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# Reading Quiz - Heat - PowerPoint PPT Presentation

Reading Quiz - Heat. 1. Heat is ___ 1. measured in joules. ___ 2. a form of energy. ___ 3. measured in calories. ___ 4. all of the above. 2. When a material changes phase, ___ 1. heat is given off. ___ 2. heat is absorbed. ___ 3. its temperature remains constant.

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Presentation Transcript

1. Heat is

___ 1. measured in joules.

___ 2. a form of energy.

___ 3. measured in calories.

___ 4. all of the above.

2. When a material changes phase,

___ 1. heat is given off.

___ 2. heat is absorbed.

___ 3. its temperature remains constant.

___ 4. it changes some physical characteristics.

___ 5. all of the above.

3.Energy from the sun reaches us through

___ 1. conduction.

___ 2. convection.

___ 4. all of the above.

Heat
• The quantity that flows from a hot object to a cold object is termed heat.
• Through the work of Joule, we see that heat is another form of energy. Heat is the energy transferred as a result of a difference in temperature between two bodies - it flows from higher to lower temperature.
• The calorie (cal) is the amount of heat needed to raise the temperature of 1 gram of water by 1 Celsius; ordinary usage Calorie is actually 1000 cal.
The SI unit for energy is the joule (J); the mechanical equivalent of heat is 1 cal = 4.186 J.
• Temperature (T) - a measure of the average KE of the individual molecules in an object.
• Internal (thermal) energy (U) - the total energy of all the molecules in an object.
• Heat (Q) - the transfer of energy between objects at different temperatures.
• Internal energy of an ideal gas: U=NkT=nRT for an ideal monatomic gas. This is dependent only on temperature and the number of gas molecules.
Conceptual Questions

1) The natural direction of the heat flow between two objects depends on

____ a) their temperatures.

____ b) their internal energy contents.

____ c) their pressures.

____ d) whether they are in the solid, liquid, or gaseous state.

2) Of the following statements, which one uses incorrect terminology or principle?

____ a) During the last 3 minutes, 15 J of heat was transferred to the cold drink from the room.

____ b) This cup of coffee has 450 calories of heat.

____ c) A cup of hot water contains less internal energy than a cold swimming pool.

____ d) Mechanical work can be converted into heat.

Quantitative Questions

1) An 80 kg weight-watcher wishes to climb a mountain to work off the equivalent of a large piece of chocolate cake rated at 700 (food) Calories. How high must the person climb?

Specific Heat Capacity
• Specific Heat Capacity - amount of heat that must be added (removed) from a unit mass of a substance to raise (lower) its temperature by 1°C.
• (heat transferred) = (mass)(specific heat capacity) (temperature change)
• Unit of c: J/kg·C or kcal/kg·C
• Specific heat capacity is analogous to inertia; we call this thermal inertia.
Latent Heat
• Change of phase: term used to describe the changes in some physical property of a substance when heat is applied/removed.
• Example: Ice --› Water --› Steam
• Latent Heat - heat required to change the phase of 1.0 kg a substance: heat of fusion (LF ) -- from solid to liquid heat of vaporization (LV) -- from liquid to vapor heat of sublimation (LS) -- from solid to vapor
Note: phase changes in the opposite direction involves the same latent heat but the heat is now given off instead of absorbed.
• In problems that involve bodies at different temperatures, and/or changes in phase, there will be exchanges of heat. Conservation of energy is the key to solving these problems: heat lost = heat gained
• Properties of water: Pressure dependence of boiling point. Pressure dependence of melting point. Triple point of water.
Conceptual Questions

1) You accidentally let an empty iron skillet get very hot on the stove (200°C). When you dunk it into a couple of inches of water (about the same mass as the skillet), the temperature of the water

____ a) rises by about 10 to 20 degrees.

____ b) rises to boiling point.

____ c) rises to 200°C.

____ d) remains the same as before.

Note: Specific heat of iron is less than that of water.

2) One gram of steam of 100°C causes a more serious burn than one gram of water at 100°C because the steam

____ a) is less dense.

____ b) strikes the skin with greater force.

____ c) has a higher specific heat capacity.

____ d) contains more internal energy.

3) Turning up the flame under a pan of boiling water causes

____ a) the water to boil away faster.

____ b) the temperature of the boiling water to increase.

____ c) both the water to boil away faster and the temperature of the boiling water to increase.

____ d) none of the above.

4) The pressure cooker cooks food more rapidly than an ordinary pot with a loose lid because

____ a) the pressure forces heat into the food.

____ b) the higher pressure lowers the boiling point of water.

____ c) the higher pressure raises the boiling point of water.

____ d) the higher pressure increases the specific heat capacity of water.

Quantitative Problems

1) How much heat must be removed from 1.4 kg of aluminum in order to cool it from 80°C to 15°C?

2) If 0.20 kg of coffee at 90°C is poured into a 0.30 kg cup at 20°C, and we assume that no heat is transferred to or from the outside, what is the final temperature of the coffee?

3) What is the heat needed to raise the temperature of the water from 60°C to its boiling point of 100°C?

4) What is the minimum amount of ice at -10°C that must be added to 0.50 kg of water at 20°C in order to bring the temperature of the water down to 0°C?
Heat Transfer
• Three ways by which heat can be transferred from one point to another: Conduction - energy transferred through molecular collisions with no movement of molecules from one point to the other. Convection - energy transferred through actual movement of molecules from one point to the other. Radiation - energy transferred in the form of electromagnetic waves (heat) that does not need a medium.
For conduction: where A is the cross-sectional area of the conducting object l is the distance between the two ends T1 is the temperature of the hotter end T2 is the temperature of the colder end kT is the thermal conductivity
• Examples - touching a hot pan using thermometer heat loss through a window
Convection - forced convection through fans. - natural convection through changes in density (hot air/water rises resulting in circulation, convection currents, weather).
Radiation - A body at temperature T and surface area A can be shown to give off radiant energy in the form of electromagnetic waves: where e is the emissivity (a number between 0 and 1) that is characteristic of the material.  is the Stefan-Boltzmann constant equal to 5.67 x 10-8 W/m2·K4
• A good absorber is also a good emitter.
Conceptual Question

1) An object with a black surface usually heats up more than one with a white surface when both are in sunlight. Such is true of the robes worn by Bedouins in the Sinai desert: black robes heat up more than white robes. Why then would a Bedouin ever wear a black robe?

2) The giant hornet Vespa mandariniajaponica preys on Japanese bees. However, if one of the hornets attempts to invade a bee hive, several hundred of the bees quickly form a compact ball around the hornet to stop it. After about 20 minutes the hornet is dead, although the bees do not sting, bite, crush, or suffocate it. Why then does the hornet die?
3) When cooking pasta, why do you turn the heat down once the pasta is in and the water boiling? Would it cook faster if you kept the heat up high?

4) How does perspiration give the body a means of cooling itself?

5) How does the thermos bottle (consists of two glass vessel, each coated with a thin film of silver, one inside the other with the space between them evacuated) work? Can we use this for cold, as well as hot, drinks?

6) A spaceship is drifting in an environment where the acceleration of gravity is zero. As the air on one side of the cabin is heated by an electric heater, what is true about the convection currents caused by this heating?

____ a) The hot air around the heater rises and the cooler air moves in to take its place.

____ b) The hot air around the heater drops and the cooler air moves in to take its place.

____ c) The convection currents move about the cabin in a random fashion.

____ d) There are no convection currents.

Quantitative Problems

1) A major source of heat loss from a house is by conduction loss through the windows. Calculate the rate of heat flow through a glass window 2.0 m x 1.5 m in area and 3.2 mm thick, if the temperatures at the inner and outer surfaces are 15.0°C and 14.0°C, respectively. The thermal conductivity of glass k is 0.84 J/s·m·C°.

2) Radiant energy from the sun arrives at the earth at a rate of about 1.4 kW per meter squared of area perpendicular to the sun’s rays. The average radius of the earth’s orbit is 1.5 x 1011 m, and the radius of the sun is 7.0 x 108 m. From these figures find the surface temperature of the sun under the assumption that it radiates like a blackbody (which is approximately true). That is, take the emissivity e of the sun to be 1.0.