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Fig showthe fieldliinesof a singlepositive and negative charge respectively. (a) Give the signs of potential difference (V_(p) - V_(Q)) and (V_(B) - V_(A)). (b) Give the sign of potential energy difference of a small negativecharge betweenthe points Q adn P , A and B. (c) Give the sign of the work doen by the field in movinga small positivechargefrom Q to P. (d) Give the sign of the workdone by externalagency in movinga small negativecharge from B to A. (e) Does the Kineticenergy of a smallnegativechargeincreaseor decrease in goingfrom b and A ? |
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Answer» Solution :As `V prop (1)/( r):. V_(p) gt v_(Q) or (V_(P) - V_(Q))` is positive Also `V_(B)` is less negativethan `V_(A)` (`:'`distancefo B is more than distance of A) Thus, `V_(B) gt V_(A) :. (V_(B) - V_(A))` is positive. (b) A negativechargeheld at Q would move towards P. As charge moves from higherpotentialenergyto LOWER potential energy `:.(P.E)_(Q) gt (P.E)_(P)` Therefore, sign of potential energydifferenceof a smallnegativecharge betweenQ and P is positive. SIMILARLY , `(P.E)_(A) gt (P.E)_(B)` Sign of potential energy difference is positive. (c) In movinga small positive charge from Q to P, WORKHAS to be done by US againstthe electric filed. Therefore, work has to be doneby the fieldis negative. (d) In movinga smallnegative charge from B to A, work done by externalagencyis positive. (e) Due to force of repulsion on the negative charge, velocity DECREASES and henceK.E. decreases in goingfrom B to A. |
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