Page 1 of 7 KEY CONCEPT Electric current is a flow of charge. BEFORE, you learned NOW, you will learn • Charges move from higher to • About electric current lower potential • How current is related to • Materials can act as conductors voltage and resistance or insulators • About different types of • Materials have different levels electric power cells of resistance VOCABULARY EXPLORE Current electric current p. 28 How does resistance affect the flow of charge? ampere p. 29 Ohm’s law p. 29 PROCEDURE MATERIALS electric cell p. 31 • pencil lead 1 Tape the pencil lead flat on the posterboard. • posterboard 2 Connect the wires, cell, bulb, and bulb • electrical tape holder as shown in the photograph. • 3 lengths of wire 3 Hold the wire ends against the pencil lead • D cell battery about a centimeter apart from each other. • flashlight bulb Observe the bulb. • bulb holder 4 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? Electric charge can flow continuously. Static charges cannot make your television play. For that you need a different type of electricity. You have learned that a static charge contains a specific, limited amount of charge. You have also learned that a static charge can move and always moves from higher to lower VOCABULARY potential. However, suppose that, instead of one charge, an electrical Don’t forget to make a four square diagram for the pathway received a continuous supply of charge and the difference in term electric current. potential between the two ends of the pathway stayed the same. Then, you would have a continuous flow of charge. Another name for a flow of charge is electric current. Electric current is the form of electricity used to supply energy in homes, schools, and other buildings. E 28 Unit: Electricity and Magnetism Page 2 of 7 Current, Voltage, and Resistance Electric current obeys the same rules as moving static charges. Charge can flow only if it has a path to follow, that is, a material to conduct COMBINATION NOTES it. Also, charge can flow only from a point of higher potential to one In your notes, try making a sketch to of lower potential. However, one concept that does not apply to a help you remember moving static charge applies to current. Charge that flows steadily has how current, voltage, a certain rate of flow. This rate can be measured. The standard unit of and resistance differ. measure for current is the ampere, or amp. An amp is the amount of charge that flows past a given point per unit of time. One amp equals one 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 of water through a pipe. Electric potential, or voltage, is like pressure pushing the water through the pipe. Resistance, or ohms, is like the diameter of the pipe, which controls how much water can flow through. Water pressure and pipe size together determine the rate of water flow. Similarly, voltage and resistance together determine the rate of flow of electric charge. How Potential Affects Current How Resistance Affects Current Current increases with potential, just as water Current decreases as resistance increases, flow increases with water pressure. just as water flow decreases as resistance to flow increases. low pressure and high pressure and low resistance and high resistance and low rate of flow high rate of flow high rate of flow low rate of flow Ohm’s Law You now have three important measurements for the study of electricity: SIMULATION volts, ohms, and amps. The scientist for whom the ohm is named dis- CLASSZONE.COM covered a mathematical relationship among these three measurements. See Ohm’s law in action. The relationship, called Ohm’s law, is expressed in the formula below. Voltage V Current ϭ ᎏᎏ I ϭ ᎏ Res istance R 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? Chapter 1: Electricity 29 E Page 3 of 7 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 and determine the exact amount of current in amps. Use the formula for current to solve the sample problem below. Calculating Current Sample Problem What is the current in an electrical pathway with an electric potential of 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 V Write the formula: I = ᎏ R ᎏ120ᎏV Substitute into the formula: I = 60 ⍀ Calculate and simplify: I = 2 A Check that your units agree: Unit is amps. Unit of current is amps. Units agree. Answer: 2 A Practice the Math 1. What is the current in an electrical pathway in which the voltage is 220 V and the resistance is 55 ⍀? 2. An electrical pathway has a voltage of 12 volts and a resistance of 24 ohms. What is the current? reading tip Measuring Electricity The terms voltmeter , Volts, ohms, and amps can all be measured using specific electrical ohmmeter , ammeter , and multimeter are all made instruments. Volts can be measured with a voltmeter. Ohms can be by adding a prefix to the measured with an ohmmeter. Amps can be measured with an word meter . ammeter. These three instruments are often combined in a single electrical instrument called a multimeter. To use a multimeter, set the dial on the type of unit you wish to measure. For example, the multimeter in the photograph is being used to test the voltage of a 9-volt battery. The dial is set on volts in the 0–20 range. The meter shows that the battery’s charge has an electric potential of more than 9 volts, which means that the battery is good. A dead battery would have a lower voltage. check your reading What does an ohmmeter measure? E 30 Unit: Electricity and Magnetism Page 4 of 7 Electric Cells SKILL FOCUS How can you produce electric current? Inferring PROCEDURE 1 Insert the paper clip and the penny into the lemon, as shown in the photo- MATERIALS graph. The penny and paper clip should go about 3 cm into the lemon. • paper clip • penny They should be close, but not touching. • large lemon • multimeter 2 On the multimeter, go to the DC volts (V—) section of the dial and select the For Challenge 0–2000 millivolt range (2000 m). • additional fruits 3 Touch one of the leads of the multimeter to the paper clip. Touch the other or vegetables lead to the penny. Observe what is shown on the display of the multimeter. • metal objects TIME WHAT DO YOU THINK? 20 minutes • 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? Electric cells supply electric current. Electric current can be used in many ways. Two basic types of device have been developed for producing current. One type produces electric current using magnets. You will learn more about this technology in Chapter 3.The other type is the electric cell, which produces electric current using the chemical or physical properties of different materials. flow of electrons Electrochemical Cells An electrochemical cell is an electric cell that produces current by means of chemical reactions. As you can see in the diagram, an electrochemical cell contains two strips made of different materials. The strips are called electrodes. positive negative The electrodes are suspended in a third material called the terminal terminal electrolyte, which interacts chemically with the electrodes to separate charges and produce a flow of electrons from the negative terminal to the positive terminal. electrode electrolyte 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 electrode D cells, are often referred to as batteries. Chapter 1: Electricity 31 E Page 5 of 7 RESOURCE CENTER Primary Cells The electrochemical cell shown on page 31 is called a CLASSZONE.COM wet cell, because the electrolyte is a liquid. Most household batteries in Learn more about use today have a solid paste electrolyte and so are called dry cells. Both electrochemical cells. wet cells and dry cells are primary cells. Primary cells produce electric current through chemical reactions that continue until one or more of the chemicals is used up. The primary cell page 33 is a typical zinc-carbon dry cell. It has a negative electrode made of zinc. The zinc electrode is made in the shape of a can and has a terminal—in this case, a wide disk of exposed 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 of the rod. A paper separator prevents the two electrodes from coming into contact inside the cell. When the two terminals of the cell are connected—for example, when you turn on your flashlight—a chemical reaction between the zinc and the electrolyte produces electrons and positive zinc ions.