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Chapter 4

Chapter 4

Chapter

“[Because] the cathode rays are deflected by an electrostatic force as if they were negatively electrified, and are acted on by a magnetic force in just the way in which this force would act on a [moving] negatively electrified body, . . . I can see no escape from the conclusion that they are charges of negative electricity carried by particles of matter.” ~ J. J. Thompson (1897) Electrical Forces ► The famous physicist Benjamin Franklin decided to call the two types of charge negative and positive. • rubber/fur leaves negative charge on rod • glass/silk leaves positive charge on rod ► objects with a like charge repel each other (two rubber or two glass rods) ► objects with opposite charge attract each other (one rubber and one glass rod) Chapter 4 - The Electromagnetic Interaction 1 Chapter 4 - The Electromagnetic Interaction 2 The Electrical Model of Matter ► At first electricity was thought to be a fluid, like water. ► Experiments done by J. J. Thompson showed this “fluid” to be a stream of charged particles. • He broke atoms into positive and negative parts. Electrical Model of Matter ►normal normal (neutral) matter consists of equal numbers of positively charged particles (protons) and negatively charged particles (electrons) ►electrons can be moved from one object to another by rubbing, producing a net charge ►protons protons remain in the solid object Chapter 4 - The Electromagnetic Interaction 3 Chapter 4 - The Electromagnetic Interaction 4 Relative Sizes of Electrical and Gravitational Forces ►Put Put the three fundamental forces that we deal with in this class in order of strength. • Two protons: electrical repulsion is 10 36 times stronger than the gravitational attraction. • Protons in a nucleus are held together by the strong force in spite of the electric repulsion. The Electric Force Law ► Gravity only pulls. And it works on objects with mass. ► The electromagnetic force both pulls and pushes and it works on objects with charge. ► But the force law is otherwise the same: F = kQq/d 2 Q and q represent charge. d which can be positive or negative. As with gravity, k is just a number. Q q Chapter 4 - The Electromagnetic Interaction 5 Chapter 4 - The Electromagnetic Interaction 6 1

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