Thermal Energy & Equilibrium
Today’s Goal: To understand how thermal energy (heat) is transferred from one to
another.
Discussion:
>Why does metal feel cold?
> Compare the colors of the red dot and yellow dot temperature sensing sheets as you
warm them up and cool them down. If the red dot sheet turns brown, what does it imply?
Or can’t this happen?
Is fur warm? Is glass cool?
Furry mittens or woolly socks make you warm. What effect do they have on a
thermometer?
On Thursday we wrapped up some thermometers in various ways. Now we will see
what temperatures they read.
One thermometer was wrapped up in a sock.
One thermometer was sealed in a bag with some water.
One thermometer was wrapped with aluminum foil.
One thermometer was covered with sand in a glass jar.
One thermometer was left unwrapped, lying on a paper towel.
What effect did wrapping the thermometers have?
23- 1
What to do:
1. Before you start, read the procedure and predict what you think will happen, by
drawing a graph for each cup. This will make you more aware of what actually happens.
Draw your prediction graphs below. Make 3 graphs on the same grid.
a.
b.
c.
d.
e.
Procedure:
Read the entire procedure, a – e before starting:
Label each calibrated cup (cup with scale) as 1, 2 or 3
Pour 30 cc of cold water in cup 1, 30 cc of room temperature water in cup 2, and
30 cc of hot water in cup 3.
Read the temperature in each cup and record in the table at 0 min.
Leave the cups sitting on the table top. Stir gently with the thermometer before
each reading. Measure and record their temperatures every minute.
Continue collecting data for 11 minutes.
2. Follow the procedure and record your data in the table.
Time (min)
Cup 1
Temperature (°C)
Cup 2
Temperature (°C)
0
1
2
3
4
5
6
7
8
9
10
23- 2
Cup 3
Temperature (°C)
3. Make line graphs of cup 1, cup 2 and cup 3 data, ending up with three lines (or
curves) on the grid below.
4. Interpret your graphs. That means look for general trends that each cup shows and
summarize how they compare to each other.
5. If you left the cups for another hour or more, what does your graph suggest their
temperatures would be?
6. If you wanted to speed this experiment up, what could you do differently?
23- 3
7. Are you in thermal equilibrium with the air in the room? Explain your answer.
8. Metal feels cold while paper does not. Does this mean that the metal has a lower
temperature than the paper, or is there another explanation?
Check #1: Discuss your results with an instructor.
Heating by Hand
How long does it take to warm something, just by holding it?
1. Ann and Barb are each holding a cup of cold water in their hands. Barb’s cup
contains three times as much water as Ann’s. Predict what you think will happen to the
temperature in each cup, by drawing a graph for each one on this grid.
23- 4
Procedure:
Read the entire procedure before you start:
a. Measure the temperature of the cold water with both thermometers, so that they
start out at the temperature of the cold water.
b. Put 60 cc (2 ounces) of cold water in one cup (small cup).
c. Put 20 cc (2/3 ounce) of cold water in the other cup (small cup).
d. Measure the starting water temperature in each cup, and record for Time.
e. Have a team member hold both cups, one in each hand, for the entire time.
f. Have another student measure the temperature every minute for 7 minutes.
g. Gently stir the water in each cup before each measurement.
3. Enter your data in the table.
Time
Cup 1
Cup 2
(minutes)
Volume of water: 20 cc
Volume of water: 60 cc
Temperature (°C)
Temperature (°C)
0
1
2
3
4
5
6
7
4. Using your data for each cup, make a line graph that shows how the temperature of
the cup changed over time. (You will have two lines or curves on this grid, one for each
cup.)
23- 5
5. Ask the person who was holding the cups to describe how his/her hands felt while the
cups were warming. Include in the description whether both hands felt the same or
different, and how.
23- 6
6. The cups and water get warmer because the person holding them added thermal
energy to them. Which cup needed the most energy to get to 10 °C?
Explain your reasoning.
Check #2: Discuss your results with an instructor.
23- 7
Sharing Energy
What temperature do you get when you mix hot water and cold water? What does the
temperature of the mixture depend on?
Materials
 2 thermometers
 1 foam cup
 thermos cup containing hot water
 thermos cup containing cold water
 a measuring scoop, with handle
 shallow pan (to use as the waste water container)
1. Measure the temperatures of the hot and cold water in the thermos cups and record
them below. Put the lid back on as soon as you complete a measurement to keep the
water at constant temperature.
2. Before you start, predict what temperature you would get if you mixed equal amounts
of your hot and cold water together. (Don’t mix them yet, just predict)
3. Follow the procedure. Enter your data in the table
Procedure
Read the whole procedure before you start.
a. One at a time, make mixtures of hot and cold water as described in the table.
Measure and record.
b. Always use a full scoop when measuring out your water, and keep the
reservoirs covered.
c. Always use a total of 4 scoops to make your mixtures
d. After each measurement dump the mixture out into your waste water container
and make the next mixture.
23- 8
Amount of hot water Amount of cold water Temperature of mixture
(number of scoops) (number of scoops)
ºC
0
4
1
3
2
2
3
1
4
0
4. Make a graph that shows how the temperature of the mixture depends on the number
of cups of hot and cold water in the mixture.
5. How is this activity related to the second one we did (with the coffee pot)?
23- 9
Each group should hand in one copy of this page at the end of class
Group:
Names of group members present:
1. The wind certainly makes you feel cold. Yet a fan blowing on a thermometer does not
change the reading! (See for yourself! It has been set up at the front of the room). Even
though both thermometers and people have red noses, they differ in the effect that the
wind has on them. Discuss the other respects in which you differ from a thermometer
that that might explain this.
2. Cooking something mainly involves getting it to the right temperature. It takes a lot
longer to roast a turkey than to roast a chicken. Why?
3. Are you in thermal equilibrium with the air in the room? Explain your answer.
23- 10