CHAPTER 20 SOLUTIONS TO PROBLEMS AND QUESTIONS

Citation: Jacqueline D. Spears and Dean Zollman, Instructor's Guide for The Fascination of Physics, (The Benjamin /Cummings Publishing Company, Inc., Menlo Park, CA 1985). Permission granted by the publisher.

A. Review of Chapter Material

A1. The answers to the "A1" questions are found directly in the text. Look for each term printed in boldface type.

A2. As with electrical charge, we define two types of magnetic poles to explain the repulsive and attractive forces. Different from electrical models we never find an isolated magnetic pole.

A3.See Figure

A4. In permanent magnets the magnetic domains remain lined up even in the absence of an external magnetic field. The fields of neighboring domains help keep the other domains in place. A temporary magnet has the domains lined up only when an external field is present to cause them to align. When the field is removed, the domains return to their random orientations.

A5. Oersted noticed that a compass needle would be deflected when a current was moving through a wire which was located near the compass. The deflection of the compass needle showed that a magnetic force acted on the needle when a current was in the wire. Thus, electric current must be related to magnetism.

A6. Moving electric charges (a current) create a magnetic field. Tracing this to the atomic level we can say that moving electric charges in the atom can cause a magnetic field. In the atom, moving electric charges are electrons orbiting the nucleus. These tiny currents give rise to the magnetic domains used to explain magnetism in materials.

A7. The current carrying wire has a magnetic field associated with it. This field can interact with other magnetic fields such as those produced by a magnet. An external magnetic field can exert a force on a current carrying wire.

A8. Faraday discovered that only when a magnetic field changed did it induce a current in a coil of wire. He connected a circuit to a battery and placed it near another circuit which had no battery but contained an instrument to measure currents moving in it. He found that a current could be induced in the second circuit only when he turned the current on or off in the first circuit.

A9. The induced voltage increases if the number of turns in the coil increases or if the rate of change of the magnetic field increases.

A10. Both generators and motors work on the basic principle that currents and magnetic fields interact. They both consist of coils of wire and magnets. In the generator a coil is turned in a magnetic field and a current is induced. In the motor a current is introduced into a coil which is in the presence of a magnetic field. The interaction between the coil and the current causes the coil to turn.

A11. The ratio of the number of turns in the secondary and the primary coils is the important factor in determining if a transformer is a step-up or step-down transformer. If the secondary has more coils than the primary, the transformer steps up the voltage. If it has fewer turns, the transformer steps down the voltage.

A12. The force exerted by a magnetic field on a moving electric charge will be in a direction which is perpendicular to the direction of the charges' motion and to the direction of the magnetic field. If the charges are inside a wire moving in the magnetic field, then the force will be along the direction of the wire and will create a current in it.

A13. A changing electric field creates a changing magnetic field which in turn creates a changing electric field, and so forth. This process causes a continual propagation of the changing fields through space. This propagation is the electromagnetic wave.

B. Using the Chapter Material

B1. (a) The two magnets attract each other. (b) The two magnets repel each other. (c) The unmagnetized iron becomes a temporary magnet and is attracted to the north pole. (d) Aluminum does not become a temporary magnet so no interaction occurs.

B2. You could move the permanent magnet around the piece of iron. When you round a point at which the iron and the magnet were repelled, you would know that you had found like poles. Iron filings would line up with the magnetic field of the magnetized iron and point to the poles of the magnetized material. The iron filings would become temporary magnets so they could not identify the type of pole. However, the permanent magnet could identify the pole. The part of the iron repelled by the north pole of the permanent magnet would be a north pole.

B3. Permanent magnets are composed of magnetic domains which are all lined up. When a material containing aligned magnetic domains is heated, the molecules in the domains begin moving more rapidly. This motion could cause the domains to lose their alignment, and thus cause the material to lose its magnetic properties.

B4. The compass lies in the same plane as the wire. The magnetic field produced by the current carrying wire is circular. In the plane of the wire, this field points upward. The compass needle is not free to rotate in the upward direction.

B5. We could increase the strength of the magnets in the motor, increase the current in the wire, or replace the simple loop of wire with a coil of many turns.

B6. While twirling the wire in the earth's magnetic field, you are moving charges (the electrons in the wire) through the earth's magnetic field. This motion creates a force on the electrons which causes them to begin moving along the wire. This motion produces a small electric current.

B7. Each of the electrons in this atom produces a magnetic field. However, because the electrons are moving in opposite directions, the magnetic fields are in opposite directions. Thus, the fields cancel and no net field is detected outside of the atom.

B8. As the magnet is dropping, it is accelerating due to the force of gravity. When it reaches the second coil it will be moving more rapidly than it was through the first coil. The rate of change of the magnetic field through the second coil will be greater because of this greater speed. Thus, the induced voltage will be greater as the magnet passes through the second coil.

B9. (Primary voltage/number of loops in the primary) = (secondary voltage/number of loops in the secondary). For this situation: 120 V/500 loops = 12 V/ ? loops. The number of loops in the secondary is 50.

B10. The generator produces electricity by moving a coil of wire through a magnetic field. The process is one of electromagnetic induction. One of the variables in determining the voltage produced by the generator is the rate of change of the magnetic field inside the coil. When the coil is turning fast, the rate of change is large, and high current and voltage are produced. When the coil is turning slowly, the rate of change is slow and the current and voltage are low. Thus, the lamp is bright at high turning rates and dim at slow turning rates.

C. Extensions to New Situations

C1. (a) When the switch is closed, a current moves through the wire and creates a magnetic field around it. In the coil the field is greater because the magnetic domains of the iron line up with the coil's field. (b) The magnetic field of the coil and the iron inside it cause the L-shaped iron to become a temporary magnet and be attracted to the iron in the center of the coil. (c) Each time one pushes down the switch, the L-shaped iron is attracted to the coil. Thus, one could send a message by sending a pattern of currents which caused the L-shaped iron to move up and down.

C2.(a)See Figure

(b) A coil of wire can create the same magnetic field as a magnet. We can replace each of the magnets above with a coil which is located at the same position as the magnet and which is oriented so that the coil's center lies in the same direction as the poles of the magnet. The electrical connections are such that the current moves through the wire to create fields in the same direction in each coil.

(c)See Figure

(d) In order for this motor to be used as a generator, the rotating coils would have to be moved in an external magnetic field. To create such a field, an external current would have to be supplied to the coils. Then, this motor could be used as a generator.

C3. (a) When the switch in circuit (a) is closed, a magnetic field exists inside the iron rod. This field causes the L-shaped iron to become a temporary magnet and to be attracted toward the iron rod. (b) The light will be on only when the switch at S is closed. When the switch is closed, the L-shaped iron is attracted to the rod. This action closes the circuit which includes the lamp. Thus, the lamp will light. (c) A low current can be used to activate the electromagnet which acts as a switch. Thus, the high current passes through the electromagnet but not the switch. We still have control over the current through the high-current circuit because we have control over whether the electromagnet is on or off.

C4. When the current moves through the coil, the iron becomes a temporary magnet. This magnet will be attracted by the magnetic field of the coil of wire. Thus, it will tend to move along the field lines which are in the center of the coll. As the fields line up, the "north" end of the coil and the "north" end of the rod will be at the same end of the system. These poles will repel each other so the rod would tend to move out of the coll.

C5. (a) A current will be induced in coil 2 only by a changing magnetic field. The current through coil 1 must change in order to produce the changing magnetic field. (b) The domains of the iron will line up with any field which is created by the current in coil 1. This field will add to the field of the current and produce a much stronger magnetic field than that created by just the current itself.

C6. The moving electrons are creating magnetic fields and thus will interact with other magnetic fields. Coils can be used to create magnetic fields which can apply forces to the electrons as they move by on their way to the TV screen. By positioning the coils carefully and controlling the current in them one can control the motion of the electrons.

C7. (a) The moving electrons can interact with the field of the magnet. This interaction causes them to deviate from the path which would produce a normal picture. (b) The material on the screen of a color television can become permanently magnetized. Once the magnet is removed, a magnetic area will remain on the screen. This area will cause the electrons to deviate from their normal path until the magnetism is removed from the screen. (c) The vacuum sweeper contains an electric motor. This motor produces magnetic fields as it is operating. The magnetic fields of the motor can cause a permanent magnetic field to be created on the surface of the screen.

C8. The steel can is sitting in the earth's magnetic field. The interaction between the earth's field and the magnetic domains of the steel can cause a gradual alignment of the domains with the earth's field. After a long enough time the steel will have some permanent magnetism.