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# Feb16note - PHYS142 Lecture 18 Magnetic Fields Chapter 29...

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Magnetic Fields • Chapter 29 All sections except 6 29.1 Magnetic Fields and Forces 29.2 Motion of a Charged Particle in a Uniform Magnetic Field 29.3 Applications Involving Charged Particles Moving in a Magnetic Field 29.4 Magnetic Force Acting on a Current-Carrying Conductor 29.5 Torque on a Current Loop in a Uniform Magnetic Field PHYS142 Lecture 18 1

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Magnets produce/generate magnetic fields (Chap. 30) The magnetic field is a vector quantity The letter B is used to denote the magnetic field 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. Magnetic fields 5
Using a compass we can draw the magnetic field lines of a magnet. 7 Magnetic field line

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N S N S N S 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 8
There is a magnetic field inside the magnet The magnetic field lines are continuous 9 Magnetic field line

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Magnetic Force Similar to the treatment or definition for the electric field , where the E-field was given as: We can define the B-field in the same way by replacing the “electric charge”, q , by the “ moving electric charge”, qv . qvB F qv F B or This is the magnetic force applied on the charge q which is moving with velocity v perpendicular to the magnetic field B . q F E el charge Force 11
. quantities vector are and , B v F + q θ Direction of Magnetic Force v F B 12

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B v v = v sin The magnitude of the magnetic force is given by sin qvB F Magnitude of Magnetic Force Experimental results show that: 13
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Feb16note - PHYS142 Lecture 18 Magnetic Fields Chapter 29...

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