12 Weeks to TAKS Week 4
Obj. 5-6a and 6b Describe the law of conservation of energy. Investigate and demonstrate the movement of
heat through solids, liquids, and gases by convection, conduction, and radiation.
Thermal energy: the total potential and kinetic energy associated with the random motion and arrangement of the particles of a
material. Heat is the transfer of kinetic energy from the particles of one object to the particles of another object due to a
temperature difference.
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Heat always transfers from the warmer to the cooler object.
The greater the difference in temperature of two objects, the more energy that will transfer.
When a material is hot, it has more thermal energy than when it is cold.
Central concept of thermodynamics is temperature. Temperature is a measure of the average kinetic energy of all the particles
within an object. Therefore, the temperature of a substance will increase if the average kinetic energy of its particles is increased.
! Our “temperature sense” is often unreliable. The same quantity of thermal energy in different bodies does not give each the
same temperature.
! On a cold winter day, an iron railing seems much colder to the touch than a wooden fence post, even though both are at the
same temperature. This error in perception results because the iron removes energy from our fingers more quickly than the
wood does. Temperature is the hotness or coldness of a material.
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The quantity of thermal energy in a body affects its temperature.
The same quantity of thermal energy in different bodies does not give each the same temperature.
The ratio between temperature and thermal energy is different for different materials.
Thermometer is a device that measures temperature.
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Thermometers are based on the principle that liquids expand when their temperature increases and contract when their
temperature falls.
Specific heat Every substance gains or loses heat based on it’s identity. This physical property of the substance is called the
specific heat capacity of the object. The specific heat capacity, c, of a solid or liquid is defined as the heat required to raise a unit
mass of the substance by one degree of temperature.
• Objects with high specific heat capacities heat up slower than those with lower specific heat capacities.
• Heat Energy =(mass) x (specific heat) x (change in temperature) or Q = m c "t
Law of Heat Exchange
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For a closed system in which heat energy cannot enter or leave, the heat lost by objects at a higher temperature is equal to
the heat gained by objects at lower temperature until thermal equilibrium is reached (at which point the final temperature of
both objects is the same).
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The final temperature will be somewhere between the initial low temperature and the initial high temperature.
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Conservation of Energy: Q lost = Q gained
• To avoid problems with signs, for Q lost = Q gained problems, it is best to make "T = Thi – Tlo
• Q is negative when heat energy is transferred to the environment from the system.
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Q = 0 J when the transfer of heat energy between the system and the environment is equal.
Q is positive when heat energy is transferred to a system from the environment.
Methods of Thermal Energy Transfer
Conduction
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Conduction is the transfer of thermal energy without any net movement of the material itself. There must be direct contact
between particles of objects for conduction to occur.
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When a metal poker is put in a hot fire, the exposed end of the poker soon becomes hot as well, even though it is not directly
in contact with the source of heat. We say that heat has been conducted from the hot end to the cold end.
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Heat conduction in many materials can be visualized as the result of molecular collisions. As one end of the object is heated,
the molecules there move faster and faster. As they collide with their slower-moving neighbors, they transfer some of their
energy to these molecules whose speeds thus increase. These in turn transfer some of their energy by collision with molecules
farther along the object. Thus the energy of thermal motion is transferred by molecular collision along the object. In metals, it
is the collisions of free electrons within the metal with each other and with metal atoms that is responsible for conduction.
Most metals are good conductors of electricity because some electrons can leave their parent atoms and wander through the
crystal lattice. These “free” electrons can carry energy from the hotter to the cooler regions of the metal. Good thermal
conductors such as silver, copper, aluminum, and gold are also good electrical conductors.
Convection
• Convection is the process of heat transfer through the mass motion or flow of some fluid, such as air or water.
• When a pot of water is heated, convection currents are set up as the heated water at the bottom of the pot rises because of
its reduced density and is replaced by cooler water from above.
• Although liquids and gases are generally not very good conductors of heat, they can transfer heat quite rapidly by convection.
Convection is the process whereby heat is transferred by the mass movement of molecules from one place to another.
Whereas conduction involves molecules (and/or electrons) moving only over small distances and colliding, convection involves
the movement of molecules over large distances.
• Convection currents are due to hot fluid rising and cold fluid sinking
Radiation
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Radiation is a more rapid transfer of thermal energy in the form of electromagnetic radiation accomplished by a process that
requires neither contact nor mass flow.
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A hot object also loses heat energy by radiation. This radiation is similar to light and can pass through empty space. The
warmth you fell when you warm yourself by a fire is due to this radiation. If the object is hot enough, some of the radiation is
visible and can indeed be seen.
Conductors readily transfer heat energy.
Insulators are poor conductors of heat energy.
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Gases are the poorest conductors
Solids are the best conductors
Fiberglass batting is better than air alone because it
traps air.
Heats of Transformation
Phase Changes
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When energy is absorbed as heat by a solid or liquid, the temperature of the
object does not necessarily rise.
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The thermal energy may cause the mass to change from one phase, or state, to
another.
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The amount of energy per unit mass that must be transferred as heat when a
mass undergoes a phase change is called the heat of transformation, L. Heat is
gained from the surroundings when a substance melts, evaporates or sublimes.
This process cools the surroundings. Heat is given off to the surroundings when
the same substance freezes or condenses which warms the surroundings.
Thermal Expansion of Solids
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Solids expand when heated and contract when cooled (with a few exceptions).
– Heated solids increase or decrease in all dimensions (length, width, and thickness).
– When a solid is heated, the increase in thermal energy increases the average distance between the atoms and molecules of
the solid and it expands.
Abnormal Expansion of Water
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When water freezes the formation of hydrogen bonds in the ice crystals causes the ice to expand.
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As the water is heated and its temperature rises, it continues to contract until it reaches a temperature of 4#C.
Increase the temperature of any common liquid and it will expand. Water at the temperature of melting ice, 0# C, contracts
when the temperature is increased.
With further increase in temperature, the water then begins to expand and the expansion continues all the way to the
boiling point, 100# C.
Air inside Refrigerator?
Your turn! Identify the method of heat transfer in each of the following
Cooling coil
Water in beaker?
Handle of Pan?
1.
2.
Sun
3.
Space heater
4.
Bottom of Pan?
5.
6.