13.5 Conductor in a Magnetic Field The Motor Principle

Transcription

1 13.5 Conductor in a Magnetic ield igure 1 Determining the force on an electric conductor in a magnetic field Magnetic field of the permanent magnet Magnetic field of the current-carrying conductor (c) hape of the magnetic field when the fields in and are superimposed (d) The direction of the force on the conductor is away from the region of concentrated field lines Electric motors are used in cooling fans for computers, in refrigerators, in electric cars, in power tools such as electric drills, and even in many remote-controlled toys. Many subway systems use electric motors as a clean and silent way to power their trains. But what do electric motors have to do with magnetic fields? In 1821, following Oersted s discovery of electromagnetism, English physicist Michael araday ( ) set out to prove that, as a wire carrying electric current could cause a magnetized compass needle to move, so in reverse a magnet could cause a current-carrying wire to move. uspending a piece of wire above a bowl of mercury in which he had fixed a magnet upright, araday connected the wire to a battery, and the wire began to rotate. araday determined that the magnetic field of a permanent magnet can exert a force on the charges in a current-carrying conductor. igure 1 shows how the direction of this force is related to the magnetic field of the conductor and to the external magnetic field. (c) concentration of field lines (d) force on conductor motor principle: A current-carrying conductor that cuts across external magnetic field lines experiences a force perpendicular to both the magnetic field and the direction of electric current. The magnitude of this force depends on the magnitude of both the external field and the current, as well as the angle between the conductor and the magnetic field it cuts across. To the left of the conductor, the field lines point in the same direction and tend to reinforce one another, producing a strong magnetic field. To the right, the fields are opposed and, as a result, tend to cancel one another, producing a weaker field. This difference in field strength results in a force to the right on the conductor. If either the external field or the direction of the electric current were reversed, the force would act in the opposite direction. A more detailed investigation would show that the actual magnitude of the force depends on the magnitude of both the current and the magnetic field. These effects are summarized in the motor principle. Motor Principle A current-carrying conductor that cuts across external magnetic field lines experiences a force perpendicular to both the magnetic field and the direction of electric current. The magnitude of this force depends on the magnitude of both the external field and the current, as well as the angle between the conductor and the magnetic field it cuts across. 490 Chapter 13

2 13.5 The direction of the force on the conductor depends on the direction of electric current and the direction of the external magnetic field (igure 2). The direction of the force can be determined using what is called the right-hand rule for the motor principle. Right-Hand Rule for the Motor Principle If the fingers of the open right hand point in the direction of the external magnetic field, and the thumb represents the direction of electric current, the force on the conductor will be in the direction in which the right palm faces. right-hand rule for the motor principle: If the fingers of the open right hand point in the direction of the external magnetic field, and the thumb represents the direction of electric current, the force on the conductor will be in the direction in which the right palm faces. external magnetic field The motor principle is also used in the definition of the ampere. If two long wires are parallel to each other as in igure 3, and electric current is in each, the wires will experience a force, as shown in igure 4. One ampere is the current that, when flowing through each of two straight parallel wires placed 1 m apart in a vacuum, produces a force of between the wires for each 1 m of their length. direction of current force igure 2 The right-hand rule for the motor principle switch igure 3 automobile storage battery igure 4 If the electric currents in the parallel wires are in opposite directions, the force is repulsive. If the electric currents in the parallel wires are in the same direction, the force is attractive. The motor principle allows us to introduce the I definition of the coulomb, which states that 1 C is the amount of charge flowing past a point in 1 s when the current is 1 A. 1 C = 1 A s Electromagnetism 491

3 Questioning Hypothesizing Predicting Planning Conducting IQUIRY KILL Recording Analyzing Evaluating Communicating Investigation As its name suggests, the motor principle is the basis of operation of all electric motors, from the tiny ones used in toys to the massive ones used to propel electric commuter trains. To help you visualize how an electric motor actually works, it is wise to start by performing an investigation involving the motor principle. Question Under what conditions does a conductor experience an electromagnetic force? electric current metre stick clamp insulated wire Materials insulated wire (fine) utility stand, clamp, and metre stick 5-cm length of bare 12-gauge copper wire pair of bar magnets 6-V battery or DC power supply + utility stand bare copper wire Prediction Predict what will happen to the bare copper wire when it is placed between two bar magnets (i) perpendicular to the magnets, and (ii) parallel to the magnets when a current passes through the copper wire. igure 5 etup for Investigation igure 6 + Procedure 1. Using the insulated wire, retort stand, clamp, metre stick, and bare copper wire, set up the apparatus as shown in igure 5. Remove some insulation from the wire before attaching it to the bare copper wire. 2. Place the bar magnets so that the bare copper wire lies between opposite poles of the magnets, and parallel to a line joining them. 3. Connect the battery momentarily, noting any effect this has on the conductor. 4. Rotate the magnets by 90 o, so that the conductor now lies between the poles but perpendicular to the line joining them with one magnet above the conductor and one below it. 5. Reconnect the battery and observe any effect this has on the conductor. 6. Reverse the poles of the magnets. What effect does this have on the conductor? Reverse the connections to the battery and note any effects. 7. Wind the wire into a rectangular coil of 15 turns as shown in igure 6 and suspend the coil between the poles of the magnets. Connect the battery and note the effect this has on the coil. Reverse the battery connections and repeat. Analysis What happens to a current-carrying conductor when it is placed in a magnetic field so that it is (i) parallel to the magnetic field lines? (ii) perpendicular to the magnetic field lines? (c) What factors affect the direction of the force on the conductor? (d) What factors will affect the magnitude of this force? 492 Chapter 13

4 13.5 (e) What happens to the rectangular coil when there is electric current in it? Do all four sides of the coil experience a force? Explain. (f) What device does this simple coil and magnet simulate? Evaluation (g) How did your observations compare with your predictions? (h) Describe the usefulness of the right-hand rule for the motor principle in predicting the direction of the force on the conductor in this investigation. Can you apply the same right-hand rule to determine the direction of the force on the suspended coil? Explain. UMMARY The motor principle states that a current-carrying conductor that cuts across external magnetic field lines experiences a force perpendicular to both the magnetic field and the direction of electric current. The magnitude of this force depends on the magnitude of both the external field and the current, as well as the angle between the conductor and the magnetic field it cuts across. The right-hand rule for the motor principle states that if the fingers of the open right hand point in the direction of the external magnetic field, and the thumb represents the direction of electric current, the force on the conductor will be in the direction in which the right palm faces. ection 13.5 Questions Understanding Concepts 1. Copy each of the diagrams in igure 7 into your notes and then use them to do the following: Draw the magnetic fields of the permanent magnet and the conductor. Determine the direction of the force on the conductor. 2. Copy each of the diagrams in igure 8 into your notebook and then use them to do the following: Draw the magnetic fields around each conductor. Determine the direction of the force on each conductor. igure 8 Applying Inquiry kills 3. A student sets up a successful demonstration of the motor principle, but notices that the force on the conductor is very weak. What two changes could the student make to increase the force? (c) electric current (continued) igure 7 or question 1 Electromagnetism 493

5 4. When you look at the apparatus used to demonstrate the motor principle using a straight conductor (igure 5), you can imagine that the suspended bare copper wire might act like a swing. What would you do to get the wire to swing back and forth with a regular period of vibration? (With your teacher s approval, you might be able to try your design.) Reflecting 5. The explanation of the definition of the ampere is included in this section that discusses the motor principle. Explain why this is logical Applications of the Motor Principle The motor principle refers to a force acting on a conductor carrying a current in a magnetic field. It is the most important principle used in the development of electric motors. However, the development of electric motors is not the only application of the motor principle. The motor principle has also been applied in the development of devices such as loudspeakers for stereos and in ammeters and voltmeters. The Moving-Coil Loudspeaker A loudspeaker reproduces sound waves by rapidly moving a paper or plastic sound cone back and forth in response to electrical signals from an amplifier. igure 1 shows side and front views of a magnetically driven speaker. movable voice coil (attached to speaker cone) ring pole speaker cone electric current in voice coil igure 1 In a moving-coil loudspeaker, a movable coil is attached to the sound cone and placed over the central shaft of a tubular permanent magnet. The external magnetic field lines run radially from the outer tubular magnet to the central shaft. As a result, when electric current runs through the voice coil, it is in a magnetic field that is always perpendicular to it. central pole side view field lines of magnet end view (ield lines of the permanent magnet are always perpendicular to the current in the coil.) 494 Chapter 13