Physics 1809 Minilab 2: Heat and Temperature Physics 1809: The Relationship Between Heat and Temperature Purpose Understand how the total energy in a closed

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.


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)


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.


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


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.


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)


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


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


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


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


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




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.


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 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!


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


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


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


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.

Documents

Physics 1809 Minilab 2: Heat and Temperature Physics 1809: The Relationship Between Heat and Temperature Purpose Understand how the total energy in a closed