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# ism_ch20 - Chapter 20 Electric Potential and Electric...

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Chapter 20 Electric Potential and Electric Potential Energy Answers to Even-numbered Conceptual Questions 2. The electric potential energy of the system decreases. In fact, it is converted into the kinetic energy gained by the electron. 4. The electric potential energy of the system decreases. In fact, the kinetic energy gained by the proton is equal to the decrease in the electric potential energy. 6. The two like charges, if released, will move away from one another to infinite separation, converting the positive electric potential energy into kinetic energy. The two unlike charges, however, attract one another – if their separation is to be increased, a positive work must be done. In fact, the minimum amount of work that must be done to create an infinite separation between the charges is equal to the magnitude of the original negative electric potential energy. 8. The electric potential energy of the proton-electron system is negative, as required to produce a bound atom. (a) If the electron is replaced with a proton, the two protons will repel one another and move off to infinite separation. This means that the initial electric potential energy of this system is positive, and that this positive potential energy is converted to kinetic energy. Therefore, the electric potential energy of the system increased by changing the electron to a proton. (b) If the proton is replaced with an electron the electric potential energy of the system increases, for exactly the same reason given in part (a). 10. Point 2 is closer to than to by a factor of the square root of 2. Therefore, if the electric potential is to be zero at point 2, it is necessary that be negative, and have a magnitude that is less than the magnitude of by a factor of q 2 q 1 q 2 q 1 2 . We conclude, then, that q 2 = Q /2 . Notice that point 1 is closer to the positive charge +Q than to the negative charge Q , and that the negative charge has a smaller magnitude. It follows that the electric potential at point 1 is positive. 12. Not necessarily. The electric field is related to the rate of change of electric potential, not to its value. Therefore, if the electric field is zero in some region of space, it follows that the electric potential is constant in that region. The constant value of the electric potential may be zero, but it may also be positive or negative. 31

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Chapter 20: Electric Potential and Electric Potential Energy Physics: An Introduction 14. An equipotential surface must always cross an electric field line at right angles. Therefore, the equipotential surfaces in this system must have the shapes indicated here: 1 2 3 E field lines equipotential surfaces 16. We know that the value of the electric potential must decrease as we move in the direction of the electric field (see, for example, Figure 20-3.) It follows that the electric potential decreases in value as we move from point 1 to point 2 to point 3.
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## ism_ch20 - Chapter 20 Electric Potential and Electric...

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