Review 3

# Review 3 - Soft and hard magnetic materials Magnetism...

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1 Magnetism • Chapter 20 (Giancoli) • All sections except section 4 (Ampère’s law) Soft and hard magnetic materials A compass is a permanent magnet. The word permanent implies that it is hard to destroy the magnetic properties of the material. A compass needle should be made from a hard magnetic material. On the other hand, it is easy to lose the magnetic properties of a soft magnetic material. A magnet, such as a compass has a north (N) and a south (S) pole. S If the magnet is free to rotate, its N-pole will point towards the Earth’s geographic north pole and its S-pole will point towards the Earth’s geographic south pole. N Magnetic poles: An important rule in magnetism: “Opposite poles attract, like poles repel. N S N S N S N S Magnets produce/generate magnetic fields The magnetic field is a vector quantity The letter is used to denote the magnetic field B K How to determine the direction of the magnetic field at any point in space? We place a small ( imaginary ) compass at the point of interest. The direction of the magnetic field at that point is the direction in which the N-pole of the compass points. Example : Determine the direction of the B-field at point P magnet P P S N Place the small compass at point P S N S N Since like poles attract the compass will rotate until its S-pole is near the N-pole of the magnet. The direction of the B-field at point P is given by the direction of the N-pole of the compass. B

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2 Using a compass we can draw the magnetic field lines of a magnet. N S N S N S Is there a magnetic field inside the magnet? If we cut the magnet in half we get two magnets. An imaginary compass placed between the two magnets gives the direction of the B-field inside the magnet. S N B One can make the cut anywhere, in each case the B-field points from the north pole towards the south pole. N S N S N S N S There is a magnetic field inside the magnet The magnetic field lines are continuous The magnetic field of the Earth The N-pole of a compass points toward the geographic north pole of the Earth. This means that the Earth’s magnetic south pole is near its geographic north pole. Earth compass N S Magnetic North Pole Magnetic South Pole Geographic South Pole Axis of rotation Geographic North Pole B Origin of Magnetism Magnetic fields are generated by moving charges. i.e. an electric current For a straight current, the B-field produced is in a plane perpendicular to the current The direction of the B-field (clockwise or counter-clockwise) is determined from the simple right hand rule (RHR) Simple RHR Imagine holding the wire carrying the current with your right hand, the thumb in the direction of the current. Your fingers will then give the sense of direction of the magnetic field B.
3 The simple right hand rule applies to many current geometries.

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## This note was uploaded on 04/07/2008 for the course PHI 102 taught by Professor Altonian during the Winter '08 term at McGill.

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Review 3 - Soft and hard magnetic materials Magnetism...

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