Specific Heat and Latent heat
Internal energy: it is all the energy of a system that is associated
with its microscopic components, atoms and molecules. Internal
energy includes kinetic energy and potential energy of molecules.
Heat: it is defined as the transfer of energy across the boundary of
a system due to a temperature difference between the system and
its surroundings. When you heat a substance, you are transferring
energy into it by placing it in contact with surroundings that have a
higher temperature. When you cool a substance, you are
transferring energy from it by placing it in contact with
surroundings that have a lower temperature.
The heat is measured in calorie (cal), which is defined as the
amount of energy transfer necessary to raise the temperature of 1 g
of water from 14.5°C to 15.5°C.
In SI unit of heat is measured in joule, where:
1 cal = 4.186 J
The specific heat (c):
The specific heat of a substance is the heat capacity per unit mass.
We can define it as the amount of energy needed to raise the
temperature of 1kg of that sample by 1°C.
Specific Latent heat of fusion Lf is the energy needed to change
1 Kg of solid into liquid at constant temperature.
Specific Latent heat of vaporization Lv is the energy needed to
change 1 Kg of liquid into gas at constant temperature.
The zeroth law of thermodynamics (the law of equilibrium):
If objects A and B are separately in thermal equilibrium with a
third object C, then A and B are in thermal equilibrium with each
other.
1) A combination of 0.250 𝑘𝑔 of water at 20.0°𝐶, 0.400 𝑘𝑔 of aluminum
at 26.0°𝐶, and 0.100 𝑘𝑔 of copper at 100°𝐶 is mixed in an insulated
container and allowed to come to thermal equilibrium. Ignore any energy
transfer to or from the container. What is the final temperature of the
mixture?
1
2) What mass of water at 25.0°𝐶 must be allowed to come to thermal
equilibrium with a 1.85 𝑘𝑔 cube of aluminum initially at 150°𝐶 to lower
the temperature of the aluminum to 65.0°𝐶? Assume any water turned to
steam subsequently condenses.
2
3) A 50.0 𝑔 sample of copper is at 25.0°𝐶. If 1200 𝐽 of energy is added to
it by heat, what is the final temperature of the copper?
4) An aluminum cup of mass 200 𝑔 contains 800 𝑔 of water in thermal
equilibrium at 80.0°𝐶. The combination of cup and water is cooled
uniformly so that the temperature decreases by 1.50°𝐶 per minute. At what
rate is energy being removed by heat? Express your answer in watts.
4
6) A 50.0 𝑔 copper calorimeter contains 250 𝑔 of water at 20.0°𝐶. How
much steam at 100°C must be condensed into the water if the final
temperature of the system is to reach 50.0°𝐶?
6
7) A 75.0-g ice cube at 0°C is placed in 825 g of water at 25.0°C. What is
the final temperature of the mixture?
Tf =
8) A 3.00 𝑔 lead bullet at 30.0°𝐶 is fired at a speed of 240 𝑚/𝑠 into a large
block of ice at 0°𝐶, in which it becomes embedded. What quantity of ice
melts?
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9) A 1.00 𝑘𝑔 block of copper at 20.0°𝐶 is dropped into a large vessel of
liquid nitrogen at 77.3 𝐾. How many kilograms of nitrogen boil away by
the time the copper reaches 77.3 𝐾? (The specific heat of copper is
0.092 0 𝑐𝑎𝑙/𝑔. °𝐶, and the latent heat of vaporization of nitrogen is
48.0 𝑐𝑎𝑙/𝑔.)
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11) Find the amount of heat required to convert 20 gm of water at 50.0°𝐶
to steam at 120.0°𝐶. If the specific heat capacity of water equal (4168 J/kg
°𝐶), the specific heat capacity of steam equal (2010 J/kg °𝐶), and the latent
heat of vaporization of water is (2.26X106J/kg).
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H.W
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