Chapter 5 Temperature and Heat Another Kind of Energy

What is temperature? Temperature is what we measure with a thermometer. What is a thermometer? A thermometer is any devise that has an observable and measurable property that changes in response to a change in the temperature of the device… Thermometric Property How does a thermometer measure temperature? The operation of a thermometer is based on the Zeroth Law of Thermodynamics A thermometer can only measure its own temperature.

Zeroth Law of Thermodynamics “If two objects initially at different temperatures are placed in thermal contact, their temperatures will change until they are at the same equilibrium temperature” This condition is called thermal equilibrium.” thermal equilibrium reached

To measure the temperature of an object a thermometer is placed in thermal contact with the object. Once the thermometer and object have reached thermal equilibrium it can be assumed that the temperature of the thermometer is equal to the temperature of the object. How do we measure the temperature of the thermometer? It must be calibrated with systems whose temperatures are known or defined. Two systems that are commonly used are: 1. A mixture of ice and water. 2. Boiling water

These systems are used because it has been observed that as long as ice and water are present together the temperature remains constant, and as long as water is boiling its temperature remains constant. The temperature of a mixture of ice and water is defined to be: 0 degrees Celsius, 0°C or 32 degrees Fahrenheit, 32°F. The temperature of boiling water is defined to be: 100 degrees Celsius, 100°C or 212 degrees Fahrenheit, 212°F. Consider the calibration of a liquid-in-glass thermometer.

Liquid-in-Glass Thermometer The thermometric property is the height of the liquid in the capillary As the temperature of the thermometer changes, the liquid in the reservoir expands or contracts causing the level of the liquid in the capillary to rise or fall. Once the temperature of the thermometer becomes constant the level of the liquid will be constant. What temperature corresponds to this level of liquid?

Calibration of a Liquid-in-Glass Thermometer Insert the thermometer into a container holding a mixture of ice and water and wait for thermal equilibrium to be reached. Height of liquid has fallen. Place a “mark” on the thermometer where the liquid stopped moving and label with the defined temperature of ice/water (0°C or 32°F) Thermal Equilibrium reached

Insert the thermometer into a container holding boiling water and wait for thermal equilibrium to be reached. Height of liquid has risen. Source of heat To keep water boiling Place a “mark” on the thermometer where the liquid stopped moving and label with the defined temperature of boiling water (100°C or 212°F) Thermal Equilibrium reached

Determine how the height of the liquid relates to the temperature. The space between the two marks can be divided into equally spaced divisions: 100 divisions for Celsius (each division = 1C°) or 180 divisions for Fahrenheit (each division = 1F°).

Temperature Scales

Temperature Conversions Celsius to Fahrenheit Fahrenheit to Celsius Celsius to Kelvin Kelvin to Celsius

Example Convert 77K (temperature of liquid nitrogen) to Fahrenheit.

What causes the temperatures of two objects placed in thermal contact to change? Something must move from the high temperature object to the low temperature object. Is it matter or energy? If it is matter the mass of the high temperature object would decrease while the mass of the low temperature object would increase. This is not observed. It must be energy flowing between the two objects that causes a change in their temperatures.

The energy flowing between the two objects must be on the microscopic level because we can not see it. Energy Review Kinetic energy, KE-the macroscopic energy an object has due to its motion, measured in Joules, J. Gravitational potential energy energy, GPE-the macroscopic energy an object has due to its position, measured in Joules, J. Total mechanical energy, E-the sum of an object’s kinetic and potential energies, measured in Joules, J. Work, W-the process by which the total mechanical energy can be changed, measured in Joules, J.

Work and the Related Changes in Macroscopic Energy

All macroscopic objects are composed of microscopic objects: atoms and molecules These atoms and molecules are moving so they have a microscopic kinetic energy. These atoms and molecules are subject to conservative forces (gravitational and electrical) so they have a microscopic potential energy. The sum of these microscopic kinetic and potential energies is called Thermal Energy, U.

When two objects with different temperatures are placed in thermal contact, thermal energy flows from the higher temperature object to the lower temperature object until thermal equilibrium is reached. The thermal energy that flows between two objects because of a difference in temperature is called heat, Q. Since heat is a form of energy it is measured in Joules, J. For historical reasons another unit of thermal energy or heat is sometimes used: calorie, cal or kilocalorie, kcal. Conversion Factor 1kcal = 4186J

When work is done on or by an object there is a change in the object’s kinetic energy or its gravitational potential energy or both. The change in kinetic energy is perceived as a change in object’s velocity. The change in gravitational potential energy is perceived as a change in object’s position (height above the reference level). When heat flows into or out of an object there is a change in the object’s thermal energy. How is the change in the thermal energy of an object perceived? A change in thermal energy is perceived as a change in the object’s temperature or phase (solid, liquid, or gas). It has been observed that the change in temperature and the change in phase never occur at the same time.

When the temperature is changing the phase remains constant and when the phase is changing the temperature remains constant.

What variables determine the magnitude of the change in temperature? More heat results in a larger change in temperature: change in temperature is directly proportional to the amount of heat.

When the same quantity of heat flows into (or out of) a larger mass the change in temperature is less.The change in temperature is inversely proportional to the mass.

When the same quantity of heat flows into (or out of) equal masses of different substances the change in temperature is different. The change in temperature depends on the specific heat, c of the substance. The specific heat of a substance is the amount of heat required to change the temperature of 1kg of the substance by 1C°.

The substance with the higher specific heat experiences a smaller change in temperature. The change in temperature is inversely proportional to the specific heat.

What causes the temperatures of two objects placed in thermal contact to change? Something must move from the high temperature object to the low temperature object. Is it matter or energy? If it is matter the mass of the high temperature object would decrease while the mass of the low temperature object would increase. This is not observed. It must be energy flowing between the two objects that causes a change in their temperatures.

The energy flowing between the two objects must be on the microscopic level because we can not see it. Energy Review Kinetic energy, KE-the macroscopic energy an object has due to its motion, measured in Joules, J. Gravitational potential energy energy, GPE-the macroscopic energy an object has due to its position, measured in Joules, J. Total mechanical energy, E-the sum of an object’s kinetic and potential energies, measured in Joules, J. Work, W-the process by which the total mechanical energy can be changed, measured in Joules, J.

Work and the Related Changes in Macroscopic Energy

All macroscopic objects are composed of microscopic objects: atoms and molecules These atoms and molecules are moving so they have a microscopic kinetic energy. These atoms and molecules are subject to conservative forces (gravitational and electrical) so they have a microscopic potential energy. The sum of these microscopic kinetic and potential energies is called Thermal Energy, U.

When two objects with different temperatures are placed in thermal contact, thermal energy flows from the higher temperature object to the lower temperature object until thermal equilibrium is reached. The thermal energy that flows between two objects because of a difference in temperature is called heat, Q. Since heat is a form of energy it is measured in Joules, J. For historical reasons another unit of thermal energy or heat is sometimes used: calorie, cal or kilocalorie, kcal. Conversion Factor 1kcal = 4186J

When work is done on or by an object there is a change in the object’s kinetic energy or its gravitational potential energy or both. The change in kinetic energy is perceived as a change in object’s velocity. The change in gravitational potential energy is perceived as a change in object’s position (height above the reference level). When heat flows into or out of an object there is a change in the object’s thermal energy. How is the change in the thermal energy of an object perceived? A change in thermal energy is perceived as a change in the object’s temperature or phase (solid, liquid, or gas). It has been observed that the change in temperature and the change in phase never occur at the same time.

When the temperature is changing the phase remains constant and when the phase is changing the temperature remains constant. or

What variables determine the magnitude of the change in temperature? More heat results in a larger change in temperature: change in temperature is directly proportional to the amount of heat.

When the same quantity of heat flows into (or out of) a larger mass the change in temperature is less.The change in temperature is inversely proportional to the mass.

When the same quantity of heat flows into (or out of) equal masses of different substances the change in temperature is different. The change in temperature depends on the specific heat, c of the substance. The specific heat of a substance is the amount of heat required to change the temperature of 1kg of the substance by 1C°.

The substance with the higher specific heat experiences a smaller change in temperature. The change in temperature is inversely proportional to the specific heat.

Combing All of the Proportions Heat flow that results in a phase change can not be described by the equation above since during a phase change the temperature remains constant…  T=0. It has been observed that phase changes can only occur at certain temperatures which depend on the particular substance.

Temperature remains constant Ice begins melting 0°C 0°C is the melting (or freezing) point of water. As more heat is added the temperature will remain constant as more ice melts. Once all of the ice has melted the addition of more heat will result in an increase in the temperature of the water. Temperature increases

Temperature remains constant Water begins boiling 100°C is the boiling point of water. As more heat is added the temperature will remain constant as more water converts to steam. Once all of the water has converted to steam the addition of more heat will result in an increase in the temperature of the steam. Temperature increases

What determines how much ice melts or water converts to steam? More heat results in a larger mass of ice melting. The mass of ice melted is directly proportional to the quantity of heat.

When the same quantity of heat flows into (or out of) equal masses of the solid phase of different substances, each at its melting point, different masses will melt. The mass that melts depends on the latent heat of fusion, Q f of the substance.

The latent heat of fusion of a substance is the quantity of heat required to change 1kg of the solid phase of the substance, at its melting point, to 1kg of liquid at the same temperature. The substance with the greater heat of fusion experiences less melting. The mass melted is inversely proportional to the heat of fusion

Combing All of the Proportions There is a similar equation describing the phase change between liquid and gas. Q v is the latent heat of vaporization, the quantity of heat required to change 1kg of the liquid phase of the substance, at its boiling point, to 1kg of gas at the same temperature.

Summary of Thermodynamic Properties and Relationships I) Phase Change Temperatures- °C, K, °F A) Melting / Freezing Point, T f B) Boiling Point, T b A) Solid Phase, c solid B) Liquid Phase, c liquid C) Gas (vapor) Phase, c gas 1. c p, constant pressure 2. c v, constant volume

A) Latent Heat of Fusion, Q f B) Latent Heat of Vaporization, Q v IV) Thermodynamic Relationships

Example: Water T f = 0°C, 273K, 32°F T b = 100°C, 373K, 212°F

Graphical Representation of Heat Flow, Temperature Change, and Phase Change TfTf TbTb T initial T final Step #1 Raise the temperature to the melting point Step #2 Melt all of the solid Step #3 Raise temperature of liquid to boiling point Step #4 Convert the liquid to gas Step #5 Raise the temperature of the gas to the final temperature

Graphical Representation of Heat Flow, Temperature Change, and Phase Change In Converting 2kg of -20°C to 120°C T f = 0°C T b = 100°C T initial = -20°C T final = 120°C Step #1 Raise temperature of the ice to the melting point. Step #2 Melt all of the ice Step #3 Raise the temperature of the water to the boiling point Step #4 Convert all of the water to steam Step #5 Raise temperature of steam to final temperature, 120°C

Calculation of Total Heat Required Step #1 Step #2 Step #3 Step #4 Step #5

Total Heat Required = kcal kcal