Download Physics 10 chapter 22 HW Solution

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.)