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PHY108 - Chapter 28

# PHY108 - Chapter 28 - Ch 28 Magnetic Fields Up to this...

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Ch. 28: Magnetic Fields Magnetic fields Force due to a magnetic field, Lorentz Force both for a single charged particle and for a current in a conductor Torque on a current loop due to a magnetic field. Up to this point we have only discussed the electrostatic force. We will now begin to discuss magnetism, starting from the magnetic force through which we observe magnetism. In this chapter we will discuss:

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Magnet Demo - Fe filings. What do we notice? Magnet Demo - Fe filings. What do we notice? A force force acts on the iron filings or the compass needles that aligns them and then they come to rest. The filings organize themselves along lines. These lines look similar to electric field lines we mapped out with grass seed for electric dipoles. By analogy with the electric field, we will associate a magnetic field magnetic field with these lines, and just as in the electric field, the density of magnetic field indicates the the density of magnetic field indicates the strength of the magnetic field. strength of the magnetic field. Magnetic materials have been known for almost 12,000 years. Previous observations of the attraction of stones for each other and for objects made of iron. Mysterious force was quantified beginning around 1600 by William Gilbert.
Magnetic Field Lines Magnetic Field Lines Like in case of electric field lines, magnetic filed lines are useful graphical representation of magnetic field in space. B The direction of the tangent to a magnetic field line at any point gives the direction of at that point. B P P B magnetic field line magnetic field lines P Q P B Q B The spacing of the lines represents the magnitude of -- the magnetic field is stronger where the lines are closer together, and conversely. B

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Magnetic Demo: compasses - what do we notice? Magnetic Demo: compasses - what do we notice? The field lines always begin and end on the same object. Recall how this occurs for an electric dipole . Here we have a MAGNETIC dipole with two “magnetic charges”. The positive field called the North Pole and the negative field called the South Pole . If we start to divide the material into smaller pieces, we always have BOTH a north pole AND south pole for each smaller piece. THERE ARE NO KNOWN MAGNETIC MONOPOLES. N S
Earth’s Magnetic Field Earth’s Magnetic Field Earth has magnetic field. The origin of Earth’s magnetic field is still unknown. Earth’s magnetic field can be detected using a compass, which represents a narrow, thin bar magnet. A compass’s north pole points towards geomagnetic North pole of Earth (in Arctic), and its south pole points towards geomagnetic South pole (in Antarctic). In northern hemisphere, the magnetic field lines of Earth generally point down into Earth. In southern hemisphere, the line generally point out of Earth.

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PHY108 - Chapter 28 - Ch 28 Magnetic Fields Up to this...

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