Review 3A - Physics 122 Spring 2009 Document#19 Cycle 3A...

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Physics 122 Spring 2009 – Document #19: Cycle 3A Review Sheet page 1 of 13 PHYS 122: Cycle 3A Review Sheet March 16, 2009 Cycle 1 and Cycle 2 Materials Physics is a cumulative subject and the this is especially so for the Cyclic approach. Therefore is it explicitly the case that each student are responsible for ALL materials delineated in the Cycle 1 Review Sheet and the Cycle 2 Review Sheet. As part of your preparations for the Third Hour Exam be sure you are completely comfortable with all materials presented in the these first two Review Sheets since any question on any of these topics is “fair game” for the next exam. Electric Dipoles: If we have two point charges of equal but opposite magnitude, then we define the electric dipole moment as the vector: vector p Q vector d where vector d is the position vector from the negative to the positive charge. Note that the (confusingly) the dipole vector runs in the opposite direction relative to the electric field. A dipole will exert a field that falls as 1 r 3 at large distances in any direction. A dipole placed in a uniform electric field will experience zero net force. However it will experience a net torque according to: vector τ vector p = vector p × vector E There is an associate potential energy associated with this: U dipole = vector p · vector E Note that the potential energy can be zero, positive, or negative. Dielectric materials: Many insulators are also dielectric materials. This means that although charges do not move, the material has an impact on the electric field. Specifically, a dielectric reduces the magnitude of the field: E net = χE applied where E net is the net electric field in the capacitor, E applied is the field that was applied by the capacitor in the absence of the dielectric material, and χ is the factor which gives a reduced field: 0 < χ < 1 . We can also calculate the impact on the capacitance: C net = 1 χ C original
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Physics 122 Spring 2009 – Document #19: Cycle 3A Review Sheet page 2 of 13 By convention, if we define a constant κ that is the inverse of χ , then, when we are working within a dielectric substance, in all of the electrostatic equations that include ǫ 0 , this term is re- placed by κǫ 0 where κ is called the dielectric constant. The dielectric always acts so as to reduce the strength of the electric field. This always has the impact of increasing the capacitance when a dielectric is used between the plates: C = κC 0 where C 0 was the capacitance in the absence of the dielectric. Be sure you understand how dielectric properties can be explained by considering the molecules as dipoles which tend to line up in the applied electric field. The resultant alignment of dipoles results in a second (induced) electric field that partially cancels the applied one.
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