Date Period 22 Use with Chapter 22. Current Electricity Understanding Concepts Part A Use each of the following terms once to complete the statements below. ampere electric current potential difference resistance electric circuit kinetic energy power 1. A charge pump creates a flow of charged particles, or. 2. A closed loop through which charges can flow is a(n). 3. When a water wheel drives a generator, the generator converts the of the water to electric energy. 4. The rate at which energy is transferred is. 5. The unit used to measure the rate of flow of electric current is a(n). 6. The of a conductor can be determined if potential difference and current are known. 7. A device that has constant resistance and appears to be independent of the is said to obey Ohm s law. Write the letter of the choice that best completes each statement. 8. The current flowing in an electric circuit can be increased by. a. increasing voltage or decreasing resistance b. decreasing voltage or increasing resistance c. increasing voltage and increasing resistance d. decreasing voltage and decreasing resistance 9. A device that measures the amount of current in a circuit is a(n). a. potentiometer b. resistor c. voltmeter d. ammeter 10. Space heaters convert most of the electric energy in a circuit into energy. a. light b. thermal c. mechanical d. sound 11. Electricity is carried long distances at high voltages because. a. this reduces the current and less power is lost as thermal energy b. this reduces the resistance and less power is lost as thermal energy c. current cannot be changed to reduce thermal energy d. capacitance cannot be changed to reduce thermal energy. 12. Utility companies measure energy used in. a. joules b. kilowatt-hours c. watt-seconds d. watts Physics: Principles and Problems 103
22 Understanding Concepts Part B Answer each of the following questions, showing your calculations. 1. A portable compact-disk player receives its energy from a 9.0-V cell. The current used to operate the player is 135 A. a. How many joules of energy does the cell deliver to the CD player each second? b. How much power in watts does the CD player use? c. How much energy does the CD player use to play a selection 3.0 min long? 2. What voltage is applied to a 6.80-Ω resistor if the current is 3.20 A? 3. An electric buzzer is connected across a 4.2-V difference in potential. The current through the buzzer is 1.8 A. a. What is the power rating of the buzzer? b. How much electric energy does the buzzer convert in 1.5 min? 4. An electric blanket with a resistance of 8.6 Ω is connected to a 120-V source. a. What is the current in the circuit? b. How much heat is produced if the blanket is turned on for 15 min? 104 Physics: Principles and Problems
22 Applying Concepts Answer the following sentences, using complete sentences. 1. What is the difference between an ampere and a volt? 2. Identify the parts of this schematic. Will current flow through the circuit? Give a reason for your answer. 3. Could a voltmeter be substituted for the ammeter shown in the schematic in problem 2? Give a reason for your answer. 4. What would happen to current if voltage and resistance were doubled? 5. Which has a greater effect on the amount of thermal energy produced in a heater, the current or the resistance of the heater? Give a reason for your answer. Physics: Principles and Problems 105
22 Answer the following questions, showing your calculations. 6. An electric motor operates an elevator the mass of which is 2.0 10 3 kg. The elevator rises 120 m in 32 s. The motor has a resistance while operating of 34.0 Ω and is connected across a 2.4 10 3 -V source. What percentage of electric energy is converted to kinetic energy? 7. A three-pack of 1.5-V hearing-aid batteries costs $4.26. Each battery puts out 8.0 ma of current. Each battery lasts for 15 days. What is the cost per kilowatt hour to operate the hearing aid with one battery? 8. While waiting for the school bus, you keep your hands warm in a pair of electric gloves. The heating element in each glove has a resistance of 8.0 W. Each glove operates from a 12-V source. The thermal energy produced by each glove is 640 J. You wait for the bus for 3.0 min. When the bus arrives, are the electric gloves still producing heat? 9. A model electric train makes one complete pass around a circular track every 15 s. The train s motor has a resistance of 6.0 Ω and is connected to a 70.0-V source. How much energy will the train use in 12 complete passes around the track? 106 Physics: Principles and Problems
Date Period 23 Use with Chapter 23. Series and Parallel Circuits Understanding Concepts Part A Write the letter of the choice that best completes the statement or answers the question. 1. If four electric devices are connected in a series circuit, then the number of current paths is equal to. a. one b. two c. three d. four 2. A series circuit contains a generator, an ammeter, and a lamp. The current in the lamp is. a. equal to the current in the ammeter, but greater than the current in the generator b. less than the current in the ammeter c. equal to the current in the generator and equal to the current in the ammeter d. less than the current in the generator 3. A series circuit contains four resistors. What is the equivalent resistance of the circuit? a. 4R c. R/4 b. R 1 R 2 R 3 R 4 d. (R 1 R 2 R 3 R 4 )/4 4. A series circuit has a 120-V generator but requires a 60-V potential source. To achieve the desired potential, a can be used. a. photoresistor c. voltage divider b. sensor d. semiconductor 5. If three resistors are connected in parallel, there are current paths in the circuit. a. one b. two c. three d. four 6. In an electric circuit, are switches that act as safety devices. a. fuses and circuit breakers c. ammeters b. fuses and voltage dividers d. combined circuits For each of the statements below, write true or false. 7. To measure the current through a resistor, an ammeter should be connected in series with the resistor. 8. The equivalent resistance of a parallel circuit is always less than the resistance of any resistor in the circuit. 9. A voltmeter should have a very low resistance so that it causes the largest possible changes in currents and voltages in the circuit. 10. The resistance of an ammeter should be as low as possible. 11. To measure the current across a resistor, connect a voltmeter in parallel with the resistor. Physics: Principles and Problems 107
23 Understanding Concepts Part B Answer the following questions, showing your calculations. 1. Two resistors of 3.0 and 8.0 are connected in series across a 9.0-V battery. a. What is the equivalent resistance of the circuit? b. What is the current through the 3.0- resistor? c. What is the current through the 8.0- resistor? d. What is the voltage drop across each resistor? 2. A 15.0- bell and an 8.0- lamp are connected in parallel and placed across a difference in potential of 42 V. a. What is the equivalent resistance of the circuit? b. What is the current in the circuit? c. What is the current through each resistor? d. What is the voltage drop across each resistor? 108 Physics: Principles and Problems
23 Applying Concepts Answer the following questions, using complete sentences. 1. A string of holiday lights has 15 bulbs connected in series. If one of the bulbs burns out, what happens to the other bulbs? Give a reason for your answer. 2. What happens to resistance when a resistor is added in parallel to a circuit that already has two resistors? 3. How is it possible to use more than one electric appliance at a time in a house? 4. A circuit has five identical resistors, A, B, C, D, and E. Resistors A, D, and E have the same potential difference across them. What kind of circuit is this? Give a reason for your answer. 5. What would happen to the current in a circuit if a voltmeter were substituted for an ammeter? 6. Explain why a ground-fault interrupter is often required by law in electric outlets in bathrooms and kitchens, but not in other rooms in a house. 7. Why does turning on additional appliances on the same circuit breaker increase the current through the wires? Physics: Principles and Problems 109
23 Answer the following questions, showing your calculations. 8. Find the reading of each ammeter and each voltmeter in the diagram below. The equivalent resistance of the resistors in parallel is For the entire circuit, the equivalent resistance is The voltage drop across each 8.0- resistor is 9. What is the power in watts used by each resistance in the diagram above? 110 Physics: Principles and Problems