Name Period Date

Electric Current

Concept-DevelopmentPractice 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 waterwill not flow in the pipe when both endshave the same potential energy (PE).Similarly, charge will not flow in a conductorif both ends of the conductor are at the sameelectric potential. But tip the water pipe andincrease the PE of one side so there is a differencein PE across the ends of the pipe, as in (b), and waterwill flow. Similarly, increase the electric potential of oneend of an electric conductor so there is a potential difference across the ends, and charge willflow.

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)

2. Complete the statements:

a. A current of 1 ampere is a flow of charge at the rate of

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 ?

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

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Ohm's Law

1. How much current flows in a 1000-ohm resistorwhen 1.5 volts are impressed across it?

2.

3.

If the filament resistance in an automobileheadlamp is 3 ohms, how many amps does it drawwhen connected to a 12- volt battery?

The resistance of the side lights on an automobileare 10 ohms. How much current flows in themwhen connected to 12 volts?

4. What is the current in the 30-ohm heating coil of acoffee maker that operates on a 120-volt circuit?

VOLTAGERESISTANCE

OR

USE OHM'S LAWIN THE TRIANGLETO FIND THE /

QUANTITY / IYOU WANT,

COVER THELETTER WITH YOURFINGER AND THEREMAIN1N6 TWOSHOW YOU THE

FORMULA ?

CONDUCTORS AND RESISTORSHAVE RESISTANCE TO THECURRENT IN THEM.

5. During a lie detector test, a voltage of 6 V is impressed across two fingers. When a certain questionis asked, the resistance between the fingers drops from 400 000 ohms to 200 000 ohms. What is thecurrent (a) initially through the fingers, and (b) when the resistance between them drops?

/ ^O / » / ? 3 QO *>i,/(a) (b)

6. How much resistance allows an impressed voltage of 6 Vto produce a current of 0.006 A?

1. What is the resistance of a clothes iron that draws a currentof 12 A at 120V?

/ O

8. What is the voltage across a 100-ohm circuit element that drawsa 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 whenconnected to a 10-V source. Does the resistance of the lamp change in these cases? Explain youranswer and defend it with numerical values.

Oh*, c

->pt+rc^» 4v r g.

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Name Period Date

Concept-DevelopmentPractice Page 34-2

Electric Power

Recall that the rate energy is converted from one form to another is power.

energy converted voltage x charge chargepower = 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 -

1. What is the power when a voltage of 120 V drivesa 2-A current through a device?

2. What is the current when a 60-W lamp is connectedto 120V?

Q. G fr^ps

3. How much current does a 100-W lamp draw whenconnected to 120V?

Q, 81> ft^s

4. If part of an electric circuit dissipates energyat 6 W when it draws a current of 3 A, what voltage is impressed across it?

SO ENERGY* VOLTAGE "CHARGE-

A 100-WATT BULB CONVERTSELECTRIC ENERGY INTO HEATAND LIGHT MORE QUICKLY THANA 25-WATT 6ULB. THAT'S WHYFOR THE SAME VOLTAGE A IQO-WATT

BULB GLOWS BRIGHTER THAN A25-WATT BULB'

WHICH DRAWS MORECURRENT-THE 100-WATT

OR THE 25-WATT BULB?

5. The equationenergy 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 containsa 60-W bulb at 120 V, and your local power utility sells energy at 8 cents per kilowatt-hour, howmuch will it cost to leave the bulb on for the whole month? Show your work on the other side ofthis page.

Conceptual PHYSICS

3

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Name Period Date

Series Circuits

Concept-DevelopmentPractice Page 35-1

2/1

1. In the circuit shown at the right, a voltage of 6 Vpushes 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

THE EQUIVALENTRESISTANCE OFRESISTORS IN

SERIES isSJMPtV THEIR

2.VWv—,

>3JI>

If a second identical lamp is added, as on the left, the 6-V batterymust 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, thenthe other, and finally the last in turn? /

~ '

6. Circuits a and b below are identical with all bulbs rated at equal wattage (therefore equal resis-tance). The only difference between the circuits is that Bulb 5 has a short circuit, as shown.

"4.5Va. 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 of6 V across each 2-11 resistor.

a. By Ohm's law, the current in each

resistor is _ _A.

b. The current through the battery isthe sum of the currents in the

znresistors,. A.

'6V

c. Fill in the current in the eight blankspaces in the view of the samecircuit shown again at the right.

2. Cross out the circuit below that is not equivalent tothe circuit above.

THE SUM OF THf ClMtEMTS (H THETWO 6RAMCH PATHS K3UW.S THECUMEHT KfOK a WVIWS.

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

b. The current through the batteryequals the sum of the currents which

equals / *- A.

c. The equivalent resistance of the circuitequals /2- n.

THE EQUIVALENTRESISTANCE OF APAIR OF RESISTORSIN PARAUEL ISTHEIR PRODUCT

DIVIDED 6YTHEIR

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Name Period Date

Compound Circuits

Concept-DevelopmentPractice Page 35-2OJ «.

1. The initial circuit, below left, is a compound circuit made of a combination of resistors. It isreduced to a single equivalent resistance by the three steps, the circuits to its right, a, b, c. In stepa, show the equivalent resistance of the parallel 4-fl resistors. In step b combine this in series withthe 3-fl resistor. In step c, combine the last parallel pair to obtain the equivalent resistance of thecircuit. (Note the similarity of this circuit and Figure 35.10 in your textbook.)

R =—Wv-

L_AMr-

4.H.

SflA/W

Initial Circuit

5A

- => —-VW—

a. b.

2. The circuit below is similar to Figure 35.11 in your textbook. In three successive steps, as inQuestion 1, replace each pair of resistors by a single resistor of equivalent resistance.

3-*.

(AM/vV-l

3. Find the equivalent resistance of these three circuits.

R =

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4. The table beside circuit a below shows the current through each resistor, the voltage across eachresistor, and the power dissipated as heat in each resistor. Find the similar correct values forcircuits b, c, and d, and put your answers in the tables shown.

2SI

I2v

2/1

' 6 V

fe/L

Conceptual

RESISTANCE

2A4/16-rt.

CURRENT'

2A

2A2A

« VOLTAGE = POWER

4 V

8V12V

8W

16 W2<4W

RESISTANCE CURRENT" VOLTAGE = POWER

2/1

RESISTANCE

6J13/L

CURRENT" VOLTAGE = POWER

6

RESISTANCE CURRENT" VOLTAGE = POWER

I/5/? *,.*>»'

132 Chapter 35 Electric Circuits OAddlson-Wesley Publishing Company, Inc. All rights reserved.