L 41 Magnetism 1.docx

1、L 41 Magnetism 1Electromagnetism 1FerromagnetismAll early studies of magnetism concentrated on the properties of permanent and temporary magnets.Magnetic iron oxide or lodestone occurs naturally. The fact that this mineral possesses the property of attracting iron has been known for many centuries.W

2、hen an iron object is being attracted it temporarily becomes a magnet - like the lodestone - and some harder materials, such as steel, can be magnetised permanently. Magnetic materials (or, more correctly, ferromagnetic materials) are those which are attracted by a magnet and can be made into a magn

3、et. These include iron, cobalt and nickel as well as certain alloys, which may include no ferromagnetic elements. Materials, which are not ferromagnetic, do have some magnetic properties but these are so small as to be difficult to measure. For example, inserting a sheet of copper, plastic or glass

4、between magnets makes almost no difference to the force between them. For most practical purposes non-ferromagnetic materials can be regarded as non-magnetic.The magnetic powers of a magnetised bar appear to be concentrated at points (or at least small regions) near its ends. These regions are calle

5、d magnetic poles. If a bar magnet is supported so as to be free to rotate about a vertical axis it aligns itself in a constant direction at a given point on the earth. This direction is approximately north-south geographically and so one basic application of magnetism is the magnetic compass.The pol

6、e near the end indicating north is called the north-seeking pole - normally abbreviated to north pole. The other pole is of course south. Thus there are two kinds of magnetic pole and it is easily shown experimentally that like poles repel and unlike poles attract.The Magnetic FieldA magnet can be t

7、hought of as being surrounded by a magnetic field - a region in which magnets (both permanent and temporary) will experience magnetic forces. The direction of this field is defined as the direction of the magnetic force on the north (seeking) pole of a compass needle. Thus the earths magnetic field

8、is directed approximately south to north geographically.A magnetic field can be represented diagrammatically by drawing lines of magnetic flux (also known as lines of magnetic force or just magnetic field lines).A magnetic flux line is one whose shape is such that the field is tangential to it at an

9、y point. To these lines we add arrows to show the general direction. The diagram shows the magnetic field around a bar magnet.In accordance with our definition of the direction of the field the flux lines go inwards to south poles and emerge from north poles. The lines are also shown more closely sp

10、aced where the field is stronger. Not shown is the field within the magnet. Internally the lines will run from S to N, the field will be at its strongest and approximately uniform. (Equally spaced parallel lines of flux).ElectromagnetismIn 1820 Oersted showed that a current-carrying wire is surround

11、ed by a magnetic field. Thus it was established that there is a close connection between electricity and magnetism. After Oersteds discovery development of the subject of electromagnetism was very rapid. Important advances were made by a number of people - most notably Ampere and Faraday. By the mid

12、 1860s Maxwell was able to account for all the known phenomena of electricity and magnetism in one unified theory. However, the modern explanation of electromagnetism was only possible after the publication of Einsteins theory of relativity. It is now known that:All magnetic effects are electrical i

13、n origin and are caused by moving charges. That is, the fundamental source of magnetic fields and forces is moving electric charges and the fundamental effect of a magnetic field is to exert a force on a moving chargeThe familiar bar magnet turns out to be a very complex example of magnetic behaviou

14、r. It was well into the twentieth century before a satisfactory explanation of ferromagnetism, using physics beyond the scope of this course, was possible.Definition of Magnetic Field VectorSuppose a particle carrying a charge +Q travels into a magnetic field with velocity v at an angle to the field

15、 as shown.The moving charge experiences a magnetic force F.The magnitude of this force is found experimentally to be proportional to the magnitude of the charge and to the component of the velocity of the charge at right angles to the field. That is,.We define the magnitude of the magnetic induction

16、, B, as the magnitude of the magnetic force per unit charge moving with unit velocity at right angles to the field.That is,.Magnetic force on a moving charge The SI unit of B is the tesla (T).The direction of B was defined earlier as the direction of the magnetic force on an N pole placed in it.B is

17、 a measurable quantity whose value tells us just how strong a magnetic field is. A much better name for B would be magnetic field strength or magnetic intensity.Some basic books do use these terms for B. However they are not strictly correct as magnetic intensity and magnetic field strength are name

18、s used for another magnetic vector introduced in more advanced work. Correct names for B are magnetic induction and magnetic flux density.Direction of the Magnetic ForceThe direction of the magnetic force is at right angles to both v and B. That is, it is normal to the plane defined by v and B.In th

19、e case shown in the diagram above F would be normal to the plane of the paper and directed inwards.It may seem strange that the force produced by a magnetic field is at right angles to the direction of the field. (No, lets face it, it is strange.) This is an unfortunate result of defining the direct

20、ion of the field in terms of magnetic poles rather than moving charges. This is the historical definition and it would be too confusing to change it.There are a number of equivalent rules for remembering how magnetic field direction, charge motion and magnetic force are related. One such rule is:Rig

21、ht-hand palm rule for direction of magnetic force on moving charge or currentPoint the fingers of your right hand in the direction of the magnetic field component perpendicular to the motion and your thumb in the direction of positive charge motion (or conventional current). The direction of the mag

22、netic force is found by pushing with the palm of your hand.If you have already mastered a different rule and feel comfortable with it you should continue to use it. The diagram illustrates the application of the right hand palm rule.Magnetic Force on a Wire Carrying a CurrentConsider a wire carrying

23、 a current I at an angle to a magnetic field B as shown.Using palm (or other) rule there is a magnetic force straight up out of the page.Suppose the moving charge per unit length is q, moving with an effective velocity (the mean drift speed).Then the force per unit length on the wire is .However is

24、the charge that flows past a fixed point on the wire per unit time.i.e. Torque on a Current-carrying LoopSuppose a single rectangular loop of wire carrying a current, I, is situated with its plane in a magnetic field B as shown.We see that the loop is subject to a torque that is anticlockwise as vie

25、wed from above because of the couple produced by the magnetic forces.The turning moment or torque of a couple is the same about all points in its plane.eg. For P = (d + x) F x F = d F And this does not depend on x.FFdxPIf there are N loops each one experiences this torque and = NAIBIf the coil rotat

26、es under the action of this couple the forces do not change but the distance between their lines of action falls to zero as the angle becomes 90o. After some oscillation the coil ends up in equilibrium with its plane perpendicular to the field. Examples1. Determine the magnitude and direction of the

27、 magnetic force on the moving charge in each of the following cases.(a) (b) (c) (d) (e) Note: An particle is 2. Terrestrial MagnetismThe earths magnetic field is not usually horizontal or true north-south. The following diagram shows the usual situation in the southern hemisphere. The angle between

28、the horizontal and the earths magnetic field (an angle in the vertical plane) is called the inclination. In the northern hemisphere this angle is usually below the horizontal and is called the dip.The angle between true north and magnetic north (the direction of the horizontal component of the earth

29、s magnetic field) is called the deviation or declination.In the diagram the deviation or declination is to the east.In Sydney the earths magnetic field is specified as follows: Flux density 56.0T inclination 63.0 declination 11.0E.Calculate:(a) the magnitude and direction of the horizontal component

30、 of the earths magnetic field;(b) the magnitude and direction of the vertical component of the earths magnetic field.(c) the component of the earths magnetic field in the direction true north.3. An apparatus to demonstrate the magnetic force on a current carrying wire consists of a swing made out of

31、 copper wire, through which a current passes as shown in the diagram.The swing is deflected inwards towards the back of the magnet.(a) Which pole of the magnet is north? Explain your answer.(b) A very sensitive spring balance attached to the horizontal part of the wire registers a pull of 0.05 N per

32、pendicular to the wire when the current is 1.5A. The width of the magnet is 40 mm. What is the magnetic induction in the gap between the magnets poles?4. A rectangular coil, XYZW has a side WZ of length 0.075 m. The coil is suspended from a spring balance with its plane at right angles to a uniform magnetic field as shown.The table shows how the reading on the spring balance varies with current through the coi