Physics 202 Magnetism PDF

14/10/2024 1 At the end, students should be able to 1. Describe properties of magnets. 2. Describe magnetic fields around a magnet and two magnets, including the direction of the field lines, the density and the spacing between fields. 3. Explain how magneti...

14/10/2024 1 At the end, students should be able to 1. Describe properties of magnets. 2. Describe magnetic fields around a magnet and two magnets, including the direction of the field lines, the density and the spacing between fields. 3. Explain how magnetic fields are produced. 4. Distinguish between an electromagnet from a permanent magnet. 5. Distinguish between ferromagnetic, paramagnetic, and diamagnetic materials. 6. Identify that a magnetic field is a vector quantity. 7. Analyse and apply the relationship between force magnitude F , charge q, speed v, field magnitude B, and the angle ϕ between the directions of the velocity vector v and the magnetic field vector B, for a charged particle moving through a uniform magnetic field F q v B sin ∅. 8. Recall that the force in a magnetic field is always perpendicular to the velocity of the particle. 9. Describe the effect of the magnetic force on the particle’s speed and kinetic energy. 10. Apply the right-hand rule to find the direction of the magnetic force F, for a charged particle sent through a uniform magnetic field. 2  14/10/2024 At the end, students should be able to 11. Compare electric and magnetic fields in terms of the force on a charge particle, work on a charge particle and electric fields. 12. Describe the application of magnetic field to the earth. 13. Describe the magnetic field around a straight current carrying wire. 14. Identify that the magnetic field due to the current in a long straight wire is directly proportional to the current in the wire and inversely proportional to the distance from the wire B ∝ 15. Apply magnetic field due to a long straight wire B to solve problems. 16. Describe the magnetic field due to current in a loop. 17. Apply current in loops B to solve problems. 18. Distinguish between solenoids and electromagnets. 19. Describe the magnetic fields around solenoids and electromagnets. 20. Apply magnetic field for a solenoid or electromagnets B to solve problems. 21. Describe Maxwell equations basic equations for all electromagnetisms. 3 At the end, students should be able to 22. Describe Ampere’s law. 23. Identify that Ampere’s law relates magnetic fields and current in a general way. 24. Identify that a current is induced in a conducting loop when the number of magnetic field lines intercepted by the loop is changing. 25. Analyse Ampere’s law to a loop that encircles current. ∮ B · ds μ i 26. Explain motional emf. 27. Describe how electric fields develop inside a moving conductor in a magnetic field. 28. Analyse motional emf ε vlB 29. Analyse magnetic flux Φ AB cosθ 30. Describe Gauss’s law in magnetism. 31. Describe Faraday’s law in words as the magnitude of the induced emf is the rate of change of the magnetic flux through the loop. ∆( 32. Analyse Faraday’s Law ε ∆) 33. Extend Faraday’s law from a loop to a coil with multiple loops 4  14/10/2024 At the end, students should be able to 34. Identify that the net magnetic flux through a Gaussian surface (which is a closed surface) is zero. 35. Describe the magnetic flux based on the orientation of the magnetic field and the plane. 36. Describe induced electric fields. 37. Identify three general ways in which the magnetic flux through a coil can change. 38. Identify that an electric potential cannot be associated with an induced electric field. 39. Identify that a changing magnetic field induces an electric field, regardless of whether there is a conducting loop. 40. Use a right-hand rule for Lenz’s law to determine the direction of induced emf and induced current in a conducting loop. 41. Identify that when a magnetic flux through a loop changes, the induced current in the loop sets up a magnetic field to oppose that change. 42. Describe eddy currents. 43. For power transmission lines, identify why the transmission should be at low current and high voltage. 44. Identify the role of transformers at the two ends of a transmission line. 45. Distinguish between a step-down transformer and a step-up transformer. 5  Any magnet, whether it is in the shape of a bar or a horseshoe, has two ends or faces, called poles, which is where the magnetic effect is strongest.  If a bar magnet is suspended from a fine thread, it is found that one pole of the magnet will always point toward the north, that is, a freely suspended magnet aligns is N-S direction.  This is the principle of a compass. 6  14/10/2024 N S S N Repulsive S N N S Repulsive N S N S Attractive Opposite magnetic poles attract each other, and like magnetic poles repel each other. 7 What happens if a magnet N S breaks? Physicists have searched for N S N S isolated single magnetic poles (monopoles), but no magnetic monopole has ever been N S N S N S N S observed. 8  14/10/2024  Ferromagnetic: strong magnetic fields, can be made into strong permanent magnets.  Non-ferromagnetic materials fall into two principal classes:  paramagnetic materials consist of atoms that have a net magnetic dipole moment which can align slightly with an external field.  weakly attracted by the magnetic fields  diamagnetic materials have atoms with no net dipole moment, but in the presence of an external field electrons revolving in one direction increase in speed slightly whereas electrons revolving in the opposite direction are reduced in speed; the result is a slight net magnetic effect that opposes the external field.  weakly repelled by the magnetic fields Additional reading: https://www.nde-ed.org/Physics/Magnetism/ferromagmaterials.xhtml 9  Moving electrically charged particles, such as a current in a wire, to make an electromagnet.  The magnetic fields of the electrons in certain materials add together to give a net magnetic field around the material, giving rise to a permanent magnet. Video source: https://www.nde- ed.org/Physics/Magnetism/electronpairing.xhtml 10  14/10/2024 Iron, cobalt, nickel, gadolinium, and some of their oxides and alloys, show strong magnetic effects. Iron filings line up along magnetic field lines due to a permanent magnet. 11  Just as electric field lines, magnetic field lines are drawn  the direction of the magnetic field is tangent to a field line at any point  the number of lines per unit area is proportional to the strength of the magnetic field  number of lines per unit area, the field line density.  The direction of the magnetic field at a given location can be defined as the direction that the north pole of a compass needle would point if placed at that location.  Magnetic field lines do not intersect.  Magnetic field lines continue inside a magnet.  Magnetic field lines always form closed loops, unlike electric field lines that begin on positive charges and end on negative charges. 12  14/10/2024 Earth's Magnetic Field - Earth itself is a huge magnet – Magnetosphere https://www.youtube.com/watch?v=Gea4cEA5Ris 13 A magnetic field * is defined  In general,  +, / -* sin ∅ as a vector quantity that is where ∅ is the angle between the directions of velocity -⃗ and directed along the zero-force axis, that is, the magnitude of magnetic field *. +, when -⃗ is directed perpendicular to that axis. +,  This force is often called * the Lorentz force. /- +, is the force, / is the  The strength of a magnetic field charge of the particle and - is is measured by its flux density, the particle’s velocity measured in Tesla (T) 14  14/10/2024 Magnetism: Question1  A magnetic field exerts a force of 8 *10-14 N toward the west on a proton moving vertically upward at a speed of 5 *106 m/s. When moving horizontally in a northerly direction, the force on the proton is zero. Determine the magnitude of the magnetic field in this region. Note the charge on a proton is 1.6 *10-19 C. +, * /- 8 ∗10−14 N 11.6 ∗10−19 C215 ∗106 m/s2 0.10 6 15 Magnetism: Question 2 When potassium ions 17 8 2 cross a membrane through an ion channel, they acquire a velocity on the order of 109 :; 9 T), when the angle between Earth’s field and the direction of motion of the 7 8 is 90°? Assuming the charge of the 7 8 is 1.6 109 R< , increases the voltage  Step-down transformer: R < R< , lowers the voltage. 61  By considering energy conservation, one can relate the currents in the secondary and primary. E< R< E R  A step-up transformer raises voltage but lowers current  A step-down transformer lowers voltage but raises current  This must be the case in order to conserve energy. 62