DO PHYSICS ONLINE MOTORS AND GENERATORS MAGNETIC FIELDS

Transcription

1 DO PHYSICS ONLINE MOTORS AND GENERATORS MAGNETIC FIELDS Powerful magnets are essential components in motors and generators. Some electric motors and generators rely upon a combination of a permanent and an electromagnet. An electric motor converts electrical energy into mechanical energy by the torque acting on a conductor carrying an electric current in a magnetic field. A generator converts mechanical into electrical energy by moving a conductor through a magnetic field. ut, what is a magnet and what is meant by a magnetic field? Magnetic phenomena have been known for thousands of years. Certain elements are magnetic such as iron, cobalt and nickel. Strong magnets can now be made from certain alloys including (neodymium iron boron) and (alnico - aluminium, iron, cobalt and nickel). Flexible magnets can be made from ceramic material. Magnets can attract ferromagnetic materials such as iron. Magnets can attract or repel other magnets as shown in figure (1). N S N S opposite poles attract each other N S S N S N N S similar poles repel each other Fig. 1. Forces between a pair of bar magnets. 1

2 To explain these magnetic forces, we introduce the concept of magnetic field. A field is a region of influence. For example, an object because of its mass experiences a force in a gravitational field. A charged object experiences a force in an electric field. A field for a vector quantity is shown by a pattern of lines. The lines indicate the field pattern and the density of the lines indicates the strength of the field. The closer the field lines are together, then the stronger the field and hence the stronger the force. A magnet has a north pole and south pole. The magnetic field of a magnet can be shown by a set of continuous loops that exit from the north pole of the magnet and enter at the south pole as shown in figure (2), (3) and (4). The - field lines indicate how a small magnet (or compass) will align itself in the field. strong field: high density of -lines S N weak field: low density of -lines Fig. 2. Magnetic field lines for a bar magnet. The magnetic field lines are continuous loops. The field lines exit away from the north pole and enter the magnet at the south pole. The magnetic field is strongest where the field lines are closest together at the two poles. S N compass needles / magnets align along -field 2

3 Fig. 3. Magnetic field lines for a bar magnet showing the external magnetic field only and the magnetic field pattern due to iron filings. The iron filings align themselves as if they were small magnets and more of them accumulate where the -field is strongest. N S the magnetic field in the region between the poles is nearly uniform Fig. 4. Magnetic field lines for a horse-shoe magnet. The magnetic field lines are continuous loops. The field lines exit away from the north pole and enter the magnet at the south pole. The Earth has a magnetic field surrounding it. The field lines are similar to a huge bar magnet. The south pole of the magnet is located near the geographical north pole and the north magnetic pole near the south geographical pole as shown in figure (5). south magnetic hole north magnetic hole Fig. 5. The Earth is surrounded by a magnetic field. 3

4 Figure 6 show that if you break a magnet into pieces you create smaller magnets. Fig. 6. reaking a magnet produces new smaller magnets. 4

5 ELECTRIC CURRENTS AND MAGNETISM When electric charges are in motion they exert forces on each other that can t be explained by Coulomb s law. If two parallel current carrying conductors are near each other they attract each other when the currents are in the same direction and repel each other when the currents are in opposite directions. Such forces are called magnetic forces. Hans Oersted ( ) placed a wire near a compass needle and switch on the current. When the wire was parallel to the compass needle, the compass needle was deflected by the current. When the wire was perpendicular to the compass needle, there was no deflection when the current was switched on. He made two conclusions: (1) the electric current somehow exerts a twisting force on the magnet near it and (2) the magnitude of the force depends upon the relative orientation of the current and the magnet. A current (moving charges) through a wire alters the properties of space near it such that a piece of iron will experience a force. Hence, surrounding a wire carrying a current is a magnetic field. It is the interaction of the magnetic field and the iron that leads to the force, rather than the current and iron acting upon each other. The magnetic field surrounding a straight conductor carrying a current I can be visualized as a series of circles. The closer the lines are together, the stronger the magnetic field. A compass placed near the wire will align itself with the field lines. The direction of the magnetic field is determined by the right hand screw rule. Using the right hand: the direction of the thumb represents the current (direction in which positive charges would move) and the curl of the fingers represents the direction of the magnetic field as shown in figure (7). The magnetic field strength is given by the vector quantity where stands for the -field or magnetic induction or magnetic flux density. The S.I. unit for the - field is the teslas [T]. I Fig. 7. The right hand screw rule is used to determine the direction of the magnetic field produced by moving charges. 5

6 The -field surrounding a conductor carrying a current I at a distance R from the conductor is given by equation (1) as shown in figure (8) 0 I (1) = 0 = T.m.A -1 2 R where 0 is a constant called the permeability of free space. direction of -field given by right hand rule -field increases with increasing I R -field decreases with increasing R I current I into page Fig. 8. -field surrounding straight conductor carrying a current. R A conductor carrying a current produces a magnetic field similar to a bar magnet as shown in figure (9). current I -field Fig. 9. Magnet field due to a current loop. The direction of the -field is given by the right hand screw rule. -field stro strong ng -field - field A solenoid is a conductor wound into a long set of coils 6

7 Solenoids are often used as electromagnets where a ferromagnetic substance placed inside the coils greatly increases the strength of the magnetic field. Figure (10) shows the magnetic field patters for an air filled solenoid and when a rod of ferromagnetic material is placed inside the coils. Fig. 10. Magnetic field patterns for a solenoid (air and ferromagnetic cores). The magnetic field of a solenoid is very similar to that of a bar magnet as shown in figure (11). Fig. 11. Magnetic field patterns for a bar magnet and solenoid. 7

8 MAGNETIC FLUX [T.m -2 ] A useful quantity is to consider the number of magnetic field lines crossing an area. This concept is called the magnetic flux and is illustrated and defined in figure (12). Magnetic flux for constant -field Acos Each point on a surface is associated with a direction, called the surface normal; the magnetic flux through a point is then the component of the magnetic field along this direction A 90 o cos 1 A Acos 0 cos 0 0 Fig. 12. Magnetic flux when the magnetic field is uniform over an area A. o ORIGIN OF THE MAGNETIC FORCE Permanent Magnets A moving charge gives rise to a magnetic field. An electron is not a spinning or orbiting particle, but to account for the magnetism of materials it is useful to view the electron as a charged particle spinning as it orbits the nucleus. Every electron, on account of its spin, is a small magnet. In most materials, the countless electrons have randomly oriented spins, leaving no magnetic effect on average. However, in a bar magnet many of the electron spins are aligned in the same direction, so they act cooperatively, creating a net magnetic field. In addition to the electron's intrinsic magnetic field, there is sometimes an additional magnetic field that results from the electron's orbital motion around the nucleus. This effect is analogous to how a current-carrying loop of wire generates a magnetic field. Ordinarily, the motion of the electrons is such that there is no average field from the material, but in certain conditions, the motion can line up so as to produce a measurable total field as in iron. 8

9 Origin of magnetic effects Consider a charged particle A with its electric field surrounding it. Another charged placed in this electric field will experience an electric force as described by Coulomb s law. ut what happens when the charge particle A moves. About 1880 the famous English scientist James Clerke Maxell stated that the charge was not instantaneous aware that charge A had moved but the change in the electric field due to A moving, spreads out from A at the speed of light. The time required for the influence of the electric field to travel causes a time lag. This time lag for the change to be travel or due to the fact that one electric charge does not influence another instantaneously gives rise to the magnetic force. Thus, the force between two moving becomes dependent upon their speeds. At any instant of time one charge is feeling the electric field of influence of the other, not where it is now, but where it was a short time before. Hence, all magnetic effects are simply magnetic effects. 9

10 Common uses of magnets Magnetic recording media: VHS tapes and audio cassettes contain a reel of magnetic tape. The information that makes up the video and sound is encoded on the magnetic coating on the tape. Computers, floppy and hard disks record data on a thin magnetic coating. Credit, debit, and ATM cards: have a magnetic strip on one side. Common televisions and computer monitors: TV and computer screens containing a cathode ray tube employ an electromagnet to guide electrons to the screen. Speakers and microphones: Most speakers employ a permanent magnet and a current-carrying coil to convert electric energy (the signal) into mechanical energy (movement which creates the sound). The coil is wrapped around a bobbin attached to the speaker cone, and carries the signal as changing current which interacts with the field of the permanent magnet. The voice coil feels a magnetic force and in response moves the cone and changes the pressure the neighbouring air, thus generating sound. Dynamic microphones employ the same concept, but in reverse. A microphone has a diaphragm or membrane attached to a coil of wire. The coil rests inside a specially shaped magnet. When sound vibrates the membrane, the coil is vibrated as well. As the coil moves through the magnetic field, a voltage is induced across the coil. This voltage drives a current in the wire that is characteristic of the original sound. Medicine: Hospitals use Magnetic Resonance Imaging (MRI) to spot problems in a patient's organs without invasive surgery. Transformers: are devices that transfer electric energy between two windings of wire that are electrically isolated but are coupled magnetically. Compasses: is a magnetized pointer free to align itself with a magnetic field, most commonly Earth's magnetic field. Art: Vinyl magnet sheets may be attached to paintings, photographs, and other ornamental articles, allowing them to be attached to refrigerators and other metal surfaces. Toys Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold. Some screwdrivers are magnetized for this purpose. Magnets can be used in scrap and salvage operations to separate magnetic metals (iron, steel, and nickel) from nonmagnetic metals (aluminium, non-ferrous alloys, etc.). The same idea can be used in the so-called magnet test, in which an auto body is inspected with a magnet to detect areas repaired using fiberglass or plastic putty. Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels trains through electromagnetic force. The maximum recorded speed of a maglev train is 581 km.h -1. Health: human tissues have a very low level of susceptibility to static magnetic fields, there is little mainstream scientific evidence showing a health hazard associated with exposure to static fields. Dynamic magnetic fields may be a different issue however; correlations between electromagnetic radiation and cancer rates have been postulated due to demographic correlations. If a ferromagnetic foreign body is present in human tissue, an external magnetic field interacting with it can pose a serious safety risk. A different type of indirect 10

11 magnetic health risk exists involving pacemakers. If a pacemaker has been embedded in a patient's chest (usually for the purpose of monitoring and regulating the heart for steady electrically induced beats), care should be taken to keep it away from magnetic fields or metal detectors at airports. It is for this reason that a patient with the device installed cannot be tested with the use of an MRI, which is a magnetic imaging device. Children sometimes swallow small magnets from toys; and this can be hazardous if two or more magnets are swallowed, as the magnets can pinch or puncture internal tissues; one death has been reported. Motors and generators P6354 P6428 P