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 INTRODUCTORY PHYSICS HUNTER COLLEGE CIRCUITS R=V/IIn preparing for this lab you are encouraged to review the basics in Chapter 18 of Giancoli(7th edition). In addition, The Physics Classroom a comprehensive tutorial on Electric Circuits.The operational base of our modern civilization is the electric circuit. This lab explores the parts of an electrical circuit and its mathematical operation known as Ohm’s law. Because of its apparent mathematical simplicity, Ohm’s law can be easily learned and applied by most students. This empirical rule is so embedded within our scientific intuitions it is hard to imagine that its introduction was met with some resistance ( no pun intended). George Ohm,studying the relationship between voltage and current, in varying lengths of wire, first published his famous formula in 1827. The German Minister of Education, apparently believing that we need only reason, not experimentation, to study nature, stated that "a professor who preached such heresies was unworthy to teach science." (Hart, IB, Makers of Science, London, Oxford University Press, 1923. p. 243). This lab experience is designed to demonstrate just how wrong this minister must be. To better appreciate the operation of a circuit we need to examine the components and parameters that are part of its operation. As you will see, all elementary circuits minimally contain sources of energy( batteries), conductors to convey the energy of the electric field (wires) and some type of load that uses the energy (lights, machines etc). Inevitably, in all circuits, there is opposition to the transmission of electrical energy. This is designated as resistance. You will be given opportunities to manipulate all these parameters. Properties of a ConductorFirst examine resistance in a wire. Go to this simulation: Manipulate the controls you see on the right and vary resistivity, length and cross sectional area of the wire. Resistivity (?) is the innate property of the wire to oppose or resist electric current. Mathematically, it is defined as the ratio of the strength of the Electric Field over the Current Density at that point In this exercise we are simply going to see its function in relation to the length and cross sectional area of the wire. Explore three conditions. A. Vary length and hold resistivity and area constant B. Vary resistivity and hold length and area constant C. Vary the Area and hold length and resistivity constant. Note that In all cases, it may not be possible to adjust the sliders to obtain the exact value. Try to get within 0.03 places from the value itself. For A and B, sketch the results. For C create a graph that displays a plot for Radius ( Not Area) and Resistance. Label the scales on the axes. Set Resistivity ? = 0.5 ohm cm and Area=5.00sq cm. ( within +/-0.03 sq. cm.). Vary the length to the values shown and record the Resistance Set Area =5 cm2 Length = 10 cm. and vary resistivity .Describe the relationship of length and resistivity to Resistance. Set ? = 0.5 ohm cm., Length = 10 cm and vary Area. Calculate the radius of the specified areas. Area cm2Radius cmResistanceΩ15.0012.009.006.003.001.50GRAPH THIS TABLE.. Resistance on the Y axis and Radius on the x axis. Plot the points and draw a curve of best fit.What type of a curve is it? Why?How significant are changes in the radius to changes in the current in the wire?Exploring Ohm’s LawNow go to this simulation: the two control levers on the right for Volts and Resistance and the white box in the center that indicates currentSet the Volts at 6.0 volts and vary the Resistance from the minimum of 10.0 ohms to maximum 1000 ohms. Record the values of current when resistance is set at the following values: 10 ( minimum), 100, 250, 500, 750 and 1000 ( maximum) ohms.Plot the points on the graph below (set to the appropriate scale of values). Sketch the CURVE OF BEST FIT. DO NOT JUST CONNECT THE DOTS. ( Do not use Excel)What type of relationship does this represent? Where else in Physics do you find such a curve? Building a CircuitNow go to the simulation labelled “Circuit Construction Kit DC” Note there are three similar looking simulations in PhET. Make sure you go to this one. Click the box on the right that says Lab.Your screen should show this:In this simulation, you are going to actually build a circuit with the components you have been introduced to. First click and drag the various components on the left into the main area as shown.Play with the components. Note how the wire’s length can be varied by clicking and dragging the bubbles at each end. Add components by simply clicking and dragging an item from the left. On the right side , you can click and drag the two measuring instruments, voltmeter and ammeter as shown.Note the boxes on the upper right. Check the “Values” box. Also note that in our lab current flows in the conventional way rather than the direction of the electrons. So click the term “Conventional”. Do not click “Labels” and “Values”.Practice connecting wires and components to each other by clicking and dragging them. At points that components are connected click the joint to see the scissorClicking the scissor icon disconnects the two. Also note that the switch icon can be closed or opened by clicking the circle in the center Once you have practiced manipulating the components, assemble a circuit to look like this:Note the voltmeter that can be moved around as can the red (+) and black (-) leads. Obviously it measures voltage. The ammeters measure current. There is one that is wired into the circuit and a portable one with a flexible probe. On the right, click the Wire Resistivity to ensure that it is at zero. Do the same with the Battery Resistance box . Tap the battery and edit voltage at 48 volts. Tap the lamp and edit its resistance at 12 ohms.Tap the switch to close the circuit. Calculate the current in this arrangement. Compare it to the value seen in the ammeter wired into the circuit.Take the probe at the end of the portable ammeter and, moving counterclockwise, place it at five positions on the circuit: Directly to the left of the battery, just above the lamp, just below the lamp, just below the switch and just below the battery.How does the current vary?Tap the switch to open it. Tap the battery. On the bottom window, tap the yellow button with two counterclockwise arrows on the left side of the box. Tap to close that box. Close the switch.Note the new orientation of the battery. What else has changed?Compare the current reading now to the one before. Open the switch and orient the battery to its original direction. Take the red lead of the voltmeter and place it on the gold (+) end of the battery and place the black one on the black end (-) of the battery. Record the Voltage.Now close the switch. Record the voltage again. Describe and explain your observation from these two recordings: once with no current and then with current flowing. Open the switch. Place the red lead, on the wire, under the lamp, and the black lead over lamp. Record the voltage or more technically, the voltage drop.Now close the switch. Record the voltage, or voltage drop. What conservation principle must be at work here?Note the brilliance of the lamp in this situation.Now add a lamp above the first one.in the same orientation. Move the voltmeter leads and disconnect and then connect the appropriate wires to arrange for a second lamp.It should look like this.Set the top and bottom lamp to 12 ohms each.Close the switch. How has the brightness of each lamp changed?Note and record the current reading in the hard wired ammeter. Why would it have changed from before?With the probe from the portable ammeter move counterclockwise and record the current reading in five places. What conclusion can you draw about the current in this arrangement?Now arrange this new circuit with the same values of voltage and resistance..With the portable ammeter, record current in five places. Record the voltage drops across each lamp by placing the voltmeter lead above and below each lamp.. What if anything has changed in this setup?3 Lamp CombinationsEqual Resistances: Add one more lamp of 12 ohms to the circuit ( to make 3) Place the lamps at any point you wish. Make a screenshot of the circuit you created for your lab report. 1.Make a table for recording your data.2.Using the portable ammeter, record the current at five positions on the circuit. List the positions.3. In the table also record the voltage drops across each of the three lamps. 4. Then record three voltage drops across combinations of 2 lamps. How is the distribution of voltage across the circuit related to the voltage in the battery?Unequal Resistances: First click the values box on the right to see resistance and voltage values.Make another table as before and record your information. Adjust the resistances in the lamps for 3 different values: 6, 12 and 18 ohms. Repeat steps 1 through paring the previous circuit of 3 equal lamps, how do the properties of this circuit remain the same AND how do they change. 5. Record the total current through the circuit of these three bulbs like in step 4. Include Resistivity. On the right side click open the resistivity box. Move the slider from zero to somewhere in the middle of the scale. So that the current value is 1 amp. 6. Show how you would determine the value of the equivalent resistance of the conductor in the circuit.7. Assume a conductor has a resistance of 8 ohms and a radius r= 5 x 10^-4 m Let its length be 373 m tall ( think Empire State Building). Determine the resistivity and identify the conductor used in this question. ................
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