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Chapter 14: Q8Q (page 578)
Explain briefly why repulsion is a better test for the sign of a charged object than attraction is.
Repulsion is a better test for the sign of a charged object than attraction is.
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Here is a variant of “charging by induction.” Place two uncharged metal objects so as to touch each other, one behind the other. Call them front object and back object. While you hold a charged comb in front of the front object, your partner moves away the back object (handling it through an insulator so as not to discharge it). Now you move the comb away. Explain this process. Use only labeled diagrams in your explanation (no prose!).
A solid plastic ball has negative charge uniformly spread over its surface. Which of the diagrams in Figure 14.85 best shows the polarization of molecules inside the ball?
You take two invisible tapes of some unknown brand, stick them together, and discharge the pair before pulling them apart and hanging them from the edge of your desk. When you bring an uncharged plastic pen withinof either the U tape or the L tape you see a slight attraction. Next you rub the pen through your hair, which is known to charge the pen negatively. Now you find that if you bring the charged pen withinrole="math" localid="1655718752350" of the L tape you see a slight repulsion, and if you bring the pen withinrole="math" localid="1655718766744" of the U tape you see a slight attraction. Briefly explain all of your observations.
8 (a) An object can be both charged and polarized. On a negatively charged metal ball, the charge is spread uniformly all over the surface (Figure 14.42). If a positive charge is brought near, the charged ball will polarize. If any of the following quantities is zero, state this explicitly. (1) Draw the approximate final charge distribution on the ball. (2) At the center, draw the electric field due to the external positive charge. (3) At the center, draw the electric field due to the charge on the surface of the ball. (4) At the center, draw the net electric field.
(b) Next, consider a negatively charged plastic pen that is brought near a neutral solid metal cylinder (Figure 14.43). If any of the following quantities is zero, state this explicitly. (1) Show the approximate charge distribution for the metal cylinder. (2) Draw a vector representing the net force exerted by the pen on the metal cylinder, and explain your force vector briefly but completely, including all relevant interactions. (3) At the center, draw the electric field due to the external negative charge. (4) At the center, draw the electric field due to the charge on the surface of the ball. (5) At the center, draw the net electric field.
(c) Replace the solid metal cylinder with a solid plastic cylinder. (1) Show the approximate charge distribution for the plastic cylinder. (2) Draw a vector representing the net force exerted by the pen on the plastic cylinder. (3) Explain your force vector briefly but completely, including all relevant interactions.
A glass sphere carrying a uniformly distributed charge of is surrounded by an initially neutral spherical plastic shell (Figure 15.67).
(a) Qualitatively, indicate the polarization of the plastic. (b) Qualitatively, indicate the polarization of the inner glass sphere. Explain briefly. (c) Is the electric field at location P outside the plastic shell larger, smaller, or the same as it would be if the plastic weren’t there? Explain briefly. (d) Now suppose that the glass sphere carrying a uniform charge of is surrounded by an initially neutral metal shell (Figure 15.68). Qualitatively, indicate the polarization of the metal.
e) Now be quantitative about the polarization of the metal sphere and prove your assertions. (f) Is the electric field at location outside the metal shell larger, smaller, or the same as it would be if the metal shell weren’t there? Explain briefly.
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