## Studio Session 10

### Changes of phase

Substances can exist in solid, liquid or gas phases.  At a constant pressure changes of phase always occur at the same temperatures for any pure substance.

The temperature is a measure of the internal, disordered energy of a substance.  The absolute temperature of any substance is proportional to the average translational kinetic energy associated with the random motion of the molecules of the substance.

½m<v2> = (3/2)kBT

In SI units the scale of absolute temperature is Kelvin (K).  But we use different temperature scales in everyday situations.  The Kelvin scale is identical to the Celsius (oC) scale, except it is shifted so that 0 oC equals 273.15 K.

If an equation in physics contains the temperature T, this temperature is always the absolute temperature.  In SI units it is always measured in K.  If an equation contains a temperature difference ΔT, this temperature difference can be measured in K or oC in SI units, since both scales give the same ΔT.

In this session you will experimentally investigate a change of phase of water.

Equipment needed:

• temperature sensor
• electronic balance
• cups with hot water and ice
• paper towels

Open a Microsoft Word document to keep a log of your experimental procedures, results and discussions.  Address the points highlighted in blue.  Answer all questions.

Discussion:

### Specific heat capacity

Consider the experiment described below.

A 1.0 kg mass and a 3.0 kg mass with different initial temperatures are placed together inside a well-insulated container and allowed to come to thermal equilibrium.  The insulated container prevents any transfer of energy to or from the environment (including the container itself).

Mass Initial temperature 1.0 kg 3.0 kg 100 oC 160 oC 440 kJ/(kg oC) 73.33 kJ/(kg oC) 120 oC 120 oC +20 oC -40 oC

Do you agree or disagree with the following statements?  Explain why you agree or disagree.

• (a)  Heat flows from the 1.0 kg mass to the 3.0 kg mass because the 1.0 kg mass has a much larger heat capacity.
• (b)  The thermal energy lost by the 3.0 kg mass is greater than the thermal energy gained by the 1.0 kg mass.

Experiment

Make sure the Pasco 850 interface is turned on.  Open the Capstone program.

• Plug the temperature sensor into Analog Channel A of the interface.
• Click Hardware Setup, Analog Channel A and choose to add a temperature sensor (stainless steel).
• Choose a digits display and measure the temperature.  Choose Continuous Mode, Fast Monitor Mode.

Start with a known quantity of hot water in a large Styrofoam cup.  Measure its temperature, and then add ice at its melting temperature (0 oC) until the temperature of the water drops to about 10 oC.  Measure the final temperature and determine the amount of ice added.  Determine the latent heat of fusion of ice/water using the measured masses and temperatures and value of the specific heat capacity of water.

Procedure:

• Record the mass of the Styrofoam cup in a table in Excel.
• Fill the cup ~½ full with hot water and record its mass again.
• Record the initial temperature Ti of the water.
• From a supply of melting ice cubes, dry off any excess water with a paper towel and drop one or two pieces into the cup of water.  Stir and wait until all the ice has melted and measure the temperature.  Repeat until the temperature of the water drops to about 5 - 10 oC.
• Record this temperature as Tf.
• Record the mass of the cup-water-ice combination.
mass of cup (kg) mcup mcup + mw mcup + mw + mice mw mice Ti Tf Lf (measured) Lf (accepted)

Data Analysis:

• The specific heat capacity of water is cw = 1 kcal/(kg oC).
When the water cools, the amount of heat released is ΔQreleased = cwmw(Ti - Tf).
• If no energy is exchanged with the environment, then the heat released by the water is absorbed by the ice.
It is used to melt the ice and raise the temperature of the resulting water to the final temperature of the measurement.
The amount of heat absorbed by the ice is ΔQabsorbed = Lfmice + cwmice(Tf - 0 oC).
• Setting ΔQreleased = ΔQabsorbed we have cwmw(Ti - Tf) = Lfmice + cwmice(Tf - 0 oC).
Lf = cw(mw/mice)(Ti - Tf) -  cwTf  (all temperatures in oC).
• Determine the latent heat of fusion, Lf, from your data and compare with the accepted value.

Discuss:

• What are some of the experimental errors that could cause your measured value of Lf to differ from the accepted value?
• Why is it important to dry the ice before adding it to the water?

Discussion:

### Regulating the temperature of the human body

The human body has the ability to regulate its temperature so that it remains very close to 37 oC.  If the body is overheating as a result of strenuous exercise, the surface blood vessels dilate to increase the blood flow to the surface areas.  Heat is carried by the blood to the surface where it causes the skin temperature to increase.  The body also begins to produce sweat.  The rate of sweating increases strongly with body temperature above 37oC.

Conduction, evaporation, convection, and radiation can now transfer heat from the skin to the environment.  The chart below shows the relative importance of these heat transfer mechanisms at different environmental temperatures.

Questions:

• For environmental temperature below 32 oC, convection and radiation are the dominant heat removal mechanisms.  What can a person do to increase the amount of heat carried away by these mechanisms?
• Why is conduction usually not a dominant heat removal mechanism?  Can you think of situations when it becomes important?
• Why do convection and radiation no longer remove heat from the body at environmental temperature above 37 oC?

### Evaporation

The body secrets water on to the skin from sweat glands.  As this water evaporates, the latent heat of vaporization is removed from the body.  The rate of evaporation increases as the degree of saturation of the surrounding air decreases.  In warm humid weather, the rate may be so low that a layer of water accumulates on the skin.  In still air, the air near the skin will become saturated and evaporation will stop.  Evaporation will increase if this saturated air is continually removed by wind or an artificially produced air stream.

Questions:

• If there is a cool wind blowing, what would be the consequences, as far as heat loss from the body is concerned, of wearing wet clothes?  Explain your answer.
• Cloudy nights are usually warmer than cloudless nights.  Why?

Convert your log into a session report, certify with you signature that you have actively participated, and hand it to your instructor.