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# DQ2C_solution - Discussion Question 2C P212 Week 2 Forces...

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Discussion Question 2C P212, Week 2 Forces Between Point Charges 1 Q Q h d q, m Consider two charges Q placed at fixed positions a horizontal distance d apart, as shown in the figure. A third particle of mass m and unknown charge q is then placed a vertical distance h above the fixed charges. The position of this third particle is arranged so that it is suspended in mid-air above the other two! This equilibrium condition occurs because the electrical and gravitational forces on the charge precisely cancel each other. (a) Calculate the charge q that is required to balance the third particle in mid-air. Express your answer in terms of the given parameters plus physical constants. 22 3/2 20 (/ 2 ) 2 ) 2 ) 2 kQq h mg dh mg qd h KQh ×− = + + ⎡⎤ =+ ⎣⎦ (b) Does your answer make sense ? Find three limiting cases where you know what the answer should be and make sure your expression for q gives the right result. . Hint: The best algebraic check is probably 0 d . Does the sign of q make sense?

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DQ2C_solution - Discussion Question 2C P212 Week 2 Forces...

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