KEY CONCEPT Electric current is a flow of charge. · KEY CONCEPT Electric current is a flow of charge. EXPLORE Current How does resistance affect the flow of charge? PROCEDURE Tape

VOCABULARYelectric current p. 360ampere p. 361Ohm’s law p. 361electric cell p. 363

BEFORE, you learned

• Charges move from higher tolower potential

• Materials can act as conductorsor insulators

• Materials have different levelsof resistance

NOW, you will learn

• About electric current• How current is related to

voltage and resistance• About different types of

electric power cells

KEY CONCEPT

Electric current is a flowof charge.

EXPLORE Current

How does resistance affect the flow of charge?

PROCEDURE

Tape the pencil lead flat on the posterboard.

Connect the wires, cell, bulb, and bulb holder as shown in the photograph.

Hold the wire ends against the pencil lead about a centimeter apart from each other.Observe the bulb.

Keeping the wire ends in contact with the lead, slowly move them apart. As you move the wire ends apart, observe the bulb.

WHAT DO YOU THINK?• What happened to the bulb as you moved the

wire ends apart?• How might you explain your observation?

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Electric charge can flow continuously.Static charges cannot make your television play. For that you need adifferent type of electricity. You have learned that a static charge contains a specific, limited amount of charge. You have also learnedthat a static charge can move and always moves from higher to lowerpotential. However, suppose that, instead of one charge, an electricalpathway received a continuous supply of charge and the difference inpotential between the two ends of the pathway stayed the same. Then,you would have a continuous flow of charge. Another name for a flowof charge is Electric current is the form of electricityused to supply energy in homes, schools, and other buildings.

electric current.

360 Unit 3: Electricity and Magnetism

VOCABULARYDon’t forget to make a foursquare diagram for theterm electric current.

MATERIALS• pencil lead• posterboard• electrical tape• 3 lengths of

wire• D cell battery• flashlight bulb• bulb holder

Sunshine StateSTANDARDSSC.B.1.3.1: The studentidentifies forms ofenergy and explainsthat they can be mea-sured and compared.

Current, Voltage, and ResistanceElectric current obeys the same rules as moving static charges. Chargecan flow only if it has a path to follow, that is, a material to conduct it. Also, charge can flow only from a point of higher potential to oneof lower potential. However, one concept that does not apply to amoving static charge applies to current. Charge that flows steadily hasa certain rate of flow. This rate can be measured. The standard unit ofmeasure for current is the or amp. An amp is the amount ofcharge that flows past a given point per unit of time. One amp equalsone coulomb per second. The number of amps—or amperage—of a flowing charge is determined by both voltage and resistance.

Electric current, or amperage, can be compared to the flow ofwater through a pipe. Electric potential, or voltage, is like pressurepushing the water through the pipe. Resistance, or ohms, is like thediameter of the pipe, which controls how much water can flowthrough. Water pressure and pipe size together determine the rate ofwater flow. Similarly, voltage and resistance together determine therate of flow of electric charge.

Ohm’s LawYou now have three important measurements for the study of electricity:volts, ohms, and amps. The scientist for whom the ohm is named dis-covered a mathematical relationship among these three measurements.The relationship, called is expressed in the formula below.

Current � �R

Ves

oilsttaagnece

� I �

I is current measured in amps (A), V is voltage measured in volts (V),and R is resistance measured in ohms (�).

check your reading What two values do you need to know to calculate the amperage of electric current?

V�R

Ohm’s law,

ampere,

Chapter 10: Electricity 361

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See Ohm’s law in action.

COMBINATION NOTESIn your notes, try making a sketch to help you remember how current, voltage,and resistance differ.

How Potential Affects Current

low pressure and low rate of flow

high pressure andhigh rate of flow

How Resistance Affects Current

low resistance andhigh rate of flow

high resistance andlow rate of flow

Current decreases as resistance increases, just as water flow decreases as resistance toflow increases.

Current increases with potential, just as waterflow increases with water pressure.

You have read that current is affected by both voltage and resistance.Using Ohm’s law, you can calculate exactly how much it is affected anddetermine the exact amount of current in amps. Use the formula forcurrent to solve the sample problem below.

Measuring ElectricityVolts, ohms, and amps can all be measured using specific electricalinstruments. Volts can be measured with a voltmeter. Ohms can bemeasured with an ohmmeter. Amps can be measured with anammeter. These three instruments are often combinedin a single electrical instrument called a multimeter.

To use a multimeter, set the dial onthe type of unit you wish to measure.For example, the multimeter in the photograph is being used to test thevoltage of a 9-volt battery. The dial is set on volts in the 0–20 range. The metershows that the battery’s charge has an electricpotential of more than 9 volts, which meansthat the battery is good. A dead battery wouldhave a lower voltage.

check your reading What does an ohmmeter measure?

362 Unit 3: Electricity and Magnetism

reading tip

The terms voltmeter, ohmmeter, ammeter, andmultimeter are all made by adding a prefix to theword meter.

Sample Problem

Practice the Math

Calculating Current

What is the current in an electrical pathway with an electric potentialof 120 volts and a resistance of 60 ohms?

What do you know? voltage = 120 V, resistance = 60 �

What do you want to find out? current

Write the formula: I =

Substitute into the formula: I = �16200

�V

�

Calculate and simplify: I = 2 A

Check that your units agree: Unit is amps. Unit of current is amps. Units agree.

Answer: 2 A

1. What is the current in an electrical pathway in which the voltage is 220 Vand the resistance is 55 �?

2. An electrical pathway has a voltage of 12 volts and a resistance of 24ohms. What is the current?

V�R

Electric cells supply electric current.Electric current can be used in many ways. Two basic types of device havebeen developed for producing current. One type produces electric current using magnets. You will learn more about this technology inChapter 12. The other type is the which produces electriccurrent using the chemical or physical properties of different materials.

Electrochemical CellsAn electrochemical cell is an electric cell that produces current by means of chemical reactions. As you can see inthe diagram, an electrochemical cell contains two stripsmade of different materials. The strips are called electrodes.The electrodes are suspended in a third material called theelectrolyte, which interacts chemically with the electrodesto separate charges and produce a flow of electrons fromthe negative terminal to the positive terminal.

Batteries are made using electrochemical cells.Technically, a battery is two or more cells connected to each other. However, single cells, such as C cells and D cells, are often referred to as batteries.

electric cell,

How can you produce electric current?PROCEDURE

Insert the paper clip and the penny into the lemon, as shown in the photo-graph. The penny and paper clip should go about 3 cm into the lemon. They should be close, but not touching.

On the multimeter, go to the DC volts (V—) section of the dial and select the0–2000 millivolt range (2000 m).

Touch one of the leads of the multimeter to the paper clip. Touch the otherlead to the penny. Observe what is shown on the display of the multimeter.

WHAT DO YOU THINK?• What did you observe on the display of the multimeter?

• How can you explain the reading on the multimeter?

CHALLENGE Repeat this experiment using different combinations of fruits or vegetables and metal objects. Which combinations work best?

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Electric CellsElectric Cells

Chapter 10: Electricity 363

positiveterminal

electrode electrolyte

electrode

negativeterminal

flow ofelectrons

SKILL FOCUSInferring

MATERIALS• paper clip• penny• large lemon• multimeterFor Challenge• additional fruits

or vegetables• metal objects

TIME20 minutes

Primary Cells The electrochemical cell shown on page 363 is called awet cell, because the electrolyte is a liquid. Most household batteries inuse today have a solid paste electrolyte and so are called dry cells. Bothwet cells and dry cells are primary cells. Primary cells produce electriccurrent through chemical reactions that continue until one or more ofthe chemicals is used up.

The primary cell on page 365 is a typical zinc-carbon dry cell. Ithas a negative electrode made of zinc. The zinc electrode is made inthe shape of a can and has a terminal—in this case, a wide disk ofexposed metal—on the bottom of the cell. The positive electrode con-sists of a carbon rod and particles of carbon and manganese dioxide.The particles are suspended in an electrolyte paste. The positive elec-trode has a terminal—a smaller disk of exposed metal—at the top ofthe rod. A paper separator prevents the two electrodes from cominginto contact inside the cell.

When the two terminals of the cell are connected—for example,when you turn on a flashlight—a chemical reaction between the zincand the electrolyte produces electrons and positive zinc ions. The elec-trons flow through the wires connecting the cell to the flashlight bulb,causing the bulb to light up. The electrons then travel through the car-bon rod and combine with the manganese dioxide. When the zinc andmanganese dioxide stop reacting, the cell dies.

check your reading Why are most household batteries called dry cells?

Storage Cells Some batteries produce current through chemical reac-tions that can be reversed inside the battery. These batteries are calledstorage cells, secondary cells, or rechargeable batteries. A car batterylike the lead-acid battery shown on page 365 is rechargeable. The bat-tery has a negative electrode of lead and a positive electrode of leadperoxide. As the battery produces current, both electrodes changechemically into lead sulfate, and the electrolyte changes into water.

When storage cells are producing current, they are said to be dis-charging. Whenever a car engine is started, the battery discharges tooperate the ignition motor. A car’s battery can also be used when thecar is not running to operate the lights or other appliances. If the bat-tery is used too long in discharge mode, it will run down completely.

While a car is running, however, the battery is continually beingcharged. A device called an alternator, which is run by the car’s engine,produces current. When electrons flow into the battery in the reversedirection from discharging, the chemical reactions that produce currentare reversed. The ability of the battery to produce current is renewed.

check your reading What kind of battery can be charged by reversing chemical reactions?

364 Unit 3: Electricity and Magnetism

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Learn more about electrochemical cells.

reminder

A battery converts chemicalenergy into electrical energy. You will learn more about chemical energy in grade 8.

Content PreviewFLORIDA

Both primary cells and storagecells produce electricity throughchemical reactions.

Batteries

In which direction do electrons flow when a storage cell is being charged?

Primary Cell Storage Cell

Discharging Storage cells produce current through chemicalreactions that can be reversed in the battery.

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terminal

terminal

carbonrod

separator

manganese dioxide particles in paste electrolyte

Flashlights useprimary cells.

Car batteries and cell phonesuse storage cells.

flow of electrons

flow of electrons

leadperoxide(blue)

lead(red)

mostlysulfuric acid

–

+startermotorstartermotor

mostlywater

lead sulfate

Charging Sendingcurrent through the battery in theopposite directionreverses the chemicalreactions.

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zinccan

Primary cells produce electric current through chemical reactions.The reactions continue until thechemicals are used up.

Chapter 10: Electricity 365

alternator

366 Unit 3: Electricity and Magnetism

Solar CellsSome materials, such as silicon, can absorb energy from the Sun or othersources of light and then give off electrons, producing electric current.Electric cells made from such materials are called solar cells.

Solar cells are often used to make streetlights come on automati-cally at night. Current from the cell operates a switch that keeps thelights turned off. When it gets dark, the current stops, the switch closes,and the streetlights come on.

Many houses and other buildings now get at least some of theirpower from solar cells. Sunlight provides an unlimited source of free,environmentally safe energy. However, it is not always easy or cheap touse that energy. It must be collected and stored because solar cells donot work at night or when sunlight is blocked by clouds or buildings.

check your reading Where do solar cells get their energy?

KEY CONCEPTS1. How is electric current differ-

ent from a static charge that moves?

2. How can Ohm’s law be used to calculate the electrical resistance of a piece of wire?

3. How do rechargeable batterieswork differently from nonrechargeable ones?

CRITICAL THINKING4. Infer Electrical outlets in a

house maintain a steady voltage, even when theamount of resistance on themchanges. How is this possible?

5. Analyze Why don’t solar cells eventually run down aselectrochemical cells do?

CHALLENGE6. Apply Several kinds of electric

cells are discussed in this section. Which do you thinkwould be the most practicalsource of electrical energy on along trek through the desert?Explain your reasoning.

This NASA research aircraft is powered onlyby the solar cells on itsupper surface.

reading tip

The word solar comes fromthe Latin word sol, whichmeans the Sun.