Workshop module 1 - Physics 113, Fall 1999

Workshop module 1 - Physics 114, Spring 2000

My sources inside the White House tell me that Monica and Bill are still misleading Congress and the public about what really went on behind closed doors. My source says the current scandal is a cover-up. They were too embarrassed to tell what really happened. According to what I can gather, Monica and Bill share a fascination with electrostatics. Their time alone was spent running little experiments. Apparently, they are confused about the results of one experiment. I hope you can explain it.

Monica rubbed a plastic cigar case with the skirt of her blue silk dress. This left a positive net charge on the plastic cigar case. (Note: single, fine cigars often come in metal or plastic containers, roughly shaped like the cigar.)
Monica held the plastic case near a metal, conducting cigar case (held in the air by a nonconducting string) but did not touch the case.
While the charged, plastic cigar case was in this position, Bill touched the metal cigar case and removed his hand.
Monica removed the plastic case from the proximity of the metal one.
Then she held the edge of her blue skirt near the metal cigar case (but not touching).

What effect did the blue skirt have on the hanging metal cigar case? Please explain your reasoning. (tests concept of induction, given on last year’s exam I, the story is less timely this year, take away story and it is very similar to induction homework problems, straightforward, most students did okay on it)

What is the ratio of the electromagnetic force to the force of gravitational attraction between the electron and the proton in a hydrogen atom? (Coulomb’s law problem plus simple gravitation attraction, yet makes you think about the similarities and differences between the two forces … ideal exam problem, medium difficulty, simple to execute but requires some synthesis)

Consider two positive charges and two negative charges placed at the four corners of a square in a plane, such that charges of the same sign occupy diagonally opposite corners of the square. Draw the electric field lines in the plane containing the charges. Put a positive test charge in the plane in 5 different places (including the middle of the square) and discuss the force on that test charge (direction and magnitude). (Helps develop E field intuition, straightforward problem for exam)

Two charges of magnitude +Q are placed at the bottom of an equilateral triangle with sides of length d. A third charge of magnitude +q is placed on the third corner of the triangle. What is the force on the third charge? Calculate the force two ways … 1) calculate the force directly and 2) calculate the E field first, then the force from your answer. How would you expect things to change if the two charges at the bottom of the triangle have charges +Q and –Q, respectively? (straightforward for exam, tests basic knowledge of electric field and ability to sum vector components)

Why does clothing tend to stick together after going through the dryer? Would you expect there to be more or less clinging if all your clothing was made of the same material (say, cotton) than if you dry different kinds of clothing together? (conceptual, fine for an exam)

Young and Freedman 22-80: Positive charge Q is uniformly distributed around a semicircle of radius a. Find the electric field (magnitude and direction) at the center of curvature (Yes, we begin the promised integrals … This would be a very difficult exam problem for P114 and a medium difficulty one for P122).