Download Lab 1: Temperature and Heat

LAB 1
Temperature & Heat
OBJECTIVES
1. Predict and measure the latent heat of fusion and vaporization.
2. Test the principle of conservation of energy by determining whether the energy lost
by a resistor is equal to the energy absorbed by water.
3. Observe phase transitions and the difference between heat of transformation and
specific heat.
EQUIPMENT
Buckets of hot water and ice water, liquid nitrogen, Styrofoam cups, digital scale, DC
power supply and power resistors, Capstone and temperature probe.
THEORY
Heat is energy in transit between two or more objects. Temperature however, is a
measure of the average kinetic energy of all the particles in the object. The number for
the temperature of an object doesn’t tell you much about the actual kinetic energy of a
particles inside the object. The number comes from a temperature scale such as the kind
found on a common thermometer.
When heat is added to a substance, its temperature normally rises. However, when a
substance undergoes a change of phase, e.g., solid to liquid or liquid to gas, the heat
energy goes into doing work against the intermolecular forces and is not reflected in a
change in the temperature of the substance. This heat energy is called the latent heat of
fusion Lf and the latent heat of vaporization LV for the phase changes that occur at the
melting point temperature and boiling point temperature, respectively.
PROCEDURE
Part 1: Latent Heat of Vaporization- Lv
(a) Connect a 1.0 Ω resistor to a DC power supply, adjust the voltage to 5.0 V, and turn
off the supply.
(b) Add some liquid nitrogen to the insulated container and immerse the resistor. Once
the resistor's temperature has dropped to liquid nitrogen temperature (77 K), measure
the resistance of the resistor.
(c) Start recording the cup's mass once every 10 seconds. You will note that it drops
steadily as the nitrogen boils off due to heat gain from the room.
(d) After one minute, turn on the power supply for one minute and record the cups mass
once every 10 seconds.
(e) Turn the power supply back off and record the cup's mass once every 10 seconds for
an additional minute.
(f) Plot your mass-time data and draw straight lines through the data taken during the
first minute and the data taken during the last minute. Determine the mass of nitrogen
boiled off by the resistor from the vertical distance between the two lines near the
center of the heating period.
(g) Find the latent heat of vaporization of nitrogen by assuming that the electrical energy
supplied by the resistor equals the thermal energy gained by the nitrogen as it boiled.
(h) Put your value for the latent heat of vaporization on the white board. When all lab
groups have reported their values, calculate an average, standard deviation, and
standard error.
(i) Compare your result with the accepted value of 198 × 103 J/kg. How did your result
compare with the accepted value? What are some possible reasons for any
discrepancy between your result and the accepted value?
Part 2: Latent Heat of Fusion- Lf
(a) Put a known mass mwater (use about 100 g) of warm water in an insulated container
and measure the temperature of the warm water. If you start with water about 10oC
above room temperature and end with the water about 10oC below room temperature,
the heat that sneaks into the cooler room from the warm water will nearly cancel the
heat that sneaks into the cooler water from the warm room. This improves your
results.
(b) Add pieces (about the size of the end of your thumb) of carefully “dried ice” (dry them
with a paper towel) of known mass mice. Stir to keep the system in thermal equilibrium.
Measure the temperature T of the mixture frequently.
(c) Once the mixture reaches a constant final temperature, compute the heat of fusion
(Lf)exp from conservation of energy:
Qice + Qwater = 0
mice ( L f )exp + mice cwater ∆Tice water + mwater cwater ∆Twater =
0
(d) (Put your value for the latent heat of fusion on the white board. When all lab groups
have reported their values, calculate an average, standard deviation, and standard
error.
(e) Compare it with the accepted value, ( L f )=
333 × 103 J/kg . What were the most
thy
important sources of error in the experimental procedures? If the temperature of the
ice added to the calorimeter were less than 0oC, how would this affect the results? Is
energy conserved in the phase transition between water and ice?
Part 3: Conservation of Energy
(a) Put a known mass (about 100 g) of water into a foam cup. Use water that is about
five degrees below room temperature when data collection begins. Take data until
the temperature of the water is about five degrees above room temperature to
minimize the effects of heat transfer between the water and the room.
(b) Submerge a 1 Ω heating resistor and the temperature sensor into the water. Turn on
the power supply (use a voltage of 5 V) and begin recording data.
(c) The electrical energy dissipated by the resistor is given by E = P ⋅ t = (V 2 / R) ⋅ t where
V is the voltage and R is the resistance. The thermal energy Q absorbed by the water
is given by Q = mc∆T. Use these equations to predict how much the temperature of
the water should change after 1 min, 2 min, 3 min, 4 min, and 5 min. Compare your
predictions with your results using a percent difference. What are some possible
reasons for any discrepancies between your results and your predictions?
(d) Continue recording data until the temperature reaches five degrees above room
temperature
(e) By conservation of energy, the electrical energy dissipated by the resistor should
equal the thermal energy gained by the water, neglecting losses to the surroundings.
Compare the thermal energy gained by the water to the electrical energy dissipated by
the resistor using a percent difference. Was the thermal energy gained by the water
greater, the same as, or less than the electrical dissipated by the resistor? Why?