please follow the instruction: do lab report for file Physics 316 lab
Microsoft Word - Lab1_Lab2_214_spring_2021.docx Lab Section # 2 Resistivity & Ohm’s Law STReam LAB 3A- Resistivity of a Wire Equipment List 1m of constantan wire 1m of nichrome wire 12 V supply, 2 multimeters Introduction The resistivity of different metals is determined by finding the resistance of wires as a function of their length. Theory If a current (I) is flowing through a wire, the voltage drop (V) across a certain length of wire with resistance R is given by Ohm's Law: V=IR On a graph of V versus I, the slope is equal to R. In this experiment, you will plot V versus I to determine R for various lengths of wire. You will then make a graph of R versus length (L). The resistance of a wire depends on the length of the wire, the cross-sectional area (A), and the resistivity (ρ) of the material: Thus the slope of the R versus L graph is equal to ρ/A. Procedure 1) Take one of the pieces of constantan wire and record its diameter in three places using the micrometer screw gauge. Calculate the average. Set up the circuit as shown in the diagram. Set the power supply to 1V. This limits the current through the wire in order to reduce any heating effect which may change the resistivity. Make sure you connect ammeters in series and voltmeters in parallel to the circuit 3). Record the readings on the ammeter and voltmeter for 4 different lengths for two different wire types. Complete a table as below and calculate the resistivity for each wire. Wire type Diameter (mm) Length (m) Voltage (V) Current(A) Resistance (Ω) Resistivity Constantan Nichrome 2) Carry out the above process for the other constantan wire and both nichrome wires. Analysis For each wire plot a graph of resistance (y-axis) against length. The resistance of a wire is given by the equation: R = ρ l A Where R is resistance, ρ is resistivity, l is length and A is cross-sectional area. The gradient of each graph will give you the quantity ρ / A Therefore if you multiply the gradient by the cross-sectional area, this will give you the resistivity of the wire. (you can use the measured diameter to calculate the wire’s cross-sectional area) Use the above method of analysis to calculate the resistivity for each wire. 3A-PostLab Questions: Resistivity of a Wire 1. What are the factors affecting the resistance of an electrical conductor? 2 If the length and diameter of a wire conductor were both doubled, would the resistance be the same? Explain. 3. Why is resistivity called a material property? 4. Work out the average resistivity for each metal. How do these values compare with printed values? (you will have to research these). If there is a discrepancy, explain whether it is within reasonable experimental error of the printed value. 5)The maximum allowable resistance for an underwater cable is one hundredth of an ohm per meter. If the resistivity of copper is 1.7 x 10-8 m, find the least diameter of a copper cable that could be used? 3B-Ohm’s Law Equipment List Qty Items 1 PASCO 850 Interface 1 Circuit Board 2 Resistors (10 Ω and 100 Ω) 2 Banana Plug Patch Cord Introduction The purpose of this activity is to confirm the relationship of current, voltage, and resistance in an electric circuit. You will also explore what happens to the resistance of a light bulb’s filament as it changes temperature. Use the Capstone software to measure the current through resistors and the filament of a light bulb as the voltage across the resistors and the filament of the light bulb is changed. Background Ohm discovered that when the voltage (potential difference) across a resistor changes, the current through the resistor changes. He expressed this as where I is current, V is voltage (potential difference), and R is resistance. Current is directly proportional to voltage and inversely proportional to resistance. In other words, as the voltage increases, so does the current. The proportionality constant is the value of the resistance. Since the current is inversely proportional to the resistance, as the resistance increases, the current decreases. A resistor is ‘Ohmic’ if as voltage across the resistor is increased, a graph of voltage versus current shows a straight line (indicating a constant resistance). The slope of the line is the value of the resistance. A resistor is ‘non-Ohmic’ if the graph of voltage versus current is not a straight line. For example, if resistance changes as voltage changes, the graph of voltage versus current might show a curve with a changing slope. For a certain resistor, the value of its resistance does not change appreciably. However, for a light bulb, the resistance of the filament will change as it heats up and cools down. At high AC frequencies, the filament doesn’t have time to cool down, so it remains at a nearly constant temperature and the resistance stays relatively constant. At low AC frequencies (e.g., less than one hertz), the filament has time to change temperature. As a consequence, the resistance of the filament changes dramatically and the resulting change in current through the filament is interesting to watch. In the first part of this activity, investigate the relationship between current and voltage in simple ten-ohm (Ω) and one-hundred ohm resistors. In the second part, investigate the relationship between current and voltage in the filament of a small light bulb. Setup 1. Set up the PASCO 850 Interface and the computer and start Capstone. 2. Connect banana plug patch cords into the ‘OUTPUT’ ports on the interface. 3. Open the Capstone file: Ohm’s law The file opens with a Signal Generator window and a Scope display of voltage vs current. The Scope display will show the voltage and current across the resistor from the ‘Output’ of the interface. The Signal Generator is set to produce a triangle wave at 60 Hz. It is set to ‘Auto’ so it will automatically start or stop the signal when you start or stop measuring data. Capstone will record the ‘Output Voltage’ and the ‘Output Current’. 4. Place a ten-ohm (Ω) resistor on the circuit board is like in Fig1. 5. Connect banana plug patch cords from the ‘OUTPUT’ ports of the interface to the banana jacks on the circuit board. Procedure 1 Measure voltage and current for resistors 1. Begin measuring data. Click “Record” on Capstone. 2. Click ‘Stop’ to end data measurement. 3. Use Curve button , and then apply linear fit. . It will give you the slope. 4. Replace the ten-ohm resistor with a 100-ohm resistor and repeat the procedure. 5. Record the calculated slopes in the Lab Report. Procedure 2 Measure voltage and current for a light-bulb filament 1. After you record your data for the resistors, click ‘Stop’ to end data measurement. 2. Remove the 100-ohm resistor from the circuit board. 3. Place the BULB in a series on the circuit board is like in the illustration. 4. Change the Amplitude and Frequency of the output AC waveform. Click the Signal Generator window to make it active. Change the Amplitude value to ‘2.5 V. Change the Frequency value to ‘0.300’. Start measuring data. Press “Record”. Observe the Scope display of voltage versus current for the light bulb filament. Wait a few seconds and then stop measuring data. 5. Use the built-in coordinates in the Scope display to find the coordinates at several points on voltage versus current graph. Calculate the ratio of voltage to current at each point. 6. Make a sketch of the graph for voltage and current of the light bulb filament in the Lab Report section. 3-B Lab Report: Ohm’s Law Name: ______________________________________________________ Prediction What happens to the current in a circuit with a constant resistance as the voltage goes up? What happens to the resistance of the filament in a light bulb as the filament heats up? Data resistance (ten-ohm resistor) = ________ volt/amp resistance (100 Ω) = ________ volt/amp Make a sketch of the Scope display screen for voltage and current for the light bulb filament. Analysis Questions 1. How does the ratio of voltage and current from the Scope display compare to the resistance of the resistors? 2. Does each resistor appear to have a constant resistance? 3. Does the light bulb filament appear to have a constant resistance (constant ratio of voltage to current)? Why or why not? 4. For a circuit with a constant resistance, what happens to the current as the voltage increases?