Download Recitation 3.2 Temperature/Heat

EF 152 – Physics for Engineers
Spring, 2012
Recitation 3.2 Temperature/Heat
Task 1. Concept Questions.
1. Two objects are made of the same material, but have different masses and temperatures. If the objects are
brought into thermal contact, which one will have the greater temperature change?
a. the one with the higher initial temperature
b. the one with the lower initial temperature
c. the one with the greater mass
d. the one with the smaller mass
e. the one with the higher specific heat
2. Two equal-mass liquids, initially at the same temperature, are heated for the same time over the same stove.
You measure the temperatures and find that one liquid has a higher temperature than the other. Which liquid has
a higher specific heat?
a. the cooler one
b. the hotter one
c. both the same
3. The specific heat of concrete is greater than that of soil. A baseball field (with real soil) and the surrounding
parking lot are warmed up during a sunny day. Which would you expect to cool off faster in the evening when the
sun goes down?
a. the concrete parking lot
b. the baseball field
c. both cool off equally fast
4. You put 1 kg of ice at 0°C together with 1 kg of water at 50°C. What is the final temperature?
LF = 80 cal/g
cwater = 1 cal/g-°C
a. 0°C
b. between 0°C and 25°C
c. 25°C
d. between 25°C and 50°C
5. Given your experience of what feels colder when you walk on it, which of the surfaces would have the highest
thermal conductivity?
a. a rug
b. a steel surface
c. a concrete floor
d. has nothing to do with thermal conductivity
Task 2: Ice Melting Blocks
Hold the blocks and predict which block will melt ice the fastest.
Put some ice on each block and see what happens.
Explain the results.
EF 152 – Physics for Engineers
Spring, 2012
Task 3: Mechanical Equivalent of Heat
The tube contains 70 steel BB’s with a total mass of 23.2g. The specific heat of steel is 0.50 J/g-K. The end of
the tube has a thin aluminum plate with a mass of 1.57g. The specific heat of aluminum is 0.90 J/g-K. The tube
is 0.90 m long.
Turn the tube over so the BB’s fall on the instrumented aluminum plate five times. Measure the temperature
change of the aluminum plate.
Compare the mechanical energy to the thermal energy.
Mechanical energy: Each time the tube is turned over the BB’s fall the length of the tube. Calculate the total
amount of mechanical energy. _________ J
Thermal energy: Assume the temperature rise is the same in the BB’s and the aluminum plate. Calculate the
total thermal energy. _________ J
Lower α
Task 5: Thermostats/Bi-metallic strips
Older mechanical thermostats were made with bi-metallic
strips. Two metals with different coefficients of expansion were
bonded together. As the temperature changed, the strip would
bend due to the different rates of expansion. An example is
provided. Heat it with a hair dryer, and then put in front of the
air conditioner to see what happens. If the material with the
higher coefficient of expansion were on the left, which way
would the strip bend when heated? How could you use this
strip as a thermostat?
Higher α
Task 4: Snowball Fight
One winter, there is 26 inch snowfall recorded in Knoxville. Megan and Josh get in a snowball fight. Megan
notices that the snowballs coming from Josh contain broken pieces of black plastic. She assembles these pieces,
and realizes that they are parts of broken Hank Snow albums. Josh claims that this is fair because it is a recordbreaking snowstorm. However, Megan does not agree. Enough is enough!! Her temper overcomes her, and she
throws a snowball so hard that it completely melts when it hits the wall above Josh! Josh is so in awe that he
surrenders. If the temperature of the snowball is -10º C, and assuming that 50% of the energy of the throw is
converted to heat absorbed by the snowball, at what speed does the snowball hit the wall?
A
B
As bi-metallic strip is heated,
which way will it bend?
Task 6: Practice Problem
A container holds 0.500 kg of ice at -15°C. Assume negligible heat loss to the container and the air. Heat is
supplied to the ice at a constant rate of 8000 J/min. Plot a curve showing temperature vs. time for the length of
time it takes to boil all the ice away.