Lecture 05 - Electric fields

Lecture 05 - Electric fields - ACT: Two balls Lecture 5 Two...

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1 Lecture 5 Electric Fields Demo: Van der Graaf and pie plates 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 C AB 3 3 Same force on both (3rd law) and same mass! EXAMPLE: Two electrons Compare the gravitational attraction and the electric repulsion of two electrons. 22 ee g em Fk FG rr == 19 31 1.6 10 C 9.1 10 kg e m =− × () 2 2 2 2 91 9 2 42 2 2 11 31 2 Nm 9.0 10 1.6 10 C C 4.2 10 Nm 6.7 10 9.1 10 kg kg Fke FGm −− ⎛⎞ ×× ⎜⎟ ⎝⎠ × HUMONGOUS!! Electric forces are very strong Electric forces are VERY strong. They are very much responsible for holding the universe together!!! Electron-proton in an atom Ionic crystals Covalent bonds Metals At distances < 10 -17 m (within nuclei), the strong force takes over, but for anything >10 -17 , electric force is king!
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2 ACT: Net force Which of the three vectors best represents the direction of the net electric force on charge Q? q 1 y = 2 a = 4 μ C Q 2 x = 2 = - 1 = -4 2 = +1 μ C Q = +3 μ C = 2 cm C A B 1 = 2 Step 1: Draw the force vectors. 2 = 2 = - F 1 2 net Step 2: Compute the magnitude of the force by each charge () 1 1 2 30 N 3 e Qq Fk == 1 2 = 2 1 2 2 2 d = = 2 = - 2 2 2 2 2 2 2 13.5 N 5 k aa = +
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Lecture 05 - Electric fields - ACT: Two balls Lecture 5 Two...

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