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?

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!

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