Objective: The object of this lab is to show how temperature affects the conductivity (resistance) in various electrical materials and devices. Review of Scientific Principles: Heat: As heat is applied to a crystalline solid, we say "it gets hotter"; mean
PRELIMINARY INFORMATION
Hot and Cold
Temperature and Resistance of Electronic Materials
Objective: The object of this lab is to show how temperature affects the conductivity (resistance) in various electrical materials and devices.
Review of Scientific Principles:
Heat: As heat is applied to a crystalline solid, we say "it gets hotter"; meaning the temperature increases. On the atomic level, the kinetic energy of the atoms has increased which means the atoms are moving faster. However, in a crystalline solid, the atomic movement is limited to vibration around stable lattice positions. As the temperature increases, the atoms vibrate at a greater amplitude and move farther from their stable lattice positions. This motion has a negative effect on the ability of the material to conduct an electric current, causing it to have a greater electrical resistance.
Metals: In a metal, the valence electrons are thought of as being shared by all the positive ions. Therefore, the electrons are free to move throughout the crystalline lattice. The electrons move randomly throughout the crystal, until an electric field is applied to the material. Then the electric field forces the electrons to move in a direction opposite to the field. Actually, the electrons still move somewhat randomly, but with a superimposed "drift". This produces current. As the temperature increases, the positive ions in the crystal vibrate more, and more collisions occur between the valence electrons and the vibrating ions. These collisions hinder the "drift" motion of the valence electrons, thus reducing the current. In summary, for a metal, an increase in temperature causes an increase in resistance.
Semiconductors: In a semiconductor, at 0 K, valence electrons are in filled energy levels (bonds are formed by electron pairs filling the energy levels). They do not respond to an applied electric field to produce current flow. In the presence of an electric field, the electron motion is still random; no net motion in one direction occurs (no current flows). These filled energy levels, which the valence electrons occupy, form the valence band. In order for current to flow, electrons must move from the filled valence band to the empty conduction band. To make this move requires energy, which can be in the form of heat. (Important: the electrons do not move from a "place" in the crystal called the valence band to another "place" called the conduction band. The electrons have the energy associated with the valence band and acquire enough energy to have the energy associated with the conduction band. An energy change occurs, not a position change.) At room temperature, many electrons will have the energy needed to jump to the conduction band. As one electron moves out of the valence band and into the conduction band, a hole (vacancy) is produced in the valence band. Both the electrons in the conduction band and the corresponding holes in the valence band are considered charge carriers. When an electric field is applied to the material, these electrons and holes "drift". The electrons in the conduction band drift in the direction opposite to the applied field, and the holes drift in the same direction as the applied field. Thus, current is produced. As the temperature of the material is increased, more valence electrons acquire sufficient energy to move to the conduction band (producing holes), so more current flows. It is still true that as the temperature is increased, the atoms vibrate more and cause more collisions with the drifting electrons. However, this opposing effect is negligible, compared to the increase in charge carriers.
Applications:
Different types of materials respond differently to temperature changes. A computer engineer designing a circuit must be able to predict if the conductivity of each material in the device will be within an acceptable range over the expected temperature range of operation of the device.
Time: One hour
Materials and Supplies:
heat source for boiling water (hot plate preferred)
5 beakers for water baths
thermometer
choke coils or resistance spools
germanium diodes
thermistors
light emitting diodes (LEDs)
carbon resistors
glass rod (5 cm)
2 digital multimeters or a voltmeter and milliammeter
wire connectors with alligator clips
power supply (0 to 12 volts DC)
General Safety Guidelines:
* The heat source could cause burns. ...
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Time: One hour
Materials and Supplies:
heat source for boiling water (hot plate preferred)
5 beakers for water baths
thermometer
choke coils or resistance spools
germanium diodes
thermistors
light emitting diodes (LEDs)
carbon resistors
glass rod (5 cm)
2 digital multimeters or a voltmeter and milliammeter
wire connectors with alligator clips
power supply (0 to 12 volts DC)
General Safety Guidelines:
* The heat source could cause burns. Exercise caution.
* Be careful of electrical shock.
* Handle meters and samples with care.
* Wear safety glasses.
Procedure:
. Set up five water baths of about 100-200 ml of water in beakers at the following
temperatures: boiling, 75 ° C, 50 ° C, 25 ° C, and ice water.
2. Measure the temperature of each bath with a thermometer, thermistor, or
thermocouple.
3. For measuring the resistance of the device (choke or resistance coil) set up the
multimeter to read ohms and connect as in the following diagram.
4. Carefully holding on to the lead wires so as not to burn the fingers, immerse the
coil into the boiling water bath, until a stable value is received (for about one
minute) and record the resistance in the data table.
5. Follow the same procedure in the 75 °, 50 °, 25 ° and ice water.
6. Remove the coil and attach another device to the meter, following the same
procedure for measuring the resistance.
Data and Analysis:
CONDITION
TEMP. C
CHOKE COIL
GERMANIUM
DIODE
THERMISTOR
°
R
°
R
°
R
°
R
Boiling water
00
Hot water
75
Warm water
50
Room temp
25
Ice water
0
For each device, draw a graph, with the temperature (x-axis) vs. resistance (y-axis).
Questions:
. Which samples had a change in resistance as the temperature increased? What
direction was that change?
2. As their temperatures increased, what happened to the resistance of the
conductors, the semiconductors? Does the change seem to be linear?
3. Did any of the examples not follow the general guidelines explained in the
introduction to the lab? Explain.
4. Describe the motion of the atoms or ions in a crystalline solid as the temperature
increases.
5. What causes electrons to "drift"?
6. Describe the electron motion while current is flowing.
7. Explain how increasing the temperature of a semiconductor decreases the
resistance.
8. Explain how increasing the temperature of a metal increases its resistance.
Extension:
For the thermistor, plot 1/T (K-1) on the x-axis and ln R (natural log of the value of the resistance in ohms). This graph is a straight line. The equation of this line is:
ln R = (Egap / 2k) x 1/T + ln Ro
where:
k = 8.62 x 10-5 eV/K (Boltzman's constant)
Egap = band gap energy (the difference in energy between the conduction and valence bands) in electron volts.
Determine the slope of the line from the graph. (Egap / 2k) = the slope, from the equation. Solve this equation for Egap.
Teacher Notes:
*Teacher preparation time is about 30 minutes.
* Resistance coils could be used instead of a choke coil. (Short pieces of wire do not show enough resistance.)
* Other kinds of wire besides copper could be tried.
* Connector leads to devices could be extended by soldering on short lengths of wire.
* The carbon device should lose only a small percentage of its room temperature resistance, but semiconductor devices should go up appreciably at low temperature.
* As an example of a nonconductor , a length of glass rod could be tested.
* Use a Type K thermocouple for lower temperatures in conjunction with some digital multimeters.
* Thermistor probes are available form Vernier.
* The teacher should demonstrate proper hookups of meters and devices.
* Diode results work best if the temperature is taken from the boiling water and the ice water.
* For the extension activity, using the sample data, the value of Egap= 0.6 eV.
Answers to Questions:
. All the devices changed their resistance as the temperature changed. The
resistance of the choke coil (which is copper wire) increased as the temperature
increased. The resistance of the diode and the thermistor (which are made of
semiconductor material) decreased as the temperature increased.
2. The resistance of the conductor increased linearly. The resistance of the
semiconductor decreased, but not linearly.
3. Student answers will vary. The devices do react as theoretically predicted.
4. As the temperature increases, the atoms or ions vibrate with greater amplitude
around their stable lattice positions.
5. When an electric field is applied, the electrons are forced to drift .
6. The electrons are moving randomly and drifting in the opposite direction of the
applied electric field.
7. As the temperature increases, more electrons have the energy needed to move to
the conduction band (more charge carriers means more current).
8. Greater amplitude of vibration of the ions in the lattice cause more collisions with
the valence electrons, which decreases the drift velocity.
Sample Data and Analysis:
Condition
Temp ° C
Choke
Coil
Germanium
Diode
Thermistor
°
R
°
R k
°
R k
°
R k
Boiling Water
00
97
44
98
0.29
98
.0
Hot Water
75
77
41
70
0.63
76
.7
Warm Water
50
51
37
44
.2
42
5.2
Room Temp
25
22
33
22
.7
21
1
Ice Water
0
3.5
31
2.8
2.4
3
27
Ohmic Heating Coursework
Introduction-
I am researching into the effect that changes in temperature and conditions has on the resistance of a wire. I will perform a series of experiments on a wire and record the results.
From my preliminary information, I can see that when there is no electrical charge on a wire, the electrons move randomly. However, when an electrical charge is passed through the metal of the wire, the electrons move in the direction that is opposite to the field. This produces current. When the temperature increases, the atoms in the wire vibrate more and hinder the movement of the electrons. The more heat that the metal is exposed to, the more the atoms will slow the electrons down. This means that when a metal is exposed the an increase in temperature, its resistance also increases. In this experiment, there are several variables that I will keep the same:
o The material of the wire
o The thickness of the wire
o The source of power
o The amount of time that the wire is exposed to the electrical charge
o The apparatus used to read the voltage and current.
o The number of coils on the wire
However, there are also several variables that I will change:
o The voltage passing through the wire
o The temperature and conditions that the wire is placed under.
Aim-
To find out what effect changes in temperature, condition and electrical charge have on the resistance of a wire.
Apparatus-
EXPERIMENT 1
* A wire
* A pencil
* A power pack
* Crocodile clips
* A volt meter
* An Ammeter
* A bulb
EXPERIMENT 2
* A wire (same as the one used in experiment 1)
* A power pack
* Crocodile clips
* A volt meter
* An Ammeter
* A beaker
* A thermometer
* Water
* A Bunsen Burner
* A heat resistant mat
* A bulb
* A tripod
EXPERIMENT 3
* A wire (same as the one used in experiment 1 & 2)
* A power pack
* Crocodile clips
* A volt meter
* An ammeter
* A tap
Diagram-
PRELIMINARY CHECKING
EXPERIMENT 1
EXPERIMENT 2
EXPERIMENT 3
Method-
EXPERIMENT 1
. Begin your experiment by arranging some of your equipment as shown in the preliminary experiment diagram. This is to ensure that all of your equipment is working.
2. Once you are sure that all your equipment is properly working, remove the light bulb and arrange the rest of the equipment as shown in the diagram.
3. Set your power pack to 2 volts.
4. Turn the power pack on just long enough for you to get a reading from the voltmeter and ammeter. DO NOT TOUCH ANY METALLIC PART OF THE CIRCUIT WHILST THE POWER PACK IS ON.
5. Record your results.
6. Change the voltage on the power pack to 4 volts.
7. Repeat steps 4 and 5. After each time, increase the voltage by 2 volts.
8. Finish recording your results after you have recorded the 12-volt readings.
EXPERIMENT 2
. Begin your experiment by arranging some of your equipment as shown in the preliminary experiment diagram. This is to ensure that all your equipment is working.
2. Once you are sure that all your equipment is properly working, remove the light bulb and arrange the rest of the equipment as shown in the diagram.
3. Set your power pack to 4 volts but keep it switched off.
4. Turn the Bunsen burner on the safety flame and heat the water up to 20°c.
5. Turn the power pack on and record the readings on the voltmeter and ammeter. DO NOT TOUCH ANY OF THE METALLIC PARTS OF THE CIRCUIT OR THE WATER WHEN THE POWER PACK IS SWITCHED ON.
6. Turn the power pack off.
7. Wait for the Bunsen burner to heat the water up to 30°c.
8. Repeat steps 5 and 6.
9. After recording the 30°c results, wait for the temperature of the water to increase by another 10°c.
0. Repeat steps 5 and 6.
1. Continue increasing the temperature by 10°c until you get to 80°c.
EXPERIMENT 3
. Begin your experiment by arranging some of the equipment as shown in the preliminary experiment diagram. This is to ensure that all your equipment is working.
2. Once you are sure that all your equipment is properly working, remove the light bulb and arrange the rest of the equipment as shown in the diagram making sure that the coil of wire is directly under the running water.
3. Set your power pack to 1 volt.
4. Turn the power pack on and record the readings on the voltmeter and ammeter. DO NOT TOUCH ANY OF THE METALLIC PARTS OF THE CIRCUIT OR THE WATER WHEN THE POWER PACK IS SWITCHED ON.
5. Turn the power pack off.
6. Increase the power pack voltage by 1 volt.
7. Repeat steps 4-6 until you get to 13 volts.
Prediction-
EXPERIMENT 1
I predict that as the voltage is increased, the resistance will not increase. I also think that if the results are plotted in a graph, the results will be shown in a straight line. This is because when the charge is increased, the electrons in the wire move faster. But as the temperature that the experiment takes place under is an even temperature throughout. I do not expect the resistance to change much throughout either. However, if the current and the voltage both increase in the results, the resistance will have to stay the same 'to balance them out'.
EXPERIMENT 2
I predict that as the temperature is increased, the resistance will increase. I also think that if the results are plotted on a graph, the results will not be directly proportional. I think that as the slope on the graph will be much steeper. This is because, as the heat increases, the atoms vibrate faster and produce more kinetic energy. This movement of atoms will get in the way of the electrons, which are carrying the electricity. My preliminary information also shows that as the heat increase, the resistance of the wire increases.
EXPERIMENT 3
I predict that as the voltage is increased, the resistance will remain the same. This is because, as the voltage is increased, the current will increase as well. According to my preliminary information, when the heat is increased, the resistance also increases but the temperature of the tap water will stay the same. Therefore, the resistance will stay the same so as to 'balance' the equation out. The running water will provide an even temperature for the experiment to take place in so the resistance will not increase with the aid of heat, like in experiment 2.
Results-
EXPERIMENT 1
VOLTAGE
Amps.
Volts.
Resistance
2
0.14
0.67
4.79
4
0.55
2.52
4.58
6
0.95
4.26
4.48
8
.4
6.16
4.4
0
.8
7.9
4.39
2
2.16
9.55
4.42
EXPERIMENT 2
TEMP °c
Amps (a)
Volts (v)
Resistance
20
0.37
.8
4.86
30
0.28
.3
2.24
40
0.35
.5
4.29
50
0.4
.8
.29
60
0.67
.5
2.24
70
0.38
.5
3.95
80
0.38
.6
4.21
EXPERIMENT 3
VOLTAGE
Amps
Volts
Resistance
0.01
0.03
0.33
2
0.13
0.57
4.38
3
0.32
.34
4.19
4
0.55
2.29
4.16
5
0.78
3.25
4.16
6
0.47
4.05
8.62
7
.18
4.67
3.97
8
.35
5.65
4.19
9
.58
6.58
4.16
0
.81
7.51
4.15
1
2.07
8.35
4.03
2
2.24
9.24
4.13
3
2.46
0.12
4.11
Conclusion-
From my results and graph, I can see that when the wire has an electric charge through it when it is dry, the resistance does not increase as more voltage is passed through it. In fact, it decreases before it slightly increased at the end. I think this occurred because of the limited range in temperature. This evidence supports my prediction. I think that this happened because as the current increased, the voltage also increased; the resistance must have stayed down in order to balance the equation out. If the voltage increases, either the current or resistance must increase. In this case, the current and voltage both increased. Therefore, the resistance must have stayed the same to balance the equation out. From my results and graph, I can also see that when the wire placed under water that varied in temperature, the current, voltage and resistance stayed the same. This does not support my prediction. I think that this happened because we did not use a wide enough range of temperature. We only used 20°c to 80°c. According to the Kelvin scale, at absolute zero (-273°c or 0°K), the atoms stop moving al together. According to my preliminary information, any metal that conducts electricity at absolute zero is known as a super conductor. This is because there is no resistance because the atoms do not move. We did not range our temperature from absolute zero to a higher temperature; this is why we did not get a range in resistance. From my results and graph, I can see that when the wire is placed under running tap water, the resistance did not change much either. This supports my prediction. I think that this happened because the resistance did not have the aid of heat. However, I think that the 'spike' in the graph may be due to a sudden change in the temperature of the water.
Evaluation-
I am not pleased with my results. This is because I didn't not receive a range of resistance. To improve my results, I should use a wider range of temperatures, preferably going into the '- ' temperatures. This should ensure a wider range of resistance results. I could also read the voltmeter and ammeter quicker to ensure that I do not damage the wire, which could also effect the results.
