• University of Illinois, Chicago

• PHYS 142

UIC Physics Department Physics 142 Laboratory Manual

Electrostatics – Charges and Fields Page 1 of 12

Electrostatics – Charges and Fields

Objectives

‰ To make qualitative observations of electrostatic phenomena and interpret the phenomena in terms

of positive and negative electrical charges and interactions between the charges

‰ To study the fundamental concepts in electrostatics, electrical charge and electric field.

Introduction

Electrostatics is the study of static electric charges and electrical forces. In solid objects, static charges are

due to the gain or loss of electrons only. Electrons have a negative charge and are normally bound to the

positively charged nuclei in atoms consisting of protons ሺpositively charged particlesሻ and neutrons

ሺneutral particlesሻ. The positive charge in the nuclei cancels the negative charges of the electrons, and the

object is electrically neutral when its net charge is zero. Normally, materials remain electrically neutral

since it takes energy to pull electrons away from an atom. But for some materials, the action of rubbing

them together can lead to a certain amount of electrons jumping from one of the materials to the

other. Materials that can lose ሺor gainሻ electrons in this way are called triboelectric.

When triboelectric materials are rubbed together, the material which the electrons are jumping from has

more protons than electrons and therefore a positive charge, and the material the electrons are jumping to

has excess of electrons therefore a negative charge. This is what we call "static electricity”.

Unless you live in a place where the weather is extremely humid the year round, static electricity should

be very familiar to you all you have to do to build up a triboelectric charge in your body is walk across the

carpet. This is because complex organic materials such as plastics, nylon, rubber, and even paper tend to

gain and hold static electric electricity rather easily.

Static electricity is "static" in different ways for insulators and conductors. Insulators are materials such

as rubber, cloth, glass, plastic, or wood which do not allow electrons to move freely through

them. Conductors, which are virtually always metals, are materials that do allow electrons to move freely.

On an isolated conductor, where the electrons of a static charge are cut off from the Earth but can still freely

move, the electrons instantly rush all over the conductor's surface, evenly coating it as they try to move as

far apart as possible. Then they become static. There is nothing mysterious about this: like charges repel,

so the electrons repel each other. They only come to rest after they have completely filled the space

available to them.

We ሺhumansሻ mostly made up of water, so we are fairly good conductors of electricity. However, we often

wear shoes that are very good insulators, and this puts an electric barrier between us and the Earth. It is

fairly easy for us to acquire a static electric charge, exactly like a conducting ball placed on an insulating

stand or suspended from a string, as we walk about and rub against chairs, carpets, and the like. And since

we are conductors, the charge on us is free to move if we touch another conductor that is "grounded" ሺi.e.,

connected to the Earthሻ, and thus all the static electricity we have on us can zip away in a single burst,

producing a spark that can be downright painful.