Thermal Physics IDS Physics: Unit 04

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Thermal Physics IDS Physics Unit 04

• Lecture 3 Latent Heat

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Heat

• When thermal energy is transferred from one
  object to another, we called the transferred
  energy heat
• Temperature is a measure of the thermal energy.
  It is proportional to the kinetic energy of the
  moving molecules.
• The absolute temperature is a measure of the
  average thermal (kinetic) energy of a substance.

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Review

• There is a relationship between how much heat is
  transferred to an object and how much the
  temperature changes.

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Demo

• Boiling water in a kettle
• What will the temperature do as we add heat?
• Energy is obviously being added, but where is it
  going?

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States of Matter

• There are three common states of matter
• Solid
• Liquid
• Gas
• There are other, less common states of matter
• Plasma
• Boson

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Characteristics of Gas

• No interaction between molecules
• No definite shape
• no definite volume

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Characteristics Liquid

• Weak interaction between molecules
• Has a definite volume
• Has no definite shape

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Characteristics Solid

• Strong interactions between molecules
• Has a definite volume
• Has definite shape

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Phase

• The phase of a substance depends on the
  competition between the motion of the particle
  and the interaction force.
• http//www.chm.davidson.edu/ronutt/che115/Phase/Ph
  ase.htm
• What happens when steam cools?

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Puzzle Solved

• Energy is obviously being added, but where is it
  going?
• The energy is used to break the chemical bonds
  and free the molecule

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Latent Energy

• Energy is needed to overcome the chemical bonds
• This energy is called the latent heat

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Latent Energy

• If energy is added
• L gt 0
• If energy is taken removed
• L lt 0

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Latent heat of Fusion

• For solid - liquid phase changes, we use Lf

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Heat of Vaporization

• For liquid - gas phase changes, we use Lv

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Table Practice

• What value of L would you use to calculate the
  latent heat of lead as it changed from a liquid
  to a solid?

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Example 1

• Determine how much heat needs to be added to 120
  grams of ice (initially at 0 C) in order to melt
  it?

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Example 2

• 1.0 oz of gold starts at 24 C. How much heat is
  needed to completely melt it ?

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Example 3

• 2.0 kg of Al at 100 C is placed in a cup with
  0.25 kg. of ice at 0C. What is the final
  temperature of the mixture?

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Power

• Power is the rate of energy transfer.
• Thermal power is the rate of thermal energy
  transfer (heat)

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Example 4

• How long will it take a 300. W hot plate to
  completely boil away 200 grams of ethanol that
  was initially at 20 C?
• (cEtOH 430 J/kgC)