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Transcript
When one object is given a net electric charge by placing it in contact with a charged object it is called charging by contact. Ch 18 If a charged object is held close to another object, and the second object is temporarily grounded before the first object is removed; the second object is left charged opposite the initial charge. This is charging by induction. Charging by contact makes both objects the same charge. Charging by induction will make the two objects opposite charges. F = k•q1•q2/ 2 r k is a proportionality constant whose value is 9 2 2 k = 8.99 x 10 N•m /C A small point charge, called a test charge, may be used to determine the extent to which the surrounding charges generate a force. The electric field E that exists at a point is the electrostatic force F experienced by a small test charge q0 placed at that point divided by the charge itself: E = F/q0 The electric field is a vector, and its direction is the direction of of the force on the charge. The unit is the newton per coulomb (N/C). From Coulomb’s law, the force exerted on a test charge q0 by a charge q is 2 F = k•q•q0/ r . Since E = F/q0, E is also equal to k•q•q0/ r2 divided by q0. q0 cancels out, and we are left with 2 E = k•q/ r . A parallel plate capacitor has two plates of different charge with a space between them. The charges are distributed uniformly over each plate. The electric field points from the positive plate to the negative and is perpendicular to both. In a parallel plate capacitor, the field has the same value at all places between the plates. The field does not depend on the distance from the charges, as it does in a field created by an isolated point charge. Electric field lines are always directed away from positive charges and toward negative charges. Where lines are closest together, the electric field is strongest. At equilibrium under electrostatic conditions, any excess charge resides on the surface of a conductor. Free electrons within the conductor are not moving, so no electric field exists there. At equilibrium under electrostatic conditions, the electric field at any point within a conducting material is zero. Gauss’ law The electric flux through a Gaussian surface is equal to the net charge q enclosed by the surface divided by the ε0 , the permittivity of free space: FE = ∑(E cosf)∆A = q/ε0. The SI unit of electric flux: N•m2/C Ch 19 Ch 18