1. Gerald Gangaway

A current of 28.5 A flows through an electric heater operating on 220 V. What is the heater's resistance?

2.  A 650 Ω resistor is connected to a variable-voltage power supply.

(a) What voltage is necessary to cause a current of 0.2 A in the resistor?

(b) What current flows in the resistor when the voltage is 16 V?

3. A current of 14.3 A flows through an electric heater operating on 120 V. Compute the power consumption of the electric heater.

4. An electric train operates on 655 V. What is its power consumption when the current flowing through the train's motor is 2,170 A?

5. All of the electrical outlets in a room are connected in a single parallel circuit. The circuit is equipped with a 20 A fuse, and the voltage is V = 120 V.

(a) What is the maximum total power that can be supplied by the outlets without blowing the fuse?

(b) How many 1,900-W appliances can be plugged into the sockets without blowing the fuse? (Enter only a number) appliance(s)

6. Your cell phone typically consumes about 390 mW of power when you text a friend. If the phone is operated using a lithium-ion battery with a voltage of 3.5 V, what is the current flowing through the cell-phone circuitry under these circumstances?

Compare your result with the data given in the table below.

The current in the cell phone is less than a color television but greater than a 60-watt lightbulb. The current in the cell phone is less than a 60-watt lightbulb but greater than a clock radio.
The current in the cell phone is less than an 1800-watt hair dryer but greater than a color television. The current in the cell phone is less than a clock radio but greater than a spark plug.
The current in the cell phone is less than a spark plug but greater than a calculator.

7.  An electric clothes dryer is rated at 3,900 W. How much energy does it use in 30 min?

8. A certain electric motor draws a current of 29 A when connected to 120 V.

(a) What is the motor's power consumption?

(b) How much energy does it use during 8 h of operation? Express the answer in joules and in kilowatt-hours.

9. 

An electric car is being designed to have an average power output of 9,000 W for 2 h before needing to be recharged. (Assume there is no wasted energy.)

(a) How much energy would be stored in the charged batteries?

(b) The batteries operate on 40 V. What would the current be when they are operating at 9,000 W?

(c) To be able to recharge the batteries in 1 h, how much power would have to be supplied to them?

10. A solid metal cylinder has a certain resistance. It is then heated and carefully stretched to form a longer, thinner cylinder. After it cools, will its resistance be the same as, greater than, or less than what it was before?

The resistance will be the same since the resistance is independent of the length and cross-sectional area. Increasing the length and shrinking the radius will not affect the resistance.
The resistance will be greater since the resistance is proportional to length and inversely proportional to cross-sectional area. Increasing the length and shrinking the radius both act to increase the resistance.

The resistance will be less than before since the resistance is inversely proportional to length and proportional to cross-sectional area. Increasing the length and shrinking the radius both act to decrease the resistance.

11.  If a new material is found that is a superconductor at all temperatures, what parts of some common electric devices would definitely not be made out of it? (Select all that apply.)

• electric motors
• heating elements in toasters and electric stoves
• transmission lines
• resistors in electrical devices
• filaments of light bulbs

Explain.

12.  A simple electric circuit consists of a constant-voltage power supply and a variable resistor. What effect does reducing the resistance have on the current in the circuit and on the power output of the power supply?

13. What is the purpose of having fuses or circuit breakers in electric circuits? How should they be connected in circuits so they will be effective?

14. A 20-A fuse in a household electric circuit burns out. What catastrophe could occur if it is replaced by a 30-A fuse?

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