Physics 2020, Fall 2011 Lab 8 page 1 of 5 University of Colorado at Boulder, Department of Physics Circle your lab day and time. Your name: Tue Tue Tue Wed Th Th Th Fri TA name: 10-12 12-2 2-4 12-2 10-12 12-2 2-4 12-2 Lab 8: Electromagnetic Induction and Motors In today’s lab you will explore how changing magnetic fields induce a voltage across a wire loop, a principle called induction and used in many technical devices. You will use the oscilloscope again (some useful hints about using the oscilloscope can be found at the end of these instructions). In the second part of the lab, you will build a simple motor. PART I: INDUCED VOLTAGE USING TWO WIRE LOOPS Recall that magnetic fields are both created by and act on moving charges. One of the ways that this happens is by the process called induction. Simply put, if you put a loop of wire in a changing magnetic field (assuming the orientation is correct), a voltage is induced between the ends of the wire. For a simple loop of wire, this can be expressed by Faraday’s law as follows: where Vinduced is the voltage difference between the ends of the wire that made the loop, and Φ is the magnetic flux (magnetic flux is simply the area of the loop times the strength of the field, or Φ=A*B, assuming the orientation is correct). Thus, ΔΦ/Δt is the rate of change of the flux. Since the area of the loop is a constant, substituting Φ=A*B, we get: You will use two wire loops to observe the phenomenon of electromagnetic induction. The first wire is used to create a changing magnetic field by applying a time-varying voltage across its ends. If you put a second wire loop close to the first one, the second will be surrounded by the changing magnetic field generated by the first loop. What do you expect to see when you measure the voltage across the ends of the second loop?Physics 2020, Fall 2011 Lab 8 page 2 of 5 University of Colorado at Boulder, Department of Physics A. Before doing the experiment, think about the set-up and plot your expectations on the graphs. 1. Imagine we were to put a sinusoidal voltage across the ends of a loop of wire. This would drive an alternating current through the loop (call this I1), as plotted in the first graph. 2. For the second graph, plot the B-field strength at the center of the loop (call this B1). Think about the relation between B1 and I1 (magnitude, direction) and use the right-hand rule. Start with timepoints A, B, C, and D, then fill in the rest of the plot. Draw any diagrams that are helpful. Don’t worry about absolute magnitude, but pay attention to direction. 3. For the third graph, plot the local (instantaneous) slope of the B1 plot (i.e. ΔB1/Δt). Again, start with timepoints A, B, C, and D, then fill in the rest of the plot. Don’t worry about absolute numbers – just get the sign and shape of the plot right. 4. Now imagine that we put a second loop right next to the first one, so that the wire of the second loop surrounds the changing B-field produced by the first loop. For the fourth graph, use Faraday’s law to plot the voltage induced on the second loop (call it V2). Draw any diagrams that are helpful. Start with time points A, B, C, and D, then fill in the rest of the plot. Again, don’t worry about absolute numbers – just get the sign and shape of the plot right.Physics 2020, Fall 2011 Lab 8 page 3 of 5 University of Colorado at Boulder, Department of Physics B. Now we will put together the setup described above. First, take the cable from the function generator, and connect it to a banana-plug adaptor. Next, connect a loop of wire across the two banana-plug terminals. This will be loop 1, which produces the magnetic field. Then take a second cable and make an identical loop. Use any adaptors that you need, and connect the two ends of the second wire to the oscilloscope. This will allow you to measure the time-varying voltage induced on loop 2. Turn the volts/div knob fully clockwise, to maximize the oscilloscope sensitivity. Turn on the function generator, and set it to something between 10-100 kHz, at maximum amplitude. Place the two loops next to each other – what happens? Adjust the frequency higher and lower (by at least a factor of 10). Record what happens. Record actual numerical frequencies. Can you explain the behavior that you see? Based on Faraday’s law, would you expect a higher induced voltage with a higher or lower source frequency? Why? Bonus/challenge questions (if you have time…): By now you can see that the induced voltage (V2) looks very similar to the source voltage (V1), but if you look closely at your plots of V2 and I1 (a plot of V1 would look just like the I1 plot), you will see that there is a phase shift between the two plots. Namely, the peaks in V2 are not exactly lined up with the peaks in V1. Why is this?Physics 2020, Fall 2011 Lab 8 page 4 of 5 University of Colorado at Boulder, Department of Physics PART II: ELECTRIC MOTOR At your table, you will find the parts needed to make an electric motor. You should have a wire coil on a bent-wire stand, a “telegraph key” switch, a bar magnet, and a battery set. Look closely at the leads to the wire coil. You should notice that the coating is scratched off of half of the surface of the wire near the end. This is an insulating coating (called Kapton), so that when the Kapton is in contact with the wire stand, no current flows through the coil. When the scratched-off part comes in contact with the wire stand, current flows through the coil. Using the drawings as a template, sketch the magnetic field lines around the magnet. Indicate the current direction (clockwise and counter-clockwise) on each coil. Determine the direction of the force on the top and bottom parts of the coil when the current is going in each direction. What would happen if the motor was set up as pictured, but current was then allowed to flow through the coil at all times? (Namely, if the Kapton were completely stripped off rather than just half-stripped.) How do the half-exposed leads (with half of the Kapton insulation scratched
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