Idea Transcript
The Bronx High School of Science Dr. J. Donahue, Principal
Physical Science Department Dr. A. Wheeler, Assistant Principal
Name _________________________________________ Partner(s) _______________________________________ Teacher _______________________________________ Day/Periods _________ Date _______________________
Experiment 2.4 (Regents): Ohm's Law Problem: How are voltage, current, and resistance related for different devices? Hypothesis/recollection from class:_______________________________________________________________________ _____________________________________________________________________________________________________ _____________________________________________________________________________________________________ Materials and Equipment for each group: Ammeter, voltmeter, resistors (30Ω, 50Ω), lamp, connecting wires
with clips, variable DC voltage source CAUTION: Ø The resistors may get very hot. Ø DO NOT leave the power on for more than two minutes. Ø DO NOT turn the voltage up beyond the limits set by your teacher. Ø After obtaining data, allow the resistors to cool for a couple of minutes before touching. Ø Please be very careful to observe the polarity of the meters. If the needle tries to move to the left of the zero mark when power is switched on, you must turn off power immediately and check your wiring. Ø DO NOT TURN THE CIRCUIT ON UNTIL CHECKED BY YOUR TEACHER!
Procedure: 1. Connect the circuit as shown above using, first using the lamp. 2. Use the 0.5 A scale on the ammeter. Set to DC (=) not AC (~). 3. Use the 15 V scale on the voltmeter. You will keep your readings below 15V. Set to DC (=) not AC (~). DO NOT TURN THE CIRCUIT ON UNTIL CHECKED BY YOUR TEACHER! 4. Set the voltage to the lowest position (entirely counter-clockwise), and switch on the power supply. 5. As a final check of your wiring, very slowly raise the voltage. The lamp should start dim and begin to glow brightly. 6. Make sure the ammeter readings stay below 0.5A. If they increase rapidly and/or the lamp does not light, turn off the circuit and double check the wiring. Reset the voltage to the lowest setting to begin the experiment. 7. Turn the voltage of the power supply up SLOWLY until the voltage reads 2V on the voltmeter. 8. Record the current (measured by the ammeter) in your data table. 9. Continue turning the power supply up so the voltage increases by 1V increments until you reach 14V. 10. Record the values of the current for each of the steps in your data table. 11. Turn the voltage down entirely, then turn off the power supply before you change the element. 12. Allow the lamp to cool, then replace it with the 30Ω resistor and repeat the experiment (#7-11). 13. Allow the 30Ω resistor to cool, then replace it with the 50Ω resistor and repeat the experiment (#7-11).
Data Tables: Be sure to take into account the scale settings! Don't just copy the numbers!
Lamp Current Voltage (A) (V) 2 3 4 5 6 7 8 9 10 11 12 13 14
30Ω Current Voltage (A) (V) 2 3 4 5 6 7 8 9 10 11 12 13 14
50Ω Current Voltage (A) (V) 2 3 4 5 6 7 8 9 10 11 12 13 14
Graphs (2): For both graphs, assign current (amperes) to the x-axis and voltage (volts) to the y-axis. Draw one graph on each side of your graph paper. The graphs should fill most of the paper. 1. On the front of the page, make a single graph showing the data for the 30 Ω and the 50 Ω resistors (using one set of axes). Draw a separate best-fit line for each data set. 2. Draw a separate graph for the lamp data, and sketch a best-fit curve. Calculations: Submit the following calculations in your printed report. 1. For each of the two resistors, using the range shown on your graph, calculate the slope of the line. 2. For each of the two resistors, calculate the percent error between the resistance measured from the graph and the accepted values of 30 Ω and 50 Ω. 3. Calculate the maximum power that was used by each of the resistors and the lamp in this experiment. Did any of these values exceed the power rating for these devices (30Ω:25W, 50Ω:20W, lamp:5W)? Sources of Error: Identify two possible source of scientific error associated with your data collection and how each impacted your data/results. Summary Questions: 1. How can you tell from your graphs that the resistors followed Ohm’s Law, but that the lamp did not? 2. Why did the lamp not follow Ohm’s Law? 3. What happens to the resistance of the lamp as the current increases? 4. Why do you think that incandescent light bulbs generally burn out when turned on rather than while they are already on?
Lab Investigation 2.4: Ohm’s Law Spring Semester 2016 For this lab investigation, you will be doing a formal lab write-up. Your write-up should follow this format: 1. Typed, double spaced, 12 point font -- standard font such as Times New Roman. Do not exceed three pages. Your lab sheet with the data tables, as well as the graphs must be attached at the end. Optionally, a cover page including the title of this lab, your name, and your class section. Please note the lab sheet, the graph paper, and the cover page don’t count towards the three pages. 2. You should organize your report in the following format: a) An introductory paragraph stating the main hypothesis/objective of the lab. b) The body of your report must include: - The summary questions #1,#2,#3, #4. - The sources of error and the % error calculations. Focus your discussion on how each of the sources impacted your results and suggest a procedure to mitigate the error. c) Don’t discuss procedures, data collection, etc. because these have been addressed in the lab and in the lab sheet. 3. Your writing: a) Should be in prose, no bullets or listed points. b) Should be in the past tense and passive voice. Remember, you did this investigation earlier in the week. For example: "The ammeter was used to measure the current." NOT "Measure the current using the ammeter." c) Should use proper scientific and physics terms and concepts. d) Adhere to the writing standard, as defined by your English teacher, for a Bronx High School of Science student, with proper spelling, subject verb agreement, punctuation, etc. 4.
The following rubrics will be used to evaluate your report: 1. Introduction paragraph 10% 2. The data tables, completeness & neatness 10% 3. Data analysis: a) The graph: proper scaling and neatness, best fit lines 15% b) Calculation of the slopes for the two best fit lines 10% 4. Discussion Summary Questions 30% 5. Conclusion paragraph and error analysis 10% 6. Organization and grammar 15%
5.
The grade for this lab will be counted double and it is due on the date stated by your teacher:
DUE __________________________________