Electricity & Magnetism PDF

Chapter 21 Electric Charge and Electric Field PowerPoint® Lectures for University Physics, 14th Edition, Global Edition – Hugh D. Young and Roger A. Freedman Lectures by Jason Harlow © 2016 Pearson Education, Ltd....

Chapter 21 Electric Charge and Electric Field PowerPoint® Lectures for University Physics, 14th Edition, Global Edition – Hugh D. Young and Roger A. Freedman Lectures by Jason Harlow © 2016 Pearson Education, Ltd.  Electric charge Plastic rods and fur (real or fake) are particularly good for demonstrating electrostatics, the interactions between electric charges that are at rest (or nearly so). After we charge both plastic rods by rubbing them with the piece of fur, we find that the rods repel each other. © 2016 Pearson Education, Ltd.  Electric charge When we rub glass rods with silk, the glass rods also become charged and repel each other. © 2016 Pearson Education, Ltd.  Electric charge A charged plastic rod attracts a charged glass rod; furthermore, the plastic rod and the fur attract each other, and the glass rod and the silk attract each other. These experiments and many others like them have shown that there are exactly two kinds of electric charge: The kind on the plastic rod rubbed with fur (negative) and the kind on the glass rod rubbed with silk (positive). © 2016 Pearson Education, Ltd.  Electric charge and the structure of matter The particles of the atom are the negative electrons (dark blue spheres in this figure), the positive protons (red spheres), and the uncharged neutrons (gray spheres). Protons and neutrons make up the tiny dense nucleus, which is surrounded by electrons. © 2016 Pearson Education, Ltd.  Atoms and ions A neutral atom has the same number of protons as electrons. The electron “shells” are a schematic representation of the actual electron distribution, a diffuse cloud many times larger than the nucleus. © 2016 Pearson Education, Ltd.  Atoms and ions A positive ion is an atom with one or more electrons removed. © 2016 Pearson Education, Ltd.  Atoms and ions A negative ion is an atom with an excess of electrons. © 2016 Pearson Education, Ltd.  Conservation of charge The proton and electron have the same magnitude charge. The magnitude of charge of the electron or proton is a natural unit of charge. All observable charge is quantized in this unit. The universal principle of charge conservation states that the algebraic sum of all the electric charges in any closed system is constant. © 2016 Pearson Education, Ltd.  Conductors and insulators Copper is a good conductor of electricity; nylon is a good insulator. The copper wire shown conducts charge between the metal ball and the charged plastic rod to charge the ball negatively. © 2016 Pearson Education, Ltd.  Conductors and insulators After it is negatively charged, the metal ball is repelled by a negatively charged plastic rod. © 2016 Pearson Education, Ltd.  Conductors and insulators After it is negatively charged, the metal ball is attracted by a positively charged glass rod. © 2016 Pearson Education, Ltd.  Charging by induction in 4 steps: Steps 1 and 2 1. Start with an uncharged metal ball supported by an insulating stand. 2. When you bring a negatively charged rod near it, without actually touching it, the free electrons in the metal ball are repelled by the excess electrons on the rod, and they shift toward the right, away from the rod. © 2016 Pearson Education, Ltd.  Charging by induction in 4 steps: Steps 3 and 4 3. While the plastic rod is nearby, you touch one end of a conducting wire to the right surface of the ball and the other end to the ground. 4. Now disconnect the wire, and then remove the rod. A net positive charge is left on the ball. The earth acquires a negative charge that is equal in magnitude to the induced positive charge remaining on the ball. © 2016 Pearson Education, Ltd.  Electric forces on uncharged objects A charged body can exert forces even on objects that are not charged themselves. If you rub a balloon on the rug and then hold the balloon against the ceiling, it sticks, even though the ceiling has no net electric charge. After you electrify a comb by running it through your hair, you can pick up uncharged bits of paper or plastic with it. How is this possible? © 2016 Pearson Education, Ltd.  Electric forces on uncharged objects The negatively charged plastic comb causes a slight shifting of charge within the molecules of the neutral insulator, an effect called polarization. © 2016 Pearson Education, Ltd.  Electric forces on uncharged objects Note that a neutral insulator is also attracted to a positively charged comb. A charged object of either sign exerts an attractive force on an uncharged insulator. © 2016 Pearson Education, Ltd.  Electrostatic painting Induced positive charge on the metal object attracts the negatively charged paint droplets. © 2016 Pearson Education, Ltd.  Coulomb’s Law Coulomb’s Law: The magnitude of the electric force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them. © 2016 Pearson Education, Ltd.  Exp. 21.1 © 2016 Pearson Education, Ltd.  Electric field: Introduction Slide 1 of 3 To introduce the concept of electric field, first consider the mutual repulsion of two positively charged bodies A and B. © 2016 Pearson Education, Ltd.  Electric field: Introduction Slide 2 of 3 Next consider body A on its own. We can say that body A somehow modifies the properties of the space at point P. © 2016 Pearson Education, Ltd.  Electric field: Introduction Slide 3 of 3 We can measure the electric field produced by A with a test charge. © 2016 Pearson Education, Ltd.  Electric force produced by an electric field © 2016 Pearson Education, Ltd.  The electric field of a point charge © 2016 Pearson Education, Ltd.  The electric field of a point charge Using a unit vector that points away from the origin, we can write a vector equation that gives both the magnitude and the direction of the electric field: © 2016 Pearson Education, Ltd.  Electric field of a point charge A point charge q produces an electric field at all points in space. The field strength decreases with increasing distance. The field produced by a positive point charge points away from the charge. © 2016 Pearson Education, Ltd.  Electric field of a point charge A point charge q produces an electric field at all points in space. The field strength decreases with increasing distance. The field produced by a positive point charge points toward the charge. © 2016 Pearson Education, Ltd.  Superposition of electric fields The total electric field at a point is the vector sum of the fields due to all the charges present. © 2016 Pearson Education, Ltd.  Exp. 21.6 © 2016 Pearson Education, Ltd.  Exp. 21.8 © 2016 Pearson Education, Ltd.  Electric field lines An electric field line is an imaginary line or curve whose tangent at any point is the direction of the electric field vector at that point. © 2016 Pearson Education, Ltd.  Electric field lines of a point charge Electric field lines show the direction of the electric field at each point. The spacing of field lines gives a general idea of the magnitude of the electric field at each point. © 2016 Pearson Education, Ltd.  Electric field lines of a dipole Field lines point away from + charges and toward – charges. © 2016 Pearson Education, Ltd.  Electric field lines of two equal positive charges At any point, the electric field has a unique direction, so field lines never intersect. © 2016 Pearson Education, Ltd.  The water molecule is an electric dipole The water molecule as a whole is electrically neutral, but the chemical bonds within the molecule cause a displacement of charge. The result is a net negative charge on the oxygen end of the molecule and a net positive charge on the hydrogen end, forming an electric dipole. © 2016 Pearson Education, Ltd.  The water molecule is an electric dipole When dissolved in water, salt dissociates into a positive sodium ion and a negative chlorine ion, which tend to be attracted to the negative and positive ends of water molecules. This holds the ions in solution. If water molecules were not electric dipoles, water would be a poor solvent, and almost all of the chemistry that occurs in aqueous solutions would be impossible! © 2016 Pearson Education, Ltd.  Electric potential energy in a uniform field In the figure, a pair of charged parallel metal plates sets up a uniform, downward electric field. The field exerts a downward force on a positive test charge. As the charge moves downward from point a to point b, the work done by the field is independent of the path the particle takes. © 2016 Pearson Education, Ltd.  A positive charge moving in a uniform field If the positive charge moves in the direction of the field, the field does positive work on the charge. The potential energy decreases. © 2016 Pearson Education, Ltd.  A positive charge moving in a uniform field If the positive charge moves opposite the direction of the field, the field does negative work on the charge. The potential energy increases. © 2016 Pearson Education, Ltd.  A negative charge moving in a uniform field If the negative charge moves in the direction of the field, the field does negative work on the charge. The potential energy increases. © 2016 Pearson Education, Ltd.  A negative charge moving in a uniform field If the negative charge moves opposite the direction of the field, the field does positive work on the charge. The potential energy decreases. © 2016 Pearson Education, Ltd.  Electric potential Potential is potential energy per unit charge. The potential of a with respect to b (Vab = Va – Vb) equals the work done by the electric force when a unit charge moves from a to b. © 2016 Pearson Education, Ltd.  Electron volts and cancer radiotherapy One way to destroy a cancerous tumor is to aim high-energy electrons directly at it. Each electron has a kinetic energy of 4 to 20 MeV (1 MeV = 106 eV), and transfers its energy to the tumor through collisions with the tumor’s atoms. Electrons in this energy range can penetrate only a few centimeters into a patient, which makes them useful for treating superficial tumors, such as those on the skin or lips. © 2016 Pearson Education, Ltd.  Introduction In flash photography, the energy used to make the flash is stored in a capacitor, which consists of two closely spaced conductors that carry opposite charges. The energy of a capacitor is actually stored in the electric field. © 2016 Pearson Education, Ltd.  Capacitors Any two conductors separated by an insulator (or a vacuum) form a capacitor. When the capacitor is charged, it means the two conductors have charges with equal magnitude and opposite sign, and the net charge on the capacitor as a whole is zero. © 2016 Pearson Education, Ltd.  Capacitors and capacitance One common way to charge a capacitor is to connect the two conductors to opposite terminals of a battery. This gives a potential difference Vab between the conductors that is equal to the voltage of the battery. If we change the magnitude of charge on each conductor, the potential difference between conductors changes; however, the ratio of charge to potential difference does not change. This ratio is called the capacitance C of the capacitor: © 2016 Pearson Education, Ltd.  Parallel-plate capacitor A parallel-plate capacitor consists of two parallel conducting plates separated by a distance that is small compared to their dimensions. © 2016 Pearson Education, Ltd.  Parallel-plate capacitor The field between the plates of a parallel-plate capacitor is essentially uniform, and the charges on the plates are uniformly distributed over their opposing surfaces. When the region between the plates is empty, the capacitance is: The capacitance depends on only the geometry of the capacitor. The quantities A and d are constants for a given capacitor, and is a universal constant. © 2016 Pearson Education, Ltd.  Condenser microphones Inside a condenser microphone is a capacitor with one rigid plate and one flexible plate. The two plates are kept at a constant potential difference. Sound waves cause the flexible plate to move back and forth, varying the capacitance C and causing charge to flow to and from the capacitor. Thus a sound wave is converted to a charge flow that can be amplified and recorded digitally. © 2016 Pearson Education, Ltd.  Units of capacitance The SI unit of capacitance is the farad, F. 1 F = 1 C/V = 1 C2/N ∙ m = 1 C2/J One farad is a very large capacitance. For the commercial capacitors shown in the photograph, C is measured in microfarads © 2016 Pearson Education, Ltd. Energy stored in a capacitor A practical application of capacitors is their ability to store energy and release it quickly. An extreme example of the same principle cat Sandia National Laboratories in New Mexico, which is used in experiments in controlled nuclear fusion. The Z machine uses a large number of capacitors in parallel to give a tremendous equivalent capacitance. The arcs shown here are produced when the capacitors discharge their energy into a target, which is heated to a temperature higher than 2 × 109 K. Compared to Sun Surface 5700K, core= 15x106 K  Electromotive force and circuits Just as a water fountain requires a pump, an electric circuit requires a source of electromotive force to sustain a steady current. © 2016 Pearson Education, Ltd.  Electromotive force and circuits The influence that makes current flow from lower to higher potential is called electromotive force (abbreviated emf and pronounced “ee-em-eff”), and a circuit device that provides emf is called a source of emf. Note that “electromotive force” is a poor term because emf is not a force but an energy-per-unit-charge quantity, like potential. The SI unit of emf is the same as that for potential, the volt (1 V = 1 J/C). A typical flashlight battery has an emf of 1.5 V; this means that the battery does 1.5 J of work on every coulomb of charge that passes through it. We’ll use the symbol (a script capital E) for emf. © 2016 Pearson Education, Ltd.  Internal resistance Real sources of emf actually contain some internal resistance r. The terminal voltage of the 12-V battery shown at the right is less than 12 V when it is connected to the light bulb. © 2016 Pearson Education, Ltd.  Table 25.4 — Symbols for circuit diagrams © 2016 Pearson Education, Ltd.  Exp. 25.5 © 2016 Pearson Education, Ltd.  Potential changes The figure shows how the potential varies as we go around a complete circuit. The potential rises when the current goes through a battery, and drops when it goes through a resistor. Going all the way around the loop brings the potential back to where it started. © 2016 Pearson Education, Ltd.  Energy and power in electric circuits The box represents a circuit element with potential difference Vab = Va − Vb between its terminals and current I passing through it in the direction from a toward b. If the potential at a is lower than at b, then there is a net transfer of energy out of the circuit element. The time rate of energy transfer is power, denoted by P, so we write: © 2016 Pearson Education, Ltd.  Power The upper rectangle represents a source with emf and internal resistance r, connected by ideal wires to an external circuit represented by the lower box. Point a is at higher potential than point b, so Va > Vb and Vab is positive. P = VabI © 2016 Pearson Education, Ltd.  Exp. 25.8 © 2016 Pearson Education, Ltd.  Metallic conduction Electrons in a conductor are free to move through the crystal, colliding at intervals with the stationary positive ions. The motion of the electrons is analogous to the motion of a ball rolling down an inclined plane and bouncing off pegs in its path. © 2016 Pearson Education, Ltd. © 2016 Pearson Education, Ltd.  Introduction Even in a complex circuit like the one on this circuit board, several resistors with different resistances can be connected so that all of them have the same potential difference; in this case the currents through the resistors will be different. In this chapter, we will learn general methods for analyzing complex networks of resistors, batteries, and capacitors. We shall look at various instruments for measuring electrical quantities in circuits. © 2016 Pearson Education, Ltd.  dc versus ac Our principal concern in this chapter is with direct-current (dc) circuits, in which the direction of the current does not change with time. Flashlights and automobile wiring systems are examples of direct-current circuits. Household electrical power is supplied in the form of alternating current (ac), in which the current oscillates back and forth. The same principles for analyzing networks apply to both kinds of circuits, and we conclude this chapter with a look at household wiring systems. © 2016 Pearson Education, Ltd.  Resistors in series Resistors are in series if they are connected one after the other so the current is the same in all of them. The equivalent resistance of a series combination is the sum of the individual resistances: © 2016 Pearson Education, Ltd.  Resistors in parallel If the resistors are in parallel, the current through each resistor need not be the same, but the potential difference between the terminals of each resistor must be the same, and equal to Vab. The reciprocal of the equivalent resistance of a parallel combination equals the sum of the reciprocals of the individual resistances: © 2016 Pearson Education, Ltd.  Series versus parallel combinations When connected to the same source, two incandescent light bulbs in series (shown at top) draw less power and glow less brightly than when they are in parallel (shown at bottom). © 2016 Pearson Education, Ltd.  Series and parallel combinations: Example 1 Resistors can be connected in combinations of series and parallel, as shown. In this case, try reducing the circuit to series and parallel combinations. For the example shown, we first replace the parallel combination of R2 and R3 with its equivalent resistance; this then forms a series combination with R1. © 2016 Pearson Education, Ltd.  Series and parallel combinations: Example 2 Resistors can be connected in combinations of series and parallel, as shown. In this case, try reducing the circuit to series and parallel combinations. For the example shown, we first replace the series combination of R2 and R3 with its equivalent resistance; this then forms a parallel combination with R1. © 2016 Pearson Education, Ltd.  Exp. 26.1 © 2016 Pearson Education, Ltd.  Power distribution systems The figure below shows the basic idea of house wiring. The “hot line” has an alternating sinusoidal voltage with a root-mean-square value of 120 V. The “neutral line” is connected to “ground,” which is usually an electrode driven into the earth. © 2016 Pearson Education, Ltd.  Circuit overloads A fuse (Figure a) contains a link of lead–tin alloy with a very low melting temperature; the link melts and breaks the circuit when its rated current is exceeded. A circuit breaker (Figure b) is an electromechanical device that performs the same function, using an electromagnet or a bimetallic strip to “trip” the breaker and interrupt the circuit when the current exceeds a specified value. Circuit breakers have the advantage that they can be reset after they are tripped, while a blown fuse must be replaced. © 2016 Pearson Education, Ltd.  Why it is safer to use a three-prong plug © 2016 Pearson Education, Ltd.  Chapter 27 Magnetic Field and Magnetic Forces PowerPoint® Lectures for University Physics, 14th Edition, Global Edition – Hugh D. Young and Roger A. Freedman Lectures by Jason Harlow © 2016 Pearson Education, Ltd.  Learning Goals for Chapter 27 Looking forward at … the properties of magnets, and how magnets interact with each other. how to analyze magnetic forces on current-carrying conductors and moving charged particles. how magnetic field lines are different from electric field lines. some practical applications of magnetic fields in chemistry and physics, including electric motors. how current loops behave when placed in a magnetic field. © 2016 Pearson Education, Ltd.  Introduction The most familiar examples of magnetism are permanent magnets, which attract unmagnetized iron objects and can also attract or repel other magnets. A compass needle aligning itself with the earth’s magnetism is an example of this interaction. But the fundamental nature of magnetism is the interaction of moving electric charges. How can magnetic forces, which act only on moving charges, explain the behavior of a compass needle? © 2016 Pearson Education, Ltd.  Magnetic poles If a bar-shaped permanent magnet, or bar magnet, is free to rotate, one end points north; this end is called a north pole or N pole. The other end is a south pole or S pole. Opposite poles attract each other, and like poles repel each other, as shown. © 2016 Pearson Education, Ltd.  Magnetism and certain metals An object that contains iron but is not itself magnetized (that is, it shows no tendency to point north or south) is attracted by either pole of a permanent magnet. This is the attraction that acts between a magnet and the unmagnetized steel door of a refrigerator. © 2016 Pearson Education, Ltd.  Magnetic field of the earth The earth itself is a magnet. Its north geographic pole is close to a magnetic south pole, which is why the north pole of a compass needle points north. The earth’s magnetic axis is not quite parallel to its geographic axis (the axis of rotation), so a compass reading deviates somewhat from geographic north. This deviation, which varies with location, is called magnetic declination or magnetic variation. Also, the magnetic field is not horizontal at most points on the earth’s surface; its angle up or down is called magnetic inclination. © 2016 Pearson Education, Ltd.  Magnetic monopoles Magnetic poles always come in pairs There is no experimental evidence for magnetic monopoles. © 2016 Pearson Education, Ltd.  Electric current and magnets A compass near a wire with no current points north. However, if an electric current runs through the wire, the compass needle deflects somewhat. © 2016 Pearson Education, Ltd.  The magnetic field A moving charge (or current) creates a magnetic field in the surrounding space. The magnetic field exerts a force on any other moving charge (or current) that is present in the field. Like an electric field, a magnetic field is a vector field—that is, a vector quantity associated with each point in space. We will use the symbol for magnetic field. At any position the direction of is defined as the direction in which the north pole of a compass needle tends to point. © 2016 Pearson Education, Ltd.  The magnetic force on a moving charge The magnitude of the magnetic force on a moving particle is proportional to the component of the particle’s velocity perpendicular to the field. If the particle is at rest, or moving parallel to the field, it experiences zero magnetic force. © 2016 Pearson Education, Ltd.  Magnetic force as a vector product The magnetic force is best represented as a vector product. © 2016 Pearson Education, Ltd.  The magnetic force on a moving charge © 2016 Pearson Education, Ltd.  Right-hand rule for magnetic force The right-hand rule gives the direction of the force on a positive charge. The next slide shows three steps involved in applying the right-hand rule: 1. Place the velocity and magnetic field vectors tail to tail. 2. Imagine turning toward in the plane (through the smaller angle). 3. The force acts along a line perpendicular to the plane. Curl the fingers of your right hand around this line in the same direction you rotated. Your thumb now points in the direction the force acts. © 2016 Pearson Education, Ltd.  Right-hand rule for magnetic force © 2016 Pearson Education, Ltd.  Right-hand rule for magnetic force If the charge is negative, the direction of the force is opposite to that given by the right-hand rule. © 2016 Pearson Education, Ltd.  Equal velocities but opposite signs Imagine two charges of the same magnitude but opposite sign moving with the same velocity in the same magnetic field. The magnetic forces on the charges are equal in magnitude but opposite in direction. © 2016 Pearson Education, Ltd.  Cathode-ray tube (CRT) The electron beam in a cathode-ray tube, such as that in an older television set, shoots out a narrow beam of electrons. If there is no force to deflect the beam, it strikes the center of the screen. The magnetic force deflects the beam, and creates an image on the screen. © 2016 Pearson Education, Ltd.  Magnetic field lines We can represent any magnetic field by magnetic field lines. We draw the lines so that the line through any point is tangent to the magnetic field vector at that point. Field lines never intersect. © 2016 Pearson Education, Ltd.  Magnetic field lines are not lines of force It is important to remember that magnetic field lines are not lines of magnetic force. The force on a charged particle is not along the direction of a field line. © 2016 Pearson Education, Ltd.  Magnetic field of a straight current-carrying wire © 2016 Pearson Education, Ltd.  Magnetic field lines of two permanent magnets Like little compass needles, iron filings line up tangent to magnetic field lines. Figure (b) is a drawing of field lines for the situation shown in Figure (a). © 2016 Pearson Education, Ltd.  Units of magnetic field and magnetic flux The SI unit of magnetic field B is called the tesla (1 T), in honor of Nikola Tesla: 1 tesla = 1 T = 1 N/A ∙ m Another unit of B, the gauss (1 G = 10−4 T), is also in common use. The magnetic field of the earth is on the order of 10−4 T or 1 G. The SI unit of magnetic flux ΦB is called the weber (1 Wb), in honor of Wilhelm Weber: 1 Wb = 1 T ∙ m2 © 2016 Pearson Education, Ltd.  Motion of charged particles in a magnetic field When a charged particle moves in a magnetic field, it is acted on by the magnetic force. The force is always perpendicular to the velocity, so it cannot change the speed of the particle. © 2016 Pearson Education, Ltd.  Helical motion If the particle has velocity components parallel to and perpendicular to the field, its path is a helix. The speed and kinetic energy of the particle remain constant. © 2016 Pearson Education, Ltd.  The Van Allen radiation belts Near the poles, charged particles from these belts can enter the atmosphere, producing the aurora borealis (“northern lights”) and aurora australis (“southern lights”). © 2016 Pearson Education, Ltd.  Chapter 28 Sources of Magnetic Field PowerPoint® Lectures for University Physics, 14th Edition, Global Edition – Hugh D. Young and Roger A. Freedman Lectures by Jason Harlow © 2016 Pearson Education, Ltd.  Introduction The immense cylinder in this photograph is a current- carrying coil, or solenoid, that generates a uniform magnetic field in its interior as part of an experiment at CERN, the European Organization for Nuclear Research. What can we say about the magnetic field due to a solenoid? What actually creates magnetic fields? We will introduce Ampere’s law to calculate magnetic fields. © 2016 Pearson Education, Ltd.  Magnetic fields for MRI MRI (magnetic resonance imaging) requires a magnetic field of about 1.5 T. In a typical MRI scan, the patient lies inside a coil that produces the intense field. The currents required are very high, so the coils are bathed in liquid helium at a temperature of 4.2 K to keep them from overheating. © 2016 Pearson Education, Ltd.  MRI © 2016 Pearson Education, Ltd.  Paramagnetism and diamagnetism When an external magnetic field permeates a paramagnetic material, the result is that the magnetic field at any point is greater by a dimensionless factor Km, called the relative permeability of the material, than it would be if the material were replaced by vacuum. If an external magnetic field permeates a diamagnetic material, the result is a magnetic field that is slightly less than it would be if the material were replaced by vacuum. The amount by which the relative permeability differs from unity is called the magnetic susceptibility, denoted by χm: χm = Km − 1 © 2016 Pearson Education, Ltd.  Magnetic susceptibilities of certain materials Material χm (×10−5) Iron ammonium alum 66 Paramagnetic Aluminum 2.2 Oxygen gas 0.19 Bismuth −16.6 Silver −2.6 Diamagnetic Carbon (diamond) −2.1 Copper −1.0 © 2016 Pearson Education, Ltd.  Ferromagnetism In ferromagnetic materials (such as iron), atomic magnetic moments tend to line up parallel to each other in regions called magnetic domains. When there is no externally applied field, the domain magnetizations are randomly oriented. When an external magnetic field is present, the domain boundaries shift; the domains that are magnetized in the field direction grow, and those that are magnetized in other directions shrink. © 2016 Pearson Education, Ltd.  Chapter 29 Electromagnetic Induction PowerPoint® Lectures for University Physics, 14th Edition, Global Edition – Hugh D. Young and Roger A. Freedman Lectures by Jason Harlow © 2016 Pearson Education, Ltd.  Learning Goals for Chapter 29 Looking forward at … how Faraday’s law relates the induced emf in a loop to the change in magnetic flux through the loop. how to determine the direction of an induced emf. how a changing magnetic flux generates a circulating electric field. the four fundamental equations that completely describe both electricity and magnetism. the remarkable electric and magnetic properties of superconductors. © 2016 Pearson Education, Ltd.  Introduction The card reader at a gas station scans the information that is coded in a magnetic pattern on the back of your card. Why must you remove the card quickly rather than hold it motionless in the card reader’s slot? Energy conversion makes use of electromagnetic induction. Faraday’s law and Lenz’s law tell us about induced currents. Maxwell’s equations describe the behavior of electric and magnetic fields in any situation. © 2016 Pearson Education, Ltd.  Induction experiment: Slide 1 of 4 During the 1830s, several pioneering experiments with magnetically induced emf were carried out. In the figure shown, a coil of wire is connected to a galvanometer. When the nearby magnet is stationary, the meter shows no current. © 2016 Pearson Education, Ltd.  Induction experiment: Slide 2 of 4 When we move the magnet either toward or away from the coil, the meter shows current in the circuit, but only while the magnet is moving. We call this an induced current, and the corresponding emf required to cause this current is called an induced emf. © 2016 Pearson Education, Ltd.  Induction experiment: Slide 3 of 4 In this figure we replace the magnet with a second coil connected to a battery. When we move the second coil toward or away from the first, there is current in the first coil, but only while one coil is moving relative to the other. © 2016 Pearson Education, Ltd.  Induction experiment: Slide 4 of 4 Using the two-coil setup of the previous slide, we keep both coils stationary and vary the current in the second coil by opening and closing the switch. The induced current in the first coil is present only while the current in the second coil is changing. © 2016 Pearson Education, Ltd.  Chapter 32 Electromagnetic Waves PowerPoint® Lectures for University Physics, 14th Edition, Global Edition – Hugh D. Young and Roger A. Freedman Lectures by Jason Harlow © 2016 Pearson Education, Ltd.  Learning Goals for Chapter 32 Looking forward at … how electromagnetic waves are generated. how and why the speed of light is related to the fundamental constants of electricity and magnetism. how to describe the propagation of a sinusoidal electromagnetic wave. what determines the amount of energy and momentum carried by an electromagnetic wave. how to describe standing electromagnetic waves. © 2016 Pearson Education, Ltd.  Introduction Why do metals reflect light? We will see that light is an electromagnetic wave. There are many other examples of electromagnetic waves, such as radiowaves and x rays. Unlike sound or waves on a string, these waves do not require a medium to travel. © 2016 Pearson Education, Ltd.  James Clerk Maxwell and electromagnetic waves The Scottish physicist James Clerk Maxwell (1831–1879) was the first person to truly understand the fundamental nature of light. He proved in 1865 that an electromagnetic disturbance should propagate in free space with a speed equal to that of light. From this, he deduced correctly that light was an electromagnetic wave. © 2016 Pearson Education, Ltd.  Electricity, magnetism, and light According to Maxwell’s equations, an accelerating electric charge must produce electromagnetic waves. For example, power lines carry a strong alternating current, which means that a substantial amount of charge is accelerating back and forth and generating electromagnetic waves. These waves can produce a buzzing sound from your car radio when you drive near the lines. © 2016 Pearson Education, Ltd.  The electromagnetic spectrum The frequencies and wavelengths of electromagnetic waves found in nature extend over such a wide range that we have to use a logarithmic scale to show all important bands. The boundaries between bands are somewhat arbitrary. © 2016 Pearson Education, Ltd.  Visible light Visible light is the segment of the electromagnetic spectrum that we can see. Visible light extends from the violet end (400 nm) to the red end (700 nm). © 2016 Pearson Education, Ltd.  Ultraviolet vision Many insects and birds can see ultraviolet wavelengths that humans cannot. As an example, the left-hand photo shows how black-eyed Susans look to us. The right-hand photo (in false color), taken with an ultraviolet-sensitive camera, shows how these same flowers appear to the bees that pollinate them. Note the prominent central spot that is invisible to humans. © 2016 Pearson Education, Ltd.  A simple plane electromagnetic wave To begin our study of electromagnetic waves, imagine that all space is divided into two regions by a plane perpendicular to the x-axis. At every point to the left of this plane there are uniform electric field magnetic fields as shown. The boundary plane, which we call the wave front, moves in the +x-direction with a constant speed c. © 2016 Pearson Education, Ltd.  Gauss’s laws and the simple plane wave Shown is a Gaussian surface, a rectangular box, through which the simple plane wave is traveling. The box encloses no electric charge. In order to satisfy Maxwell’s first and second equations, the electric and magnetic fields must be perpendicular to the direction of propagation; that is, the wave must be transverse. © 2016 Pearson Education, Ltd.