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Lab01-PreLab

# Lab01-PreLab - PHYS 2212 Lab Exercise 01 Force Field...

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PHYS 2212 Lab Exercise 01: Force, Field, & Superposition PRELIMINARY MATERIAL TO BE READ BEFORE LAB PERIOD I. Force & Coulomb’s Law: Coulomb’s Law is an empirical rule describing the force between charged particles. (By “empirical”, we mean that the law is a principle inferred from experimental observation and measurement.) The two most important preliminary issues we must realize are: (1) Coulomb’s Law only applies between pairs of charged particles ; and (2) Coulomb’s Law is only accurate if the charges involved are pointlike . These issues will have a number of important consequences when we try to make practical use of Coulomb’s Law. A third key point that we must keep in mind is: (3) the forces implied by Coulomb’s Law are always along the direct line joining the two charges. If they are like charges, the force is repulsive, while if they are unlike charges, the force is attractive. What impact do these constraints have in practice? First of all, when more than two point charges are present, we must compute the net force on any particular charge by summing all of the individual force vectors due to the pairwise interactions with each of the other charges. (This is a vector summation, not a simple scalar addition of magnitudes.) In order to accomplish this summation we take advantage of the fact that the force between any pair of charges is independent of the possible presence of any other charges nearby. This means, for example, that when we try to compute the net force on particle B in the diagram at right, we do so via three separate and utterly independent applications of Coulomb’s Law: once to get the force vector F AB (ignoring C and D ), once to get the force vector F CB (ignoring A and D ), and once to get the force vector F DB (ignoring A and C ) A B C D Force due to A A B F AB Force due to C B C F CB Force due to D F DB B D Net Force on B A B C D F net This process—determining a net effect due to multiple causes by first determining the separate effects and then adding them together—is the essence of the Superposition Principle . This principle allows us to turn complex problems into smaller, “bite-sized” chunks, which can be solved independently and their answers added back together to give a final result (in what would otherwise be an very difficult calculation). We must also keep in mind that we are only allowed to apply Coulomb’s Law when we are dealing with “pointlike” charges. What does this mean? Essentially, a charge is pointlike if it “looks” like a point particle—the size of the charge should be something so small as to be negligible. More practically, we say that charges can be treated as pointlike (and hence, Coulomb’s Law can be applied) in situations where the sizes of the charged particles are very small compared to the distance that separates them.

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