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1、General Physics (PHY 2140) Electricity and MagnetismMagnetismApplication of magnetic forces Amperes law Chapter 19http:/www.physics.wayne.edu/apetrov/PHY2140/ Last lecture: Review Problem: How does the aurora borealis (the Northern Lights) work? sinF qvB sinF BIl How does your credit card work?The s
2、tripe on the back of a credit card is a magnetic stripe, often called a magstripe. The magstripe is made up of tiny iron-based magnetic particles in a plastic-like film. Each particle is really a tiny bar magnet about 20-millionths of an inch long. The magstripe can be written because the tiny bar m
3、agnets can be magnetized in either a north or south pole direction. The magstripe on the back of the card is very similar to a piece of cassette tape .A magstripe reader (you may have seen one hooked to someones PC at a bazaar or fair) can understand the information on the three-track stripe. A duck
4、 flying horizontally due north at 15 m/s passes over Atlanta, where the magnetic field of the Earth is 5.010-5T in a direction 60 below a horizontal line running north and south. The duck has a positive charge of 4.010-8C. What is the magnetic force acting on the duck? B=0.50 x 10-4 T. I = 22 Al = 3
5、6 mFmax = BIl max 420.50 10 22 364.0 10F BIl T A mN B I ba B a/2FF FF B I ba B a/2FF FF 1 2F F BIb max 1 22 2 2 2a a a aF F BIb BIb max BIba BIA sinBIA sinNBIA In a motor, one has “N” loops of current B30.0or = 0.500 m = 30oB = 0.50 TI = 2.0 AN = 1 2sin0.50 2.0 0.50.39 0 sin30.0oNBIAT A mNm A 2.00m
6、long wire carrying a current of 2.00A forms a 1 turn loop in the shape of an equilateral triangle. If the loop is placed in a constant magnetic field of magnitude 0.500T, determine the maximum torque that acts on it. MagnetCurrent loop or coil Spring Scale Galvanometer 60 WRp Galvanometer 60 WRp Let
7、s convert a 60 W, 1 mA full scale galvanometer to an ammeter that can measure up to 2 A current. Rp must be selected such that when 2 A passes through the ammeter, only 0.001 A goes through the galvanometer. 0.001 60 1.999 0.03002 pp A A RR W W Rp is rather small! The equivalent resistance of the ci
8、rcuit is also small! Galvanometer used as Voltmeter Finite internal resistance of a galvanometer must also addressed if one wishes to use it as voltmeter. Must mounted a large resistor in series to limit the current going though the voltmeter to 1 mA. Must also have a large resistance to avoid distu
9、rbing circuit when measured in parallel. Galvanometer 60 WRs Galvanometer 60 WRsMaximum voltage across galvanometer: max 0.001 60 0.06V A V W Suppose one wish to have a voltmeter that can measure voltage difference up to 100 V: 100 0.001 6099940 ppV A RR W W Large resistance Fv q r Bin The magnetic
10、force produces a centripetal acceleration.2mvF qvB r mvr qBThe particle travels on a circular trajectory with a radius: r = 0.14 mB = 0.35 Tm = 1.67x10-27 kgq = 1.6 x 10-19 Cmvr qB 19 22761.6 10 0.35 14 101.67 104.7 10 msqBrv m C T mkg Consider the mass spectrometer. The electric field between the p
11、lates of the velocity selector is 950 V/m, and the magnetic fields in both the velocity selector and the deflection chamber have magnitudes of 0.930 T. Calculate the radius of the path in the system for a singly charged ion with mass m=2.1810-26 kg. I=0 I I2oIB r Magnitude of the field2oIB r Ir Bo c
12、alled the permeability of free space 74 10 /o Tm A Andre-Marie AmpereAmperes LawConsider a circular path surrounding a current, divided in segments l, Ampere showed that the sum of the products of the field by the length of the segment is equal to o times the current. Ir BloB l I 2 oB l B l B r I Consider a case where B is constant and uniform.Then one finds: 2oIB r
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