Physical Science 1421
Department of Physics and Geology
Static Electricity
Equipment Needed
Clear Rod
Fur
Pith Ball
Electroscope
Qty
2
1
1
1
Equipment Needed
Opaque Rod
Silk Cloth
Rotating Support
Paper towel
Qty
2
1
1
1
Background
The word electricity comes from the Greek word electron which means amber (a tree resin that has
been fossilized). When rubbed, a piece of amber can attract small objects like tiny pieces of paper.
Later, other materials were found which exhibited this same behavior. Today we know that the source of
this attraction is due to the presence of static electric charge.
Electric charge comes in two forms: positive and negative. Electrons carry negative charge and
protons carry positive charge. Most materials are electrically neutral – they have an equal number of
positive and negative charges (thus an equal number of protons and electrons). Static electricity occurs
whenever you have an accumulation of one type of charge. This can happen when two dissimilar
materials are rubbed together – for example plastic and fur. A static charge is produced because some
electrons are transferred from one object to the other. The material losing electrons now has fewer
electrons than normal and becomes positive. The material gaining electrons now has more electrons and
becomes negative.
Surrounding a charge there is an invisible electric field which gets weaker the further away one gets
from the charge. For a positive charge, the field is directed outward from the charge. For a negative
charge, the field is directed inward towards the charge. As a result, like charges repel each other and
unlike charges attract each other.
This field will interact with any charge it encounters applying a force to any charge. The fields
being generated by charge leads to like charges being repelled and opposite charges being attracted.
Charged Object
Polarized Object
Non Polarized
Object
If a neutral material is placed inside a weak electric field, it will not experience a net force, because
it contains an equal number of positive and negative charges. However if a neutral object is placed
inside a strong electric field the material will become polarized. Inside a polarized material, positive and
negative charges are no longer randomly distributed and are slightly separated (positive on one side,
negative on the other). Since the electric field surrounding a charged object gets weaker with distance,
the object will have a stronger attractive force on its closest charges of opposite sign and a weaker
repulsive force on the like charges further away. Now the polarized material will experience a net
attractive force. For example, if you charge a balloon by rubbing it on your hair, it can stick to a neutral
wall.
In a metal, a charge can move freely and stay on the surface. In an insulator, a charge can not move
very far (if at all). Once a charge is placed on an insulator it will remain there until something else takes
it.
SAFETY REMINDER
•
Do not drop the magnets and make sure that like poles on the
magnets are not left touching each other.
•
Follow the directions for using the equipment.
For You To Do
The purpose of this laboratory activity is to study the concepts of electrical force, charge, and
polarization. We will investigate both types of charge (positive and negative) and how they interact with
one another and with common objects.
PART I: Equipment Setup and Data Collection
I. Accumulation of Charge
To charge rods you must use the correct pairs Clear Rod with Silk, Opaque Rod with Fur.
1. Briskly rub a clear rod with a piece of silk and Briskly rub an opaque rod with a piece of fur and
answer the questions located in the lab report section that corresponds to this part.
II. Force Between Charged Objects
1. For each of the following cases indicate if there was attraction or repulsion and the charge of
each rod in the lab report section.
A. Charge clear rod with a piece of silk and then put it on the rotating support. Next rub a second
clear rod with a piece of silk. Bring this second rod near the rod on the support.
B. Charge a clear rod and put it on the rotating support. Bring a charged opaque rod near the
charged clear rod on the support and record your observations.
C. Charge an opaque rod and then put it on the rotating support. Bring a second charged opaque rod
near the opaque rod on the support and record your observations.
D. Charge an opaque rod and put it on the rotating support. Bring a charged clear rod near the
charged opaque rod on the support and record your observations.
III. Polarization
A. Small Paper Pieces
1. Paper is neutral. Tear a paper towel into small pieces (the smaller the better).
2. Briskly rub a clear rod with a piece of silk and then bring it near the pieces of paper and record
what happens.
3. Do the same with the opaque rod; which has been rubbed with a piece of fur.
B. Pith Ball
1. Neutralize the pith ball by holding it few seconds with your hand before each of the following
cases.
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A. Bring a charged clear rod near the pith ball. (Make sure not to touch the pith ball with the
rod.) Describe what happens and mark the corresponding type of charge (+ or −) in the figure
in the lab report section.
B. Bring a charged opaque rod near the pitch ball. (Make sure not to touch the pith ball with the
rod.) Describe what happens and mark the corresponding type of charge (+ or −) in the figure
in the lab report section.
C. Electroscope
1. Record your observations and draw charges in the figure for each of the following cases.
A. Briskly rub a clear rod with a piece of silk and bring it near, but not touching, the
electroscope terminal.
B. Briskly rub an opaque rod with a piece of fur and bring it near, but not touching, the
electroscope terminal.
C. Briskly rub an opaque rod with a piece of fur and touch it to the electroscope terminal.
Record your results in the Lab Report section.
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