From john.campbell at canterbury.ac.nz Wed Sep 3 16:13:16 2008 From: john.campbell at canterbury.ac.nz (John Campbell) Date: Thu, 04 Sep 2008 08:13:16 +1200 Subject: Ask A Scientist response re bar magnets Message-ID: Kirsten Franlin of Otago University responded in the ask a scientist programme as below. John Campbell Dr J A Campbell Physics Dept University of Canterbury Christchurch New Zealand john.campbell at canterbury.ac.nz www.phys.canterbury.ac.nz/people/campbell Author of Rutherford Scientist Supreme, Rutherford's Ancestors, and www.rutherford.org.nz aas p212 John Bull of St John's College, asked:- If I place two bar magnets with like poles facing they repel. However, if I place a 2cm wide bar of unmagnestised steel (a keeper) between them then both are attracted to the keeper, ie the attractive force to the keeper is greater than the repulsive force between the opposite poles. Why is this? Kirsten Franklin, a physicist at Otago University, responded. The question you?re asking here is really a very old one. The ability of magnetic materials like lodestone to attract iron has been known of and puzzled over since ancient times. The Arabian Nights tales mention a mountain that would tear the nails out of ships that came near. The Greek wise man Thales of Miletus believed that the attraction was due to the stones having a soul. A superstition from the Middle Ages was that a magnet could cure gout but garlic removed its power, although this could be restored with goat?s blood. It is a basic property of magnets that they have two poles, which we refer to as north and south. Opposite poles attract and like poles repel, so as you say, the bar magnets normally repel each other if you try to force the like poles together. However, a magnet will always attract an unmagnetised piece of iron or steel, and this is what is happening when both magnets are attracted to the keeper. The iron in the magnets and the steel keeper is special because it is what we call ?ferromagnetic?. A ferromagnetic material acts like a whole lot of tiny little bar magnets that tend to line up the same way as their neighbours. These form into little regions with the same alignment, called magnetic domains. The magnetisation of the material depends on the degree of alignment of the domains. In a permanent magnet, like the bar magnets, these domains are largely pointing in the same direction and stay that way; in unmagnetised iron there are as many domains pointing one way as the other. You?re no doubt familiar with how a magnetic field can move other magnets ? just think of how a compass needle twists to line up with the Earth?s magnetic field. If we put unmagnetised iron near another magnet, it will become magnetised as the iron atoms change their alignment, and the domains that line up with the magnetic field start to dominate. If you put the north pole of a bar magnet against the steel keeper, it will produce a south pole on the keeper?s surface, so the opposite poles attract and the keeper attaches to the magnet. If you can find some other bits of steel, you might like to experiment with making the middle piece thinner. How thin do you think it can be and still have both magnets stick? Send questions to Ask-A-Scientist, PO Box 31-035, Christchurch, or email -:- questions at ask-a-scientist.net From francis.Bryden at stcuthberts.school.nz Wed Sep 3 21:21:31 2008 From: francis.Bryden at stcuthberts.school.nz (Bryden, Francis) Date: Thu, 4 Sep 2008 13:21:31 +1200 Subject: teaching physics with computers Message-ID: <6AC36C5F0F8E364CBEC8178DD499F3EF01D73173@stcc-exchange.stcuthberts.local> I'm running an introductory course for physics teachers on using computers to teach physics. The emphasis is on data logging, but I'll also cover: Using animations Animated diagrams on powerpoint Using videoclips and photos. If you or anyone in your department is interested, please contact me. (details below) there are 4 places left. The course is on Friday 12th Sept, all day Thanks ................ Francis Bryden Francis Bryden HoD Physics St Cuthbert's College 122 Market Rd or Box 26 020 Epsom Epsom Auckland 1051 Auckland 1344 New Zealand New Zealand ph: 64 9 5204159 ext 7808 fbryden at stcuthberts.school.nz P Please consider the environment before printing this email -------------------------------------------------------- This message (and any associated files) is intended only for the use of the individual or entity to which it is addressed and may contain information that is confidential, subject to copyright or constitutes a trade secret. If you are not the intended recipient you are hereby notified that any dissemination, copying or distribution of this message, or files associated with this message, is strictly prohibited. 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