PHY132_ACT_3_3_Induction

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PHY 132 - Activity 3.3: Magnetic Induction and Lenz's Law (E) Preliminary: Now that you are familiar with the magnetic field about a solenoid (coil), we would like to use a solenoid to demonstrate the phenomenon of induced emf and to study Lenz's law. In his studies Faraday reasoned that if a wire carrying a current acquires its own magnetic field, then conversely, when a magnetic field is established about the wire, the wire should acquire a current. By further experimentation, he found that his reasoning was correct provided that there is relative motion between the wire and the magnetic field . You can observe this effect with a solenoid and a magnet. There should be two solenoids, wrapped on wooden cylinders, one of which fits inside the other included with your equipment. We shall refer to the larger, outer solenoid as the secondary coil and the smaller, inner solenoid as the primary coil. MATERIALS 500 µA Galvanometer Low resistance rheostat with high current capacity Several small compasses 3-A DC power supply Bar magnet Soft iron core (unmagnetized) Connecting wires Primary and secondary coils Procedure: A. Magnetic Induction: 1. Set the primary coil aside for now and connect the secondary coil to a galvanometer as shown in Figure 5. 2. Insert a magnet in the coil and observe the galvanometer. Any deflection of the galvanometer indicates a current in the coil.
Initial Response: a) When the magnet is at rest relative to the coil, is there any current present in the coil? b) What happens as the magnet is being removed from the coil? What happens as the magnet is being inserted into the coil? c) What happens as you increase the speed with which you move the magnet? d) Where does the magnetic field have to change in order to produce a current? e) What would happen if the magnetic field remained as it is but the coil moved relative to that field? Try this by holding the magnet at rest but moving the coil up and down along it. What do you conclude?
The changing magnetic flux which induces a current in the coil does not have to be produced by a bar magnet. Since a coil of wire with a current in it produces a magnetic field, this coil should be able to induce a current in another coil. 3. Connect the primary coil in series with the rheostat and the power supply and rapidly move the primary coil up and down inside the secondary coil. How do the effects that you observe compare to those produced by the bar magnet? If you have difficulty observing these effects, place the soft iron core inside the primary coil, being careful that the core does not short out the wires connecting to the primary coil. By using two coils as you now have them arranged, it is possible to induce a current in the secondary coil without moving either of the coils. 4. Let the primary coil sit at rest inside the secondary coil and turn the power supply on and off so that the current in the primary coil changes rapidly. Final Response: Carefully state a rule that would explain how magnetically-induced current is created in all the cases you’ve observed so far? How would you create a large induced current?
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A current can be produced in a loop of wire using magnetic fields. The resulting current is called an “induced current”. The magnitude of the induced current is found to obey the following rule: Faraday’s Law: “The magnitude of the induced current is proportional to the rate of change of the magnetic flux through a conductive loop. That is, the induced current in a loop of wire may be calculated with i = N R ΔΦ Δt where the induced current, i , is expressed in Amps if the magnetic flux is given in Webers (T m 2 ), the resistance of the loop in Ohms and the time in seconds.” STOP! Check with your instructor before continuing!
B. Lenz’s Law: This part of the experiment deals with Lenz's law, which describes the direction of the induced current in a coil through which there is a changing magnetic flux. Notice what when you insert the north pole of the magnet into the secondary coil, the galvanometer deflects in a different direction than when a south pole is inserted. Initial Response: a) How does the deflection produced by removing a south pole compare to that produced by inserting a north pole? b) How does removing a north pole compare to inserting a south pole?
Direction of Current on the Galvanometer 1. Disconnect the galvanometer from the secondary coil and connect it to the power supply with your body acting as a large resistor in the circuit (see Figure 6). Rest as- sured that no permanent damage will be done to you! 2. Observe which way the galvanometer deflects when the current enters via the left hand terminal. Reverse the polarity of the power supply and observe which way the galvanometer deflects when the current enters via the right hand terminal of the gal- vanometer. Record your observations. Current Enters Via Left Terminal: The galvanometer goes left. Current Enters Via Right Terminal: The galvanometer goes right. Determining polarity (N or S) of coil due to induced current 3. Make sure you know which end of your bar magnet is the north end and which is the south end. Mark each end clearly. 4. Reconnect the galvanometer to the secondary coil (see Figure 5), set the coil on its end and insert the north end of the magnet into the top of the coil. From the direction the galvanometer deflects, determine the direction of the induced current in the coil (CW or CCW looking down at the top of the coil). When putting north end of the magnet the galvanometer it goes CCW (right). And when we pull the north end of the magnet out of the galvanometer it goes CW (left). 5. Knowing the direction of the induced current, determine the magnetic polarity of the upper end of the secondary coil. (N or S, are the B-field lines due to the induced current going into the coil or coming out of the coil?) CCW means the top is North CW means the bottom is South.
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6. Is this polarity such as to attract or to repel the approaching north pole of the bar magnet? (You won’t be able to feel the attraction or repulsion as the magnet enters or leaves the coil.) It is attraction because it is north to south. 7. Repeat the above analysis (4-6) when: (a) the north pole of the magnet is removed from the coil. (b) the south pole of the magnet is inserted into the coil. (c) the south pole of the magnet is removed from the coil. Record your observations (4-7) on the following Table. Action Galvanometer Deflection (Right or Left) Direction of Induced Current (CW or CCW) Polarity of Top of Coil (N or S) Does coil Attract or Repel the magnet? (A or R) Insert N Pole Right CCW N R Insert S Pole Left CW S R Remove N Pole Left CW S A Remove S Pole Right CCW N A Final Response: Make a clear statement about the direction of the induced current based on the above observations. I have no idea.
Using an electromagnet to induce current . 8. Connect the primary coil in series with the rheostat and the power supply as before. Keep the iron core in the primary coil. From the direction the current flows around the primary coil (or using a compass (Remember the compass will point to the S pole of the coil.)), determine the magnetic polarity of each of its ends. 9. With the primary coil at rest inside the secondary (2ndary) coil, turn the power supply off and record the deflection of the galvanometer. 10. Turn the power supply back on while the primary coil is at rest within the secondary (2ndary) coil. Record the direction of the induced current on the Galvanometer. 11. Reverse the connections on the primary coil to change its polarity. Repeat steps 8 and 9 and record your observations on the table below. Action Galvanometer Deflection (Right or Left) Direction of Induced Current in 2ndary Coil (CW or CCW) Polarity of Top of 2ndary Coil (N or S) Polarity of 2ndary Coil is the Same or Opposite of the Primary (Same or Opp.) N pole of Primary Coil at the top. Primary at Rest as Power Supply is Switched from On to OFF Left Primary at Rest as Power Supply is Switched from OFF to ON Right S pole of Primary Coil at the top. Primary at Rest as Power Supply is Switched from ON to OFF Primary at Rest as Power Supply is Switched from OFF to ON Final Response:
Are your observation in this case consistent with your rule for the direction of the induced current you stated above? Restate, if necessary, your rule for the direction of the induced current that accounts for both sets of observations. The formal statement for the direction of the induced current is given by Lenz’ law below. Lenz’ Law: “ The direction of the induced current is always such that the magnetic field produced by the induced current opposes the change in magnetic flux on the inside of the coil that produced that current .” Is your rule consistent with Lenz’ law for the direction of the induced current? Why or Why not? Imagine that alternating current is flowing in the primary coil. Alternating current builds up to a maximum in one direction, decreases back to zero, builds up to a maximum in the other direction, dies back to zero and repeats the cycle over and over. How will the induced current in the secondary coil behave?
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STOP! Check with your instructor before continuing!

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