ELECTRICITY This barber employs the novel technique of cutting a customer's hair after it has been charged with static current making it stand on end and thus presumably easier to cut General Photographic Agency Hulton Archive Getty Images 12
THE FUNDAMENTALS OF ELECTRICITY seems to pull the strands upward toward the comb To understand the nature of this force it is necessary to know something about the concept of electric charge and the structure of atoms Simple experiments can illustrate how electric charge works If a glass rod is rubbed with silk and touched to a small sphere of aluminum foil suspended by a thread the sphere moves away from the glass rod The rod and sphere repel each other If the pro cess is repeated with a second sphere and the spheres are brought near each other they too repel If a plastic rod is rubbed with wool and brought near either sphere the spheres move toward the plastic rod These objects attract each other If two new alumi num foil spheres are touched to the plastic rod they are repelled by the plastic as well as by each other But they are attracted to the glass rod and the spheres touched by the glass rod These experiments can be explained by a two charge model Rubbing the glass with silk causes the glass to acquire a positive charge When touched to the spheres it shares some of its positive charge with the spheres and these objects repel Rubbing the plastic with 13
ELECTRICITY Aluminum foil spheres blue are touched by a glass rod blue that has been rubbed with silk These have positive charges and repel each other Other aluminum foil spheres orange are touched by a plastic rod orange that has been rubbed with wool These have negative charges and repel each other Spheres with like charges also repel each other Oppositely charged spheres and rods however attract each other Copyright Encyclopaedia Britannica Inc rendering for this edition by Rosen Educational Services wool causes the plastic to acquire a nega tive charge When touched to the spheres it shares some of its negative charge with the spheres and these objects repel 14
THE FUNDAMENTALS OF ELECTRICITY This interpretation of the experiment leads to the conclusion that like charges repel But when a positively charged sphere is brought near a negatively charged plastic rod or sphere the objects attract each other And when a negatively charged sphere is brought near a positively charged glass rod the objects attract each other This leads to the conclusion that unlike charges attract No matter how an object is charged if it attracts the negatively charged sphere it will also repel the positively charged sphere and vice versa This leads to the belief that there are only two kinds of electric charge ELECTRIC CHARGES IN ATOMS A model of matter is needed to explain how only two kinds of charged rods can be pro duced Atoms contain two kinds of charge which are arbitrarily called positive and negative Every atom is composed of a posi tively charged nucleus around which are distributed negatively charged electrons Each nucleus contains a specific number of protons particles that carry the positive charge With the exception of the hydro gen atom nuclei also contain uncharged 15
