Physics 101: Lecture 27 Specific Heat and Latent Heat

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Physics 101 Lecture 27Specific Heat and Latent
Heat

• Todays lecture will cover Textbook Sections 12.6
  - 12.8

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How To Change the Temperature of a System (Solids
or Liquids)

• Add or subtract heat
• Q heat energy that flows from warmer to
  cooler systems.
• Q c m ?T
• Q amount of heat that must be supplied or
  subtracted to raise or lower the temperature of
  mass m by an amount ?T.
• Units of Q Joules or calories
• 1 cal 4.186 J
• 1 kcal 1 Cal 4186 J
• c specific heat capacity Heat required to
  raise 1 kg by 1oC.
• Q c m ?T Cause inertia x effect (just
  like Fma)
• cause Q
• effect ?T
• inertia c m (mass x specific heat capacity)
• ?T Q/(cm) (just like a F/m)

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Examples of Specific Heat Capacity(see Text,
Table 12.2)
Substance c in J/(kg C) Aluminum 900 Copper 3
87 Iron 452 Lead 128 Water (15 C) 4186 Ice (-15
C) 2000
Q c m ?T
Suppose you have equal masses of aluminum and
copper at the same initial temperature. You add
1000 J of heat to each of them. Which one ends
up at the higher final temperature a)
aluminum b) copper c) the same
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Latent Heat
water temp rises
T
Q added to water
Q m L with L Latent Heat L J/kg Q is the
amount of heat needed to add or remove from a
substance with mass m to change the state of that
substance. Liquid lt-gt Solid (fusion energy)
Liquid lt-gt Gas (vaporization energy) Solid lt-gt
Gas (sublimation energy)
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Concept Question

• Summers in Phoenix Arizona are very hot (125 F is
  not uncommon), and very dry. If you hop into an
  outdoor swimming pool on a summer day in Phoenix,
  you will probably find that the water is too warm
  to be very refreshing. However, when you get out
  of the pool and let the sun dry you off, you find
  that you are quite cold for a few minutes. How
  can you explain this?

The water is evaporating off of your skin. This
means that enough heat (or energy) is entering
the water drops to break bonds between water and
allow them to evaporate. Where is this heat
coming from? Your body! Heat flows from your body
to the drops of water, making you feel cooler.
When the water is gone, no more heat will flow
from your body and you will get hot once again.