Physics 241 Lab: Oscilloscope http://bohr.physics.arizona.edu/~leone/ua_spring_2009/phys241lab.html Name:____________________________ “Viaticum” When you go ten miles away I’ll go along for nine miles. Then I’ll leave you a hairpin As a compass for your route. -Pao Yu “Dew on the Young Garlic Leaves” The dew on the garlic Is gone soon after sunrise. The dew that evaporated this morning Will descend again in tomorrow’s dawn. Man dies and is gone, And when has anybody ever come back? -T’ien Hung Important: • In this course, every student has an equal opportunity to learn and chance of success. • How smart you are at physics depends on how hard you work. Work problems daily. • Form study groups and meet as often as possible.. • Join professional organizations. • Physicists help people: science => technology => jobs.Section 1: 1.1. The following picture shows the energy bands for a generic semiconductor, which must be calculated using quantum mechanics. If the temperature is low so that each electron in the valence band of the semiconductor has an average kinetic energy much less than the band gap energy, explain whether the semiconductor acts as a conductor or insulator. Your explanation: Imagine that an external voltage source is applied across the semiconductor so that each valence electron has more kinetic energy. Approximately what must the applied external voltage be in order for the semiconductor to transition from an insulator to a conductor? (Hint: energy equals charge times electric potential.) Your answer and explanation: Diodes are layered semiconductors and in a simple circuit act as one-way components. Light emitting diodes (LEDs) have a myriad of practical uses. The photons emitted by an LED each have energy roughly equal to the band gap energy of the semiconductor. Laser diodes are conceptually similar to LEDs and have led directly to the “digitized age of music”. More and more electrical engineering programs requiring their majors to gain a firm understanding of quantum mechanics. (Not a question.)Section 2: 2.1. Before actually using the oscilloscope, you need to be able to understand and predict what will appear on the oscilloscope screen. For the next few pages you will be asked predict what would appear on the oscilloscope screen, then later you will use the oscilloscope to make measurements. Feel free to experiment with your oscilloscope as you work through the prediction-making activities of this section. An oscilloscope is a device that measures voltage differences over time. It can be used to study rapidly oscillating voltags. For example, the voltage supplied by a wall outlet oscillates at the incredibly slow rate of 60 Hz. However, the oscilloscope can easily measure an oscillation of 1MHz or more. Most DMMs indicate that they can measure an oscillating voltage. However, a DMM can only make average measurements of sinusoidal 60 Hz voltages. In other words, a DMM is only useful for alternating current measurements (AC) on household circuits, not radios or other electronics. What is the period T of one oscillation for a linear frequency f = 5 MHz sinusoidal oscillating voltage? What is the angular frequency? Remember: T=1/f and ω=2πf. Your work and answers: 2.2. The simplest way to use an oscilloscope is as a DMM measuring a constant voltage. Imagine you have a 1.5 Volt battery and you measure the voltage every second for 5 seconds. Make an imaginary data table. Then use the data table to make a graph of what you would see on the oscilloscope screen. Your oscilloscope allows you to control the size of the tick marks on its screen. FOR THIS PROBLEM ONLY you are provided a choice of axis settings and selection of origin. For all other problems, you will need to select the appropriate axis settings yourself. Connect your data points on your graph to demonstrate what the oscilloscope would really show. Fill out the data table and make your graph directly in the oscilloscope screen below.2.3. Now imagine you have a 6 Volt sinusoidal power supply with a frequency of 60 Hz. What is VMAX and VMIN for this voltage? _____________ and ___________ . What is the period of one oscillation? _________________ Mathematically, this voltage is described as ! V (t) = 6sin(2"# 60 # t) where ! 2"# 60 is often referred to as the angular frequency, ω. Use this formula and a calculator to complete the given data table and make a graph on the oscilloscope. Be sure to label your axes and choose your units per division on the time axis wisely so that all your data fits on the screen.2.4. Now imagine you have two sinusoidal voltage signals. Both have a frequency of 60 Hz, but V1 has an amplitude of 6 Volts while V2 has an amplitude of 12 Volts. Furthermore, V2 lags behind V1 out of phase by 90o. Without making a data table, sketch what would appear on the oscilloscope. Feel free to use a graphing calculator with the functions ! V1(t) = 6sin(2"# 60 # t) and V2(t) = 12sin 2"# 60 # t $"2% & ' ( ) * . Be sure to label your axes and choose appropriate time and voltage units per division for your graph.2.5. Finally, let’s imagine taking the two alternating voltage sources from the previous problem, but now graph V1(t) on the x-axis and V2(t) on the y-axis. In oscilloscope terminology, this is called an XY plot. Thus we are graphing V2 vs. V1. Note: this is a voltage vs. voltage graph NOT a voltage vs. time graph. Make a data table using some points of common time provided below and the formulas from the previous sections. Use this table to graph V2 vs. V1. Be sure to label your axes and choose appropriate voltage units per division for your graph.Section 3: 3.1. Use the oscilloscope to examine the voltage vs. time graphs of many different sine waves, square waves and saw tooth waves created by the function generator (oscillating voltage supply). Be sure to experiment with all sorts of frequencies, voltage amplitudes and DC offsets. Practice making the voltage functions fit nicely on the oscilloscope screen. Take the time to twiddle every knob and switch. Once you feel comfortable with your understanding of each
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