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# l10 - 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
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?
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”
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.
ISP209s10 Lecture 10 -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 B) It is 4 times the initial force C) It is 1/2 the initial force D) It is 2 times the initial force E) Nothing changes

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ISP209s10 Lecture 10 -10- Clicker Question If the distance between 2 charges suddenly doubles, how does the electric force change?
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l10 - Today Announcements The average on the first exam was...

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