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Average: 27.7/40 = 69% Top grade: 37/40 = 92.5% I “curve”: roughly , the corresponding letter grades are: A+ >92, A 85—92, A- 80—84, B+ 75--79 , B 65—74, B- 60— 64, C+ 55—59, C 50—54, D 45—49, F < 45 0 5 10 15 20 25 30 35 40 Make sure you go through your test and solutions carefully and understand where you went wrong. Ask me at office hours if needed. Note that your final score can go up considerably from improved performance on the final exam (repeated questions) and from lab/attendance contributions. For this, you must do this and start studying asap!

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Chapter 24: Magnetism
Magnetism Magnetic Forces What is the origin of the magnetic force? Moving charged particles (more later). Eg. Orbiting electrons in the atoms making up a magnet. Stationary charged particle – produces electric field Moving charged particle – produces both an electric field and a magnetic field. Actually, electrical and magnetic forces are intimately related through relativistic considerations – different manifestations of “electromagnetism”.

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Magnetic Poles Magnetic poles – from which magnetic forces emanate. Two types: north and south Every magnet has both a north and a south pole Like poles repel each other; unlike poles attract. Simple compass: Suspend bar magnet from its center by a piece of string. Convention is that the north pole points northward; south pole points southward. This means that the Earth’s “north pole” is actually a magnetic south pole! And vice-versa. N S Eg. Bar magnet
Magnetic Poles cont. Another type: Horseshoe magnet Fridge magnets – have narrow alternating N and S strips. Strong field near the magnet but field decays quickly with distance since N and S fields cancel. Magnetic poles cannot be isolated. (Big difference with electric charge) eg if break bar magnet in two, each half behaves as complete magnet, each with N and S poles. Even when it’s one atom thick! No magnetic monopoles .

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Magnetic Fields Iron filings sprinkled around bar magnet align with the magnetic field. (c.f. electric field lines earlier) Field is stronger where lines are more dense Actually magnetic field lines form closed loops – they continue inside the magnet (not shown in pic) Eg. Two bar magnets with opposite poles at same end Eg. Two bar magnets with like poles at same end.
Compared to the huge force that attracts an iron  tack to a strong magnet, the force that the tack  exerts on the magnet is 1. relatively small. 2. equally huge. Clicker Question

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2. equally huge. Compared to the huge force that attracts an iron  tack to a strong magnet, the force that the tack  exerts on the magnet is Answer: 2 The pair of forces between the tack and magnet comprises a single interaction and both are equal in magnitude and opposite in direction—Newton’s third law. Because of its
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