1) You are given three (3) identical positive charges and two thumbtacks. Position two (2) charges in space and hold them stationary with the thumbtacks. Where can you place the third charge so that it will remain stationary without a thumbtack? Will the location of the third charge change if it is a larger charge? Will the location of the third charge change if it is a negative charge?

2) An electroscope (this is the device we have been using in class) is positively charged so that the needle is deflected from the vertical. You might want to make a sketch of the charges on the electroscope under these circumstances. A plastic rod is brought near the sensor and the needle is observed to deflect even further (move further from the vertical). Is the plastic rod positively or negatively charged? Explain and/or illustrate your reasoning.

3) When you rub a plastic pen through your hair it will generate a static charge on the pen. With this charged pen you can now pick up little bits of (neutral, uncharged) paper (try it, it works!). Explain and illustrate why this happens. You can assume either sign of the unbalanced charge created on the pen although it tends to be negative due to the nature of plastics.

4) You are given two (2) identical metal spheres on insulating stands. One sphere is already negatively charged while the other is neutral. Using only yourself (mind and body) and the spheres how can you give the neutral sphere a positive charge that is roughly equal in magnitude to the negative charge? This is the process of charging by induction that is described in your book and that we may have had a chance to discuss in class.

5) A metal sphere has a net charge of +4 µC. 6.0^.10¹³electrons are added to the sphere. What is the net charge on the sphere after the electrons are added? Remember the charge on a single electron is 1.6^.10^-19 C.

6) Two identical point charges are held in place at opposite corners of a square. A third charge is placed at one of the empty corners of the square and the net force on it is measured. The charge is then moved to the center of the square and the net force is again measured. Where does the third charge experience a greater net force? Consider the vector nature of the forces and explain your answer. No calculations are needed!

7) In 1731 an experiment was described where Stephen Gray suspended a 9 year old boy from the ceiling by his feet. Just below the boy's head he positioned a stand on which were placed small light flakes of brass while he held a charged glass tube near the boy's feet. "Upon the tube's being rubbed, and held near his feet without touching them, the leaf-brass was attracted to the boy's face with much vigor, so as to rise to the height of eight, and sometimes ten inches". Describe the movement and distribution of charges that produce this effect.

Now I want you to perform some calculations to get a clearer sense of just how many atoms and how much charge is hiding in plain sight. This will also get you to revisit some ideas about Avogadro's number and the amount of charge on a proton and an electron.

8) Assume that a human being is just a 70 kg bag of water with a few impurities thrown in. This is a bit of an oversimplification but not so much as you might think. Determine (roughly) how many molecules of water are in this average human being.

9) Now determine how much negative charge (in the form of electrons) is contained within the human body. If a modest lightning bolt is 1 Coulomb of charge how many lightning bolts of negative charge are hiding in your body at this time?

10) If one molecule in a trillion misplaced a single electron what would the net charge on a human being be and would it be positive or negative? What would be the force between two such students sitting half a meter apart in the classroom?

11) Assume that equal amounts of unbalanced charge (one negative and one positive) are placed on the earth and the sun. How big would this charge need to be so that the Coulomb force between the sun and the earth would be equal in magnitude to the gravitational force that keep the earth in its orbit?

 

Answer Cloud

[400 µC] /[2.5^.10²⁷]/[midway, unaffected] /[touch carefully]/[5.7 kN] /[4.0^.10⁹ C]/[corner, 0 in center] /[human is conductor] /[polarization of charge] /[-5.6 µC]/ [3^.10¹⁷ C] /[+]