Electromagnetic Induction

1. Electromagnetic Induction Motional emf Faraday’s law Examples Generator

2. Reading Question Currents circulate in a piece of metal that is pulled through a magnetic field. What are these currents called? 1. Induced currents 2. Displacement currents 3. Faraday’s currents 4. Eddy currents 5. This topic is not covered in Chapter 33.

3. Reading Question Currents circulate in a piece of metal that is pulled through a magnetic field. What are these currents called? 1. Induced currents 2. Displacement currents 3. Faraday’s currents 4. Eddy currents 5. This topic is not covered in Chapter 33.

4. Reading Question Electromagnetic induction was discovered by 1. Faraday. 2. Henry. 3. Maxwell. 4. Both Faraday and Henry. 5. All three.

5. Reading Question Electromagnetic induction was discovered by 1. Faraday. 2. Henry. 3. Maxwell. 4. Both Faraday and Henry. 5. All three.

6. Reading Question The direction that an induced current flows in a circuit is given by 1. Faraday’s law. 2. Lenz’s law. 3. Henry’s law. 4. Hertz’s law. 5. Maxwell’s law.

7. Reading Question The direction that an induced current flows in a circuit is given by 1. Faraday’s law. 2. Lenz’s law. 3. Henry’s law. 4. Hertz’s law. 5. Maxwell’s law.

8. Electromagnetic Induction Motional emf

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15. Electromagnetic Induction Magnetic Flux

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19. Electromagnetic Induction Faraday’s Law • On the table you will find a bar magnet and a coil. The coil is connected to channel A on the Pasco Interface so that we can monitor the voltage across the coil. Turn on the interface and PC. Start DataStudio and configure the system by clicking and dragging the analog sensor plug icon to the channel A input. Choose the voltage sensor and connect to channel A. Next select the graph display icon and drag it to the channel A icon. Set the time scale maximum to 10 sec. and the voltage scale maximum to 1.0 V.

20. Electromagnetic Induction Faraday’s Law • On the graph below draw (dash line) your prediction for the emf due to thrusting a magnetic into the center of the coil with the north pole toward the coil. Do not withdraw the bar magnetic but stop the bar magnetic in the center of the coil after thrusting it in. coil

21. Electromagnetic Induction Faraday’s Law • Now let’s do the experiment. Start the recording and thrust the bar magnet into the center of the coil with the north pole toward the coil. Do not withdraw the magnet and stop the recording. Observe the deflection of the graph, both the magnitude and the sign. How does your prediction compare with the measurements? Draw the results on the axis above with your prediction.

22. e time (s) 1.0 2.0 3.0 Electromagnetic Induction Faraday’s Law • Now try different speeds. Draw the voltage for two different speeds on the same graph above. Describe what happens. Get actual data

23. e time (s) 1.0 2.0 3.0 Electromagnetic Induction Faraday’s Law • Now try to predict the curve if you reverse the magnetic so that the south pole is toward the coil. Draw the curve on the graph on the next page. • Reverse the poles so that the south pole is toward the coil and thrust the bar magnetic to the center of the coil. Draw the trace below.

24. e time (s) 1.0 2.0 3.0 Electromagnetic Induction Faraday’s Law • Now try this. Place the bar magnet so that the north pole is inside the coil. Start recording and pull the bar magnetic out of the coil (south pole first). Draw your prediction below. • Now do the experiment.

25. Electromagnetic Induction Faraday’s Law • This phenomena is called electromagnetic induction and is described by Faraday’s law. Write Faraday’s law. • In order to understand Faraday’s law we need to understand the concept of magnetic flux. Write the definition of magnetic flux.

26. The magnitude is the common area and the direction is normal to the area. dA normal to area Electromagnetic Induction Faraday’s Law • The magnetic flux is exactly like the electric flux we studied in Gauss’s law. The flux is defined in terms of a vector area dA. Describe the magnitude and direction of this vector.

27. Electromagnetic Induction Lenz’s Law • The direction of the emf and thus the current is given by Lenz’s law. The statement in bold in the center of page 789 is a statement of Lenz’s law. Use this to find the direction of the current. If you are looking down on the loop from above, is the current flowing clockwise or counter clockwise? Explain.

28. Electromagnetic Induction The magnetic is moving away from the coil so the magnetic field is decreasing, thus the current is in a direction to off-set the decrease. Lenz’s Law The magnetic is moving toward the coil so the magnetic field is increasing, thus the current is in a direction to off-set the increase.

29. Electromagnetic Induction Faraday’s Law Does it make a difference if it is the magnetic moving or the coil? This was a major point in Einstein’s theory of relativity.

30. Electromagnetic Induction Faraday’s Law • What about these two cases?

31. e time (s) 1.0 2.0 3.0 Electromagnetic Induction Faraday’s Law • Discuss the following experiment in your group. What will happen if you drop the bar magnet through the coil with the north pole toward the coil. Use a dash line to draw what you expect to see.

32. Electromagnetic Induction Faraday’s Law • Now do the experiment. Do not let the bar magnet hit the floor. The bar magnet will lose its magnetism if it hits the floor. Draw the results on the axis above. Use a solid line. How did you do? If your prediction was different discuss the results to make sure you all understand.

33. Electromagnetic Induction Faraday’s Law • Show that when you integrate the emf, e with respect to time you get the average change in flux in time t. Average value

34. Electromagnetic Induction Faraday’s Law • Now drop the magnet through the coil again and use Data Studio to integrate the voltage curve for the two peaks. How do the two compare? Write the answer here. • Why is the maximum for the second peak larger in magnitude than the first?

35. normal 300 Electromagnetic Induction Problem • A circular wire loop with a radius of 20 cm. is in a constant magnetic field of 0.5 T . • What is the flux through the loop if the normal to the loop makes an angle of 300 with the magnetic field?

36. Electromagnetic Induction Problem • The magnetic field increases from 0.5 T to 2.5 T in 0.8 seconds. What is the average emf, e(t) induced in the loop.

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43. Class Questions Is there an induced current in this circuit? If so, what is its direction? 1. Yes, clockwise 2. Yes, counterclockwise 3. No

44. A square loop of copper wire is pulled through a region of magnetic field. Rank in order, from strongest to weakest, the pulling forcesthat must be applied to keep the loop moving at constant speed. Class Questions 1. F2 = F4 > F1 = F3 2. F3 > F2 = F4 > F1 3. F3 > F4 > F2 > F1 4. F4 > F2 > F1 = F3 5. F4 > F3 > F2 > F1

45. Class Questions A current-carrying wire is pulled away from a conducting loop in the direction shown. As the wire is moving, is there a cw current around the loop, a ccw current or no current? 1. There is a clockwise current around the loop. 2. There is a counterclockwise current around the loop. 3. There is no current around the loop.

46. Class Questions A conducting loop is halfway into a magnetic field. Suppose the magnetic field begins to increase rapidly in strength. What happens to the loop? 1. The loop is pushed upward, toward the top of the page. 2. The loop is pushed downward, toward the bottom of the page. 3. The loop is pulled to the left, into the magnetic field. 4. The loop is pushed to the right, out of the magnetic field. 5. The tension is the wires increases but the loop does not move.

47. Electromagnetic Induction Example: Induction stove • The pan on the stove is heated by eddy currents produced by induction. • Would this stove work with a ceramic bowl? • Does the surface of the stove get hot?

48. Electromagnetic Induction Faraday’s Law • Now lets do another example of Faraday’s law. What happens when you pull a coil out of a magnetic field? Use the power supply and connect it to the large coil on the table. Turn the voltage knob to zero (counter clockwise) and turn on the supply. Make sure the current limit switch is on high and the current knob all the way clockwise to full range. Now increase the voltage to 30 V and the current should be about 0.5 A. We are using this large coil to create a magnetic field. Turn the supply off. • What direction is the field pointing when the supply is on? Use the current direction to find the field direction.

49. e time (s) 1.0 2.0 3.0 Electromagnetic Induction Faraday’s Law picture would help • Get the small coil and place the coil above the large coil and in the center. Pull the coil horizontally out of the field. Discuss what you expect the voltage in the small coil to look like. Draw what you expect below. Assume positive voltage is when the emf and current is clockwise looking down on the coil.

50. e time (s) 1.0 2.0 3.0 Electromagnetic Induction Faraday’s Law • Connect the Pasco interface voltage leads to the small coil and place the coil in the center and over the large coil. Turn on the supply, start DataStudio recording and pull the coil out of the field. Stop recording. Draw the measured voltage on the axis above. Does the measured voltage agree with your prediction?