Extra Credit 4

Work and the electric field

Please complete the exercises below and answer all questions.  Then go to Canvas, Extra credit 4, and enter your answers to selected questions.


A: The concept of work

imageThe work W done on an object by a constant force is defined as W = Fd.  It is equal to the magnitude of the force, multiplied by the distance the object moves in the direction of the force.
The SI unit of work is Nm = J.
An object travels from point A to point B while two constant forces of equal magnitude are exerted on it, as shown in the figure on the right. 

imageAn object travels from point A to point B while two constant forces of unequal magnitude are exerted on it, as shown in the figure on the right. 

B: Work and the electric field

imageIn the diagram on the right the red dot denotes a positive point charge.  Points W, X, Y, and Z and the point charge lie in the same plane.  Points W and Y are equidistant from the charge, as are points Y and Z.
Draw the electric field vectors at points W, X, Y, and Z.
A particle with charge +qe travels along a straight line from point W to point X.

imageA particle travels from point X to point Z along the circular arc shown. 

C: Electric potential difference

A potential energy function is a function of the position of an object.  It can only be defined for conservative forces.  A force is conservative if the work it does on an object depends only on the initial and final position of the object and not on the path. 
Suppose the charge in part B increases from +qe to +1.7 qe.

The electric potential difference  ∆VWX between two points W and X is defined to be the negative of the work done by the electric field on a charge q, divided by q, as q travels from W to X.

When a net force does work on an object, its kinetic energy changes.
Wnet = ½m(vf2 - vi2) = ∆K.
A particle of charge |qe| = 2*10-6 C and mass m = 3*10-8 kg is released from rest at point W.  The speed of the particle is measured to be 40 m/s as it passes point X. 


Now go to Canvas, Assignments, Extra Credit 4.  Use the simulation to help you answer questions 1 - 5.  You can submit twice and the highest score counts.