Chapter 1 – Section 4 Temperature in Thermal Systems

Objectives Define thermal energy. Name the property that determines the temperature of an object. Convert Fahrenheit temperatures to Celsius & vice versa. Explain the difference between heat & thermal energy. Explain the relationship between heat transfer and temperature change. Solve heat transfer problems.

Energy Energy is the ability to do work. Potential energy is energy of position. When you lift an object, it has potential energy because you lifted it higher. Kinetic energy is energy of motion. A moving object has energy because of its speed.

Thermal Energy The atoms of a substance are in constant motion, even if the substance is frozen. Because the atoms are moving, they have kinetic energy. The total energy of all the atoms in a substance is its thermal energy.

Temperature The temperature of an object is a measure of the average kinetic energy of all the particles in the object. As energy is added to the object, the particles speed up thus increasing the kinetic energy of the particles. As a result, the temperature rises. The object gets hotter.

Measuring Temperature A thermometer uses expansion & contraction of a liquid to measure temperature (usually colored alcohol or mercury). When placed in contact with a hot object or substance, heat flows from the object to the thermometer. The liquid in the thermometer expands.

Measuring temperature - cont The heat flows until the thermometer and the object are in thermal equilibrium, i.e. they are the same temperature. Likewise, for a cold object, heat flows from the thermometer to the object until thermal equilibrium is reached. The liquid contracts.

Temperature Scales Celsius (formerly centigrade) is based on the freezing point (FP) and boiling point (BP) of water. FP = 0 o C BP = 100 o C Fahrenheit is still used in the US. FP = 32 o F BP = 212 o F

Converting temperatures To convert Celsius to Fahrenheit use the formula: T F = 9/5 T C +32 To convert Fahrenheit to Celsius use the formula: T C = 5/9 (T F - 32)

Kelvin temperature scale Also known as the absolute temperature scale. To convert Celsius to Kelvin: T k = T C +273 Thus 0 K = -273 o C 0 K is known as absolute zero.

Heat Heat is a form of energy. Heat flows spontaneously from a high temperature source to a low temperature source. The rate at which heat flows increases as the temperature difference between the objects or areas increases.

Prime Movers In mechanical systems, force is the prime mover. In fluid systems, pressure difference is the prime mover. In electrical systems, potential difference or voltage is the prime mover. In thermal systems, temperature difference is the prime mover.

Heat transfer methods Conduction – objects are in direct contact. Heat (kinetic energy) is transferred as atoms collide with other atoms. Examples– iron in a fire; metal spoon in a hot liquid. Convection – heat is transferred by moving large quantities of fluid (liquid or gas). Examples – weather systems, forced air heaters, convection ovens, hair dryers. Radiation – the only heat transfer method that does not require a medium. Example – sunshine.

Units The SI unit of heat is the joule (J) Other commonly used units are the calorie (cal) and the British thermal unit (BTU). 1 cal is the amount of heat that must be added to 1 gram of water to raise its temperature 1 o C. 1 cal = J 1 BTU is the amount of heat that must be added to 1 lb of water to raise its temperature 1 o F.

Specific heat The specific heat of a substance is the amount of heat needed to raise a unit of mass a unit of temperature. By the definition of the calorie, the specific heat of water is 1 cal/g.o C. By the definition of the BTU, the specific heat of water is 1BTU/lb.o F.

Heat formula Q = mC  T where Q = heat (energy) m = mass C = specific heat (see chart page 71)  T = temperature difference

Heat formula – cont. This formula assumes that there is no phase change. A phase change is going from solid to liquid; liquid to solid or liquid to gas; gas to liquid. If a phase change occurs, a different formula is used.

Phase Change Diagram

Phase change formulas Hv – heat of vaporization – the amount of heat needed to change state from liquid to gas. Hf – heat of fusion – the amount of heat needed to change state from solid to liquid. Q = mHv or Q = mHf Chart pg 75 lists Hf & Hv for some materials.

Summary The thermal energy of a body is the total kinetic energy of all the particles in it. The temperature of a body is the average kinetic energy of all the particles in it. Three temperature scales are Celsius, Fahrenheit and Kelvin. Heat flows due to a temperature difference.

More Summary Heat flows from high temperature to low temperature. The heat needed to raise the temperature of a substance is given by Q = mC  T, assuming no phase change. For a phase changes, Q = mHf for solid – liquid change; or Q = mHv for liquid – gas change. During phase change, temp does not change.