Purpose

1
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.

2
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.

Heat added (in Joules)
Change in Temperature (in Kelvin)
Heat capacity (in Joules/Kelvin)
If Q gt 0 then Tfinal gt Tinitial
(temperature rises) If Q lt 0 then Tfinal lt
Tinitial (temperature drops)
3

• 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
The specific heat capacity only depends on the
material, not on the amount of the material.
4
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.

Heat added (in Joules)
Change in Temperature (in Kelvin)
Specific Heat capacity
mass of the object
5
Example and Implications of Specific Heat Capacity
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/(cm) 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.

6
Heat Transfer Between Two Objects(assume no heat
is lost to the environment)
Before contact
After reaching thermal equilibrium they both have
the same temperature
m1 c1
T1, final
T1, final T2, final Tfinal
Given m1, m2, c1, c2, T1,initial, T2,initial
(Tfinal unkown)
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Because no heat is lost to (or gained from) the
environment
The originally colder object gains energy (a
positive Q) The originally hotter object looses
energy (a negative Q)
? Solve for Tfinal
8
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
9
Activity 2 Specific Heat Capacity / Power Output
of Heater
Styrofoam cup filled with 150 ml water. Make sure
heater doesnt touch styrofoam !!!!!
Stand clamp
Red LED Heater is ON
Computer Data Studio Switches Heater on/off
heater
Heater Switch Box
Temp. Probe
750 Interface
Make sure this is plugged in the right way
(ground to ground) ground is marked on tape
10
Activity 2 Specific Heat Capacity
Switch heater on and monitor the rise of the
temperature
First run with heater power connected.
Temperature
DT
Second run with heater power disconnected
from outlet.
time
Heater off
Heater on
Dt
Note The temperature may still rise after you
turn the heater off (it gets turned off when you
hit the STOP button in Data Studio). Problem
Data Studio stops monitoring the temperature
after STOP is hit. Solution After you hit the
STOP button, unplug the main power (outlet)
from the heater box. Then hit START again to
monitor the temperature without further heating.
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Activity 2 Specific Heat Capacity
Determine power output of the heating
element. Power Energy / time c m DT / Dt
This is the heat/energy given off to the water (
Q )
Compare your result to the power rating written
on the heating element.
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Activity 3 Measure 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.

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Activity 4 The Transfer of Heat
Caution This experiment uses liquid nitrogen,
which is extremely cold. Follow the safety
instructions in your lab manual!!!!
Step 2 Put cold brass (-197ºC) into water.
Step 1 Cool brass in the LN2 (wait until
bubbling stops)
Step 3 Monitor temperature
Brass disc on a string
Liquid nitrogen (LN2)
Water at Room Temperature
14
Activity 4 The Transfer of Heat
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)
15
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) ? Thats why you need to use your measured
  power rating in Activity 3, not the official
  rating.