1. Electromagnetic Induction Physics La Cañada High School Dr.E

2. Magnetism Field The electric field proves a useful concept to explain the effects of charge at a distance (explains how one charge knows another charge is there). Stationary charges produce only an electric field Moving charges (in flux) produce both an electric field and a magnetic field

3. Ampere’s Law: electric currents create magnetic fields Lorentz Force: Charges moving in a magnetic field experience an electromagnetic force. Concepts

4. Lenz’s Law Induced electric currents act so as to oppose the motion that caused them

5. According to Ampere’s law, electrical current generates A. A solenoid B. An increased voltage C. Magnetic fields D. A compass

6. In a Lenz Force, charges moving in a magnetic field experience what kind of force? A. Magnetic B. Electromagnetic C. Gravitational D. Applied

7. If charge particles move forward in a magnetic field, they experience a force. A. Forward B. Backward C. Sideways D. Up and down

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9. The Magnetic Field The electric field proves a useful concept to explain the effects of charge at a distance (explains how one charge knows another charge is there).The electric field proves a useful concept to explain the effects of charge at a distance (explains how one charge knows another charge is there). Stationary charges produce only an electric fieldStationary charges produce only an electric field Moving charges (in flux) produce both an electric field and a magnetic fieldMoving charges (in flux) produce both an electric field and a magnetic field

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11. Motors Motors work due to two major principlesMotors work due to two major principles 1. Opposite poles attract while like poles repel 2. Current running through a coiled wire creates a magnet

12. Theory

13. Mechanics

14. S N S N S S S S SS

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16. Faraday’s Law of Induction The voltage induced in a coil is proportional to the number of coils times the magnetic flux (rate at which the magnetic field changes)

17. N Wrap Rule to find Magnetic Field 1. Wrap your fingers in the direction of the current 2. The magnetic field points in the direction of the thumb (to the left) 3. Since the field lines leave the left end of solenoid, the left end is the N orth pole

19. An ammeter is connected in a circuit of a conducting loopAn ammeter is connected in a circuit of a conducting loop When a bar magnet is moved closer to, or farther from, the loop, an electromotive force (emf) is induced the loopWhen a bar magnet is moved closer to, or farther from, the loop, an electromotive force (emf) is induced the loop The ammeter indicates currents in different directions depending on the relative motion of magnet and loopThe ammeter indicates currents in different directions depending on the relative motion of magnet and loop When the magnet stops moving, the current returns to zero as indicated by the ammeterWhen the magnet stops moving, the current returns to zero as indicated by the ammeter

24. Randomly oriented unmagnetized iron is known as the: A. Domain B. Magnet C. Pole D. Coil

25. Magnetism originates in the: A. Attraction of poles B. Electrical Current C. Spinning of a compass D. Motion of electrons

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27. Faraday’s Law of Induction The voltage induced in a coil is proportional to the number of coils times the magnetic flux (rate at which the magnetic field changes)

28. Primary Voltage # of 1 o turns Secondary Voltage # of 2 o turns

29. (Power IN) (Power OUT)

30. (Voltage x Current) primary (Voltage x Current) secondary

31. The voltage induced in a coil is proportional to times the magnetic flux A. The current B. The resistance C. The strength of the charge D. The number of coils

32. The ratio of Primary to Secondary is 100:1. Will the voltage go higher or lower?

33. The ratio of Primary to Secondary is 1:100. If the voltage is 120V on the primary, what is it on the secondary?

34. The ratio of Primary to Secondary is 1:100. If the amperage is 10A on the primary, what is it on the secondary?

35. The ratio of Primary to Secondary is 1:100. If the primary has 120V and 10A on the, what is its power?

36. The ratio of Primary to Secondary is 1:100. If the primary has 120V and 10A on the, what is the power for the secondary side?

37. The ratio of Primary to Secondary is 100:1. If the voltage is 120,000V on the primary, what is it on the secondary?

38. The ratio of Primary to Secondary is 100:1. If the current is 10-A on the secondary, what is it on the primary?

39. The ratio of Primary to Secondary is 100:1. What is the power if the primary has 120,000V and the secondary has 10A?

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41. Power Transmission Plant 6000 V and 20Amps Local and Homes 120 V and 100 Amps Low Voltage Wires 2200 V and 54 Amps High Voltage Wires in Town 120,000 V and 1 Amp Ultra High Voltage 400,000 V and 0.3 Amps

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43. Wrap Rule to Find Magnetic Field 1. Wrap your fingers in the direction of the current 2. The magnetic field points in the direction of the thumb (to the left) 3. Since the field lines leave the left end of solenoid, the left end is the N orth pole

44. Solenoids The magnetic field of a solenoid is essentially identical to that of a bar magnet. The big difference is that we can turn the solenoid on and off ! It attracts/repels other permanent magnets; it attracts ferromagnets, etc.

46. Solenoid Applications Digital [on/off]: – Doorbells – Power door locks – Magnetic cranes – Electronic Switch “relay” Close switch  current  magnetic field pulls in plunger  closes larger circuit Advantage: A small current can be used to switch a much larger one – Starter in washer/dryer, car ignition, … Magnet off  plunger held in place by spring Magnet on  plunger expelled  strikes bell

47. Solenoid Applications Analog (deflection α I ): – Variable A/C valves – Speakers In fact, a typical car has over 20 solenoids! Solenoids are everywhere!

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49. Thanks to

50. MRI / NMR If we “bathe” the protons in radio waves at a particular frequency, the protons can flip back and forth. If we detect this flipping  hydrogen! The presence of other molecules can partially shield the applied magnetic field, thus changing the resonant frequency (“chemical shift”). Looking at what the resonant frequency is  what molecules are nearby. If a strong magnetic field gradient is produced across the sample, can look at individual slices, with ~millimeter spatial resolution. B Small B low freq. Bigger B high freq. Signal at the right frequency only from this slice!

51. Bibliography 1. 1. Magnetism: Examples of Magnetic Field Calculations, Innovations in Undergraduate Physics Education at Illinois @ online.physics.uiuc.edu/courses/phys112/spring04/ Lectures/Lect15.ppt, 4/17/04