27. Resistors & DC Circuits

The current in an electromagnet connected to a 240-V line is 21.5 A. At what rate must cooling water pass over the coils for the water temperature to rise no more than 5.50 C°?

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.

Many electric companies use time-of-day pricing in which electricity costs more during hours of high demand. Suppose electricity costs \$0.21/kWh from 10 a.m. to 6 p.m.; \$0.09/kWh at all other times. What is the annual cost of electricity for a 2.5 kW industrial pump that runs 24 hours a day?

Consider the circuit of Fig. E25.30 Show that the power output of the 16.0 V battery equals the overall rate of consumption of electrical energy in the rest of the circuit.

A 2800-W oven is connected to a 240-V source. How long will it take to bring 150 mL of 15°C water to 100°C assuming 65% efficiency?

A voltage V is applied to n identical resistors connected in parallel. If the resistors are instead all connected in series with the applied voltage, show that the power transformed is decreased by a factor n².

Lightbulb A is rated at 120 V and 40 W for household applications. Lightbulb B is rated at 12 V and 40 W for automotive applications. What is the resistance of each bulb?

Consider the circuit of Fig. E25.30. What is the total rate at which electrical energy is dissipated in the 5.0 Ω and 9.0 Ω resistors?

A microwave oven running at 72% efficiency delivers 950 W to the interior. Find the current drawn. Assume a source voltage of 120 V.

The five identical bulbs in FIGURE EX28.11 are all glowing. The battery is ideal. What is the order of brightness of the bulbs, from brightest to dimmest? Some may be equal.

A 1500 W electric heater is plugged into the outlet of a 120 V circuit that has a 20 A circuit breaker. You plug an electric hair dryer into the same outlet. The hair dryer has power settings of 600 W, 900 W, 1200 W, and 1500 W. You start with the hair dryer on the 600 W setting and increase the power setting until the circuit breaker trips. What power setting caused the breaker to trip?

Lightbulb A is rated at 120 V and 40 W for household applications. Lightbulb B is rated at 12 V and 40 W for automotive applications. What is the current through each bulb?

A microwave oven running at 72% efficiency delivers 950 W to the interior. Find the power drawn from the source.

An electric heater is used to heat a room of volume 65m³. Air is brought into the room at 5°C and is completely replaced twice per hour. Heat loss through the walls amounts to approximately 750 kcal/h. If the air is to be maintained at 22°C, what minimum wattage must the heater have? (The specific heat of air is about 0.17 kcal/kg C°.)