Problems for
Intermediate Methods in Theoretical Physics

Edward F. Redish

Work in an Electric and Gravitational Field

Suppose you have a very long line of charge with uniform charge per unit mass, λ, lying along the y -axis. (Only part of it is shown in the figure.)   Using Gauss's Law, it is fairly easy to show (Don't do it!) that the electric field vector (i.e. the electric force per unit charge) in the x-y plane due to this line charge is given by .   There is also a uniform gravitational field (i.e. the gravitational force per unit mass) given by .

If you release an object with a charge q and mass m from rest at x = 10 cm, y = 40 cm,   it will follow the curved path shown by the solid line.   The parameters of the problem are such that the object passes through the point x = 50 cm, y = 10 cm.  

(a) Starting with the fundamental definition of work, find the work done by the combined electric and gravitational forces on the object if it moves not along the curved path but along the path shown by the dotted lines.


(b) Is the result you obtained in the previous section the same or different from the work that is done by the combined electric and gravitational forces along the true (curved) path, shown by a solid line?   Explain.

(c) How fast is the object going when it reaches the point x = 50 cm , y = 10 cm?   (Assume the object moved along the solid line path.)


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This page prepared by

Edward F. Redish
Department of Physics
University of Maryland
College Park, MD 20742
Phone: (301) 405-6120

Last revision 16. October, 2005.