Direct Curr~n~ ~ an induced current, charges may flow in one direction only, or they may alternate dlrec~ions. The direction of an induced current depends on file direction in whick,~i :i~ the wire or magnet moves.You probably no~iced in Figure 12 on ~ ’ the previous page that when the direction o£ the motion o£ the wire coil changed, the ~rec~ion o£ the current reversed. A current consisting o£ charges that flow in oIl~ direction only is called direct currel~t, or DC. A direct current c~ be induced ~om a changing magnetic field or produced £ro~ an ~nergy source such as a battery. Whe~a a battery is placed in a circuit, charges flow away £rom one end o£the bakery, aroond the circuit, and into the other end o£ the battery. Thomas Eclison used direct currel~t in his first electric generatisag plan~. Several scientists were responsible for bringing electricity ~rorn the laboratory into everyday use. A~ternating Current What would happen if a wire in a r~a~netic field were moved up and down repeate~y? The " ~,:ed current ~ the wire would reverse dire~on repeatedly a~~eI!. This ~d of current is called ~ternat~g current, or AC. ~ Mt~na~g c~re~t consis~ of charges ~at move back md forth in a c~cuk. The electric c~rrent ~ the circuits in homes, schools, a~d other buildings is Mternating c~r~t. Mtema~g current has a’ major adv~tage over direct c~s-’ rent. An AC voltage c~ be easily r~sed or lowered to a higher .or lower voltage. This means ~at a high vol~ge can be used to send electricM e~er~ over great ~stances: Then ~e voltage ~n be reduced to a safer level for everyday use. Joseph Henry, or Hans Christian Oersted. Write a letter to a friend in which you describe your work as a research assistant for the ~ Reading ~ What s the advantago of using alternating scientist you choose. ~ Che~po~t~ cur~nt? ~" include descriptions of his e xperimentalproceduresand the equipment he uses. Tell how his work has led to ...... ] surprising discoveries. " I Chaptar3 N ~ 97 e~ergy ~to dectricaI ener~. ~ electric generator is the o~posite of an elecb’ic too*or. ~ e]e~ic tootor uses an electric current in a ~agnet field to produce moron. A generator uses ’ magnets Armature .... -- Current FIGURE 14 Turbines In most generators, a source of mechanical energy turns huge turbfnes such as this one. The turbine is attached to the armature of a generator, which produces current. Using Generators TI~e electric company uses giant generamrs to produce most~of the electrical energy you use in your home and school. But, instead of using a crank to supl?ly the toechanical energy to tuitn the armature, a turbine is used. Tur~ bh~es are large circ~laz devices made nt~ of many blades. Figure 14 shows how a turbine is attached to the armature in a generator.. The turbine spins as the water flows by it. As a result, the artoatare spins and generates electric current. . Reading ~ ~ What is a turbine? Primary coil Secondary coil The electrical energy generated by electric companies is transmitted over tong distance at very high v01tages. However in your hotoe, electrical energy is used at much lower voltages. What changes the vottage of the electrical energy? The answer is tralxsfortoers. What is a Trans~iormer7 A transformer is a device ~hat increases or decreases voltage. A transformer consists of two separgt~ coils of insulated ~a~re wrapped around an iron core. One coil, called the primary coil, is connected to a circuit with a voltage source and alter~ating current. The other coil, the secondary c~fl, is connected to a separate drcuit that does FIGURE 15 A Transformer The primary co!! of a transformer is connected to a wltage source, The secondary coil is not F~ua~ 16 Chancding Voltage Transformers are involved in the transmission, of elec~Hcal energy ~rom an electric plant to a ho~e, Relating Cause and Effe~ Ho~. does the number of loops in th~~ , primary and secondary colb the voltage of the induced current? A Transformer at Work W~en a current is in the primary coil of the transformer~ it produces a magnetic field. The magnetic field changes as the current alternates. This changing magnetic fidd is like a moving magnetic field. It induces a current in thq secondary coil. A transformer works only if the current in the primary coil is changing. If the current does not change, the magnetic field does not change. No current will be induced ha the secondary coil. 8o a transformer will not work with direct current. ~ypes of Transformers If the number of loops in the primary and secondary coils of a transformer is the same, the voltage of the induced current is the same as the original voltage. But if the secondary coll has more loops than the primary coil, thevoltage in the secondary coil will be greater. A transformer that increases voltage is called a step-up transformer. Step-up Transformer A step-up transformer increases voltage. The secondary coil has more loops than the primary coil. Step*do~vn Transformer A step-down transformer decreases voltage, The primary.c, oi! has mere loops than the secondary Low Voltage High Vol~age High Voltage @ Primary coil Step-up transformer plant 11,000 V Low Voltage @ Secondary coil Step~down ":kransformer Primary coil Secondary coil Step-down .transformer " Suppose there are fewer loops in the secondary coil than in i fhe primary coil. The vokage in the secondary coil will be less ~ itan in the primary coil A transformer that decreases voltage i .s called a step-dm,m transformer, Figure 16 shows both types ~f transformers, Uses of ~ransformers An important use of transformers is in the transmission of electrical energy from generating pla~ts’. The most efficient way to transmtt current over long dlstan es is to maintain high voltages--about 11,000 volts to 765,000 volts. But the high voltage must be decreased to be used safely in your home, The use of step-up and step-down transformers al!ows safe transmission of electrical energy from generating plants to the consumer. . ,.; ,~ . .. T~ansformers are also used m some elecmca! dewces. "}escant lights, teIe,~slons, and X,-r~y machines require higher voltages than the current in your home, which is about 120 volts, These devices contain step-up transformers. Other : devices, such as doorbells, electronic games, and portable CD pIayers, require lower voltages, about 6 to 12 volts. They con’i~ain step-down transformers. :~~~eading What is the voltage in your house? 1, What is the difference between AC and DC? Explain[ 2. Which one do you beheve we use today. Why is this an advantage? 3. What does a ger~erat~r do? How does a tarbine make a generator work? 4. What is a transformer? 5. Observe the image on page 100 in you~ packet. How does energy move ~omthe generating plant to our houses? What happens to the voltage? Explain! (Use step-up and step-down transformers in your answer) What’s the voltage in an average household?