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# Lec17 - TODAY Chapter 22 Electrostatics Electrical Force...

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TODAY Chapter 22: Electrostatics

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Electrical Force: Coulomb’s Law Charged particles exert forces on one another : Like charges repel each other Unlike charges attract Acts along a line connecting the charges Determined by Coulomb’s Law (18 th century): F = k q 1 q 2 d 2 c.f . Newton’s gravitational law -- Inverse-square dependence on separation -- proportional to size of each charge - c.f . grav. law (prop to each mass) -- BUT k >> G; the electrical force is much stronger than gravitational force --- Also, elec. force can be either attractive or repulsive, grav. force always attractive k = 9 x 10 9 N m 2 /C 2 C = Coulomb, unit of charge q (more next slide)
Charge Fundamental quantity in all electrical phenomena: positive and negative particles carry “charge” Recall, protons electrons Attractive force btn protons and electrons cause them to form atoms, as we saw in Ch.11. Electrical force is behind all of how atoms bond i.e. behind chemistry… Every electron has charge -1.6 x 10 -19 C, and every proton 1.6 x 10 -19 C i.e. -1 C represents the charge of 6.25 billion billion electrons ! Yet 1C is the amount of charge passing through a 100-W light bulb in just over a second. A lot of electrons! Charge is always conserved: charge cannot be created or destroyed, but can be transferred from one object to another. Eg. Rubbing a rod with fur – electrons transfer from fur to rod, leaving rod negatively charged, and the fur with exactly same magnitude of positive charge.

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More on charge Note that in everyday charging processes (like rubbing objects), it is the electrons that transfer (not the protons). A negatively charged object has an excess of e’s, whereas positively charged one has deficiency (by same amount) Which object gains the electrons depends on their electron affinity: Eg. Rod has greater affinity than fur, so rod becomes –, fur + Eg. Silk has greater affinity than rod when rubbed together, rod becomes +, silk - Eg. Combing hair Comb becomes –, hair + (e’s go from hair to comb) Charge is quantized: cannot divide up charge into smaller units than that of electron (or proton) i.e. all objects have a charge that is a whole- number multiple of charge of a single e.
Question Compare the gravitational force between an electron and proton in an H atom with the electrical force between them. Use: Average radius of H atom = 0.5 x 10 -10 m Mass of proton = 1.67 x 10 -27 kg Mass of electron = proton mass/2000 = 8.35 x 10 -31 kg

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Clicker Question So the electrical attraction is by far dominant in providing the centripetal force that keeps the electron in orbit around the proton. How about the force the electron exerts on the proton? A) it’s larger than the force on the electron B) It’s the same C) It’s smaller D) It depends Answer: B, Newton’s 3 rd law The electrical force is an interaction (as any force is) and each pulls on the other equally.
Conductors and Insulators How easy is it to get an electric current to flow across a material?

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Lec17 - TODAY Chapter 22 Electrostatics Electrical Force...

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