Man's physical universe

xanabras

486 MAGNETISM AND ELECTRICITY

an excess of protons.

It is a fundamental concept of our modern electrical

theory of matter that opposite charges attract each other,

whereas like charges repel each other.

It is easy to explain by the modern electron theory the phenomena

just observed. According to this theory, glass, when rubbed by silk,

loses electrons and therefore becomes positively charged. On the

other hand, hard rubber, when rubbed by wool, gains electrons from

the wool and therefore becomes negatively charged. The pith ball,

when touched by the glass rod, is repelled because it gives up some

of its electrons to the glass rod; and, thus becoming

positively charged, the ball is repelled

by the glass rod, which still possesses an excess

of protons.

The concept that electrostatic fields surround

all electric charges is very useful in explaining

Fig. 229. The lines of , ^,

• • ,. rr

r 1 . c J many phenomena. 1 he imagmary Imes oi force

• I

force in an electric held

,. . . ,, ,• • •

r

between two opposite radiating in all directions from a single electric

charges. charge, either positive or negative, terminate

on other opposite charges. Figure 229 shows

the field between two opposite charges. The conception of tension

along these lines and of a compression at right angles to them,

explains electric attraction and repulsion, just as it served to explain

magnetic attraction and repulsion using magnetic lines of force.

A Static Charge May Be Induced by Bringing a Charged Body Near an

Uncharged Body.

Perhaps you have already raised the question : why was the pith ball

originally attracted to the charged rod? This is explained by assuming

that the charges on the pith ball corresponding to the charge on the

rod were driven to that surface of the pith ball farthest away from

the rod, thus leaving an excess of opposite charges on the nearest side.

This idea can well be illustrated by the electroscope. The electroscope

is an instrument used to identify and indicate the charge on a

body. One form of the electroscope consists of a pair of gold leaves

suspended in a metal box to shut out air drafts, and provided with

windows to look through.

If a charged rod is brought near the electroscope, the leaves will

repel each other because they acquire a like charge by induction.

This charge is not permanent, however, for the electrons have only

shifted their position in

the leaves and have not been transferred to

another object. When the charged rod is removed, the leaves fall

back next to each other.

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