chapter29

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

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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 arrowrightnosp q v Ampere-Maxwell Law Faraday’s Law Summarized in Maxwell equations = arrowrightnosp arrowrightnosp q F E = × arrowrightnosp arrowrightnosp arrowrightnosp q F v B
Math = × arrowrightnosp arrowrightnosp arrowrightnosp C A B Vector cross product: θ arrowrightnosp A arrowrightnosp B = × arrowrightnosp arrowrightnosp arrowrightnosp C A B Vector cross product: = × = - arrowrightnosp arrowrightnosp arrowrightnosp arrowrightnosp D B A C θ arrowrightnosp A arrowrightnosp B = × arrowrightnosp arrowrightnosp arrowrightnosp D A B ( ) sin = arrowrightnosp C AB θ C Determine the direction. If Magnitude of the vector : arrowrightnosp C = × arrowrightnosp arrowrightnosp arrowrightnosp 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 circle6 The field surrounds a magnet is called the magnetic field. The field is a vector, and is symbolized by circle6 Magnet exists in nature. circle6 Any magnets have two poles, called the north pole and the south pole. Like poles (from different magnets) repel, unlike poles attract. B arrowrightnosp 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 circle6 The SI unit of magnetic field is the tesla (T) circle6 Wb is a weber circle6 A non-SI commonly used unit is a gauss (G) circle6 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? circle6 Current (or moving charges, or changing electric field) generates magnetic field. circle6 We will get back to this topic in the following chapter. circle6 Magnet can take the form of a permanent magnet (ex. the bar magnet) or a solenoid. circle6 The Earth itself is also a big magnet.

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Magnetic force on circle6 Moving charge circle6 CRTs (old TV tube) circle6 Particle accelerator circle6 Particle mass spectrometer circle6 Particle detection and homeland security circle6 Current carrying conductor.
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