Thermal Force
Unit 1.4
Objectives
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Define thermal energy
Name the property of a body that determines its
temperature.
Given Celsius or Fahrenheit temperature and the
formula for conversion, find the equivalent
temperatures.
Explain the difference between heat and thermal
energy
Explain the relationship between heat transferred to
an object and the change in the object’s temperature
Use specific heat, heat of fusion, and heat of
vaporization to solve problems involving heat
transfer
Terms
(We will study energy in the next unit)
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Energy – the ability to do work.
Potential energy – the stored or build up of energy;
the capacity/ “potential” to do work.
Kinetic energy – energy of motion.
Thermal motion – the random motion of vibration
of an object’s atoms and molecules.
Thermal energy – the total energy of the thermal
motion of all particles that make up an object
Temperature
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Temperature is the “hotness” property of a body; it
is the average kinetic energy of the random motion
of the atoms and molecules in a body.
Heat flows from regions of higher temperature to
regions of lower temperature.
Measured in degrees Fahrenheit or degrees Celsius.
Thermal Energy is measured in Joules, Calories and
BTU’s
Measuring Temperature
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Thermometer – uses expansion and
contraction of a liquid usually colored
alcohol or mercury.
Celsius scale – developed by Anders
Celsius and based on the properties of
water. (0° = freezing; 100° = boiling)
Fahrenheit scale - used in the US (32° =
freezing and 212° = boiling)
Temperature scales
TC =
5
(TF – 32)
9
TF =
9
TC + 32
5
Heat
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Conduction – thermal energy flows from the hotter
object to the cooler object as kinetic energy is
transferred when particles collide.
Heat is the energy that flows from one object to
another because of temperature difference.
Heat flow depends on temperature difference.
As the temperature difference increases so does the
heat transfer rate. (no temp. diff = no heat transfer)
Temperature difference is the prime mover in
thermal systems.
Analogies to Prime Movers
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Mechanical – Force
Fluid – Pressure
Electrical – Voltage
Thermal – Temperature Difference
Specific Heat
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The specific heat (C) of a substance is the
amount of energy that must be added to
raise the temperature of a unit mass of the
substance one unit of temperature.
cal
Specific heat of water = 1 g • C°
energy
Unit of temp.
mass
Change of State
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States of matter – Solid, Liquid, Gas, and
Plasma.
Changes of state
Changes in State
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Freezing Point – Temperature at which a
substance changes from a liquid to a solid.
Boiling Point – Temperature at which a
liquid changes to a gas.
Heat of Fusion (Hf)– the amount of energy
required to change a melt a solid.
Heat of Vaporization (Hv) – the amount of
energy required to vaporize a liquid.
Changes of State
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The amount of heat (Q) needed to melt a
solid of mass (m) is
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Q = m Hf
The mount of heat (Q) needed to vaporize a
liquid of mass (m) is
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Q = m Hv
Summary
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The thermal energy of a body is the total kinetic energy of motion
of all the particles that make up the body.
The temperature of a body is determined by the average kinetic
energy of the particles that make up the body.
A thermometer measures temperature in degrees Celsius or
degrees Fahrenheit.
Heat is the energy that flows from one body to another because of
a temperature difference.
Whenever two bodies are brought together, heat flows from the
body with higher temperature to the body with the lower
temperature.
The amount of heat transferred to an object varies linearly with
the object’s temperature change, as long as there is no change of
state: Q=mcT
If heat is transferred to a substance and it changes state, its
temperature does not change.
Measurements
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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.
Heat Transfer
Object’s
mass
Q = m C T
Heat
transferred
Specific
heat
Temperature
Change