1. ENGINEERING PHYSICS SEMESTER 2 2011/2012

2. ENGINEERING PHYSICS SUBTOPICS ● Magnets, magnetic poles and magnetic field direction ● Magnetic field strength and magnetic force ● Force and current-carrying conductor ● Magnetic material: Ferromagnetic SEMESTER 2 2011/2012

3. ENGINEERING PHYSICS When thinking about magnetism, most people tends to think of an attraction – certain things can be picked up with a magnet. INTRODUCTION SEMESTER 2 2011/2012

4. ENGINEERING PHYSICS In physics, magnetism is one of the phenomena by which materials exert attractive or repulsive forces on other materials. Materials that exhibit easily detectable magnetic properties (called magnets) are nickel, iron and their alloys; however, all materials are influenced to greater or lesser degree by the presence of a magnetic field. Magnetism also has other manifestations in physics, particularly as one of the two components of electromagnetic waves such as light. SEMESTER 2 2011/2012

5. ENGINEERING PHYSICS Magnetostatics is the study of static magnetic fields. In electrostatics, the charges are stationary, whereas here, the currents are stationary. As it turns out magnetostatics is a good approximation even when the currents are not static as long as the currents do not alternate rapidly. SEMESTER 2 2011/2012

6. ENGINEERING PHYSICS MAGNETS, MAGNETIC POLES AND MAGNETIC FIELD DIRECTION Magnets have two distinct types of poles; we refer to them as north (N) and south (S). SEMESTER 2 2011/2012

7. ENGINEERING PHYSICS By using two bar magnets, the nature of forces acting between them can be determined. - pole – force law or law of poles: - Like magnetic poles repel and unlike poles attract. SEMESTER 2 2011/2012

8. ENGINEERING PHYSICS Two magnetic poles of opposite kind form a magnetic dipole. All known magnets are dipoles (or higher poles); magnetic monopoles could exist (in theory) but have never been observed (experimentally). A magnet creates a magnetic field B : The direction of a magnetic field (B) at any location is the direction that the north pole of a compass would point if placed at that location. SEMESTER 2 2011/2012

9. ENGINEERING PHYSICS North magnetic poles are attracted by south magnetic poles, so the magnetic field points from north poles to south poles. The magnetic field may be represented by magnetic field lines. The closer together (that is, the denser) the B field lines, the stronger the magnetic field. At any location, the direction of the magnetic field is tangent to the field line, or equivalently, the way the north end of a compass points. SEMESTER 2 2011/2012

10. ENGINEERING PHYSICS MAGNETIC FIELD STRENGTH AND MAGNETIC FORCE A magnetic field B can exert a force on a moving charged particle. A horseshoe magnet created by bending bar magnet, produces a fairly uniform field between its poles. When a charged particle enters a magnetic field, the particle is acted on by a force whose direction is obvious by the deflection of the particle from its original path. SEMESTER 2 2011/2012

11. ENGINEERING PHYSICS The magnitude of the force is proportional to the particle’s charge and its speed. When the particle’s velocity v is perpendicular to the magnetic field B, the magnitude of the field is valid only when v is perpendicular to B Where, F is force and q is charges SI unit of magnetic field: newton/ampere-meter (N/(A.m) or commonly used - tesla, T. SEMESTER 2 2011/2012

12. ENGINEERING PHYSICS In general, a particle’s velocity may not be perpendicular to the field. Then the magnitude of the force depends on the sine of the angle θ between velocity vector and the magnetic field vector, thus it may be represents as, The force is perpendicular to both the velocity and to the field. The magnetic force F = 0 when v and B is parallel ( θ = 0 ° or 180 ° ), The maximum when v and B is perpendicular ( θ = 90 ° ), where F = qvB sin (90 ° ) = qvB Magnetic force on charged particle NOTE: SEMESTER 2 2011/2012

13. ENGINEERING PHYSICS The right-hand Force Rule for moving charges A right-hand rule gives the direction of the force. SEMESTER 2 2011/2012

14. ENGINEERING PHYSICS EXAMPLE 1 The figure above shows three situations in which charged particle with velocity v travels through a uniform magnetic field B. In each situation, what is the direction of the magnetic force FB on the particle? + y x z - y x z - y x z v B B v v B (a) (b) (c) SEMESTER 2 2011/2012

15. ENGINEERING PHYSICS EXAMPLE 2 A uniform magnetic field B, with magnitude 1.2 mT, points vertically upward throughout the volume of a laboratory chamber. A proton with kinetic energy 5.3 MeV enters the chamber, moving horizontally from south to north. What magnetic deflecting force acts on proton as it enters the chamber? [the proton mass, m = 1.67 x 10^-27kg] SEMESTER 2 2011/2012

16. ENGINEERING PHYSICS Solution: The magnetic deflecting force depends on the speed of the proton, which we can find from K = ½ mv2, solving for v, we find It may seem like a small force, but it acts on a particle of small mass, producing a large acceleration SEMESTER 2 2011/2012

17. ENGINEERING PHYSICS Applications: Charged Particles in Magnetic Fields A cathode-ray tube, such as a television or computer monitor, uses a magnet to direct a beam of electrons to different spots on a fluorescent screen, creating an image. SEMESTER 2 2011/2012

18. ENGINEERING PHYSICS A velocity selector consists of an electric and magnetic field at right angles to each other. Ions entering the selector will experience an electric force: and a magnetic force: These two forces will be perpendicular to each other. B E SEMESTER 2 2011/2012

19. QUIZ (open book) 1. Give the function of right-hand force rules and sketch the components involved regarding the rules. 2. Determine the force on a particle of mass and charge if it enters a magnetic flux density 5 mT with an initial speed of 83.5 km/s.

20. 3. A uniform magnetic fieldmagnitude 1.2 mT, is directed vertically upward throughout the volume of a laboratory chamber. A proton with kinetic energy 5.3 MeV enters the chamber, moving horizontally from south to north. Define magnetic deflecting force acts on the proton as it enters the chamber. The proton mass is 1.67 x 10 -27 kg. (Neglect the Earth's magnetic field) Given : 1eV = 1.60 x 10 -19 Joule

21. 4. A charge of 0.050 C moves vertically in a field of 0.080 T that is oriented 45° from the vertical. What speed must the charge have such that the force acting on it is 10 N? 5. An electron travels at a speed of 2.0 x 10^4 m/s through a uniform magnetic field whose magnitude is 1.2 mT. What is the magnitude of the magnetic force on the electron if its velocity and the magnetic field (a) are perpendicular, (b) make an angle of 45° (c) are parallel

22. 6. a)Define poles-force law or law of poles b)State the function of Right Hand Rules Law c)Sketch the magnetic field lines for the poles as shown in Figure below. i) ii) iii) NN SN SS

23. 7. A 2.0-m length of straight wire carries a current of 20 A in a uniform magnetic field of 50 mT whose direction is at an angle of 37° from the direction of the current. Find the force on the wire.

24. ENGINEERING PHYSICS The magnetic force on a current-carrying wire is a consequence of the forces on the charges. The force on an infinitely long wire would be infinite; the force on a length L of wire is: MAGNETIC FORCES ON CURRENT- CARRYING WIRES θ is the angle between I and B. If current, I, and magnetic field, B, is perpendicular to each other ( θ = 90 ° ), thus F = ILB SEMESTER 2 2011/2012

25. ENGINEERING PHYSICS The direction of the force is given by a right-hand rule: When the fingers of the right hand are pointed in the direction of the conventional current I and then curled toward the vector B, the extended thumb points in the direction of the magnetic force on the wire. Can you used “Left-Hand Rule”? SEMESTER 2 2011/2012

26. ENGINEERING PHYSICS Additional: Fleming’s Left-Hand Rule Fleming's left hand rule (for electric motors) shows the direction of the thrust (F) on a conductor carrying a current (I) in a magnetic field (B). John Ambrose Fleming (1849 – 1949) Named after British engineer John Ambrose Fleming who invented them. SEMESTER 2 2011/2012

27. ENGINEERING PHYSICS EXAMPLE 4 A straight, horizontal stretch of copper wire has a current I = 28A through it. What are the magnitude and direction of the minimum magnetic field B needed to suspend the wire, that is, balance its weight? Its linear density is 46.6 g/m. mg FBFB B Length, L [side view] SEMESTER 2 2011/2012

28. ENGINEERING PHYSICS Solution: The wire with length L, and the current is out of page. If the field is to be minimal, the force FB that is exerts on the section must be upward. B to be horizontal. In order to balance the weight of the section, FB must have the magnitude FB = mg, then The direction of FB is related to the direction of B and wire's length, L, thus SEMESTER 2 2011/2012

29. ENGINEERING PHYSICS Solution: SEMESTER 2 2011/2012

30. ENGINEERING PHYSICS Applications: Current-Carrying Wires in Magnetic Fields The Galvanometer: The Foundation of the Ammeter & Voltmeter A galvanometer has a coil in a magnetic field. When current flows in the coil, the deflection is proportional to the current. Sensitive Galvanometer Large Volt/Ampere Galvanometer SEMESTER 2 2011/2012

31. ENGINEERING PHYSICS The dc Motor An electric motor converts electric energy into mechanical energy, using the torque on a current loop. SEMESTER 2 2011/2012

32. ENGINEERING PHYSICS The Electronic Balance An electronic balance uses magnetic force to balance an unknown mass. The amount of current required is proportional to the mass. SEMESTER 2 2011/2012

33. ENGINEERING PHYSICS ELECTROMAGNETISM: THE SOURCE OF MAGNETIC FIELDS Magnetic Field near a Long, Straight, Current-Carrying Wire The magnitude of the field is given by: d is the distance B from current carrying wire The constant μ0 is called the permeability of free space. SEMESTER 2 2011/2012

34. ENGINEERING PHYSICS The field lines form circles around the wire; the direction is given by a right-hand rule. SEMESTER 2 2011/2012

35. ENGINEERING PHYSICS The magnetic field at the center of a current loop: Magnetic Field at the Center of a Circular Current-Carrying Loop For N loops SEMESTER 2 2011/2012

36. ENGINEERING PHYSICS A solenoid is a wire coiled into a long cylinder. The magnetic field inside is given by: Magnetic Field in a Current-Carrying Solenoid Magnetic field near the center of a solenoid. Solenoid field Depends on how closely packed (N/L) n = N / L SEMESTER 2 2011/2012

37. ENGINEERING PHYSICS A straight, horizontal wire 100 cm long carries a current of 12 A is at angle 45 ° to the direction of the horizontal magnetic field. Find the magnitude of the magnetic field, given force on the wire is 500 mN. EXAMPLE 5 SEMESTER 2 2011/2012

38. ENGINEERING PHYSICS A) Calculate magnetic field for 10 loops wire with radius of 0.5 m and carries 3 A current. B) The magnetic field at d distance from a long wire is 4 uT, the wire carries a current of 6.0 A. Find magnitude of d. EXAMPLE 6 SEMESTER 2 2011/2012

39. ENGINEERING PHYSICS Atomic electrons have a property called “spin” that gives them a small magnetic moment. In multielectron atoms, the electrons are usually paired with an electron of the opposite spin, leaving no net magnetic moment. However, this is not always the case, and some atoms do have a permanent magnetic moment. They will experience a torque in a magnetic field, and will tend to align with it. MAGNETIC MATERIALS SEMESTER 2 2011/2012

40. ENGINEERING PHYSICS In ferromagnetic materials, the forces between neighboring atoms are strong enough that they tend to align in clusters called domains. These domains are macroscopic in size. SEMESTER 2 2011/2012

41. ENGINEERING PHYSICS When a ferromagnet is placed in a magnetic field, the domains tend to align with it. SEMESTER 2 2011/2012

42. ENGINEERING PHYSICS When the external magnetic field is removed, the domains tend to stay aligned, creating a permanent magnet. The most common ferromagnetic materials are iron, nickel, and cobalt. Some rare earth alloys are also ferromagnetic. SEMESTER 2 2011/2012

43. ENGINEERING PHYSICS Electromagnets and Magnetic Permeability Ferromagnetic materials can be used to form electromagnets. Putting this material within a solenoid greatly enhances the magnetic field: Here, κ m is the magnetic permeability of the material; for ferromagnets, κ m is typically several thousand. SEMESTER 2 2011/2012

44. ENGINEERING PHYSICS For commercially useful ferromagnets, a type of iron is used that does not retain its magnetization when the current is turned off SEMESTER 2 2011/2012

45. ENGINEERING PHYSICS A “permanent” magnet can lose its magnetization through impact or heating. Every ferromagnetic material has a Curie temperature, above which the thermal motion immediately destroys any magnetic alignment (called paramagnet). Lava flows “freeze” a record of the Earth’s magnetic field at the point where they cooled below the Curie temperature. In this way, historical values of the Earth’s field may be determined. SEMESTER 2 2011/2012

46. Exercise: 1. A 2.0-m length of straight wire carries a current of 20 A in a uniform magnetic field of 50 mT whose direction is at an angle of 37° from the direction of the current. Find the force on the wire. 2. A straight wire 50 cm long conducts a current of 4.0 A directed vertically upward. If the wire is acted on by a force of in the eastward direction due to a magnetic field at right angles to the length of the wire, what are the magnitude and direction of the magnetic field?

47. 3. A horizontal magnetic field of is at an angle of 30° to the direction of the current in a straight, horizontal wire 75 cm long. If the wire carries a current of 15 A, what is the magnitude of the force on the wire? 4. The magnetic field at the center of a 50-turn coil of radius 15 cm is 0.80 mT. Find the current in the coil. 5. A long, straight wire carries a current of 2.5 A. Find the magnitude of the magnetic field 25 cm from the wire. 6. In a physics lab, a student discovers that the magnitude of the magnetic field at a certain distance from a long wire is 4.0 µT. If the wire carries a current of 5.0 A, what is the distance of the magnetic field from the wire?

48. 7. A solenoid is 0.2 m long and consists of 100 turns of wire. At its center, the solenoid produces a magnetic field with strength of 1.5 mT. Calculate the current in the coil. 8. A straightwire 40 cm long conducts a current 4.0 A directed vertically upward. If the wire is acted on by a force of 1.0 x 10 -2 N in the eastward direction due to a magnetic filed at the right angles to the length of the wire, find the magnitude and direction of the magnetic field.

49. 9. A student discovers that the magnitude of the magnetic field at a certain distance from a long wire is 4.0 μT. Calculate: i.the distance of the magnetic field from the wire carries a current of 5.0 A. ii.the number or turns for the wire that had been winding into coil with radius of 5 m and carries the same amount of current. 10. A 30 cm short solenoid has 100 turns of wire and carries a current of 0.95A which has ferromagneticcore completely filling its interior where the fields is 0.71 T. Determine the relative magnetic permeability of the material.