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Physics 241 Lab: Vulture Iguana Rabbit http://bohr.physics.arizona.edu/~leone/ua_spring_2009/phys241lab.html Name:____________________________ “Summer Day” Reading in the heat of noon I grow sleepy, put my head On my arms and fall asleep. I forget to close the window And the warm air blows in And covers my body with petals. -Yuan Mei “In a Station of the Metro” The apparition of these faces in the crowd; Petals on a wet, black bough. -Ezra Pound 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 resistance of a circuit is defined to be the amount of voltage applied to the circuit divided by the total current through the circuit, ! Rtotal"#Vtotal appliedItotal. When someone speaks of the resistance of a component of a circuit, they mean the voltage drop across the component divided by the current flowing through the component, ! Rcomponent"#Vdrop across componentIthrough component. Express the SI unit of resistance Ω (ohms) in terms of other SI units. Your answer: If you apply a large voltage difference across a light bulb but only get a trickle of current through the light bulb, what can you qualitatively say about the light bulb’s resistance? Justify this using the definition of resistance. Your answer and justification: Imagine you are in the desert and there is a vulture flying in the sky and a rabbit and iguana on the ground. The rabbit sees the vulture flying over the iguana! This is a pneumonic device to remember the definition of resistance. What would the iguana see? What would the vulture see? Write two new equations relating V, I and R based upon this pneumonic device. This is merely a way to avoid silly errors when trying to rearrange the equation ! R ="VI. Your two new equations: 1.2. There is no reason to suspect a priori that the ratio of ! "VI should be the same for different voltages. Therefore, a particular component may have different resistances for different applied voltages. For example, a diode will have a very large resistance until the applied voltage potential difference reaches a certain value and then drop to nearly zero for larger voltages. Components that have a resistance that depends on the voltage applied are called non-Ohmic. For introductory students, we usually work with Ohmic resistors in most (but not all) the labs. For an Ohmic resistor, the equation ! Rconstant="VI implies the resistance is the same value no matter what voltage is applied. If a 1.5 Volt battery is discharged through a 2.5 Ω resistor, what is the current through the resistor? Your work and answer in SI units:1.3. Answer the questions about each of the following graphs. Does this graph describe an Ohmic resistor? What does the slope of this graph represent? Explain your answers. Does this graph describe an Ohmic resistor? Explain your answer. Does this graph describe an Ohmic resistor? Explain your answer. 1.4. Make a sketch of the small board of resistors provided to you and use your DMM to measure the resistance of each with as much accuracy as possible. Label the values on your sketch. (There is never any reason to trust the values written on the resistors or the color codes!) Your sketch and labeled values:Section 2: 2.1. Experimentally verify that the “1,000” Ω resistor on your resistor board is Ohmic at room temperature. Do this by gathering (voltage, current) data and making the appropriate graph. Your graph of your data should quite nicely show the linear behavior of your Ohmic resistor. The correct choice of (V vs. I) or (I vs. V) should give your resistance as the slope. WARNING: Do not apply such a large voltage that the resistor becomes very hot, dangerous and non-Ohmic. Conduct this experiment now, collect and appropriately graph your data on graph paper. Determine the experimentally observed value for resistance from your graph and record it here in SI units. Measure the resistance of your “1,000” Ω resistor with your DMM (which gives the true value) and compare this to your experimental value by finding the percent error: ! Rexperimental-" RDMMRDMMx100%. Your answers: 2.2. When two resistors are put into series, it is often useful to treat them as a single composite resistor and to find the total resistance. The formula for this is ! Rtotal= R1+ R2. You will learn to derive this by answering the following questions. Label the current through each resistor as I1 and I2. How are these two currents related (as a simple equation)? What is the reason for your answer? Your answers: Label the voltage difference across each resistor as V1 and V2. Write an equation relating V to V1 and V2. Here you must use the idea that the sum of all voltage differences in a circuit must be zero or else one could extract an infinite amount of energy from the electrons whirling around the loop. Your work and answer: Now use Ohm’s law in the equation from part b to substitute R1, R2, I1, and I2 for V1 and V2. Your work and answer: At this point, your equation should read ! V = I1R1+ I2R2. Now use your answer from part a by substituting ! I = I1= I2 (there is no reason to differentiate between the current in the two resistors so just write I). Distribute the I from the addition on the right hand side of the equation to get ! V = I R1+ R2( ). Explain why this tells you that two resistors in series have an effective resistance of ! R1+ R2( ). Your explanation:2.3. Experimentally verify that the 100 Ω resistor and the 200 Ω resistor in series produce an effective resistance of 300 Ω by taking (voltage, current) data and making the appropriate graph. Your voltage data should be gathered from across both resistors simultaneously since you want to treat them as a single resistor. Remember that the current is the same through both resistors. WARNING: Do not apply such a large voltage that the resistor becomes very hot, dangerous


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