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Transcript
Physics 10 Chapter 10 HW Solution E: 3, 6, 12, 16, 18, 24, 30, 42, 44, 50 P: 2, 10, 12, 13, 17 Exercises 3. Excess electrons rubbed from your hair leave it with a positive charge; excess electrons on the comb give it a negative charge. 6. The penny will be slightly more massive with a negative charge, for it will have more electrons than when neutral. If it were positively charged, it would be slightly lighter because of missing electrons. 12. At twice the distance the field strength will be 1/4, in accord with the inverse-square law. 16. 10 joules per coulomb is 10 volts. When released, its 10 joules of potential energy will become 10 joules of kinetic energy as it passes its starting point. 18. As the current in the filament of a lightbulb increases, the bulb glows brighter. 24. An electric device does not “use up” electricity, but rather energy. And strictly speaking, it doesn’t “use up” energy, but transforms it from one form to another. It is common to say that energy is used up when it is transformed to less concentrated forms—when it is degraded. Electrical energy ultimately becomes heat energy. In this sense it is used up. 30. Damage generally occurs by excess heating when too much current is driven through an appliance. For an appliance that converts electrical energy directly to thermal energy this happens when excess voltage is applied. So don’t connect a 110-volt iron, toaster, or electric stove to a 220-volt circuit. Interestingly enough, if the appliance is an electric motor, then applying too little voltage can result in overheating and burn up the motor windings. (This is because the motor will spin at a low speed and the reverse “generator effect” will be small and allow too great a current to flow in the motor.) So don’t hook up a 220-volt power saw or any 220-volt motor-driven appliance to 110 volts. To be safe, use the recommended voltages with appliances of any kind. 42. How quickly a lamp glows after an electrical switch is closed does not depend on the drift velocity of the conduction electrons, but depends on the speed at which the electric field propagates through the circuit—about the speed of light. 44. Most of the electric energy in a lamp filament is transformed to heat. For low currents in the bulb, the heat that is produced may be enough to feel but not enough to make the filament glow red or white hot. Problems 2. By the inverse-square law, twice as far is 1⁄4 the force; 5 N. The solution involves relative distance only, so the magnitude of charges is irrelevant. 10. power = current × voltage, current =Power/voltage =1200W/120V = 10 A. resistance =voltage/current =120V/10A = 12 Ω. 13. First, 100 watts = 0.1 kilowatt. Second, there are 168 hours in one week (7 days × 24 h/day = 168 hours). So 168 hours × 0.1 kilowatt = 16.8 kilowatt-hours, which at 20 cents per kWh comes to $3.36. 17. The resistance of the toaster is R = V/I = (120 V)/(10 A) = 12 Ω. So when 108 V is applied, the current is I = V/R = (108 V)/(12 Ω) = 9.0 A and the power is P = IV = (9.0 A)(108 V) = 972 W, only 81% of the normal power. (Can you see the reason for 81%? Current and voltage are both decreased by 10%, and 0.9× 0.9 = 0.81.)