Electric Current Practice Page 34-1 1. Water doesn't flow in the pipe when (a) both ends are at the same level. Another way of saying this is that water will not flow in the pipe when both ends have the same potential energy (PE). Similarly, charge will not flow in a conductor if both ends of the conductor are at the same electric potential. But tip the water pipe and increase the PE of one side so there is a difference in PE across the ends of the pipe, as in (b), and water will flow. Similarly, increase the electric potential of one end of an electric conductor so there is a potential difference across the ends, and charge will flow. a. The units of electric potential difference are (T>oltsn (amperes) (ohms) (watts) b. It is common to call electric potential difference --- ==a^ (voltage)\) (wattage) - ----- c. The flow of electric charge is called electric -- ~ (voltage) ({current}) (power), and is measured in (volts) ^amperest) (ohms) (watts) IS A UNIT OF AND AN AMPERE IS A UHIT OF DOES VOLTAGE CAUSE CURRENT, OR DOES CURREMT CAUSE VOLTAGE? WHICH IS THE CAUSE AND WHICH IS THE EFFECT? 2. Complete the statements: a. A current of 1 ampere is a flow of charge at the rate of coulomb per second. b. When a charge of 15 C flows through any area in a circuit each second, the current is A c. One volt is the potential difference between two points if 1 joule of energy is needed to move f - coulomb of charge between the two points. d. When a lamp is plugged into a 120-V socket, each coulomb of charge that flows in the circuit is raised to a potential energy of / 2- O joules. e. Which offers more resistance to water flow, a wide pipe or a narrow pipe? & ^ ro c '_ - Similarly, which offers more resistance to the flow of charge, a thick wire or a thin wire? Conceptual CAddison-Wesley Publishing Company, Inc. All rights reserved. Chapter 34 Electric Current 128
Ohm's Law 1. How much current flows in a 1000-ohm resistor when 1.5 volts are impressed across it? 2. 3. If the filament resistance in an automobile headlamp is 3 ohms, how many amps does it draw when connected to a 12- volt battery? The resistance of the side lights on an automobile are 10 ohms. How much current flows in them when connected to 12 volts? 4. What is the current in the 30-ohm heating coil of a coffee maker that operates on a 120-volt circuit? VOLTAGE OR USE OHM'S LAW IN THE TRIANGLE TO FIND THE / QUANTITY / I YOU WANT, COVER THE LETTER WITH YOUR FINGER AND THE REMAIN1N6 TWO SHOW YOU THE FORMULA? CONDUCTORS AND RESISTORS HAVE TO THE CURRENT IN THEM. 5. During a lie detector test, a voltage of 6 V is impressed across two fingers. When a certain question is asked, the resistance between the fingers drops from 400 000 ohms to 200 000 ohms. What is the current (a) initially through the fingers, and (b) when the resistance between them drops? (a) / ^O /»/? (b) 3 QO *>i,/ 6. How much resistance allows an impressed voltage of 6 V to produce a current of 0.006 A? 1. What is the resistance of a clothes iron that draws a current of 12 A at 120V? / O 8. What is the voltage across a 100-ohm circuit element that draws a current of 1 A? OHM MY GOODNESS 9. What voltage will produce 3 A through a 15-ohm resistor? 10. The current in an incandescent lamp is 0.5 A when connected to a 120-V circuit, and 0.2 A when connected to a 10-V source. Does the resistance of the lamp change in these cases? Explain your answer and defend it with numerical values. Oh*, c ->pt+rc^» 4v r g. 126 Chapter 34 Electric Current CAddlson-Wesley Publishing Company. Inc. All rights reserved.
Electric Power Practice Page 34-2 Recall that the rate energy is converted from one form to another is power. energy converted voltage x charge charge power = e «L_ a voltage x ^ = voltage x current The unit of power is the watt (or kilowatt). So in units form, Electric power (watts) - current (amperes) x voltage (volts), where 1 watt= 1 ampere x 1 volt. THAT'S RIGHT- VOLTAGE - SO ENERGY* VOLTAGE "CHARGE- 1. What is the power when a voltage of 120 V drives a 2-A current through a device? 2. What is the current when a 60-W lamp is connected to 120V? Q. G fr^ps 3. How much current does a 100-W lamp draw when connected to 120V? Q, 81> ft^s A 100-WATT BULB CONVERTS ELECTRIC ENERGY INTO HEAT AND LIGHT MORE QUICKLY THAN A 25-WATT 6ULB. THAT'S WHY FOR THE SAME VOLTAGE A IQO-WATT BULB GLOWS BRIGHTER THAN A 25-WATT BULB' WHICH DRAWS MORE CURRENT-THE 100-WATT OR THE 25-WATT BULB? 4. If part of an electric circuit dissipates energy at 6 W when it draws a current of 3 A, what voltage is impressed across it? 5. The equation energy converted p wer= rearranged gives energy converted 6. Explain the difference between a kilowatt and a kilowatt-hour. 7. One deterrent to burglary is to leave your front porch light on all the time. If your fixture contains a 60-W bulb at 120 V, and your local power utility sells energy at 8 cents per kilowatt-hour, how much will it cost to leave the bulb on for the whole month? Show your work on the other side of this page. Conceptual PHYSICS 3 «Addison-Wesley Publishing Company. Inc. All rights reserved. Chapter 34 Electric Current 127
Series Circuits 1. In the circuit shown at the right, a voltage of 6 V pushes charge through a single resistor of 2 n. According to Ohm's law, the current in the resistor (and therefore in the whole circuit) is Practice Page 35-1 2/1 THE EQUIVALENT OF RESISTORS IN SERIES is SJMPtV THEIR 2. VWv, > 3JI> If a second identical lamp is added, as on the left, the 6-V battery must push charge through a total resistance of ( > n. The current in the circuit is then / A. 3. The equivalent resistance of three 4-Q resistors in series is. 4. Does current flow through a resistor, or across a resistor? Is voltage established through a resistor, or across a resistor?. 5. Does current in the lamps occur simultaneously, or does charge flow first through one lamp, then the other, and finally the last in turn? / ~ ' 6. Circuits a and b below are identical with all bulbs rated at equal wattage (therefore equal resistance). The only difference between the circuits is that Bulb 5 has a short circuit, as shown. "4.5V a. In which circuit is the current greater? u b. In which circuit are all three bulbs equally bright?_ c. What bulbs are the brightest? / r & d. What bulb is the dimmest? ^ e. What bulbs have the largest voltage drops across them?. f. Which circuit dissipates more power? C? g. What circuit produces more light? & conceptual 4.5 M Addison-Wesley Publishing Company. Inc. All tights reserved. Chapter 35 Electric Circuits 129
Parallel Circuits 1. In the circuit shown below, there is a voltage drop of 6 V across each 2-11 resistor. THE SUM OF THf ClMtEMTS (H THE TWO 6RAMCH PATHS K3UW.S THE CUMEHT KfOK a WVIWS. a. By Ohm's law, the current in each resistor is A. zn b. The current through the battery is the sum of the currents in the resistors,. A. '6V c. Fill in the current in the eight blank spaces in the view of the same circuit shown again at the right. 2. Cross out the circuit below that is not equivalent to the circuit above. 3. Consider the parallel circuit at the right. a. The voltage drop across each resistor is <& y. b. The current in each branch is: 2-tl resistor _J _A 2-ii resistor -^ A 1-n resistor Je_A THE EQUIVALENT OF A PAIR OF RESISTORS IN PARAUEL IS THEIR PRODUCT DIVIDED 6Y THEIR b. The current through the battery equals the sum of the currents which equals / *- A. c. The equivalent resistance of the circuit equals /2- n. 130 Chapter 35 Electric Circuits OAddison-Wesley Publishing Company, Inc. All rights reserved.
Compound Circuits Practice Page 35-2 OJ «. 1. The initial circuit, below left, is a compound circuit made of a combination of resistors. It is reduced to a single equivalent resistance by the three steps, the circuits to its right, a, b, c. In step a, show the equivalent resistance of the parallel 4-fl resistors. In step b combine this in series with the 3-fl resistor. In step c, combine the last parallel pair to obtain the equivalent resistance of the circuit. (Note the similarity of this circuit and Figure 35.10 in your textbook.) R = Wv- L_AMr- 4.H. - => -VW Sfl A/W Initial Circuit a. b. 5A 2. The circuit below is similar to Figure 35.11 in your textbook. In three successive steps, as in Question 1, replace each pair of resistors by a single resistor of equivalent resistance. R = (A M/vV-l 3-*. 3. Find the equivalent resistance of these three circuits. CAddison-Wesley Publishing Company, Inc. All rights reserved. Chapter 35 Electric Circuits 131
4. The table beside circuit a below shows the current through each resistor, the voltage across each resistor, and the power dissipated as heat in each resistor. Find the similar correct values for circuits b, c, and d, and put your answers in the tables shown. 2SI 4/1 6-rt. CURRENT' «VOLTAGE = POWER 4V 8V 12V 8W 16 W 2<4W I2v 2/1 CURRENT" VOLTAGE = POWER 2/1 '6V fe/l 6J1 3/L CURRENT" VOLTAGE = POWER 6 CURRENT" VOLTAGE = POWER I/5/? *,.*>»' Conceptual 132 Chapter 35 Electric Circuits OAddlson-Wesley Publishing Company, Inc. All rights reserved.