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# Lect_31 - ACT Two balls Two balls of equal mass are...

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1 Lecture 31 Electric Fields ACT: Two balls Two balls of equal mass are suspended from the ceiling with non- conducting wire. One ball is given a charge q and the other is given a charge 3 q. Which of the following best represents the equilibrium position? 3 q q C A B 3 q 3 q q q Same force on both (3rd law) and same mass! What is a “field”? Very basic definition: Assignment of a scalar or a vector to each point in space. Example of scalar field: Temperatures The shown temperatures are samples of the field 7 7 82 83 68 55 66 83 75 80 90 91 75 71 80 72 84 73 82 88 92 77 88 88 73 64

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2 Example of vector field: Winds The shown temperatures are samples of the field 77 82 83 68 55 66 83 75 80 90 91 75 71 80 72 84 73 57 88 92 77 56 88 73 64 Electric field Definition: If the electric force on a test charge q located at point P is F , then the electric field at point P is: F E q = G G Because the force is always proportional to q , the electric field is independent of the test charge ! Units: N/C Electric field by a point charge The electric force by Q on q is 2 ˆ Qq F k r r = G q F G + Q q F G + Q + E G Independent of q (test charge) 2 ˆ F Q E E k r q r = = G G G The electric field by Q (source charge) is: E G + Electric field by a negative/positive point charge
3 + If you place a test charge q near the source charge, you get a force. q + If you place a test charge q in a different place, you might get a different force. q + If the test charge is 2 q, the force is twice as large: 2 q + Positive charges feel a force in the same direction as the electric field .

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