Reading Quiz - Heat

1
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

• 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

• 3. Energy from the sun reaches us through
• ___ 1. conduction.
• ___ 2. convection.
• ___ 3. radiation.
• ___ 4. all of the above.

4
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.

5

• 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.

6
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.

7

• 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.

8
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?

9
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
  1C.
• (heat transferred) (mass)(specific heat
  capacity) (temperature change)
• Unit of c J/kgC? or kcal/kgC?
• Specific heat capacity is analogous to inertia
  we call this thermal inertia.

10
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

11

• 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.

12

• Phase diagram of water

13
Conceptual Questions

• 1) You accidentally let an empty iron skillet get
  very hot on the stove (200C). 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 200C.
• ____ d) remains the same as before.
• Note Specific heat of iron is less than that of
  water.

14

• 2) One gram of steam of 100C causes a more
  serious burn than one gram of water at 100C
  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.

15

• 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.

16

• 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.

17
Quantitative Problems

• 1) How much heat must be removed from 1.4 kg of
  aluminum in order to cool it from 80C to 15C?
• 2) If 0.20 kg of coffee at 90C is poured into a
  0.30 kg cup at 20C, 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 60C to its boiling
  point of 100C?

18

• 4) What is the minimum amount of ice at -10C
  that must be added to 0.50 kg of water at 20C in
  order to bring the temperature of the water down
  to 0C?

19
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.

20

• 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

21

• Convection - forced convection through fans.
  - natural convection through
  changes in density (hot air/water
  rises resulting in circulation,
  convection currents, weather).

22

• 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/m2K4
• A good absorber is also a good emitter.

23
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?

24

• 2) The giant hornet Vespa mandarinia japonica
  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?

25

26

• 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?

27

• 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.

28
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
  15.0C and 14.0C, respectively. The thermal
  conductivity of glass k is 0.84 J/smC.

29

• 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 suns rays. The
  average radius of the earths 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.