Physics 1809 Minilab 2: Heat and Temperature Physics 1809: The Relationship Between Heat and...
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Transcript of Physics 1809 Minilab 2: Heat and Temperature Physics 1809: The Relationship Between Heat and...
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Purpose
• Understand how the total energy in a closed system is conserved during heat exchange.
• Learn how to determine specific heat capacities of certain materials.
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
The Heat Capacity of an Object
Amount of heat (energy) that needs to be added to the object in order to raise its temperature by 1 degree Kelvin.
initialfinal TTCTCQ
Heat added(in Joules)
Heat capacity(in Joules/Kelvin)
Change in Temperature(in Kelvin)
If Q > 0 then Tfinal > Tinitial (temperature rises)If Q < 0 then Tfinal < Tinitial (temperature drops)
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
The heat capacity depends on:
Type of Material Amount of the material (more water has more heat
capacity……… you need more energy to raise its temperature………
The specific heat capacity is defined as
and has units of or
mass
Cc
Kkg
J
Kg
cal)19.41( Jcal
The specific heat capacity only depends on the material,not on the amount of the material.
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
The Specific Heat Capacity
Amount of heat (energy) per unit mass that needs to be added to a material in order to raise its temperature by 1 degree Kelvin.
initialfinal TTmcTmcQ
Heat added(in Joules)
Specific Heat capacity
Change in Temperature(in Kelvin)
mass of the object
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Example and Implications of Specific Heat Capacity
Kg
J
Kg
calcwater 180.41
A calorie is defined as the amount of heat that needs to be added to 1 gram of water in order to raise its temperature by 1 degree Kelvin.
Water has a relatively high heat capacity, which is important in biology and engineering:
Prevents your body (= mostly water) from heating up too quickly during exercise (an apple that contains 60Kcal of energy has the potential to raise the temperature of a 60Kg person by only
DT = Q/(c*m) = 60000cal/(1 cal g-1K –1 * 60000g)=1Kelvin (assuming all the energy in the apple would go to heat and none to work performed)
Is a good coolant for engines (can absorb a lot of heat without having its temperature rise a lot.
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Heat Transfer Between Two Objects(assume no heat is lost to the environment)
m1 c1
T1, initial
T2, initialm2 c2Before contact:
After reachingthermal equilibriumthey both have the same temperature
m1 c1
T1, final
T2, finalm2 c2
T1, final = T2, final = Tfinal
Given: m1, m2, c1, c2, T1,initial, T2,initial (Tfinal unkown)
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
initialfinal TTmcQ ,1111
initialfinal TTmcQ ,2222
Because no heat is lost to (or gained from) the environment:
021 QQ
The originally colder object gains energy (a positive Q)The originally hotter object looses energy (a negative Q)
0,222,111 initialfinalinitialfinal TTmcTTmc
Solve for Tfinal
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Activity 1: Calibration of Temperature Probe
Themocouples are very easily built temperature sensors.
Two dissimilar conductors touching each other produce a voltage V that changes with temperature T.
Over certain temperature ranges T is proportional to V and one can calibrate them by finding the voltage for two temperature points and then making a linear approximation. Thermocouple voltages can then be related to temperature.
V
TTcold
Vcold
Vhot
Thot
Two calibration points
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Activity 1: Calibration of Temperature Probe
Alcohol thermometer (read off temperature here)
Temp. Probe 750 Interface
Use ice bath and warm water bath for the two calibration points
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Activity 2: Determining the Power Output of the Heater
Temp. Probe 750 Interface
Styrofoam cup filled with 150 ml water. Make sure heater doesn’t touch styrofoam !!!!!
Computer:CapstoneSwitchesHeater on/off
Heater Switch Box
Red LED: Heater is ON
Make sure this is plugged in the right way (ground to ground); ground is marked on tape
Stand +clamp
heater
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Observe total temperature change DT due to heating:
time
Temperature
Heater on Turn Heater off (Turn off “Signal Generator” in Capstone)
DT
Dt
After switching heater off.
First part with heater on
Activity 2: Determining the Power Output of the Heater
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Activity 2: Determining the Power Output of the Heater
Click here to open signal generator panel
• First click on “Auto” : This ensures that when you hit “Record” the heater automatically starts with the recording.
• Click on “Record” (lower left corner of Capstone).
• After a fixed (known) amount of time stop just the heater (but not temperature measurement) by clicking “OFF”.
• To stop temperature recording, click on the “Stop” button in the lower right corner.
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Power = Energy / time = c m DT / Dt
Compare your result to the power rating written on the heating element.
This is the heat/energy given off to the water( = “Q” )
Activity 2: Determining the Power Output of the Heater
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Activity 3: Determine Specific Heat Capacity of Isopropyl Alcohol
Design an experiment to measure c isopropyl alcohol
Use your measured power rating of the heating element.
• DO NOT DRAIN THE ISOPROPYL ALCOHOL INTO THE SINK !!!!! It is illegal to do that and we also do not want to waste the alcohol – it costs money.
• Instead, please pour it back into the container from which you got it.
Caution: Isopropyl alcohol is a flammable liquid. If you accidentally spill it over a power outlet, do not unplug the equipment. Sparks may ignite the alcohol!
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Activity 4: The Transfer of Heat
Caution: This experiment uses liquid nitrogen, which is extremely cold. Follow the safety instructions!
Liquid nitrogen (LN2)Water at Room Temperature
Brass disc on a string
Step 1:Cool brass in the LN2(wait until bubbling stops)
Step 2:Put cold brass (-197ºC) into water.
Step 3:Monitortemperature
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Step 4: Determine the specific heat capacity of brass
Step 5: Compare your value of cbrass to that in the literature (you can surely find that value on the internet)
Activity 4: The Transfer of Heat
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Hints
• Do not be surprised if the power rating of the heater element disagrees with what you measured. When we measured the resistance of the heating elements with wires, some were as high as 2 Ohms.
• Therefore, a more realistic power rating may be about
• …and it may be even lower if the supplied voltage is less than 12 Volts (on some of the heater boxes) That’s why you need to use your measured power rating in Activity 3, not the official rating.
Watt
V
R
UP 72
2
12 22
Physics 1809 Minilab 2: Heat and TemperaturePhysics 1809: The Relationship Between Heat and Temperature
Liquid Nitrogen Safety• Wear goggles when using liquid nitrogen.• Pour liquid nitrogen slowly and very carefully.• Do not deliberately spill liquid nitrogen.• When putting the brass object into liquid nitrogen, suspend it on a
string and carefully lower it into the liquid nitrogen (the nitrogen will boil rapidly).
• Do not touch liquid nitrogen or metal surfaces that are in direct contact with liquid nitrogen.
• Never put liquid nitrogen into a closed container (except into specially designed containers with pressure relief and safety valves). It will explode and you may be seriously injured or killed by the exploding fragments.
• When carrying liquid nitrogen to your table, walk carefully and make sure nobody bumps into you (or you into them).
• When done with your liquid nitrogen, you can carefully pour it back into the dewar.