Voltage Current and Resistance II Equipment: Capstone with 850 interface, analog DC voltmeter, analog DC ammeter, voltage sensor, RLC circuit board, 8 male to male banana leads 1 Purpose This is a continuation of the previous lab, which entails how voltages and currents are generated and measured with the 850 interface and other equipment. Also, to investigate how voltage and current flow in resistors that are in series and parallel. 2 Voltage vs Current for a Resistor - AC Measurements In the previous lab three discrete measurements of voltage and current for a resistor were made. Using the AC capabilities of the signal generator, almost continuous measurements of voltage and current can be obtained. This saves time and allows measurements on non-ohmic circuit elements to be made quickly. Start the software. First click the Signal Generator icon, then 850 Output 1. Program the interface voltage output for a waveform of a triangular wave, an amplitide of 7 V amplitude, and a frequency of 0.2 Hz. Next, click on Auto. Select the Hardware Setup icon in the displays column. Click on the output terminals of the interface. Select Output Voltage - Current Sensor. See image below. Connect the interface output across the 100 Ω resistor on the RLC circuit board. Open a graph display. Click on the tack to resize the windows. Add a second plot to the graph by using Add 1
new plot area to the graph display. The graph display should have 2 vertical axes, make one the output voltage, and the other the output current. The horizontal axis should be the default value of time. Look at the image below. Click Record, run for about 10 s, and click STOP. Click the scale axis button. Located on the top left of the graph. Print out your graph (as well as other relevant results taken later). Discuss your graph. Open a new graph display, select output voltage for the vertical axis and output current for the horizontal axis. The data you just took will appear on this graph. Is Ohms law being obeyed? How? Click the down arrow of the apply selected curve fit icon located on top of the graph, and select Linear. Then click on the button next to the down arrow to display the linear graph information. What are the values of the slope and intercept of the fitted line? What does it represent? Are they what you expect? 3 Voltage vs Current for a Filament The resistors used so far are pretty good Ohm s law resistors. In this section you will examine the relationship between voltage and current for a filament. As a filament gets hot its resistant increases, and it does not obey Ohm s Law. Remove the 100 Ω resistor from across the interface terminals and place it over the #50 bulb on the RLC circuit board. Program the interface voltage output for a triangular wave, 7 V amplitude, and a frequency of 0.01 Hz. Click on Data Summary in the tool column. In the Sensor data Summary window delete the previous recorded runs of the output voltage and output current. Then remove the linear graph fit that you applied in the previous section. Use the graph display 2
with voltage on the vertical axis and current on the horizontal axis. Next, set up a recording delay by clicking on the Recording Conditions icon in the bottom center of the screen. Then, select Start condition. In condition type select Time Based and set a time delay for 0.400 seconds. Look at the following image. Click Record and take data for at least one full period (100 s) of the triangular voltage wave. Keep an eye on the bulb. Discuss your results. The rather low frequency of 0.01 Hz in the previous data was chosen so that the filament temperature was approximately in equilibrium with the current being applied. What would occur if the voltage were swept very quickly? Change the frequency to 15 Hz, and change the sampling rate to 500 Hz. The sampling rate adjustment is located at the bottom center of Capstone. Click Record and take data until the data points settle down. Is the graph of the filament an Ohmic function? Explain this result? The first data points differ from the rest of the data points because the filament needs time to heat up to the average power. You can avoid most of this problem by clicking Record, waiting for a short while, then taking data. 4 The Filament and Thermal Hysteresis Hysteresis is the lagging of one variable behind another. It leads to functions which are not single valued. If the voltage across the filament is increasing, the temperature is greater at a given current than when the current is constant. If the voltage across the filament is decreasing, the temperature is less at a given current than when the current is applied steadily. Investigate this effect by applying a 1 Hz triangular 7 V voltage wave to the filament and observing the data on a voltage versus current graph display. Keep the sampling rate at 500 Hz. Which part of the curve was obtained by sweeping the voltage up and which part by sweeping the voltage down? 3
5 Measuring Resistors in Series and Parallel 5.1 In Series Connect the 33 Ω and 100 Ω resistors on the RLC circuit in series across the output of the interface. Calculate the resistance values you obtain for the applied voltages of 1 thru 7 volts in 1 volt increments. Compare to the theoretical value for resistors in series. Measure the resistance in the simplest way you can think of using the equipment you have. You can use the internal DC output voltage + output current digits display or the analog voltage and analog current meters. To measure the voltage of a device you need to place the voltmeter in parallel across the device. To measure the current you need to place the current meter in series with the device you want to determine the current. 5.1.1 Resistors in series using analog current meter You will use the internal output voltage display and the external analog current meter. Connect the external analog meter in series to the same two resistors and set the meter to 100 ma DC. Determine the overall resistance value of the series circuit using the current you obtained using the analog current meter for the applied DC voltages of 1 thru 8 in 1 volt increments. Compare and explain the differences. 5.2 In Parallel Calculate the resistance values of the same resistors in parallel and compare to the theoretical value for resistors in parallel. (You should not use the 10 Ω resistor. It will draw too much current.) Use the applied DC voltages of 0.5 Volts to 4 volts in 0.5 volt increments. You have three options to 4
General Physics II determine the current in the circuit. Below is an image that will give you a hint on how to to setup a parallel circuit. You still have to figure out on how to apply the voltage! 6 Voltage Sensor and Analog Current Meter In this section the internal resistance of the analog current meter will be measured and the effect on the current in the circuit measured. The analog current meter is of the moving coil type. Turn its sensitivity knob to the least sensitive position (1 A) before inserting it into the circuit. Connect the 10 Ω resistor and the DC current meter in series. Connect this series combination across the interface voltage terminals. Connect the 2 leads of the voltage sensor across the 10 Ω resistor and plug the sensor into analog channel A. Click channel A and choose voltage sensor. Use a digits display to measure the output of the voltage sensor. 5
Program the interface voltage output for 0.1 V DC. Internally sample the output voltage and output current of the interface using two digital displays and using an appropriate number of decimal places. Below is an illustration on how to adjust for appropriate number of decimal points in a digits display. You will determine the resistance values of the current meter for the following scales 30, 100, and 300 ma scale. Click Record. For the various sensitivities of the analog current meter record the voltage output of the interface, the voltage across the resistor, and the currents measured by the analog meter and the digital meter. Discuss your results. Does the analog current meter affect the current in the circuit? What is the resistance of the analog current meter on the three different scales? How does turning the sensitivity knob change the sensitivity of the current meter? 7 Comment Parts of these labs have dealt with ohmic and non-ohmic resistors, while other parts have been tutorials on how to use the voltage and current capabilities of Capstone. If you needed to check on the value of a resistor, do not use the procedures in these labs. Pick up a digital multimeter. 8 Finishing Up Please leave your bench in a tidy fashion. Thank you. 6