1. Magnetism & Induction (and Electricity = Electromagnetism) The First Unified Theory

2. Magnetism Vocabulary – Magnetic pole- emf – Electromagnet- Electromagnetic induction – Tesla (T)- Weber – Cathode- Magnetic flux – Anode – Magnetic permeabilityQUIZ: THURSDAY 1/21 – Diamagnetic – Paramagnetic – Ferromagnetic – Magnetic domains

3. Magnetism i-Research: – Describe magnetic domains – Are they in all materials? – How do magnetic and non-magnetic material differ? – If an object is magnetized, is it permanent? – If so, why? If not, how does it lose its magnetism?

4. Faraday and Maxwell Michael Faraday was a Scottish scientist whose experiments in electricity lead to the electric motor, which works as a result of electrical and magnetic forces. James Clerk Maxwell was a Scottish scientist whose theoretical work unified the forces of electricity and magnetism. The first time forces were unified.

5. Magnetism Since we know that magnetism and electricity are related, it is sensible to recognize that magnetism is the result of moving charges. An electric field produces a magnetic field when the charges are moving relative to the observer. – Recall the Pauli exclusion principle in that electrons of opposite spin produce opposite magnetic fields, thus allowing them to occupy the same orbital, despite the repulsion of similar electric charge.

6. Magnetism Magnetic domains act like tiny magnets within materials that align to produce a magnetic filed. – In Ferromagnetic materials, all the atoms, or molecules, align to produce a strong magnetic field. – In Paramagnetic materials, some of them align to produce a weak field. – In Diamagnetic materials, none of them align, and thus produce no magnetic field

7. Magnetism i-Lab: – Using a compass, a magnet, and a large sheet of paper, plot the lines of magnetic force Extending from each end of a magnet Between two opposite magnetic poles Between to similar magnetic poles – Where is the force the strongest? – How are these similar to electric field lines? – How are they different? – In what direction does the arrow point?

8. Magnetism B fields are in the direction that the N compass needle points (i.e. toward the magnetic south pole, which is at the geographic north pole) Like electric fields, B fields have greater intensity where they are denser and are stronger near the pole and weaker away. Unlike electric fields, you CANNOT have magnetic monopoles! The strength of B fields are measured in Tesla (T)

9. i-Lab: Force of a Current Carrying Wire Use the small paper clips to build the apparatus as instructed. Place the copper hanger on the clips. Place the magnet below the low hanging copper. Connect the battery with alligator clips. Observe what happens. Reverse the polarity and repeat. Add an additional battery in series and repeat.

10. i-Lab: Force of a Current Carrying Wire We observe… Because… So that…

11. i-Lab: Force of a Current Carrying Wire We observe…that the current carrying wire in a magnetic field will interact with the magnetic field. The direction it will be deflected will be reversed if the polarity (i.e. direction of current) is reversed. Because… So that…

12. i-Lab: Force of a Current Carrying Wire We observe…that the current carrying wire in a magnetic field will interact with the magnetic field. The direction it will be deflected will be reversed if the polarity (i.e. direction of current) is reversed. Because…current in a wire produces a magnetic field. The direction of the field is determined by the direction of the current. So that…

13. i-Lab: Force of a Current Carrying Wire We observe…that the current carrying wire in a magnetic field will interact with the magnetic field. The direction it will be deflected will be reversed if the polarity (i.e. direction of current) is reversed. Because…current in a wire produces a magnetic field. The direction of the field is determined by the direction of the current. So that…you can produce electromagnets, elctric field can disrupt magnetic fields (compasses), etc.

14. Magnetism Current carrying wires produce a magnetic field Use the Right Hand Rule #1 (RHR1) to determine the direction of the B field produced by a wire. The B field lines occur in concentric circles around the wire.

15. Magnetism Current carrying wires produce a magnetic field As a result of this field, current carrying wire can exert forces on each other. To determine the direction of the force, you can use RHR #2. Due to Newton’s 3 rd, the force is the force the first wire exerts on the second wire and v.v. (i.e. you only have to calculate one)

16. Magnetism Current carrying wires produce a magnetic field Application: – Using the RHR2, determine the force exerted on two wires whose current is flowing: …in the same direction. …in the opposite direction.

17. Magnetism Current carrying wires produce a magnetic field Notice the way in which the field direct in is noted. Applying the RHR, we see that wire 2 exerts a force on wire 1 toward the right, thus pulling the wires together.

18. Magnetism Current carrying wires produce a magnetic field Remember, the magnitude of the B field is measured in Tesla (T), and this can be calculated for a current carrying wire by the equation: B = (  I) / (2  r)

19. Magnetism B = (  I) / (2  r) – Where  is the magnetic permeability of the material around the wire or inside the coil loop, I is the current in A, and r is the distance from the wire or the radius of the coil loop. – Materials that are ferromagnetic have higher permeability, paramagnetic are lower. –    is for a vacuum and equals 4  x 10 -7 T*m/A

20. Magnetism Assume a wire, 0.7m long with a current of 3 A. What is the magnetic filed 6cm from the wire?

21. Magnetism Force of a magnetic field on current carrying wire The magnitude of the force can be calculated using a formula: F = I L B – where I is current in A, L is length in m, and B is magnetic field intensity in T.

22. Magnetism Force of a magnetic field on current carrying wire You may see the formula as: F = I L B sin  – This includes the angle between the current and the B field force lines. Usually this will be 90 or 0, so that sin  is 1 or 0 respectively, but be aware, just in case.

23. Magnetism Assume a wire, 0.7m long with a current of 3 A. What is the force on the wire if it is in a B field of 2.5T at an angle of 30 degrees?

24. Magnetism A coil in a magnetic field can be turned by the field. This is the principle behind electric motors.

25. Magnetism We know: F = I L B sin  Since the coil is turned, it is torqued. Remember that t = F x, so T = I L B sin  x or T=I A Bsin 

26. Magnetism What is the torque on a coil of radius 4 cm in a B field of 5T, if 2A of current is in the wire?

27. Magnetism Current carrying wires produce a magnetic field If the wire is coiled, it can produce a B field almost identical to a magnet.

28. Magnetism A single 8cm diameter loop of wire in a vacuum, has a current of 3A passing through it clockwise. What is the direction and intensity of the B field produced?

29. Magnetism If a material other than air or vacuum in inside the loop, and multiple loops are used, then you can produce a solenoid that can be used as an electromagnet.

30. Magnetism Do Set A: p. 654-658 – M/C – 1-10 – Conceptual - 11, 12, 14, 17, 18, 19, 20, 21, 22, 24, 26-30. – Problems – 1, 2, 3, 4, 6, 8, 9, 12, 13, 15, 16.

31. Magnetism Current and magnetism are related. Since current is simply the movement of charges through a conductor, it is safe to say that magnetic field will affect moving charged particles.

32. Magnetism To determine the affect of the magnetic field on a charged particle we can use... You guessed it! A right- hand-rule. A Positive particle is pushed with the palm, a negative is back-handed.

33. Magnetism The affects on the charged particle can be calculated with the equation: F = qvB (sin  ) Where F is force, q is charge, B is magnetic filed intensity and  is the angle at which the charge approaches the field.

34. Magnetism An electron travels at a velocity of 5 x 10 6 m/s directly to the right through a magnetic field of strength is 0.4 T directed into the page. Determine the magnitude and direction of the electron’s acceleration? What kind of acceleration would this be? What does this tell you about its trajectory? (m = 9.11x10 -31 kg; q = 1.6 x10 -19 C)

35. Magnetism For the electron described before, what would be the radius of its path?

36. Magnetism Magnetic flux (  ) is a measure of the magnetic field density, i.e. the number of field lines that pass through an area.  B = BA (cos  ) Where  B is flux, B is field strength, A is area, and  is the angle at which the loop passes through the field. Note: measure angle from vertical. The unit for Flux is the Weber (Wb) which is Tesla * m 2

37. Magnetism As a loop passes through a magnetic field, the number of lines of force passing through the loop is changing, i.e. the flux changes. A change in flux causes charges in a loop of wire to move, i.e. current.

38. Magnetism and Induced Current Another term you may see relating electricity to magnetism is electromotive force (emf or  ) Its definition is: “the work done by a power source per unit charge to move charge from one terminal to the other.” Since this is measured as J/C its unit is the volt (V). So, from a practical stand point, the emf is like a battery’s voltage and can be treated as such. An emf may be induced in a wire by passing it through a magnetic field, i.e. changing flux.

39. i-Lab: All in a Faraday’s Work Michael Faraday showed the practical relationship between electricity and magnetism. He did so by doing a series of experiments. Connect the galvanometer to a coil of wire. Move the magnet into and out of the coil. Record your observations. Make a rule to describe your observations.

40. i-Lab: All in a Faraday’s Work We observe …. Because … So that …

41. We Observe (as Faraday did)… Position of magnetDeflection in galvanometer Magnet at restNo deflection in galvanometer Magnet moves towards the coil Deflection in galvanometer in one direction Magnet is held stationary at same position (near the coil) No deflection in galvanometer Magnet moves away from the coil Deflection in galvanometer but in opposite direction Magnet is held stationary at same position (away from the coil) No deflection in galvanometer

42. Because… Faraday’s Laws #1: Any change in the magnetic field of a coil of wire will cause an emf to be induced in the coil. This emf induced is called induced emf and if the conductor circuit is closed, the current will also circulate through the circuit and this current is called induced current.magnetic fieldcurrent #2: the magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil. The flux linkage of the coil is the product of number of turns in the coil and flux associated with the coil. This can be calculated by: –  = -  /  t or –  = -  (BA cos  ) /  t

43. Magnetism One equation you may run into that involves a variation of Farady’s Law:  = B L v – Where B is field strength in T, L is wire length in m, and v is velocity in m/s.

44. So that … Magnetic fields can be used to generate electric power.

45. Lenz’s Law When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant.Faraday's Law

46. Lenz’s Law In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

47. Example A circular wire of radius 0.2m is in a 3 T magnetic field. What is the induced emf if the wire is rotated from 20 degrees to 70 degrees in 5 seconds? What is the direction of the emf?. B

48. Another variation If the coil is considered as a rectangle in a constant B field, and one side to the rectangle can slide such that the area changes, the flux is changing, thus an emf can be induced.

49. Example

50. i-Lab: Electric Motor Build the electric motor as described in the instruction. Look up electric motors. Sketch your motor and label the parts. Why do you have to “paint” the ends of the wire? How can you change the magnitude of the B field? How does the intensity of the B field affect the motor? How does the current affect the motor?

51. Electromagnetism Set B: – Page 657-659: Problems: 19, 21, 41, 42; as a group answer 24, 32, 33, 34 – Page 688-691: M/C: 1-13 Conceptual: 14, 16, 17, 18 Problems:2, 3, 9, 10, 17, 18, 19, 25, 28, 37, 42