Permanent Magnets Examples of permanent magnets include refrigerator magnets and magnetic door latches They are all made of compounds of iron nickel or cobalt Physics for Scientists Engineers 2 If you touch an iron bar to a piece of magnetic lodestone the iron bar will be magnetized Spring Semester 2005 If you then float this iron bar in water the iron bar will align with the north pole of the Earth Lecture 20 We call the end of the magnet that points north the north pole of the magnet and the other end the south pole of the magnet February 20 2005 Physics for Scientists Engineers 2 1 Permanent Magnets Poles 2 If we break a permanent magnet in half we do not get a separate north pole and south pole When we break a bar magnet in half we always get two new magnets each with its own north and south pole If we bring together a north pole and a south pole the magnets will attract each other Physics for Scientists Engineers 2 Physics for Scientists Engineers 2 Broken Permanent Magnets If we bring together two permanent magnets such that the two north poles are together or two south poles are together the magnets will repel each other February 20 2005 February 20 2005 Unlike electric charge that exists as positive proton and negative electron separately there are no separate magnetic monopoles an isolated north pole or an isolated south pole Scientists have carried out extensive searches for magnetic monopoles and none have been found 3 February 20 2005 Physics for Scientists Engineers 2 4 1 Magnetic Field Lines Magnetic Field Lines 2 The magnetic field line from a permanent bar magnet are shown below The permanent magnets just discussed interact with each other at a distance without visibly touching In analogy with the electric field we define the abstract concept of a magnetic field to describe the magnetic force As we did for the electric field we can represent the magnetic field using magnetic field lines The magnetic field is always tangent to the magnetic field lines February 20 2005 Physics for Scientists Engineers 2 Two dimensional computer calculation 5 February 20 2005 Magnetic Field Lines Three dimensional real life Physics for Scientists Engineers 2 6 Magnetic Field Lines 2 For the electric field the electric force pointed in the same direction as the electric field and the electric force was defined in terms of a positive test charge Let s take a compass needle Let s place the needle in various places around a bar magnet with a north and a south pole However because there is no magnetic monopole we must employ other means to define the magnetic force We can define the direction of the magnetic field in terms of the direction a compass needle would point S A compass needle with a north pole and a south pole will orient itself such that its north pole points in the direction of the magnetic field N Thus the direction of the field can be measured at any point by moving a compass needle around in a magnetic field and noting the direction that the compass needle points February 20 2005 Physics for Scientists Engineers 2 7 February 20 2005 Physics for Scientists Engineers 2 8 2 The Earth Earth s Magnetic Field Earth Earth s Magnetic Poles The Earth itself is a magnet The north and south magnetic poles are not located at the north and south geographic poles It has a magnetic field like a bar magnet The magnetic north pole is located in Canada The magnetic south pole is located on the edge of Antarctica The poles of the Earth s magnetic field are not aligned with the Earth s geographic poles defined as the endpoints of the axis of the Earth s rotation The magnetic poles move at a rate of 40 km per year By the year 2500 the magnetic north pole will be located in Siberia The Earth s magnetic field is not a simple as drawn here because it is distorted by the solar wind There are indications that the Earth s magnetic field flips on the time scale of 1 million years Protons from the Sun moving at 400 km s February 20 2005 Physics for Scientists Engineers 2 9 February 20 2005 Magnetic Declination Physics for Scientists Engineers 2 10 Magnetic Declination 2 A compass needle points toward the magnetic north pole rather than the geographic or true north pole The difference between the direction a compass needle points and true north is called the magnetic declination The magnetic declination is defined to be Because the positions of the Earth s magnetic poles move with time they change the magnetic declination for locations on the Earth s surface For example here is the estimated magnetic declination for Lansing Michigan for the period 1900 2004 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 through central 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 in Seattle East of this line the declination is negative up to 18 in Maine February 20 2005 Physics for Scientists Engineers 2 11 February 20 2005 Physics for Scientists Engineers 2 12 3 Magnetic Force Right Hand Rule We define the magnetic field in terms of its effect on a moving charged particle The direction of the magnetic force on a moving charged particle is given by the right hand rule Remember that an electric field exerts a force on a particle with charge q given by FE qE To apply the right hand rule A magnetic field exerts no force on a stationary charge The force that a magnetic field exerts on a charge moving with velocity v is given by FB qv B Use your right hand Align thumb in the direction of v Align your index finger with the magnetic field The direction of the force is perpendicular to both the velocity of the moving charged particle and the magnetic field February 20 2005 Physics for Scientists Engineers 2 Your middle finger will point in the direction of the magnetic force 13 Magnitude of Magnetic Force February 20 2005 Physics for Scientists Engineers 2 14 Units of Magnetic Field Strength The magnitude of the magnetic force on a moving charge is FB qvBsin where is the angle between the velocity of the charged particle and the magnetic field The magnetic field strength has received its own named unit the tesla T named in honor of Croatian born American physicist and inventor Nikola Tesla 1856 1943 1T 1 One can see that there is no magnetic force on a charged particle moving
View Full Document
Unlocking...