1February 20, 2005 Physics for Scientists&Engineers 2 1Physics for Scientists &Physics for Scientists &EngineersEngineers 22Spring Semester 2005Lecture 20February 20, 2005 Physics for Scientists&Engineers 2 2Permanent MagnetsPermanent Magnets Examples of permanent magnets includerefrigerator magnets and magnetic door latches They are all made of compounds of iron, nickel, orcobalt If you touch an iron bar to a piece of magneticlodestone, the iron bar will be magnetized If you then float this iron bar in water, the ironbar will align with the north pole of the Earth We call the end of the magnet that points norththe north pole of the magnet and the other endthe south pole of the magnetFebruary 20, 2005 Physics for Scientists&Engineers 2 3Permanent Magnets - PolesPermanent Magnets - Poles If we bring together twopermanent magnets suchthat the two north polesare together or two southpoles are together, themagnets will repel eachother If we bring together anorth pole and a southpole, the magnets willattract each otherFebruary 20, 2005 Physics for Scientists&Engineers 2 4Broken Permanent MagnetsBroken Permanent Magnets If we break a permanentmagnet in half, we do notget a separate north poleand south pole When we break a barmagnet in half, we alwaysget two new magnets, eachwith its own north and south pole Unlike electric charge that exists as positive (proton) andnegative (electron) separately, there are no separate magneticmonopoles (an isolated north pole or an isolated south pole) Scientists have carried out extensive searches for magneticmonopoles and none have been found2February 20, 2005 Physics for Scientists&Engineers 2 5Magnetic Field LinesMagnetic Field Lines The permanent magnets just discussed interactwith each other at a distance without visiblytouching In analogy with the electric field, we define theabstract concept of a magnetic field to describethe magnetic force As we did for the electric field, we can representthe magnetic field using magnetic field lines The magnetic field is always tangent to themagnetic field linesFebruary 20, 2005 Physics for Scientists&Engineers 2 6Magnetic Field Lines (2)Magnetic Field Lines (2) The magnetic field line from a permanent bar magnet areshown belowTwo dimensional computer calculation Three dimensional real-lifeFebruary 20, 2005 Physics for Scientists&Engineers 2 7Magnetic Field LinesMagnetic Field Lines For the electric field, the electric force pointed in thesame direction as the electric field and the electric forcewas defined in terms of a positive test charge However, because there is no magnetic monopole, we mustemploy other means to define the magnetic force We can define the direction of the magnetic field in termsof the direction a compass needle would point A compass needle, with a north pole and a south pole, willorient itself such that its north pole points in the directionof the magnetic field Thus the direction of the field can be measured at anypoint by moving a compass needle around in a magnetic fieldand noting the direction that the compass needle pointsFebruary 20, 2005 Physics for Scientists&Engineers 2 8 Let’s take a compass needle Let’s place the needle in various places around abar magnet with a north and a south poleMagnetic Field Lines (2)Magnetic Field Lines (2)S N3February 20, 2005 Physics for Scientists&Engineers 2 9The EarthThe Earth’’s Magnetic Fields Magnetic Field The Earth itself is a magnet It has a magnetic field likea bar magnet The poles of the Earth’smagnetic field are not alignedwith the Earth’s geographicpoles defined as the endpoints of the axis of theEarth’s rotation The Earth’s magnetic field is not a simple as drawnhere because it is distorted by the solar wind• Protons from the Sun moving at 400 km/sFebruary 20, 2005 Physics for Scientists&Engineers 2 10EarthEarth’’s Magnetic Poless Magnetic Poles The north and south magnetic poles are notlocated at the north and south geographic poles• The magnetic north pole is located in Canada• The magnetic south pole is located on the edge ofAntarctica The magnetic poles move at a rate of 40 km peryear• By the year 2500 the magnetic north pole will be locatedin Siberia• There are indications that the Earth’s magnetic fieldflips on the time scale of 1 million yearsFebruary 20, 2005 Physics for Scientists&Engineers 2 11Magnetic DeclinationMagnetic Declination A compass needle points toward the magnetic north pole rather thanthe geographic or true north pole The difference between the direction a compass needle points and truenorth is called the magnetic declination The magnetic declination is defined to be• positive when magnetic north is east of true north• negative when magnetic north is west of true north The magnetic north pole currently resides on a line that passes throughcentral Missouri, Eastern Illinois, Western Iowa, and Eastern Wisconsin Along this line the magnetic declination is zero West of this line the magnetic declination is positive and reaches 18° inSeattle East of this line the declination is negative, up to -18° in Maine.February 20, 2005 Physics for Scientists&Engineers 2 12Magnetic Declination (2)Magnetic Declination (2) Because the positions of the Earth’s magnetic poles move with time,they change the magnetic declination for locations on the Earth’ssurface For example, here is the estimated magnetic declination for Lansing,Michigan for the period 1900 – 20044February 20, 2005 Physics for Scientists&Engineers 2 13Magnetic ForceMagnetic Force We define the magnetic field in terms of its effect on amoving charged particle Remember that an electric field exerts a force on aparticle with charge q given by A magnetic field exerts no force on a stationary charge The force that a magnetic field exerts on a charge movingwith velocity v is given by The direction of the force is perpendicular to both thevelocity of the moving charged particle and the magneticfield !FE= q!E !FB= q!v !!BFebruary 20, 2005 Physics for Scientists&Engineers 2 14Right Hand RuleRight Hand Rule The direction of the magneticforce on a moving chargedparticle is given by theright hand rule To apply the right hand rule• Use your right hand!• Align thumb in the direction of v• Align your index finger with the magnetic field• Your middle finger will point in the direction of
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