Heat, Specific Heat, and Phase Change

� Heat and Internal Energy

� Specific Heat

� Calorimetry

� Phase Changes

� Latent Heat

� Homework

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Heat and Internal Energy

� Heat is the energy transferred between a system and its environment because of a tempera-ture difference between them.

� Before scientists realized that heat is transferred energy, they measured heat in a unit calledthe calorie (cal), defined as the heat required to raise the temperature of 1 g of water from14.5 � C to 15.5 � C.

� In 1948, the scientific community decided that since heat (like work) is transferred energy,the SI unit for heat should be the joule (J).

� The relationship between the calorie and the joule is known as the mechanical equivalentof heat and is given by

� �� � � �� � �

� Internal energy � �� is the energy associated with the microscopic components of a system- the atoms and molecules

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Specific Heat

� The heat � transferred between a system of mass � and its surroundings when the system undergoes atemperature change �� is given by

� � �� ��

where c is the specific heat of the system. The SI units of specific heat are � ��� ����� � .

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Calorimetry

� When two objects at different temperatures are placed in thermal contact and are insulated from their sur-roundings, the principle of conservation of energy requires that

� ! �" � #

� � �� ! � " � " �� " � #

� Example 1 - A copper slug whose mass � $ is 0.0750 kg is heated in a laboratory oven to a temperature� $ of312� C. The slug is then dropped into a glass beaker containing a mass � % = 0.220 kg of water. The mass ofthe glass beaker is � & = 0.225 kg. The initial temperature� ' of the water and beaker is 12.0� C. Assuming thatthe slug, water, and beaker are an isolated system and the water does not vaporize, find the final temperature

� ( of the system at equilibrium.

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Phase Changes

� Shown below is a graph of temperature versus heat added when 1 g of ice initially at -30� C is converted tosteam at 120� C.

� Notice that the temperature doesn’t necessarily have to change as heat is transferred to the system

� Instead, the substance can undergo a phase change

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Latent Heat

� The heat required to change the phase of a mass � of a pure substance is

� � ) �*

–* is called the latent heat and depends on the type of phase change and the substance

– The sign is determined by the direction of the heat flow - � is positive if energy is added to the substanceand negative if it is removed

– The heat of fusion* ( is the heat absorbed per kg in a solid-to-liquid conversion

– The heat of vaporization* + is the heat absorbed in a liquid-to-vapor conversion

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Example 2

Four ice cubes at 0.0 � C, each of mass 0.0400 kg, are dropped into a 0.0500 kg glass containing0.300 kg of water at 30.0 � C. What is the mass of ice left when the water and glass have cooledto 0.0 � C?

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Homework Set 4 - Due Fri. Jan. 16

� Read Sections 17.1 - 17.3

� Answer Questions 17.1, 17.4 & 17.10

� Do Problems 17.1, 17.6, 17.11 & 17.15

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