Chapter 14 Heat

*The use of "heat" in the first paragraph of this chapter is misleading. Heat does not flow.

The title of this section is still somewhat misleading. Energy is not an object that can be transferred. The model that energy is transferred is useful for calculations and we will use this terminology. The "energy transfer" is accomplished by one object doing work on another object.

What can one physics calorie accomplish?

* 5. Heat is a form of energy.

When objects have a thermal contact (conduction, convection or radiation may be the mechanism of energy transfer) the hotter object will lose internal energy to the cooler object and the objects will eventually reach the same temperature. This state is called thermal equilibrium.

The metric unit for heat is the Joule, but we will sometimes use the kilocalorie. 1 kcal = 4186 Joules. A kilocalorie is the energy required to raise the temperature of 1 kg of water by 1^o C (when the water is at 14 ^o C). The English standard of heat is the BTU. One BTU will raise the temperature of 1 pound of water by 1^o F. The first person to send me e-mail, before the day this material is discussed in class, with the correct birth year for James Prescott Joule, will receive 1 bonus point.

An object is made up of atoms and molecules. These vibrate, rotate and move at rates that are related to the temperature of an object. The sum of the potential and kinetic energies in an object is called the internal energy. Heat is the amount of internal energy flowing from a body at a higher temperature to a body at a lower temperature. * Heat is not a description of the temperature of an object.

What does temperature measure?

What does internal energy refer to?

What does heat refer to?

What is the value of the rotational Kinetic Energy of an ideal gas? Why?

What is the value of the vibrational energy of an ideal gas? Why?

Is it possible for an ideal gas to have translational KE?

What is the internal energy (U ) of an ideal gas? U = N x Average KE

U = ( 3/2 )NkT or U = ( 3/2 ) nRT

What is the value of the internal energy for the air in our classroom?

14.4 Specific Heat

* The specific heat of an object measures how the temperature of a given substance responds to the addition or removal of thermal energy. The relationship between energy added or removed from the object, specific heat, mass of the object, and temperature change is

Q = m c D T .

Q represents the heat value. m is the mass of the object. c is the specific heat value. The table on page 421 gives the specific heat values for some common substances.

It is important to note that this equation cannot be used if the substance changes phase (e.g. freezes or boils). We will study a different equation in section 14.6 that deals with the energy involved in a phase change.

Calculate the temperature change that occurs when the thermal energy of a 150 gram aluminum object is decreased by 300 Joules.

Suppose your thumb has a mass of 150 grams. Calculate the temperature change that occurs when the thermal energy in your thumb is decreased by 300 Joules.

Comment on the benefit of a relatively high value of specific heat for the human body.

Calorimetry is the measurement of the exchange of internal energy. Conservation of energy applies to calorimetry problems.

What is a calorimeter?

A calorimeter is a device in which objects are allowed to come to thermal equilibrium with no transfer of energy from the room. The amount of thermal energy lost by the hot object(s) will equal the amount of thermal energy gained by the colder object(s). Suppose that the inner cup of a calorimeter is aluminum and has a mass of 50 grams. It contains 30 grams of water that has a temperature of 20^o C.

Later, a 50 gram iron object at 80^o C is placed in the inner calorimeter cup. What is the final temperature of the system? Is conservation of energy used to solve this problem?

During a phase change (e.g. ice melting) the temperature of the object will be constant even though energy is entering or leaving the object. The equation Q = m c D T cannot be used if the D T range includes a phase change. e.g. water T1 = 20^o C T2 = -5^o C.

The energy required to change the phase of a substance is called the latent heat. The latent heat of fusion refers to the energy per kg required to melt or freeze a substance. The latent heat of fusion for water is 3.33 x 10⁵ J/kg or 79.7 kcal/kg. The latent heat of vaporization refers to the energy per kg required to vaporize or condense a substance. The latent heat of vaporization for water at 100^o C is 22.6 x 10⁵ J/kg or 539 kcal/kg. Table 14.3 gives values for the latent heat, L. Q = mL

Suppose that a 40 gram aluminum calorimeter cup contains 20 grams of ice at a temperature of

-10^o C. A 200 gram lead object having a temperature of 110^o C is placed in the cup. Ice has a specific heat of 2100 J/kg/ ^o C. Find the equilibrium temperature. Find the amount of ice or water or both at this time.

14.7 Heat Transfer:Conduction

Video CC 1B Chp 52

In the subject of calorimetry we said that hot objects lose energy and cooler objects gain energy. This section discusses the process of energy transfer.

* The three modes of energy transfer are: 1) conduction, 2) convection, and 3) radiation.

Heat is the transfer of energy due to temperature difference.

Conduction is the transfer of thermal energy through the process of collisions between neighboring atoms or molecules. In conduction there is no mass transport of the material. The kinetic energy of motion is transferred from neighbor to neighbor each time a collision occurs. Suppose that you must stir spaghetti that is in boiling water. Which tool would be most comfortable: a wooden spoon, a metal spoon? Why? Why?

We will limit our conduction calculations to the case of energy transfer through a solid wall.

equation 14.3

Estimate the Q by conduction for the north wall of our classroom. The wall has three parts: 1) concrete blocks, 2) glass windows, 3) metal trim around the windows.

R Factor: The insulating ability of materials is usually stated by giving the R Factor for the material. R = thickness/ thermal conductivity R = L/K

14.8 Heat Transfer:Convection

When convection occurs there is mass motion of the material. The thermal energy is moved to a new location because the hot material itself moves to a new location. Why is convection most important for liquids and gases, not solids?

Convection is the least understood of the three mechanisms. There are two difficulties in accurately describing convection: 1) as the hot material moves it loses energy to the surroundings, 2) there is usually some amount of turbulence in the motion and exchange of mass between the hotter and cooler substances. Convection occurs naturally in fluids when there is a large temperature difference between two locations. Transfer of energy by convection can be increased by blowing ( or pumping, etc.) air (or water, etc.) such as is done in a common home heating installation (forced convection).

14.9 Heat Transfer:Radiation

Radiation is the transfer of thermal energy by light (electromagnetic waves). The term radiation in this chapter is not the same term used in discussing radioactivity. All objects (with T > 0 K) emit light. Light carries energy from the emitting object to the absorbing object.

Stefan-Boltzmann Law

D Q/D t = s e A T⁴ A = surface area

s = 5.67 x 10^-8 W/m²/K⁴ e = emissivity, values range from 0 to 1 depending on the material

e is about 1 for black surfaces and about 0 for shiny metal surfaces

Objects absorb radiation and emit radiation. The net Q is given by

D Q_net/D t = s e A (T⁴ - T⁴_surroundings )

The first person to send me e-mail that correctly describes a Dewar flask will receive 1 bonus point.

Why does the earth have an average surface temperature of about 59 ^oF?

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