# week2 - All administrative/Blackboard/iClicker CONNECT...

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All administrative/Blackboard/iClicker/ CONNECT PLUS/ attendance/scores questions should be directed to David Koltick, [email protected] 1 Thursday, September 2, 2010

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We have a negative charge q=-e. How electric field is directed at point X? IClicker question 2 q=-e (negative charge) X A: B: C: D: E: E=0 Thursday, September 2, 2010
3 q=-e (negative charge) X Place positive charge q0 Force E=F /q0 F E A: Thursday, September 2, 2010

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4 +q E Thursday, September 2, 2010
5 -q E Thursday, September 2, 2010

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5 -q E Thursday, September 2, 2010
A charge creates an electric field around itself and the other charge feels that field. August 12, 2010 University Physics, Chapter 22 3 + The Electric Field (2) Electric field at a given point in space: place a positive test charge q at the point and measure the electrostatic force that acts on the test charge; then Thursday, September 2, 2010

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A charge creates an electric field around itself and the other charge feels that field. August 12, 2010 University Physics, Chapter 22 3 + The Electric Field (2) Electric field at a given point in space: place a positive test charge q at the point and measure the electrostatic force that acts on the test charge; then + Test charge q Thursday, September 2, 2010
We define the electric field in terms of the force it exerts on a positive point charge: Unit of the electric field: N/C (Newtons per Coulomb) Note that the electric force is parallel to the electric field and is proportional to the charge The force on a negative charge will be in the opposite direction August 12, 2010 University Physics, Chapter 22 5 Precise Definition of Electric Field Thursday, September 2, 2010

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EM fields are real physical substance Electromagnetic fields are real, these are not just mathematical concepts . EM fields is another material substance: one can assign energy-density to electromagnetic field. Empty space with EM field has more energy than just empty space. There is no “substance” that “transports” EM forces (like mechanical forces are transported by the medium, sound is transported by medium), EM wave propagate “on their own”, no medium to carry. (Actually, gravitational force is not “real”). 8 Thursday, September 2, 2010
Question: What is the field created by a point charge q? Answer: Consider a “test charge” q 0 at point x . Force on q 0 : Electric field at x August 12, 2010 University Physics, Chapter 22 6 Example: Field of a Point Charge q>0, E along r q<0, E counter r Thursday, September 2, 2010

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Vector addition of force Electric force in vector form 10 q 1 q 2 q 3 r 23 r 13 F 13 F 23 F tot =F 13+ F 23 Thursday, September 2, 2010
The electric force at a given point in space is tangent to the electric field line through that point. August 12, 2010 University Physics, Chapter 22 8 Electric Field Lines F2 F1 Thursday, September 2, 2010

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