Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Todays special HW check Finish Einsteins Big Idea Assignment: Write a two page summary of the ideas of the video; you may include current ideas; MLA format, please (12 pt font, double space, etc.) SAT QOTD Notes I; HW I due next time Test results Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company The only job where you start on top is digging a hole. Unknown Fact: The longest river in Europe is the Volga River. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Electromagnetic Induction Pd.9:Explain the effects of magnetic forces on the production of electrical currents and on current carrying wires and moving charges. Pd.10:Distinguish between the function of motors and generators on the basis of the use of electricity and magnetism by each. Pa.10:Use appropriate safety procedures when conducting investigations. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Preview Section 1 Electricity from MagnetismElectricity from Magnetism Section 2 Generators, Motors, and Mutual InductanceGenerators, Motors, and Mutual Inductance Section 3 AC Circuits and TransformersAC Circuits and Transformers Section 4 Electromagnetic WavesElectromagnetic Waves Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company What do you think? The loop of wire is rotating in a counterclockwise direction. Electrons in metal are free to move. The magnetic field is horizontal and to the left. See the next slide for questions. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company What do you think? Will there be a force on the electrons in the left and right segments of the loop? If so, in what direction is that force? In which direction will the electrons flow if the two wires from the ends are connected? Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Electromagnetic Induction Imagine a wire moving to the right as shown. In what direction is the force on the negative charge? Upward This force will separate the charges. As negative charges move upward, the wire will develop a potential difference. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Electromagnetic Induction Electromagnetic induction is the process of creating a current in a circuit loop by changing the magnetic flux in the loop. This can be accomplished by moving the loop, moving the field, or changing the strength of the field. If the magnetic flux does not change, no current is induced. The current is increased if the loop size or magnetic field strength are increased. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Ways of Inducing a Current in a Circuit Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Lenzs Law As the magnet enters the coil, a force pushes the electrons around the loop, inducing a current. The induced current creates a magnetic field that opposes the motion of the magnet. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Lenzs Law Now the magnet is being removed from the coil as it moves to the right. This induces a current in the opposite direction. Once again, the magnetic field in the coil opposes the motion of the magnet. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Lenzs Law The magnetic field of the induced current is in a direction to produce a field that opposes the change causing it. This rule can be used to find the direction of the current in the loop. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Lenz's Law for Determining the Direction of the Induced Current Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Faradays Law The magnitude of the induced emf depends on the number of loops (N), the magnetic flux ( M ), and the rate of change. M = AB cos Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Classroom Practice Problem A coil with 25 turns of wire is moving in a uniform magnetic field of 1.5 T. The magnetic field is perpendicular to the plane of the coil. The cross- sectional area of the coil is 0.80 m 2. The coil exits the field in 1.0 s. Find the induced emf. Determine the induced current in the coil if the coils resistance is 1.5 . Answers: 3.0 10 1 V, 2.0 10 1 A Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Now what do you think? The loop of wire is rotating in a counterclockwise direction. Electrons in metal are free to move. The magnetic field is horizontal and to the left. The ends of the coil are connected to a load such as a light bulb (not shown). See the next slide for questions. Electromagnetic InductionSection 1 Houghton Mifflin Harcourt Publishing Company Now what do you think? In which direction will the electrons flow around the loop? What is the direction of current in the loop? Use the right-hand rule to find the magnetic field created by the current in the loop. Does this magnetic field oppose the motion of the loop? Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company What do you think? The flow of water downstream from Niagara Falls is used to generate electric current. Briefly describe how water at the top of the falls could eventually produce over 4 million kilowatts of electric power. List the steps that you think are involved and the energy conversions that accompany each step. Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Generators If the coil rotates as shown, in what direction will the current (I) exist through the external connections? Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Generators A generator is a device that converts mechanical energy into electrical energy. Motion (KE) of a coil of wire through a magnetic field creates a potential difference in a wire, causing electrons to flow. Power plants use falling water or high-pressure steam to spin a turbine that is connected to a generator. Similar to the wind blowing through a fan Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company AC Generators The current alternates as a sin curve with the maximum occurring at points (b) and (d) in the rotation. The emf max depends on the number of loops (N), the area of the loop (A), the magnetic field (B), and the rate of rotation ( ). emf max = NAB Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company AC Generators Slip rings and brushes allow electrical connections without impeding rotation. How might this be modified so that the current only exists in one direction through the bulb? Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Function of a Generator Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company DC Generators The ring is split so that the brushes switch sides of the loop every time it is vertical. The current still varies but no longer changes direction (DC). Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Comparing AC and DC Generators Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Electric Motors What way would the current exist through the loop shown? From + to - A force on these moving charged particles will cause the loop to rotate. A motor is just a generator with the load replaced by an emf. Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept DC Motors Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Electric Motors The motion of the coil causes it to generate a current of its own. Called a back emf This current opposes the current from the battery. The two emfs produce a net emf. The faster it turns, the lower the net emf and the lower the current. Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Mutual Inductance What happens in the primary coil when you close the switch? There is current in the primary coil. What effect does this current have in the primary coil and in the iron ring? A magnetic field is generated in the primary coil and in the iron ring. Continued on the next slide Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Mutual Inductance What happens when the field is constant? Once the field is constant, the current in the secondary coil goes to zero. What happens in the secondary coil as a result of the magnetic field in the iron ring? The magnetic field induces a current in the secondary coil that is subjected to the changing magnetic field. Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Mutual Inductance Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Now what do you think? The flow of water downstream from Niagara Falls is used to generate electrical current. Briefly describe how water at the top of the falls could eventually produce over 4 million kilowatts of electrical power. List the steps that you think are involved and the energy conversions that accompany each step. Electromagnetic InductionSection 2 Houghton Mifflin Harcourt Publishing Company Todays special Start working on lab as soon as you arrive! HW check; turn in papers on front desk Q & A Notes II HW II due next! Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company What do you think? In order to solve problems using I = V/R and P = V 2 /R you must know the value for the potential difference ( V) or emf. The graph shows the emf in an AC circuit. Since the emf is changing, how might the value for V be determined? Suppose the maximum emf is 12 V. What value would you use in the equations above? Why? Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company AC Potential Difference and Current Since current and potential difference change, a method for finding an equivalent current must be used. The root-mean-square value for current and potential difference is used. It is about 71% of the maximum. Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company RMS Values The value often used for household potential difference is 120 V. If this is the rms value, what is the maximum potential difference in household circuits? Answer: 170 V Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Classroom Practice Problems An AC generator supplies a 110 V rms to a 25 resistor. What is the rms current in the circuit? What is the maximum current in the circuit? Answers: 4.4 A, 6.2 A Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Transformers The primary coil is connected to the AC source The iron core enhances the changes in the magnetic field. The secondary coil is connected to the output. A transformer is a device that increases or decreases the emf of alternating current. Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Transformers If N 2 > N 1 then V 2 > V 1 Called a step-up transformer If N 2 < N 1 then V 2 < V 1 Called a step-down transformer The potential difference on the two sides is proportional to the number of turns. How much greater is V 2 than V 1 for the transformer shown? Answer: 1.5 times greater (3/2) Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Transformers In ideal transformers, P output = P input. If potential difference is increased, the current is decreased by the same factor. No energy is gained or lost. Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Transformer Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Real Transformers Real (not ideal) transformers are generally over 90% efficient. Less than 10% of the input power is lost due to currents and heating of the iron core. Utility companies use step-up transformers to send power over long distances at high emfs. V is common High values for V -----> low values for I Since P = I 2 R, less heat is lost in the transmission lines. Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Classroom Practice Problems A transformer has 75 turns on the primary coil and 1500 turns on the secondary coil. If the potential difference across the primary coil is 120 V, what is the potential difference across the secondary coil? If the transformer has 1625 turns on the secondary side instead of 1500, what is the potential difference across the secondary? Answers: 2400 V, 2600 V Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Applications of Transformers Ignition coils (transformers) for spark plugs Producing a spark requires a very high emf Since the battery has an emf of 12 V, a step-up transformer is used to increase the emf across the spark plug. Electromagnetic InductionSection 3 Houghton Mifflin Harcourt Publishing Company Now what do you think? In order to solve problems using I = V/R and P = V 2 /R you must know the value for the potential difference ( V) or emf. The graph shows the emf in an AC circuit. Since the emf is changing, how might the value for V be determined? Suppose the maximum emf is 12 V. What value would you use in the equations above? Why? Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company What do you think? What are electromagnetic (EM) waves? Based on the name, how do you think they are produced? How do they propagate? EM waves fall into many broad groups (radio, micro, infrared, visible, ultraviolet, X rays, and gamma rays). In what ways are they similar? In what ways are they different? Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Electromagnetic Waves Electromagnetic waves consist of electric and magnetic fields. Electric forces and magnetic forces are two aspects of electromagnetic forces. A changing magnetic field produces a changing electric field and vice versa. James Clerk Maxwell predicted their existence. Heinrich Hertz produced EM waves in his laboratory. Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Click below to watch the Visual Concept. Visual Concept Electromagnetic Waves Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Electromagnetic Waves EM waves are produced by accelerating charged particles. Electrons moving within atoms Electrons in an antenna EM waves are transverse. All EM waves travel at the speed of light. The energy transported is called EM radiation. Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Photons At high energies, EM waves behave more like particles. Called photons Low-energy photons behave more like waves. Some aspects of visible light are more like waves while others are more like particles. Called wave-particle duality Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company The Electromagnetic Spectrum Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Properties of EM Waves Radio Waves Diffraction around objects due to long wavelengths Pass through the atmosphere Large antennae collect radio waves from deep space. Used to detect hydrogen clouds, pulsars, quasars, and so on Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Properties of EM Waves Microwaves Used to communicate with satellites and transmit cell phone conversations Heat your food by causing certain molecules to vibrate Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Properties of EM Waves Infrared The frequency given off by anything warm such as your body, heat lamps, or an electric hot plate Mosquitoes see infrared waves. Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Properties of EM Waves Visible Light Narrow range of frequencies and wavelengths Color is determined by frequency Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Properties of EM Waves Ultraviolet Causes sunburns Largely absorbed by the ozone layer Used for sterilization in hospitals because it kills bacteria Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Properties of EM Waves X rays Discovered by Wilhelm Conrad Roentgen He created an X ray of his wifes hand. Soft tissue is transparent to X rays. Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Properties of EM Waves Gamma Rays Produced by radioactive atoms Produced from the hottest parts of the universe Useful in fighting cancer because it kills cells Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Now what do you think? What are electromagnetic waves? How are they are produced? How do they propagate? How are the different types similar? Different? List the EM spectrum from highest frequency to lowest frequency. Name at least one property of each of the seven types of EM waves. Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Todays special Turn in labs on front table Hard work pays off! Secret lives of scientists: Stephon Alexander HW check; Q & A Review for test Test next class! Electromagnetic InductionSection 4 Houghton Mifflin Harcourt Publishing Company Todays special Test 20 Electromagnetic induction Enter answers in Smart Response and turn in paper test on front table Get Vocab 21; due next time Only 20 percent of Americans surveyed believe in Big BangOnly 20 percent of Americans surveyed believe in Big Bang