Ch. 1.4 Temperature in Thermal Systems

ThermalEnergy Thermal Energy The property that enables a body to do work or cause change is called energy. You increase the energy of an object when you do work on it or cause it to move. When an object gains energy as a result of changing its position, it has the potential for doing work or exerting energy. This energy is called potential energy. When an object gains energy as a result of changing its position, it has the potential for doing work or exerting energy. This energy is called potential energy.

ThermalEnergy - 1 Thermal Energy - 1 An object in motion is also capable of doing work or transferring energy. Energy of motion is called kineticenergy. An object in motion is also capable of doing work or transferring energy. Energy of motion is called kinetic energy. We use the concept of kinetic energy to model the energy of atoms and molecules. Atoms and molecules are always vibrating randomly. The rate of this vibration determines whether an object exists in the solid, liquid, gas or plasma state. This random motion of the particles is called thermal motion.

ThermalEnergy - 2 Thermal Energy - 2 The total energy of the thermal motion of all the particles that make up an object is called the thermal energy of the object.

Temperature When two objects of different temperatures are brought together, the thermal energy always flows from the hotter object to the cooler object. The property of “hotness” or “coolness” is called temperature and is determined by the average kinetic energy of the particles in the object.

Measuring Temperature Thermal energy, since it is energy, is measured in joules. Temperature, on the other hand, is measured in degrees. Devices for measuring temperature use the property of thermal expansion in order to indicate temperature change. A thermometer uses the expansion and contraction of a liquid, usually colored alcohol or mercury, to measure temperature.

Measuring Temperature - 1 As thermal energy travels between objects, all of the objects will eventually reach the same temperature. When this occurs, we say that the objects have reached thermal equilibrium. There are three major temperature scales. Here in the United States, we normally use the Fahrenheit scale. On the Fahrenheit scale, water freezes at 32  and boils at 212 .

Measuring Temperature - 2 Scientists in the lab commonly use the Celsius temperature scale. The Celsius scale is based on properties of water. Zero on the Celsius scale is defined as the freezing point of pure water at sea level. One hundred degrees Celsius is defined as the boiling point of water. Zero on the Celsius scale is defined as the freezing point of pure water at sea level. One hundred degrees Celsius is defined as the boiling point of water. The third temperature scale is the Kelvin scale, which is based on absolute zero. We will study the Kelvin scale in more detail in Chapter 5.

Measuring Temperature - 3 You can convert from Celsius to Fahrenheit using the following formulas: To convert from degrees Fahrenheit (T F ) to degrees Celsius (T C ). T C = 0.56(T F - 32 ) T C = 0.56(T F - 32 ) To convert from degrees Celsius (T C ) to degrees Fahrenheit (T F ). T F = 1.8(T C ) + 32 T F = 1.8(T C ) + 32

Heat Heat is not another word for thermal energy. Heat is thermal energy that flows from one object to another due to a temperature difference. So, temperature difference is the prime mover in thermal systems. Conduction occurs when heat is transferred through direct contact between the objects. An example of conduction would be an iron touching the shirt being ironed.

Specific Heat In SI, the units of thermal energy and heat are the joule (J). Another commonly used unit that you may be familiar with is the calorie. One calorie is equal to J. In some applications involving thermal energy and heat, the British thermal unit (Btu) is used.

Specific Heat - 1 One calorie is the amount of thermal energy that must be added to water to change the temperature of 1 gram of water by 1 degree Celsius. One Btu is the amount of thermal energy that must be added to water to change the temperature of 1 pound of water by 1 degree Fahrenheit. In science, the calorie is more commonly used than the Btu.

Specific Heat - 2 Specific heat is the amount of energy needed to raise the temperature of 1 gram of a material by 1 degree Celsius. A table listing the specific heats of common substance is on page 71 in your text. Changes in thermal energy can be calculated by using the following formula: Change in thermal energy (Q) = mass (m) x change in temperature (  T) x specific heat (C) Q = m x  T x C OR Q = mC  T Q = m x  T x C OR Q = mC  T

Changeof State Change of State When substances change from one state or phase to another, there is a temperature on the thermometer at which the change in state occurs. The temperature at which a solid begins to liquefy is called the melting point. The temperature at which a liquid begins changing to a gas is called the boiling point.

Changeof State - 1 Change of State - 1 When observing the temperature as changes in state take place, you will notice that the temperature rises until the change of state begins. When the change in state occurs, the temperature stays the same until the change of state has occurred. After the change of state, the temperature will again rise until the next change of state. This occurs because the change in state requires energy, so the energy used to cause the temperature to rise is, instead, being used to change the state.

Changeof State - 2 Change of State - 2 The amount of energy needed to melt one gram of a solid substance is called the heat of fusion. The amount of energy needed to vaporize one gram of a liquid substance is called the heat of vaporization.

Heating Curve of Water State Changes vs Temperature of Water Temperature (°C) Heat (kJ) a b – solid changes to liquid c d – liquid changes to gas