chapter29

# chapter29 - Chapter 29 Magnetic Fields 1. 2. 3. 4....

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Chapter 29 Magnetic Fields 1. Introduction to magnetic field. 2. The forces on moving charges and currents inside a magnetic field. 3. The math that will needed is vector product. 4. The source of magnetic field will be discussed in later chapters.

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A whole picture helps Charge q as source Current I as source Electric field E Magnetic field B Gauss’s Law Ampere’s Law Force on q in the field Force on or I in the filed a q v Ampere-Maxwell Law Faraday’s Law Summarized in Maxwell equations = a a q F E = × a a a q F v B
Math = × a a a C A B Vector cross product: θ a A a B = × a a a C A B Vector cross product: = × = - a a a a D B A C θ a A a B = × a a a D A B ( ) sin = a C AB θ C Determine the direction. If Magnitude of the vector : a C = × a a a B F v B The right-hand rule: 1. Four fingers follow the first vector. 2. Bend towards the second vector. 3. Thumb points to the resultant vector.

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The Magnetic Field c The field surrounds a magnet is called the magnetic field. The field is a vector, and is symbolized by c Magnet exists in nature. c Any magnets have two poles, called the north pole and the south pole. Like poles (from different magnets) repel, unlike poles attract. B a Like field lines in electric field, magnetic field lines are used to illustrate the field. Outside a magnet , field lines start from the north pole, end at the south pole. Field lines can be traced out by a small compass.
Units of Magnetic Field c The SI unit of magnetic field is the tesla (T) c Wb is a weber c A non-SI commonly used unit is a gauss (G) c 1 T = 10 4 G 2 ( / ) Wb N N T m C m s A m = = =

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Magnetic field lines of bar magnets, shown by iron filings. Field lines of one magnet of N and S poles of N and N poles Magnetic field Electric field Comparison: there exist electric monopoles, the point charges. Magnet monopoles do not exist (have not been found). No matter how small a magnet is, it has two poles, N and S.
What generates the magnetic field? c Current (or moving charges, or changing electric field) generates magnetic field. c We will get back to this topic in the following chapter. c Magnet can take the form of a permanent magnet (ex. the bar magnet) or a solenoid. c The Earth itself is also a big magnet.

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Magnetic force on c Moving charge c CRTs (old TV tube) c Particle accelerator c Particle mass spectrometer c Particle detection and homeland security c Current carrying conductor. c Electric motor c Hall effect A CRT (Cathode ray tube) A cyclotron accelerator
Here is the magnetic force Magnetic force on moving charge B q = × F v B a a a B F a is the magnetic field B a The formula : q is the charge a v is velocity of the charge The direction of the force is determined by the charge and the

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## This note was uploaded on 05/06/2008 for the course PHYS 1304 taught by Professor Ye during the Spring '08 term at SMU.

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chapter29 - Chapter 29 Magnetic Fields 1. 2. 3. 4....

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