lecture02A - Michael Faraday 1791-1867 The Electric Field r...

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1 1 Lecture 3: The Electric Field Michael Faraday 1791-1867 This lecture: HRW 22.1-22.5,22.8-22.9 For next time: HRW 22.6-22.7 Coulomb’s Law This lecture The Electric Field Electric field lines Last Lecture 2 The Electric Field Recall Coulomb’s Law: It gives the force on a charge, due to others at some distance r r j j 2 j 0 j j Qq r F = 4 πε r r r j j 2 j 0 j j q r =Q 4 πε r r Units: Force per unit charge, N/C Faraday’s Idea: define a local “field”, r r F =QE The Electric Field r r F E = Q Ä It is a vector at every point in space. Its magnitude and direction are proportional to the force that would be felt by a test charge at that point. 3 There need not be a force to have a field r r r 2 0 q F R E = = Q 4 πε R R Eg., the field due to a point charge a distance R away: Ä Q (+ve) q r R + r F r E, Q (-ve) q + r R r F r E r r r 2 0 q F R E = = Q 4 πε R R Ä q + r R r E r r 2 0 q R E = 4 πε R R Ä E has a value everywhere. If you know the field, you know what force would be exerted on any charge. q - r R r E The sign of q gives the direction of E 4 -q -2q +2q +q y x Example: superposition E field is in the direction of total force on positive charge Force on a positive charge at the center is the sum of forces from charges at corners (Superposition) Along +y Place four charges at the corners of a square. Interactive Lecture Question #3.1 What is the direction of the electric field at the center of the square?
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2 5 (note spherical symmetry in this case) Electric Field Lines
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