Physics Lab 212P-5



Physics Pre-lab 212P-5

Template

Resistance and Ohm's Law

Name:__________________________

Section:_____ Date:__________

Pre-lab Questions:

These prelab questions require you to carry out some simple experiments. It may be convenient to enlist the help of a friend for an extra pair of hands.

Q1. How to make really cheap ammeter ("RCA").

Deflection with two batteries:

Deflection with one battery:

Deflection with reverse polarity:

Q2. Resistance of a nichrome wire

Do you get a higher temperature with a short wire or a long wire?

______________________________________________________________________________

Do you get a larger current with a short wire or a long wire?

______________________________________________________________________________

Repeat the above experiment with the thick nichrome wire, and compare your results with an identical length of thin nichrome wire.

Do you get a higher temperature with a thick wire or a thin wire?

______________________________________________________________________________

Do you get a larger current with a thick wire or a thin wire?

______________________________________________________________________________

Q3. Is the resistivity of the Nichrome in the short wire less than, equal to, or greater than in the long wire? Explain your answer.

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Is the resistivity of the Nichrome in the thick wire less than, equal to, or greater than in the thin wire? Explain your answer.

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Q4. Note that you have two types of bulbs in the kit: a round one and a long one. Set up the circuit shown in Q1 and obtain a relative measure of the current in the circuit when each of the bulbs is used in the circuit.

|Measure of current |Round Bulb |Long Bulb |

|Brightness of bulb | | |

|(qualitative comparison) | | |

|RCA reading (degrees) | | |

| | | |

Once you've measured the data for the table above, examine each of the bulbs carefully. Qualitatively explain your experimental observations based upon the construction of the bulb filaments and using your empirical observations in Q2.

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Physics Lab 212P-5

Template

Resistance & Ohm's Law

NAME: ____________________________________

LAB PARTNERS: ____________________________________

____________________________________

LAB SECTION: __________________________

LAB INSTRUCTOR: __________________________

DATE: __________________________

EMAIL ADDRESS: __________________________

Lab Activity 1: Resistance and Resistivity of Nichrome

Find the two nichrome wires in your kit and use some tape to anchor a straight length of each wire flat onto the lab table (a straight segment of 20 - 25 cm should suffice).

Set the digital multimeter (DMM) provided to measure "resistance." Note that a DMM measures "resistance" by sending a known current I through the object to be measured and then measuring the voltage V across the probes (i.e. R = V/I). This measuring current is typically quite small.

Measure the lead and internal resistance of the DMM. On the lowest resistance scale, connect the red and black voltage leads together. Allow the reading to stabilize and note the value of the lead and internal resistance of the DMM.

Rlead,DMM = _______________________

Using a scale to measure length and the DMM to measure resistance, determine the resistance of each wire for each of 10 different lengths (e.g. from 1 cm to 20 cm in 2 cm intervals).

Enter these values in an Excel spreadsheet.

Use Excel to plot a graph of resistance versus length for each wire and fit each graph to a straight line. Include these graphs with your lab report.

Note: if you are still not too comfortable with Excel, make sure you ask for your instructor's help!

Answer the following questions:

Q1. From the two graphs and the linear fits, determine the ratio of the diameters of the two wires. Explain how you do this.

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Discuss the relationship between the y-intercept of your linear fit and Rlead,DMM measured previously. Is the lead and DMM resistance a systematic or a random error? Should your measured value of Rlead,DMM be subtracted off any subsequent resistance measurements?

______________________________________________________________________________

______________________________________________________________________________

______________________________________________________________________________

Q2. You are told that the thin wire is "30-gauge" in the American Wire Gauge (AWG) system. This means that it has a diameter = 0.0125 inches. (1 inch = 2.54 cm.) Using the graph of resistance versus length for the thin wire, calculate the resistivity of Nichrome.

Q3. Suppose a 5 cm length of the thin wire is connected to two (ideal) 1.5 V batteries in series. Assuming that the resistivity of the wire is the same as in your measurements in Q2, what is the current density in the wire?

Q4. If the density of conduction electrons in Nichrome is around 1028 electrons/m3, what is the drift speed of the electrons in the wire under the conditions described in Q3? How long does it take an electron to travel from one end of the wire to the other?

Lab Activity 2: Ohm's Law

Q5. Is the resistor an "ohmic" or a "non-ohmic" device? From the I-V plot for the resistor, determine the value of the resistance. Measure the resistance value of the resistor using the DMM. How do these values compare to the stated resistance value (brown, black, brown) of 100 Ω?

Rgraph = ___________ RDMM = ___________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

• Replace the resistor with the long light bulb included in your lab kit.

• Produce a graph that displays I versus VAB. Include this graph with your report.

• Change the voltage sweep to 0.015 V and produce a second graph. Use the linear fit function to estimate the low voltage resistance of the light bulb. Include this graph with your report.

Q6. Does the light bulb have a constant resistance? Is the light bulb an "ohmic" or "non-ohmic" device? Explain your answer.

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Q7. Why does the slope of the light bulb graph change? Explain (qualitatively) why the slope changed when a current was flowing through it.

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Q8. The slope of the graph for the light bulb is not symmetrical. Why is the slope of the current trace different when the filament is heating up compared to the trace of the current when the filament is cooling down?

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Q9. Measure the resistance of the light bulb using the DMM. How does this value compare to the value obtained from the low voltage sweep above. Is this what you expect and why?

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

• Replace the light bulb with the diode, reset the voltage sweep to 5.000 V, and produce a graph that displays I versus VAB. Please include this plot with your report.

Q10. Is the diode an "ohmic" or "non-ohmic" device? Explain your answer. Suppose someone asked you: "What is the resistance of that diode?" How would you respond?

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

_____________________________________________________________________________

Physics Post-lab 212P-5

Resistance and Ohm's Law

Name:__________________________

Section:_____ Date:__________

Q1. The (United States) National Electric Code, which sets maximum safe currents for rubber-insulated copper wires of various diameters, is given (in part) below. Plot the safe current density as a function of diameter.

|Gaugea |4 |6 |8 |10 |12 |14 |16 |18 |

|Diameter, milsb |204 |162 |129 |102 |81 |64 |51 |40 |

|Safe current, A |70 |50 |35 |25 |20 |15 |6 |3 |

Q2. Which wire gauge has the maximum safe current density?

Q3. The resistance of a resistor is measured at several temperatures, as shown below. Plot the data and a linear fit to answer the following questions.

|T, (C |50 |100 |150 |200 |250 |300 |

|R, Ω |139 |171 |203 |234 |266 |298 |

Q4. Find the value of resistance at 0 (C, 20 (C and 265 (C.

Q5. Find the temperature coefficient of resistance (instead of resistivity) with a reference temperature of 20 (C.

a A way of identifying the wire diameter.

b 1 mil = 10-3 in.

................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download