Chapter 1 – Section 4 Temperature in Thermal Systems.
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Transcript of Chapter 1 – Section 4 Temperature in Thermal Systems.
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.