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 = 0o C BP = 100o C

• Fahrenheit is still used in the US.

• FP = 32o F BP = 212o F

Converting temperatures

• To convert Celsius to Fahrenheit use the formula: TF = 9/5 TC +32

• To convert Fahrenheit to Celsius use the formula: TC = 5/9 (TF - 32)

Kelvin temperature scale

• Also known as the absolute temperature scale.

• To convert Celsius to Kelvin: Tk = TC +273

• Thus 0 K = -273o 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 1o C. 1 cal = 4.184 J

• 1 BTU is the amount of heat that must be added to 1 lb of water to raise its temperature 1o 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.oC.

• By the definition of the BTU, the specific heat of water is 1BTU/lb.oF.

Heat formula

• Q = mCT 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 = mCT, 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.