Heat, Specific Heat, and Phase Change Heat and Internal Energy - - PowerPoint PPT Presentation

Heat, Specific Heat, and Phase Change

• Heat and Internal Energy
• Specific Heat
• Calorimetry
• Phase Changes
• Latent Heat
• Homework

1

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 called

the calorie (cal), defined as the heat required to raise the temperature of 1 g of water from 14.5

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 equivalent
• f heat and is given by

• Internal energy

is the energy associated with the microscopic components of a system

• the atoms and molecules

2

Specific Heat

• The heat

transferred between a system of mass

and its surroundings when the system undergoes a temperature change

is given by

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

.

3

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

• f

312

• C. The slug is then dropped into a glass beaker containing a mass

= 0.220 kg of water. The mass of the glass beaker is

= 0.225 kg. The initial temperature

• f the water and beaker is 12.0

• C. Assuming that

the slug, water, and beaker are an isolated system and the water does not vaporize, find the final temperature

• f the system at equilibrium.

4

Phase Changes

• Shown below is a graph of temperature versus heat added when 1 g of ice initially at -30

C is converted to steam 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

5

Latent Heat

• The heat required to change the phase of a mass

• f 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 substance and 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 containing 0.300 kg of water at 30.0

• C. What is the mass of ice left when the water and glass have cooled

to 0.0

C?

7

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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