Physics 2020 Lab: AC Circuits I page 1 of 8 University of Colorado at Boulder, Department of Physics Lab: AC Circuits I INTRODUCTION: USING THE OSCILLOSCOPE In previous labs, we studied some simple DC circuits, where the voltages and currents did not change with time. What happens when the voltages and currents do change with time? It turns out that we live in a world where most circuits are in fact AC (alternating current) instead of DC (direct current). In this lab, we will explore two different applications of AC circuits: induced voltage in wire loops, and magnetic force on a wire with alternating current. The goals of this lab are to learn how to use some fundamental scientific equipment (namely, the oscilloscope), to explore the mathematical expression of alternating currents, voltages, and magnetic fields, and to show some real-world applications of alternating electrical currents and magnetic fields in action. Almost every AC measurement is done using an oscilloscope, which is a very useful tool for measuring voltages that are changing in time. Think of the oscilloscope just like the DMM we have used in previous labs – it measures voltage, but now it plots it out in time (voltage on the vertical axis and time on the horizontal axis). The grid that you see on the screen is used to measure the voltage and time of your signal – think of it like graph paper. Each little box on the grid is called a division, and you can adjust the scale of the voltage and time axes with the volts/div and the sec/div knobs, respectively. For example, if the volts/div knob is set to 5, this means that each box on the grid is equal to 5 volts. There is small knob in the center of both the volts/div and time/div knobs, called the CAL or calibration knob. This should always be in the fully clockwise position for the volt/div and sec/div scale settings to be correct. Under the volts/div knob is a 3-position switch which reads (AC - ground - DC). This should be in the AC position for AC measurements. Oscilloscope Front Panel Whenever you use an oscilloscope, pay close attention to the horizontal and vertical scales (SEC/DIV and VOLTS/DIV).Physics 2020 Lab: AC Circuits I page 2 of 8 University of Colorado at Boulder, Department of Physics PART I: MEASURING SOUND WAVES At your table you should have a speaker, a signal generator, a microphone, and an oscilloscope. The speaker is driven by a signal generator which produces a sinusoidal voltage of adjustable frequency and amplitude. Note that there are both coarse and fine adjust knobs for the frequency, as well as “decade” buttons which can adjust the frequency by factors of 10. When connecting the various components to each other, you will be using two different types of cables: coaxial cables with BNC connectors and single cables with banana-plug connectors. The different types of connectors are shown below: BNC cables are actually two cables in one. They are composed of an inner conductor, which is connected to the pin on the connector, and an outer conductor, which is connected to the metal housing. The outer conductor surrounds the inner conductor, so it is a coaxial cable. Connect the signal generator to the speaker, and connect the microphone to the oscilloscope. Turn the volts/div knob on the oscilloscope all the way up to maximize its sensitivity. Place the microphone near the speaker and adjust the signal generator amplitude up until you can see a signal. Adjust the frequency and amplitude until you can hear a mid-range tone at a quiet but audible volume. Write down the frequency setting of the signal generator. From the oscilloscope trace, calculate the period of the alternating voltage signal. From this, calculate the frequency. Does your oscilloscope measurement match the setting on the signal generator? If not, why not? BNC cable Banana plugs Banana-to-BNC adapterPhysics 2020 Lab: AC Circuits I page 3 of 8 University of Colorado at Boulder, Department of Physics Measure and record the frequency range of your hearing as follows: Adjust the frequency up as high as possible so that you can still hear it. Calculate the frequency from the oscilloscope. Then adjust the frequency as low as possible so that you can still hear it. Calculate the frequency from the oscilloscope. You will need to adjust the sec/div knob to make an accurate oscilloscope reading. PART II: 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: tVinducedΔΔΦ−= 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 – again, assuming the orientation is correct). ΔΦ/Δt is therefore the rate of change of the flux. Substituting Φ=A*B, we get: tBAVinducedΔΔ−= 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 on the next page. On the second graph, plot the B-field strength at the center of the loop (call this B1). 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. Label your vertical axis appropriately.Physics 2020 Lab: AC Circuits I page 4 of 8 University of Colorado at Boulder, Department of Physics On the third graph, plot the local (instantaneous) slope of the B1 plot (i.e. ΔB/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. 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. On 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 timepoints A, B, C, and
View Full Document
Unlocking...