Page 885 - College Physics For AP Courses

Chapter 19 | Electric Potential and Electric Field
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67. A nervous physicist worries that the two metal shelves of his wood frame bookcase might obtain a high voltage if charged by static electricity, perhaps produced by friction. (a) What is the capacitance of the empty shelves if they have
area and are 0.200 m apart? (b) What is the
voltage between them if opposite charges of magnitude 2.00 nC are placed on them? (c) To show that this voltage poses a small hazard, calculate the energy stored.
68. Show that for a given dielectric material the maximum energy a parallel plate capacitor can store is directly proportional to the volume of dielectric ( ). Note that the applied voltage is limited by the dielectric strength.
69. Construct Your Own Problem
Consider a heart defibrillator similar to that discussed in
Example 19.11. Construct a problem in which you examine the charge stored in the capacitor of a defibrillator as a function of stored energy. Among the things to be considered are the applied voltage and whether it should vary with energy to be delivered, the range of energies involved, and the capacitance of the defibrillator. You may also wish to consider the much smaller energy needed for defibrillation during open-heart surgery as a variation on this problem.
70. Unreasonable Results
(a) On a particular day, it takes of electric
energy to start a truck’s engine. Calculate the capacitance of a capacitor that could store that amount of energy at 12.0 V. (b) What is unreasonable about this result? (c) Which assumptions are responsible?
Test Prep for AP® Courses
19.1 Electric Potential Energy: Potential Difference
1. An electron is placed in an electric field of 12.0 N/C to the right. What is the resulting force on the electron?
a. 1.33×10-20 N right
b. 1.33×10-20 N left
c. 1.92×10-18 N right
d. 1.92×10-18 N left
2. A positively charged object in a certain electric field is currently being pushed west by the resulting force. How will the force change if the charge grows? What if it becomes negative?
3. A −5.0 C charge is being forced south by a 60 N force. What are the magnitude and direction of the local electric field?
a. 12 N/C south
b. 12 N/C north
c. 300 N/C south
d. 300 N/C north
4. A charged object has a net force of 100 N east acting on it due to an electric field of 50 N/C pointing north. How is this possible? If not, why not?
5. How many electrons have to be moved by a car battery containing 7.20×105 J at 12 V to reduce the energy by 1%?
a. 4.80×1027 b. 4.00×1026 c. 3.75×1021 d. 3.13×1020
6. Most of the electricity in the power grid is generated by powerful turbines spinning around. Why don’t these turbines slow down from the work they do moving electrons?
7. A typical AAA battery can move 2000 C of charge at 1.5 V. How long will this run a 50 mW LED?
a. 1000 minutes
b. 120,000 seconds
c. 15 hours
d. 250 minutes
8. Find an example car (or other vehicle) battery, and compute how many of the AAA batteries in the previous problem it would take to equal the energy stored in it. Which is more compact?
9. What is the internal energy of a system consisting of two point charges, one 2.0 μC, and the other −3.0 μC, placed 1.2 m away from each other?
a. −3.8×10-2 J b. −4.5×10-2 J c. 4.5×10-2 J d. 3.8×10-2 J
10. A system of three point charges has a 1.00 μC charge at the origin, a −2.00 μC charge at x=30 cm, and a 3.00 μC charge at x=70 cm. What is the total stored potential energy of this configuration?