Magnetism. February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 1

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1 Magnetism February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 1

The minerals are various forms of iron oxide and are called permanent magnets! Examples: refrigerator magnets and magnetic door latches! They are all made of compounds of iron, nickel, or cobalt!

2 Magnetism! The Sun has massive magnetic fields as do other stars! The Earth also has a magnetic field! In the region of Magnesia, ancient Greeks found naturally occurring minerals (lodestone) that attracted and repelled each other and attracted various material such as iron! The minerals are various forms of iron oxide and are called permanent magnets! Examples: refrigerator magnets and magnetic door latches! They are all made of compounds of iron, nickel, or cobalt! If you touch an iron needle to a piece of magnetic lodestone, the iron needle will be magnetized! If you then float this iron needle in water, the needle will point toward the north pole of the Earth (approximately!)! We call the end of the magnet that points north the north magnetic pole of the magnet and the other end the south magnetic pole of the magnet February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 2

south poles are together, the magnets will repel each other!

3 Permanent Magnets - Poles! Magnets exert forces on one another! 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! If we bring together a north pole and a south pole, the magnets will attract each other February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 3

4 Broken Permanent Magnet! 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! 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; all results are negative! Magnetism is not caused by magnetic particles! Magnetism is caused by electric currents February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 5

5 Magnetic Field Lines! Permanent magnets interact with each other at a distance, without touching! In analogy with the electric field, we define a magnetic field to describe the magnetic force! As we did for the electric field, we represent the magnetic field using magnetic field lines! As with electric field lines, closer spacing of magnetic field lines indicates a higher field strength! The field is tangent to the field lines! 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 a compass needle points February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 6 B

6 Magnetic Field Lines Bar Magnet February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 7

7 Magnetic Field Lines February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 8

8 The Earth s Magnetic Field! The Earth itself is a magnet! It has a magnetic field sort of like a bar magnet (but not exactly like a bar magnet)! 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 Earth s magnetic field is not a simple dipole field because it is distorted by the solar wind Protons from the Sun moving at 400 km/s! The Earth s magnetic field is important because it protects the Earth from these cosmic rays! Cosmic rays are diverted to the poles and sometimes captured in the Van Allen radiation belts February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 9

9 The Earth s Magnetic Field N Solar Wind S February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 10

10 Superposition of Magnetic Fields! If several sources of magnetic field are present, the magnetic field at any point in space is given by the superposition of the magnetic fields from all sources! This superposition of fields follows from the principle of superposition of forces! The superposition principle can be stated mathematically as B total r ( ) = B 1 r ( )+ B 2 ( r )+...+ B n r! just as for electric fields ( ) February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 11

11 Magnetic Force! The magnetic force exerted by a magnetic field on a moving charged particle is given by F B = qv B! The direction of the magnetic force is perpendicular to both the velocity of moving positively charged particle and the magnetic field and is given by the right hand rule! The force is in the opposite direction for a negatively charged particle February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 12

12 Magnetic Force! The magnitude of the the magnetic force on a moving charged particle is F B = q vbsinθ where θ is the angle between the velocity of the charged particle and the magnetic field! No magnetic force acts on a charged particle moving parallel or anti-parallel to the magnetic field! The maximum magnetic force occurs when the charged particle is moving perpendicular to the magnetic field F B = q vb February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 13

13 Magnetic Force and Work! The magnetic force is perpendicular to the velocity of a moving charged particle Fi v = 0 and ai v = 0! The magnetic force can change the direction of the velocity of the moving charged particle but not its magnitude! The kinetic energy remains the same! The magnetic force does no work on the moving charged particle! On the other hand, the electric field can easily be used to do work on a particle February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 14

14 Units of Magnetic Field Strength! The magnetic field strength has received its own named unit, the tesla (T), named in honor of the physicist and inventor Nikola Tesla ( ) 1 T = 1 Ns Cm = 1 N Am! A tesla is a rather large amount of magnetic field strength! Sometimes you will find magnetic field strength state in units of gauss (G), (not an official SI unit) 1 G = 10-4 T 10 kg = 1 T February 24, 2014 Physics for Scientists & Engineers 2, Chapter 27 18

15 Example: Proton in B Field! A uniform magnetic field with magnitude B=1.2mT is directed upward (green dots are the tips of the B-field vectors) in a chamber. A proton with kinetic energy E= J enters the chamber and is moving horizontally from south to north. What magnetic deflecting force acts on the proton (m= kg)?! Idea: Use F=qvBsinΘ. First, figure out the velocity v. February 24, 2014 Physics for Scientists&Engineers 2 19

16 Example: Proton in B Field! A uniform magnetic field with magnitude B=1.2mT is directed upward (green dots are the tips of the B-field vectors) in a chamber. A proton with kinetic energy E= J enters the chamber and is moving horizontally from south to north. What magnetic deflecting force acts on the proton (m= kg)?! Angle between B-field direction and velocity of the proton: Θ=90! Small force but large acceleration for light particle! Direction of F: Use Right Hand Rule February 24, 2014 Physics for Scientists&Engineers 2 20

Positively charged anodes focus the electrons into a beam!

17 Cathode Ray Tube PROBLEM! Consider a cathode ray tube! In this tube, a potential difference of ΔV = 111 V accelerates electrons horizontally in an electron gun! Positively charged anodes focus the electrons into a beam! Horizontal and vertical deflecting plates use potential differences to steer the beam! In the main part of the cathode ray tube, there is a magnetic field with magnitude B = T February 24, 2014 Physics for Scientists & Engineers, Chapter 22 21

Physics for Scientists & Engineers 2

Permanent Magnets Physics for Scientists & Engineers 2 Spring Semester 2005 Lecture 20 Examples of permanent magnets include refrigerator magnets and magnetic door latches They are all made of compounds