AP Physics Chapter 11 Heat

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AP Physics Chapter 11Heat
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Chapter 11 Heat

• 11.1 Units of Heat
• 11.2 Omitted
• 11.3 Omitted
• 11.4 Heat Transfer

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Learning Objectives

• Mechanical Equivalent of Heat
• a. Students will understand the mechanical
  equivalent of heat so they can determine how
  much heat can be produced by the performance of a
  specified quantity of mechanical work.
• Heat Transfer and Thermal Expansion
• a. Students will understand heat transfer and
  thermal expansion, so they can analyze
  qualitatively the effects of conduction,
  radiation, and convection in thermal processes.

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Homework for Chapter 11

• Read Chapter 11
• HW 11 p.383 3-7. p.386 54,55,57-59,61,62.

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11.1 Units of Heat
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Warmup Meat Platter Physics Warmup 87
Each decade seems to have its own collection of
unique kitchen items sold through television and
specialty catalog advertising. One such item sold
in the 90s was a magic defrosting plate. You
simply laid the frozen meat on the plate and in a
few minutes the meat was ready for cooking.
Manufacturers claimed to have sold several
million of these in just one year.

Several
science-oriented publications printed articles
describing the defrosting plate phenomenon.
Although the various brands differed in size and
thickness, they all were similar in that they
were simply a metal block painted black. Users
were instructed to run hot water over the plate
after a few minutes to renew its defrosting
abilities. Identify which method of heat
transfer was being used (conduction, convection,
or radiation), and explain how running hot water
over the plate renewed its ability to defrost.
Answer Being at a higher temperature, the block
transferred heat to the meat by conduction. As
the block cooled down, the rater of conduction
slowed. Hot water raised the temperature of the
block.
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Warmup Hot n Cold II Physics Warmup
88
Any material that does note conduct heat well is
a thermal insulator. Air is a poor conductor of
heat and is the main ingredient in many thermal
insulation materials.

Explain why Styrofoam cups work
equally well at keeping hot drinks hot and cold
drinks cold, while still making it possible for
you to hold onto the cup.
Answer Styrofoam is mainly air. Heat travels
through it poorly it cant leave the hot coffee
or get to the cold drink.
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• Heat is the net energy transferred from one
  object to another because of a temperature
  difference.
• Heat flows naturally from a warm body to a cool
  body in contact until they reach the same
  temperature, thermal equilibrium.
• The SI unit of heat is the joule (J).
• Other units of heat commonly used are the
• calorie (cal )
• kilocalorie (kcal)
• Calorie (C) a food Calorie, which is the same as
  a kcal.
• Note the capital C for the large food calorie.
• 1 food calorie 1Cal 1,000 cal
• British thermal unit (Btu)
• 1 Btu 252 cal

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• A kilocalorie raises the temperature of 1 kg of
  water by 1 C.
• A calorie raises the temperature of 1 g or water
  by 1 C.
• A Btu raises the temperature of 1 lb of water by
  1 F.

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• mechanical equivalent of heat - the relationship
  between unit of heat and the standard SI unit
  (J).
• The mechanical equivalent of heat, 4.186 joules,
  is the amount of work equal to the transfer of
  one calorie of heat.
• 1 kcal 4,186 J
• 4.186 kJ
• 1 cal 4.186 J

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Example 11.a A piece of candy supplies 27.5
Calories. If this energy is converted into work
for lifting a load, how much mass can be lifted
to a height of 5 ft (approx. 1.52 m) with it?
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11.4 Heat Transfer
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Warmup Hot n Cold I Physics Warmup
86
Heat can be transmitted by three different
methods. When hot objects touch cooler objects,
heat is transmitted by conduction. As a warmer
fluid moves into a cooler region, heat is
transmitted by convection. The transmission of
heat through electromagnetic waves is called
radiation. Any material that effectively reduces
the transmission of heat is called a thermal
insulator.

Thermos bottles have been used for many
years to keep coffee hot for many hours. They
basically consist of double glass walls, which
are coated with a silver-colored coating. The air
between the double walls has been pumped out,
creating a vacuum. Explain the roles that the
silver coating and the vacuum play in making the
thermos bottle such an effective thermal
insulator.
Answer The silver lining prevents loss due to
radiation and the vacuum prevents loss due to
conduction.
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Warmup The Cooking Glass Physics
Warmup 85
Certain materials not only transmit heat well
through conduction, they transmit it evenly
throughout the material. Unfortunately, regular
glass is not one of those materials. It does a
good job of transmitting heat, it just doesnt do
it evenly.

Pyrex glass was specially developed to
overcome the problem with uneven heating. As a
result, it only expands half as much when heated
as regular glass. Explain why Pyrex glass is
better than regular glass to use when cooking hot
dishes.
Answer While the more even heating is somewhat
beneficial, the fact that it is far less likely
to shatter is the main reason it is better.
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11.4 Heat Transfer
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Examples Heating water in a beaker over a
Bunsen burner Heating of the earths
atmosphere warming of air in a room Radiation
does not require material medium it can take
place through a vacuum. The radiation can also
pass through some material media such as water,
glass, and air, etc. The solar heat comes to the
earth through millions of kilometers of vacuum.
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The hands at the left are warmed by the
convection of rising hot air (and some
radiation). The gloved hand at upper right is
warmed by conduction. The hands at lower right
are warmed by radiation.
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On Gold Sheet
k
H
L
H rate of heat transfer
k
L
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• thermal conductivity (k) characterizes the
  heat-conducting ability of a material.
• The greater the value of k for a material, the
  more rapidly it will conduct heat.
• The units of k are J/(msC) or kcal /(msC).
• Thermal Conductivities of Some Substances (see
  also p.374 in text)
• Thermal conductivity (k)

Substance J/(msC) kcal /(msC)
Aluminum 240 5.73 x 10-2
Copper 390 9.32 x 10-2
Iron and steel 46 1.1 x 10-2
Water 0.57 14 x 10-5
Air 0.024 0.57 x 10-5
Brick 0.71 17 x 10-5
Glass 0.84 20 x 10-5
Styrofoam 0.042 1.0 x 10-5
Vacuum 0 0
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• Example 11.4 A window glass 0.50 cm thick has
  dimensions of 2.0 m by 1.0 m. If the outside
  temperature is -10C and the inside temperature
  is 20C,
• What is the rate of heat conduction through the
  window?
• How much heat flows through the window in 1.0
  hours due to conduction only?

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Check for Understanding

• The SI unit of heat energy is the
• a) calorie
• b) kilocalorie
• c) Btu
• d) joule
• Answer d

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• 2. The warming of the atmosphere involves
• conduction
• convection
• radiation
• all of these

Answer d

• 3. Water is a very poor heat conductor. Why can
  it be thoroughly heated relatively quickly?
• conduction
• convection
• radiation
• all of these

Answer b. The warm (less dense) water rises to
the top and the cooler (more dense) water sinks
to the bottom.
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4. A plastic ice tray and a metal ice tray are
removed from the same freezer (initial
temperature). However, once your hands touch
both, the metal one feels cooler. Why?
Answer Metal has a higher heat conductivity so
it conducts heat away from your hand faster.
5. You blow on a spoonful of hot soup to cool it.
Yet, your breath is warm. How does blowing cause
the soup to cool?
Answer This convects the heat from the hot soup
to the cooler air.
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Homework 11

• HW 11 p.383 3-7. p.386 54,55,57-59,61,62.