Inspiring experiments exploit strong attraction of magnets

More documents

Recommendations

Info

Figures 3 and 4. The rolling wire (top image): a demonstration of the force on a conductor achieved by rolling a thick piece of brass rod along a pair of brass curtain rails using an Alnico magnet held above the wire to produce a vertical field. The kicking wire (bottom): a large Alnico U magnet is used to produce a vertical field at rightangles to the current and to the subsequent motion. the expense of the large Alnico magnet, but a similar effect can be obtained with magnadur magnets. With neodymium iron boron magnets a kicking wire can be set up for less than £5 by mounting a single round neodymium magnet in a hole in a block of wood and using a trapeze-style copper frame (figure 5). Double magnet roller We can use neodymium disc magnets as both the wheels of a roller and the source of the magnetic field. In this set-up (figure 6) two discs are placed carefully on the smoothed, flat end of a nail with its head cut off. It is important that the nail axle is aligned exactly with the centres of the magnets to produce a good run. The strength of the neodymi- July 2006 F RONTLINE Figure 5. Kicking wire set-up using a neodymium magnet and a copper wire trapeze frame. The wire carries the current across the top of the magnet and the force causes the frame to move. nail shaft aluminium foil neodymium magnet magnetic field inside magnet current Figure 6. Rolling motor (shaft of nail) with two neodymium disc magnets, with explanation. ums enables them to be fixed with like poles facing each other. This is essential to produce a turning force on both wheels in the same sense. The tracks are two strips of aluminium foil, stuck to the bench with overhanging ends for electrical contacts. The current flows down the aluminium and up through one magnet, along the nail and down through the P HYSICS E DUCATION 293
F RONTLINE Figure 7. Another simple motor that uses a neodymium magnet with a bolt through its centre. Figure 9. ‘Loudspeaker’with cardboard box. other. The forces arise within the magnetic wheels and the arrangement accelerates down the track. Other motors In figure 7 a ring magnet has a bolt through its centre and lies on a copper circuit board. The magnet makes contact outside a circle etched in the board and a pin though the centre enables contact to be made there. In figure 8 a strip of aluminium foil rotates rapidly on top of the magnets. Contact is made to the central part of the rotating foil with some pencil lead, while the foil touches the outside of the magnets. Loudspeaker The strength of the neodymium magnet can be shown with a simple ‘loudspeaker’(figure 9). Acoil of thin copper wire is taped to the surface of a shoe box with the leads connected to the earphone output of a portable hi-fi. Simply bring up a neodymium magnet and the forces on the coil move it to produce sound from the box. Varying the position of the magnet and hence the strength of the field at the coil greatly affects the volume of the sound. Figure 8. A motor arrangement using a piece of aluminium foil rotating rapidly on top of magnets. Figure 10. A simple but effective loudspeaker unit that uses a piece of aluminium foil over a large neodymium magnet and passes the output from a radio through the aluminium foil layer. Even simpler is to lay a small piece of aluminium foil over a large neodymium magnet and pass the output from a radio through the foil (figure 10). The sound from the radio is clearly heard. This set-up can also be used as a simple microphone. You may by now be thinking that neodymium magnets are the best thing since sliced bread, but with their strength come disadvantages. Neodymium iron boron is susceptible to corrosion if unprotected, though this will not be of great concern to us because commercially available magnets are coated. Curie temperature Of more concern is the Curie temperature. At only 120 ºC neodymium magnets cease to be magnetic, although this loss can be recovered by remagnetization. The comparable temperature for ferrite magnets is 250 ºC and for Alnico it is 550 ºC. The loss in magnetism in neodymium is irreversible at 320 ºC compared with 860 ºC for Alnico. This low Curie temperature for neodymium can be demonstrated simply by using a tealight candle as a heat source (figure 11). A small neodymium 294 P HYSICS E DUCATION July 2006
Page 1: F RONTLINE M AGNETISM Inspi

Inspiring experiments exploit strong attraction of magnets

Create successful ePaper yourself

Delete template?

Save as template?