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0 Physics 213 General Physics Lecture 2 Last Meeting: Coulomb’s Law, Electric Forces Today: Electric Field, Gauss's Law 2 Q 3 q | E ( r ) | k 2 r Faraday introduced electric field and electric field lines which are said to exist in the region of space around a charged object. When another charged object enters this electric field, the field exerts a force on the second charged object 4 5 Coulombs force and electric fields. q1q2 F12 k 2 r̂12 r q1q2 F12 k 2 r̂12 r q1 E1 k 2 r̂12 r q2 qtest Superposition Principle q4 q3 q1 Direction of Electric Field The electric field produced by a negative charge is directed toward the charge A positive test charge would be attracted to the negative source charge 7 Direction of Electric Field, cont The electric field produced by a positive charge is directed away from the charge A positive test charge would be repelled from the positive source charge 8 Question: An electron traveling horizontally enters a region where a uniform electric field is directed upward.What is the direction of the force exerted on the electron once it has entered the field? (a) to the left (b) to the right (c) upward (d) Downward (e) out of the page, toward the reader Answer: d 9 Rules for Drawing Electric Field Lines 1. The lines for a group of charges must begin on positive charges and end on negative charges In the case of an excess of charge, some lines will begin or end infinitely far away 2. The number of lines drawn leaving a positive charge or ending on a negative charge is proportional to the magnitude of the charge 3. No two field lines can cross each other Electric Field Line Patterns Point charge The lines radiate equally in all directions For a positive source charge, the lines will radiate outward Electric Field Line Patterns For a negative source charge, the lines will point inward Electric Field Line Patterns An electric dipole consists of two equal and opposite charges The high density of lines between the charges indicates the strong electric field in this region Electric Field Line Patterns Two equal but like point charges At a great distance from the charges, the field would be approximately that of a single charge of 2q The bulging out of the field lines between the charges indicates the repulsion between the charges The low field lines between the charges indicates a weak field in this region Electric Field Patterns Unequal and unlike charges Note that two lines leave the +2q charge for each line that terminates on -q