资源描述
,*,*,*,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,Click to edit Master title style,General Physics(PHY 2140),Lecture 13,Electricity and Magnetism,Magnetism,Application of magnetic forces,Amperes law,Chapter 19,:/physics.wayne.edu/apetrov/PHY2140/,11/20/2024,1,Lightning Review,Last lecture:,Magnetism,Magnetic field,Magnetic force on a moving particle,Magnetic force on a current,Review Problem:,How does the aurora borealis(the Northern Lights)work?,11/20/2024,2,Magnetic Field of the Earth,11/20/2024,3,Review Problem 2,How does your credit card work?,The stripe 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 magnets 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,.,11/20/2024,4,Review Example 1:Flying duck,A duck flying horizontally due north at 15 m/s passes over Atlanta,where the magnetic field of the Earth is 5.010,-5,T in a direction 60 below a horizontal line running north and south.The duck has a positive charge of 4.010,-8,C.What is the magnetic force acting on the duck?,11/20/2024,5,Review Example 2:Wire in Earths B Field,A wire carries a current of 22 A from east to west.Assume that at this location the magnetic field of the earth is horizontal and directed from south to north,and has a magnitude of 0.50 x 10,-4,T.Find the magnetic force on a 36-m length of wire.What happens if the direction of the current is reversed?,B=0.50 x 10,-4,T.,I=22 A,l,=36 m,F,max,=BI,l,11/20/2024,6,19.5 Torque on a Current Loop,Imagine a current loop in a magnetic field as follows:,B,I,b,a,B,a/2,F,F,F,F,11/20/2024,7,B,I,b,a,B,a/2,F,F,F,F,11/20/2024,8,In a motor,one has“N loops of current,11/20/2024,9,Example 1:Torque on a circular loop in a magnetic field,A circular loop of radius 50.0 cm is oriented at an angle of 30.0,o,to a magnetic field of 0.50 T.The current in the loop is 2.0 A.Find the magnitude of the torque.,B,30.0,o,r =0.500 m,q,=30,o,B=0.50 T,I =2.0 A,N=1,11/20/2024,10,Example 2:triangular loop,A 2.00m 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.,11/20/2024,11,19.6 Galvanometer/Applications,Device used in the construction of ammeters and voltmeters.,Magnet,Current loop or coil,Spring,Scale,11/20/2024,12,Galvanometer used as Ammeter,Typical galvanometer have an internal resistance of the order of 60 W -that could significantly disturb(reduce)a current measurement.,Built to have full scale for small current 1 mA or less.,Must therefore be mounted in parallel with a small resistor or shunt resistor.,Galvanometer,60,W,R,p,11/20/2024,13,Galvanometer,60,W,R,p,Lets convert a 60 W,1,mA,full scale galvanometer to an ammeter that can measure up to 2 A current.,R,p,must be selected such that when 2 A passes through the ammeter,only 0.001 A goes through the galvanometer.,R,p,is rather small!,The equivalent resistance of the circuit is also small!,11/20/2024,14,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 disturbing circuit when measured in parallel.,Galvanometer,60,W,R,s,11/20/2024,15,Galvanometer,60,W,R,s,Maximum voltage across galvanometer:,Suppose one wish to have a voltmeter that can measure voltage difference up to 100 V:,Large resistance,11/20/2024,16,19.7 Motion of Charged Particle in magnetic field,Consider positively charge particle moving in a uniform magnetic field.,Suppose the initial velocity of the particle is perpendicular to the direction of the field.,Then a magnetic force will be exerted on the particle and make follow a circular path.,F,v,q,r,B,in,11/20/2024,17,The magnetic force produces a centripetal acceleration.,The particle travels on a circular trajectory with a radius:,11/20/2024,18,Example 1:Proton moving in uniform magnetic field,A proton is moving in a circular orbit of radius 14 cm in a uniform magnetic field of magnitude 0.35 T,directed perpendicular to the velocity of the proton.Find the orbital speed of the proton.,r =0.14 m,B=0.35 T,m=1.67x10,-27,kg,q=1.6 x 10,-19,C,11/20/2024,19,Example 2:,Consider the mass spectrometer.The electric field between the plates of the velocity selector is 950 V/m,and the magnetic fields in both the velocity selector and the deflection chamber have magnitudes of
展开阅读全文