Now let s look at some devices that don t have a constant resistance.

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1 Lab #3 Now let s look at some devices that don t have a constant resistance. This is the same circuit you built last time. But now, in place of the resistor first build the circuit with a light bulb, then a photoresistor, and finally a thremistor. 3.1) Build the same circuit you built in the last lab but this time replace the resistor with a light bulb. Make another plot of the current verses the voltage. Make at least 5 measurements between 0 and 3 Volts. Don t worry if the light bulb doesn t come on for the low voltage measurements. You will still get good data points. Do not exceed 10 Volts on the power supply. Does your plot look like the plot for the resistor? How is it different? Your curve will not be a straight line because the resistance of the filament in the light bulb depends on how much current is flowing through it and the temperature of the filament. 3.2) A photoresistor is like the resistor you used in Lab #1 but the material it is made of is sensitive to ambient light levels. The resistance of the photoresistor will change as the light levels change. Replace the light bulb in your circuit with a photoresistor and set the voltage on your power supply at 5 Volts. What is the current? Now cover the photoresistor with something that will make it dark. What do you measure for the current now? Now try

2 shining a flashlight on the photoresistor and measuring the current. What is the current now? Determining how the photoresistor responds to specific light levels is called calibration. When you begin testing the packages you build for the balloon you will have to calibrate the instruments. 3.3) A thermistor is device that is similar to a photoresistor, but it is sensitive to temperature. The resistance of a thermistor will change with ambient temperature. Replace the photoresistor with a thermistor and keep the power supply at 5 Volts. What current do you measure? This is the response at room temperature (approximately 70 F). Hold the thermistor between your fingers and watch the current measurement change as the thermistor warms up to your body temperature. Note the current when the value stops changing, this is a reading for a temperature of about 98 F. Now use some of the cold spray to cool the thermistor. What is the current when the ice just begins to melt? This corresponds to approximately 32 F. You can create a calibration curve for the thermistor by plotting the current on the y axis and temperature on the x axis. If you were going to use this to make temperature reading you would need more than three points though. What is the reason for this? If you had only made three measurements when the light bulb was in the circuit would you have plotted a accurate curve in section 3.1? Series verse Parallel Circuits 3.4) Every circuit you have made so far has only had one component. Now we are going to look at what happens when you have multiple components in a circuit. Connect a single light bulb to the power supply set to 5 Volts. Note how bright the light bulb is and measure the current running through the light bulb with the multimeter. Make the following table. V bulb I bulb The brightness of a light bulb is related to how much current is flowing through it.

3 3.5) Now connect two light bulbs and the power supply together like Figure 2a. This circuit is 2 light bulbs wired in series. How does the brightness of the two light bulbs compare to each other? Is the current used up Figure by the 2a first light bulb? Figure 2b What happens when you unscrew one of the light bulbs? How does the brightness of the bulbs compare to when a single light bulb was connected to the battery? How does the amount of current through the 2 bulbs in series in compare to the current through a single bulb? Measure the current running through the circuit with the multimeter. How does it compare to when only one light bulb was connected to the power supply? Measure the voltage across both bulbs. Now measure the voltage across each bulb individually and make a table like the one below. How is the voltage across a single light bulb related to the total voltage? Is it a function of the number of light bulbs? V both bulbs V bulb 1 V bulb 2 I circuit Formulate a rule for how the current relates to the number of light bulbs in series in comparison to a single bulb circuit. Formulate a rule for how the voltage across is single bulb relates to the number of light bulbs in series. 3.6) Now connect two light bulbs and the power supply together like in Figure 3. This circuit is 2 light bulbs wired in parallel. Figure 3

4 What Figure happens 4a when you unscrew one Figure of the 4b light bulbs? Figure 4c How does the brightness of the light bulbs compare to each other? To the light bulb in a single light bulb circuit? To the light bulbs in the series circuit? What happens when you unscrew a lightbulb? Measure the current in the 3 positions shown in Figures 4a, 4b, and 4c making a table like the one below. How do the measured currents compare? V power supply V bulb 1 V bulb 2 I junction I bulb1 I bulb 2 Is the ratio of the current you measure in 4a to the current in 4b related to the number of light bulbs? What happens to the current when it hits the junction point where the 2 light bulbs are connected to each other? What is the voltage across each lightbulb? Formulate a rule for how the current relates to the number of light bulbs in parallel. Ohm s Law for parallel and series circuits 3.7) In Lab #2 we learned that the voltage from the power supply (V) is equal to the current through the circuit (I) times the resistance of the circuit (R), in this case either a single light bulb or multiple light bulbs. Now let s figure out how to determine the resistance of the circuits above. If the current you measured in for Figure 2b is half the current when a single light bulb is in the circuit but the potential is the same, how much does the resistance of the total circuit increase by?

5 When resistors are in series, the resistance of all the resistors combined is equal to R total = R 1 + R 2 + R 3 +. (1) and the voltage across each component in series adds to the total voltage supplied by the power supply V total = V 1 + V 2 + V 3 +. (2) 3.8) If the current you measured in for Figure 4a is twice the current when a single light bulb is in the circuit but the voltage is the same, how much does the resistance of the total circuit decrease by? When resistors are in parallel, the resistance of all the resistors combined is equal to = (3) R total R 1 R 2 R 3 and the voltage across each component is equal to the voltage supplied by the power supply V total = V 1 = V 2 = V 3 (4) Voltage dividers 3.9) When people put a collection of instruments on a satellite, balloon, or rocket, not all the instruments will need the same input voltage. But weight limitations mean that only one set of batteries and thus one voltage will be available. You can use what we learned in sections 3.4 and 3.5 to combine resistors in series and create different voltages from a single voltage supply. This circuit is called a voltage divider. Pick 2 resistors and build the circuit in Figure 5. We can use formulas 1 and 2 to determine that the output voltage is V out = V in * R 2 R + R 1 2 (5) Confirm this with a couple different sets of resistors. Figure 5