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PHYS2153: Engineering Physics III - WS21: Coulomb’s Law: SP21.02 Name: Date: Objectives: e Apply Coulomb’s law to find the electric force in one dimension. e Understand and use the dynamics with Coulomb’s force. e Exercise how the signs of charges should be included in the calculation. Sample Problem 21.02 Equilibrium of two forces on a particle, page 618 The y figure here shows two particles fixed in place: a particle of charge q; = +8q at the origin and a particle of charge q2 = —2q at « = L. With the questions below, find the location a de for a proton to be in equilibrium. x 1) Discuss why the position of the equilibrium (the net force on it is zero) for the proton i must be on the z axis. 2) Consider three regions on the x axis, the left side of qi, the right side of g2, and between two charges. Decide which region(s) can have the equilibrium. 3) Apply Coulomb’s law with superposition principle to set up the formula for the net force on the proton. 4) By setting Fret = 0 from 3), find the position of the equilibrium. 5) Is that equilibrium stable or unstable? (That is, if the proton is displaced, do the forces drive it back to the point of equilibrium or drive it farther away?) [Extra] If the particle is an electron instead, where would be the equilibrium? Explain your answer. (4 points) 
