively charged electrons are not only drawn toward the positively charged protons; they also repel each other. Moreover, if there are any free electrons or ions between the separated charges, these free charges will move very quickly to reduce the field caused by the charge separation.

So, since electrons and protons stick together with such tenacity, only relatively small charge differentials can be sustained in the laboratory. This is so much the case that, even though the electrostatic force is more than a billion-

billion-billion-billion times as strong as the gravitational force, it takes a very delicate torsion balance to measure the electrical force, whereas we can measure the gravitational force by weighing an object with a spring balance.

NOTE: The torsion balance gives a direct and reasonably accurate measurement of the Coulomb force. The most accurate determinations of Coulomb's law, however, are indirect. It can be shown mathematically that if the inverse square law holds for the electrostatic force, the electric field inside a uniformly charged sphere must be everywhere zero. Measurements of the field inside a charged sphere have shown this to be true with remarkable accuracy. The Coulomb force can be expressed by the formula:

1q2

2+n

F = kq

Using this indirect method, it has been demonstrated

experimentally that n  2 x 10

16

012-03760E Coulomb Balance

Equipment

The Coulomb Balance and the included accessories are shown in Figure 2.

be shipped with the copper rings unattached.

COULOMB

BALANCE

torsion wire (pendulum)

coupling

Plate

retainer

spare torsion wire

(3 meters)

toolbox with: one 50 mg mass two 20 mg masses one hex key

magnetic

damping arm

index arm

calibration

support tube

(The Coulomb Balance and the slide assembly should be shipped with one of the conductive spheres unattached. See the Setup section of this manual.)

NOTE: The balance may

charging probe

conductive sphere on insulating

thread

torsion wire

clamp

Figure 2. The Coulomb Balance

Additional Equipment Recommended:

 A stable kilovolt power supply for charging the

spheresAny electrostatic charger can be used to charge the spheres, but a power supply lets you replenish the charge to a fixed value throughout an experiment. Ideally the supply would have a momentary power on button so that you can conveniently turn it off whenever you are not charging the spheres.

allen wrench for the slide

assembly

slide assembly

 An electrometer and Faraday ice pail (such as

PASCO Models ES-9054A and ES-9058) for accurately measuring the charge on the spheres.

 A spring balance capable of measuring a force of

approximately 4 newtons (400 gram mass). This is not necessary for the experiment itself, but is helpful in setting the tension of the torsion wire.

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