10 - Today Announcements: The average on the first exam was...

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ISP209s10 Lecture 10 -1- Today • Announcements: – The average on the first exam was 31/40 – Exam extra credit is due by 2:00 pm Thursday February 18th. (It opens on LONCAPA today) • The Electric Force • Work, Energy and Power
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ISP209s10 Lecture 10 -2- Exam 1 0 10 20 30 40 50 60 17 18 20 21 23 24 26 27 29 30 31 33 34 36 37 39 40 Score Number
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ISP209s10 Lecture 10 -3- Comments on exam extra credit Each question you get up to 4 tries before CAPA marks it incorrect. The online test is not exactly the same as the in-class one, but it is rather close. This is why I didn’t collect your copies of the exam. There are only 21 questions, but some of these are weighted more so that the total # of points is 40. Say you got a 30/40 on the in-class exam, and a 40/40 on the extra Credit exam. Then you get (40 - 30)*.30 = 3 extra points added to Your original score.
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ISP209s10 Lecture 10 -4- What do these pictures have in common? All are instances of Static Electricity. What is electricity?
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ISP209s10 Lecture 10 -5- A new Force! • Electrical charge is a property of matter. It is measured in Coulombs C. • It comes in 2 types (“positive” and “negative” ) • Like charges repel, unlike charges attract. • Coulomb’s Law of Electric Force 2 2 2 2 1 9 99 . 8 12 C m N E k r Q kQ F ! = = - + r 12
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ISP209s10 Lecture 10 -6- Questions to ponder • Coulomb’s law looks like Newton’s Law of gravity. Why? • Why does charge come in two types and mass only came in one type? • Why do we always get r 2 ? I hate squares. • Why is k = 8.99E9 Nm 2 /C 2 so much bigger than G = Nm 2 /kg 2 ? We’ll come back to these mundane ?’s later and see that they are actually pretty profound and “cutting edge”
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ISP209s10 Lecture 10 -7- Example 1 • Two 1 kg balls with a charge of +1 C are a distance 1 m from each other. What is the electrical force? 2 2 2 2 1 9 99 . 8 12 C m N E k r Q kQ F ! = = The math is identical to Gravitational problems!
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ISP209s10 Lecture 10 -8- Example 2 • Two 1 kg balls with a charge of +1 C are a distance 1 m from each other. What is the gravitational force? 2 2 2 2 1 11 673 . 6 ; kg Nm E G r m Gm F ! = = The Gravitational Force is 20 orders of magnitude weaker Than the electrical force! Gravity is a VERY weak force.
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-9- Clicker Question If the distance between 2 charges suddenly doubles, how does the electric force change? A) It is 1/4 the initial force
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This note was uploaded on 12/05/2011 for the course ISP 209 taught by Professor Sherrill during the Fall '07 term at Michigan State University.

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10 - Today Announcements: The average on the first exam was...

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