Phy 214 Test 1 10 pts each Spring 2000

1. Describe how you would give a positive charge to a metal ball using a negatively charged rod. You may use words, diagrams, or both.

2. a. Two small charged objects each have net charge +Q. Draw an accurate diagram showing the electric force they exert on each other.

b. Next, suppose we replace one of these charges by a particle whose charge is +3Q. Draw an accurate diagram showing the electric forces, to the same scale you used in part a.

3. Find the net force on the -1 C object in the diagram below.

4. If the distance z between the charge Q and the dipole were doubled, by what factor would the force on Q due to the dipole change ?

5. Explain briefly which is a better to use to determine whether an object is charged or not, attraction or repulsion, and why.

 Answer: Repulsion because any charged object will attract any uncharged object.

6. When a positively charged rod is brought close to an electroscope that has been charged negatively, the leaves fall together. But if it is brought even nearer, the leaves spread apart again. Explain what's happening, use appropriate diagrams.

 7. You have three metal blocks marked A, B, and C, sitting on insulating stands. Block A is charged "+" but blocks B and C are neutral.

Without using any additional equipment, and without altering the amount of charge on block A,
explain how you could make block B be charged "+" and block C be charged "-". Explain your
procedure in detail, including diagrams of the charge distributions at each step in the process.

8. Use the figure below. At a particular moment, one negative and two positive charges are located as shown: Find the x and y components of the net electric field at the point indicated.
3 pts Bonus: at what point(s) on the x axis would the electric field be zero?
4 pts Bonus: Find the equation in y that must be solved to find at what point(s) on the y axis the electric field would be zero?

9. A student has said that the electric field inside a charged sphere is always zero.
Place the single word at the proper location in this sentence to make it true.

Answer: either "charged METAL sphere" or "UNIFORMLY charged sphere"

 
10. Consider a thin plastic rod bent into an arc of radius R and angle f. The rod carries a uniformly distributed positive charge Q. Follow the steps below to obtain the components Ex and Ey of the electric field at the origin,due to the rod?
a) Use a diagram to explain how you plan to cut up the charged rod, and draw the DE contributed by a representative piece.
b) Express algebraically the contribution each piece makes to the x and y components of the electric field . Be sure to show your incremental component dE of the electric field on your drawing. (Recall that the arc length Ds of a circular arc of radius R , subtended by an angle of Du, is R Du.)
c) Write the summation as an integral for each component of E, and simplify the integral as much as possible. State explicitly what the range of your integration variable is.
d) Evaluate the integrals to find Ex and Ey.