Vern J. Ostdiek Donald J. Bord

Chapter 8

Electromagnetism and EM Waves (Section 2)

8.2 Interactions Between Electricity and Magnetism • Consider the following items that we usually take for granted: • • • electric motors in hair dryers, automobiles, computer disc drives, elevators, and countless other devices; generators that produce most of the electricity we use; speakers, audio and videotape recorders, and high-fidelity microphones; and the waves that make radios, wireless telephones, radar, microwave ovens, medical x rays, and our eyes work

8.2 Interactions Between Electricity and Magnetism • • What do all of these have in common? • They all are possible because electricity and magnetism interact with each other in basic —and very useful —ways. The word

electromagnetic,

which appears dozens of times in this chapter, is perhaps the best indication of just how intertwined these two phenomena are.

8.2 Interactions Between Electricity and Magnetism • • Let ’s summarize and review the key aspects of electrostatics and magnetism: Electric charges produce electric fields in the space around them.

8.2 Interactions Between Electricity and Magnetism • An electric field, regardless of its origin, causes a force on any charged object placed in it.

8.2 Interactions Between Electricity and Magnetism • Magnets produce magnetic fields in the space around them.

8.2 Interactions Between Electricity and Magnetism • A magnetic field, regardless of its origin, causes forces on the poles of any magnet placed in it. • These statements have been worded in a particular way because, as we shall see, it is the electric and magnetic

fields

that are involved in the interplay between electricity and magnetism.

8.2 Interactions Between Electricity and Magnetism • There are three basic observations of the interactions between electricity and magnetism: • The first of the three observations is the basis of electromagnets.

Observation 1:

A moving electric charge produces a magnetic field in the space around it. • An electric current produces a magnetic field around it.

8.2 Interactions Between Electricity and Magnetism • • A single charged particle creates a magnetic field only when it is moving. The magnetic field produced is in the shape of circles around the path of the charge.

8.2 Interactions Between Electricity and Magnetism • For a steady (DC) current, which is basically a succession of moving charges, in a wire, the field is steady, and its strength is proportional to the size of the current and inversely proportional to the distance from the wire. • The field is quite weak unless the current is large.

8.2 Interactions Between Electricity and Magnetism • A current of 10 amperes or more will produce a field strong enough to be detected with a compass.

• Reversing the direction of the current in the wire will reverse the directions of the magnetic field lines.

8.2 Interactions Between Electricity and Magnetism • • Most applications of this phenomenon use coils — • long wires wrapped in the shape of a cylinder, often around an iron core The magnetism induced in the iron greatly enhances the magnetic field of the coil.

8.2 Interactions Between Electricity and Magnetism • The magnetic field of such a coil (when carrying a direct current) has the same shape as the field around a bar magnet.

8.2 Interactions Between Electricity and Magnetism • • This device is an

electromagnet.

It behaves just like a permanent magnet as long as there is a current flowing. • One end of the coil is a north pole, and the other is a south pole. Electromagnets have an advantage over permanent magnets in that the magnetism can be “ turned off ” simply by switching off the current. • Large electromagnets are routinely used to pick up scrap iron.

8.2 Interactions Between Electricity and Magnetism • • A coil with a length that is much greater than its diameter is called a

solenoid.

If an iron rod is partially inserted into a solenoid with a hollow core, the rod will be pulled in when the current is switched on: • The magnetic pole associated with the coil ’s field nearest the rod induces a magnetic field with the opposite polarity in the rod, thus exerting an attractive force on it.

8.2 Interactions Between Electricity and Magnetism • Solenoids are used in common devices for striking doorbell chimes, opening valves to allow water to enter and to leave washing machines, withdrawing deadbolts in electric door locks, and engaging starter motors on car and truck engines.

8.2 Interactions Between Electricity and Magnetism • • • Electromagnets are used to produce the strongest magnetic fields on Earth. Two factors contribute to stronger fields: • wrapping more coils around the cylinder and using a larger electric current The former suggests the use of thinner wire so that more coils can fit into the same amount of space. • But smaller wire requires smaller electric current so the wire does not overheat and melt.

8.2 Interactions Between Electricity and Magnetism • This limitation is overcome in

superconducting electromagnets.

8.2 Interactions Between Electricity and Magnetism • • When the wire used in an electromagnet is a superconductor, it can carry huge electric currents with no ohmic heating because there is no resistance. Very small superconducting electromagnets can generate very strong magnetic fields while using much less electrical energy than a conventional electromagnet.

8.2 Interactions Between Electricity and Magnetism • • Superconducting electromagnets do have limitations, though. • The superconducting state is lost if the temperature, electric current, or magnetic field strength exceeds certain values.

Most superconducting electromagnets now found in laboratories throughout the world must be kept cold with liquid helium (

T

= 4 K). • The added cost of the liquid helium system is offset by the high magnetic fields achieved and the great reduction in use of electric energy compared to conventional electromagnets.

8.2 Interactions Between Electricity and Magnetism • The polarity of an electromagnet is reversed if the direction of the current is reversed.

8.2 Interactions Between Electricity and Magnetism • • • An alternating current in a coil will produce a magnetic field that oscillates: • It increases, decreases, and switches polarity with the same frequency as the current. Such an oscillating magnetic field will cause a nearby piece of iron to vibrate. The oscillating magnetic field of a coil with AC in it is used in many common devices.

8.2 Interactions Between Electricity and Magnetism • • This first interaction not only explains how electromagnets work, but also gives us new insight into permanent magnets as well. Because electrons in atoms are charged particles in motion about the nucleus, they produce magnetic fields.

8.2 Interactions Between Electricity and Magnetism • The electrons have their own magnetic fields associated with their spin.

• In any unmagnetized material, the individual magnetic fields of the electrons are randomly oriented and cancel each other out.

8.2 Interactions Between Electricity and Magnetism • • In ferromagnetic materials, these fields can be aligned with one another by an external magnetic field; • The material then produces a net magnetic field. So we can conclude that moving electric charges are the causes of magnetic fields even in ordinary bar and horseshoe magnets.

8.2 Interactions Between Electricity and Magnetism • • This brings us back to a statement made at the beginning of Chapter 7: • electric charges are the cause of both electrical and magnetic effects We might regard electricity and magnetism as two different manifestations of the same thing — charge.

8.2 Interactions Between Electricity and Magnetism • The second observation helps us understand how things such as electric motors and speakers work.

Observation 2:

A magnetic field exerts a force on a moving electric charge. • Therefore, a magnetic field exerts a force on a current-carrying wire.

8.2 Interactions Between Electricity and Magnetism • • A stationary electric charge is not affected by a magnetic field, but a moving charge usually is. Note that this second observation is a logical consequence of the first: • Anything that produces a magnetic field will itself be affected by other magnetic fields.

8.2 Interactions Between Electricity and Magnetism • A curious characteristic of electromagnetic phenomena is that the effects are often perpendicular to the causes. • The direction of the magnetic field from a current-carrying wire is perpendicular to the direction the current is flowing.

8.2 Interactions Between Electricity and Magnetism • • Similarly, the force that a magnetic field exerts on a moving charge or on a current-carrying wire is perpendicular to both the direction of the magnetic field and the direction the charge is flowing. • • For example, if a horizontal magnetic field is directed away from you and a wire is carrying a current to your right, the force on the wire is

upward.

If the direction of the current is reversed, the direction of the force is reversed (downward). An alternating current would cause the wire to experience a force that alternates up and down.

8.2 Interactions Between Electricity and Magnetism • Electric motors —like those in hair dryers and elevators —exploit this electromagnetic interaction. • The simplest type of electric motor consists of a coil of wire mounted so that it can rotate in the magnetic field of a horseshoe-shaped magnet.

8.2 Interactions Between Electricity and Magnetism • • A direct current flows through the coil, the magnetic field causes forces on the sides of the coil, and the coil rotates. • Once the coil has completed half of a rotation, a simple mechanism reverses the direction of the current. • • This reverses the force on the coil, causing it to rotate another half turn. This process is repeated, and the coil spins continuously. Motors designed to run on AC can exploit the fact that the direction of the current is automatically reversed 120 times each second (60-cycles-per second AC, with two reversals each cycle).

8.2 Interactions Between Electricity and Magnetism • Liquid metals, such as the molten sodium used in certain nuclear reactors, can be moved through pipes using an electromagnetic pump that has no moving parts. • • If the metal has to be moved in a pipe that is oriented north –south, for example, a large electric current can be sent across the pipe —east to west perhaps. Then, if a strong magnetic field is directed downward through the same section of pipe, the current-carrying metal will be forced to move southward.

8.2 Interactions Between Electricity and Magnetism • • Several large-scale devices used in experimental physics make use of the effect of magnetic fields on moving charged particles. • High-temperature plasmas cannot be kept in any conventional metal or glass container because the container would melt. • Because plasmas are composed of charged particles, magnetic fields can be used to contain them in what is known as a

magnetic bottle.

This is one approach being employed in the attempt to harness nuclear fusion as an energy source.

8.2 Interactions Between Electricity and Magnetism • • In the absence of other forces, a charged particle moving perpendicularly to a magnetic field will travel in a circle: • the force on the particle is always perpendicular to its velocity and is therefore a centripetal force. An electron, proton, or other charged particle can be forced to move in a circle by a magnetic field and then gradually accelerated during each revolution. • Particle accelerators used for experiments in atomic, nuclear, and elementary particle physics, as well as those used for producing radiation for cancer treatments at some large hospitals, operate on this principle.

• 8.2 Interactions Between Electricity and Magnetism The world ’s highest-energy particle accelerator is the Large Hadron Collider (LHC) located along the border between France and Switzerland near the city of Geneva. • • This device comprises a circular tunnel with a diameter of 8.6 kilometers (5.3 miles) buried 50 to 175 meters beneath Earth ’s surface in which two counter-rotating beams of charged particles travel in a vacuum guided by superconducting magnets. The head-on collisions between the particles in these oppositely moving beams yield information about the fundamental forces and interactions in Nature.

8.2 Interactions Between Electricity and Magnetism • The third observed interaction between electricity and magnetism is used by electric generators. • Recall that the first observation tells us that moving charges create magnetic fields. • The third one is a similar statement about moving magnets.

Observation 3:

A moving magnet produces an electric field in the space around it. • A coil of wire moving through a magnetic field has a current induced in it.

8.2 Interactions Between Electricity and Magnetism • The electric field around a moving magnet is in the shape of circles around the path of the magnet. This circular electric field will force charges in a coil of wire to move in the same direction —as a current.

8.2 Interactions Between Electricity and Magnetism • • • The process of inducing an electric current with a magnetic field is known as

electromagnetic induction.

All that is required is that the magnet and coil move relative to each other. • If the coil moves and the magnet remains stationary, a current is induced. If the motion is steady in either case, the induced current is in one direction. • If either the coil or the magnet oscillates back and forth, the current alternates with the same frequency —it is AC.

8.2 Interactions Between Electricity and Magnetism • Electromagnetic induction is used in the most important device for the production of electricity: the generator. • • The simplest generator is basically an electric motor.

When the coil is forced to rotate, it moves relative to the magnet, so a current is induced in it.

8.2 Interactions Between Electricity and Magnetism • • • We might call this device a “ two-way energy converter.

” When electrical energy is supplied to it, it is a motor. • It converts this electrical energy into mechanical energy of rotation.

When it is mechanically turned (by hand cranking, by a fan belt on a car engine, or by a turbine in a power plant), it is a generator. • It converts mechanical energy into electrical energy.

8.2 Interactions Between Electricity and Magnetism • • This motor –generator duality is used in dozens of

pumped-storage

hydroelectric power stations. During the night when surplus electrical energy is available from other power stations, the motor mode is used to pump water from one reservoir to another that is at a higher elevation. • Most of the electrical energy is converted into “ stored ” gravitational potential energy.

8.2 Interactions Between Electricity and Magnetism • During the peak time of electric use the next day, water flows in the opposite direction, and the generator mode is used as the moving water turns the pumps (now acting as turbines) that turn the motors (now acting as generators), thereby producing electricity. • You might say that the system functions like a rechargeable gravitational battery.

8.2 Interactions Between Electricity and Magnetism • • • • Another application of this technology is

regenerative braking,

which is used in electric and hybrid vehicles. While accelerating and cruising, electric motors turn the wheels using electricity from batteries. During braking, the motors function as generators: • The wheels turn them, and the electricity that is generated can partially recharge the batteries. Instead of all of the vehicle ’s kinetic energy being converted into wasted heat —the case with conventional friction brakes —some of it is saved for reuse.

8.2 Interactions Between Electricity and Magnetism • • In summary, when electric charges or magnets are in motion, electricity and magnetism are no longer independent phenomena. The three observations given here are statements of experimental facts that illustrate this interdependence. • They can be demonstrated easily using a battery, wires, a compass, a large magnet, and a sensitive ammeter.

8.2 Interactions Between Electricity and Magnetism • • The fact that electricity and magnetism interact only when there is motion (and then the effects are perpendicular to the causes) is somewhat startling when compared to, say, gravitation and electrostatics. • Gravitational and electrostatic forces are always toward or away from the objects causing them, and they act whether or not anything is moving or changing. These basic yet surprising interactions between electricity and magnetism are crucial to our modern electrified society.

Concept Map 8.2