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Two charges q = + 1 μC and Q = +10 μC are placed near each other as shown in the figure. Which of the following diagrams best describes the forces acting on the charges: +1 μC +10 μC a) b) c) This is an example of Newton’s third law: FA _ on _ B FB _ on _ A The Electric Field The net Coulomb force on a given charge is always proportional to the strength of that charge. F = F1 + F2 q1 F1 F q positive test charge F2 q2 The direction of the field is the direction of the force on a positive test charge. We can now define a quantity, the electric field, which is independent of the test charge, q, and depends only on position in space: r r F E Electric Field Applet q The Electric Field Electric field is the force per unit positive charge at a point in space. The SI units of electric field are: N/C F E q kQq 2 E r q kQ E 2 r Ways to Visualize the E Field Consider the E-field of a positive point charge at the origin vector map field lines + chg + chg + + Rules for Vector Maps + chg + •Direction of arrow indicates direction of field •Length of arrows local magnitude of E Rules for Field Lines + - •Lines leave (+) charges and return to (-) charges •Number of lines leaving/entering charge amount of charge •Tangent of line = direction of E •Local density of field lines local magnitude of E • Field at two white dots differs by a factor of 4 since r differs by a factor of 2 •Local density of field lines also differs by a factor of 4 (in 3D) Field Lines From Two Like Charges • There is a zero halfway between the two charges • r >> a: looks like the field of point charge (+2q) at origin Field Lines from 2 Opposite Charges l Grass seed suspended in oil lines up along the electric field.