BackAn electric car uses a 45-kW (160-hp) motor. If the battery pack is designed for 340 V, what current would the motor need to draw from the battery? Neglect any energy losses in getting energy from the battery to the motor.
A 120-V hair dryer has two settings: 1250 W and 1875 W. (a) At which setting do you guess the resistance to be higher? After making a guess, determine the resistance at (b) the lower setting, and (c) the higher setting.
You buy a 75-W lightbulb in Europe, where electricity is delivered at 240 V. If you use the bulb in the United States at 120 V (assume its resistance does not change), how bright will it be relative to 75-W 120-V bulbs? [Hint: Assume roughly that brightness is proportional to power consumed.]
At $0.12/kWh, what does it cost to leave a 25-W porch light on day and night for a year?
(II) A power station delivers 750 kW of power at 12,000 V to a factory through wires with total resistance 3.0 Ω. How much less power is wasted if the electricity is delivered at 50,000 V rather than 12,000 V?
A traditional incandescent lamp filament may have been lit to a temperature of 2700 K. A contemporary halogen incandescent lamp filament may be at around 2900 K. (a) Estimate the percent increase in efficiency of the halogen bulb over the traditional one. [Hint: See Section 19–10.] (b) To produce the same amount of light as a traditional 100-W bulb, estimate what wattage a halogen bulb should use.
A 1.0-m-long round tungsten wire is to reach a temperature of 3100 K when a current of 18.0 A flows through it. What diameter should the wire be? Assume the wire loses energy only by radiation (emissivity ∊ = 1.0, Section 19–10) and the surrounding temperature is 20°C.
What is the average current drawn by a 1.0-hp 120-V motor? (1 hp = 746 W.)
A particular household uses a 2.2-kW heater 2.0 h/day (“on” time), four 100-W lightbulbs 6.0 h/day, a 3.0-kW electric stove element for a total of 1.0 h/day, and miscellaneous power amounting to 2.0 kWh/day. If electricity costs $0.115 per kWh, what will be their monthly bill (30 d)?
An 1875-W hair dryer is designed for 117 V. (a) What will be the percentage change in power output if the voltage drops to 105 V? Assume no change in resistance. (b) How would the actual change in resistivity with temperature affect your answer?
A microwave oven running at 72% efficiency delivers 950 W to the interior. Find the power drawn from the source.
A microwave oven running at 72% efficiency delivers 950 W to the interior. Find the current drawn. Assume a source voltage of 120 V.
Determine the resistance of the tungsten filament in a 75-W 120-V incandescent lightbulb at its operating temperature of about 2800 K.
A 100-W, 120-V incandescent lightbulb has a resistance of 12 Ω when cold (20°C) and 150 Ω when on (hot). Calculate its power consumption (a) at the instant it is turned on, and (b) after a few moments when it is hot.
In an automobile, the system voltage varies from about 12 V when the car is off to about 13.8 V when the car is on and the car’s generator (or “alternator”) is in operation, a difference of 15%. By what percentage does the power delivered to the headlights vary as the voltage changes from 12 V to 13.8 V? Assume the headlight resistance remains constant.