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UA PHYS 241 - Elementary Circuits

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Physics 241 Lab: Elementary Circuits http://bohr.physics.arizona.edu/~leone/ua_spring_2009/phys241lab.html Name:____________________________ “Mind” Mind in its purest play is like some bat That beats about in caverns all alone, Contriving by a kind of senseless wit Not to conclude against a wall of stone. It has no need to falter or explore; Darkly it knows what obstacles are there, And so may weave and flitter, dip and soar In perfect courses through the blackest air. And has this simile a like perfection? The mind is like a bat. Precisely. Save That in the very happiest intellection A graceful error may correct the cave. -Richard Wilbur Important: • In this course, every student has an equal opportunity to learn and chance of success. • How smart you are at physics is a simple function of how hard you work. Your work ethic is a matter of practice. Teach yourself to work hard in E&M by setting aside voluminous amounts of study time, making a study schedule and sticking to it. • Form study groups and meet as often as possible. Trade emails with several others and start working problems together immediately. Be sure to be inclusive in creating your study groups. • Join professional organizations. Nobody in their undergraduate program really knows how “the system works.” Get information by growing your professional social network by participating in academic clubs and organizations. Don’t feel too shy to join; everyone is in the same boat. • Physicists help people. o Experimental physicists discover fundamental truths about nature and then give that knowledge away for free for the rest of the world to use to develop new technologies thereby creating jobs: science => technology => jobs. o Theoretical physicists are the masters of mathematically modeling nature, and as other fields of science become more mature, physics has become a bridge science due to the increased need to construct mathematical models in biology, chemistry, engineering, medicine, etc. o Having a technological society requires good science teachers, and physics instructors are often the best at teaching the “math inside nature.” A good teacher has the power to help students realize their potential and reach future life goals. o Branches of physics you can study at universities include engineering physics, astrophysics, quantum optics, particle physics, medical physics, biophysics, nuclear physics, materials physics, physics education, and more.Section 1. 1.A. Circuit diagrams are simple ways to represent actual physical circuits. An example is provided: (Note that most physicists are sloppy and write V for potential difference when they really mean ΔV.) Draw a circuit diagram for the following physical system. Quickly sketch your circuit diagram below: 1.B. A digital multimeter (DMM) may be used to measure constant (DC) voltage across a circuit component while current is flowing. Measure the voltage produced by several combinations of batteries using your DMM (make a quick table below). Your table of combined battery voltages in SI units: 1.C. A D-cell battery and a AAA-cell battery are both 1.5 volts. That means that for a given circuit (like a flashlight), each gives the same energy to each electron as it leaves the battery so that both produce exactly the same circuit behavior (flashlight brightness). Explain what the advantage of the D-cell battery would be versus the AAA-cell battery. Your explanation:1.D. A DMM may also measure constant current through a circuit component. Measure the current through a single light bulb powered by a 1.5 volt battery. Check your result with your neighbors’ as some DMMs do not always measure current correctly . Your measured current in SI units: 1.E. A DMM can also measure the resistance of a disconnected component. Measure the resistance of a single unpowered light bulb. This is not a useful observation since a light bulb is non-Ohmic: it’s resistance changes when used in a circuit (resistance grows with heat). You will learn to calculate the resistance of a powered bulb later in this course. Your measured resistance in SI units: 1.F. Measure the resistance of your finger:_________________ and your torso:___________________. How would you expect these measurements to change while you execised? Your observations (above) in SI units and your response:1.G. One way of thinking about a circuit with constant current is to use a sort of energy-circuit diagram. The y-axis represents voltage (potential energy per unit positive charge) while the x-axis represents the position of the circuit components. Here is an example applied to the first circuit shown in 1.A.: This diagram is an attempt to convey that the battery lifts a unit of positive charge to 1.5 Joules of electrical potential energy and that this unit of charge loses all this energy when flowing through the light bulb (and lighting it). Another way to say this is that the battery provide 1.5 volts of electrical potential to the circuit and that all this electrical potential is lost across the light bulb. Note also that the wires are drawn nearly parallel to the x-axis. This is because wires are highly conductive (very low resistance) and so there is no appreciable drop in electrical potential along a wire. However, if you were to simply hook a wire to both sides of a battery without a light bulb, then you would see that the entire voltage drop must be through this bare wire. Due to a wire’s low resistance, a large current would flow from the battery and through the wire and things would heat up! (Note: always use a resistance in your circuit when measuring amps or you will blow up your DMM). 1.H. Now imagine adding another identical light bulb in series with the first: Explain what the voltage drop across each of the light bulbs must be. Your explanation: Predict how the total resistance of this circuit would compare to that of the previous single light bulb circuit. Your comparison: Predict how the total current of the circuit (the current flowing through the battery) would be affected by adding this extra bulb in series? What would the current be as a fraction of the single light


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