Heat, Temperature, and
Thermal Energy Transfer
Ck12 Science
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Printed: November 16, 2015
AUTHOR
Ck12 Science
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C HAPTER
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Chapter 1. Heat, Temperature, and Thermal Energy Transfer
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Heat, Temperature, and
Thermal Energy Transfer
Define heat.
Define temperature.
Describe thermal energy transfer.
Define Celsius and Kelvin temperature scales.
Convert Celsius temperatures to Kelvin and vice versa.
The temperature of basalt lava at Kilauea (Hawaii) reaches 1,160 degrees Celsius (2,120 degrees Fahrenheit). A
crude estimation of temperature can be determined by looking at the color of the rock: orange-to-yellow colors are
emitted when rocks (or metals) are hotter than about 900 degrees Celsius; dark-to-bright cherry red is characteristic
as material cools to 630 degrees Celsius; faint red glow persists down to about 480 degrees Celsius. For comparison,
a pizza oven is commonly operated at temperatures ranging from 260 to 315 degrees Celsius.
Heat, Temperature, and Thermal Energy Transfer
The first theory about how a hot object differs from a cold object was formed in the 18th century. The suggested
explanation was that when an object was heated, an invisible fluid called “caloric” was added to the object. Hot
objects contained more caloric than cold objects. The caloric theory could explain some observations about heated
objects (such as that the fact that objects expanded as they were heated) but could not explain others (such as why
your hands got warm when you rub them together).
In the mid-19th century, scientists devised a new theory to explain heat. The new theory was based on the assumption
that matter is made up of tiny particles that are always in motion. In a hot object, the particles move faster and
therefore have greater kinetic energy. The theory is called the kinetic-molecular theory and is the accepted theory
of heat. Just as a baseball has a certain amount of kinetic energy due to its mass and velocity, each molecule has a
certain amount of kinetic energy due to its mass and velocity. Adding up the kinetic energy of all the molecules in
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an object yields the thermal energy of the object.
When a hot object and a cold object touch each other, the molecules of the objects collide along the surface where
they touch. When higher kinetic energy molecules collide with lower kinetic energy molecules, kinetic energy is
passed from the molecules with more kinetic energy to those with less kinetic energy. In this way, heat always flows
from hot to cold and heat will continue to flow until the two objects have the same temperature. The movement of
heat from one object to another by molecular collision is called conduction.
Heat is the energy that flows as a result of a difference in temperature. We use the symbol Q for heat. Heat, like all
forms of energy, is measured in joules.
The temperature of an object is a measurement of the average kinetic energy of all the molecules of the object. You
should note the difference between heat and temperature. Heat is the sum of all the kinetic energies of all the
molecules of an object, while temperature is the average kinetic energy of the molecules of an object. If an object
was composed of exactly three molecules and the kinetic energies of the three molecules are 50 J, 70 J, and 90 J, the
heat would be 210 J and the temperature would be 70 J.
The terms hot and cold refer to temperature. A hot object has greater average kinetic energy but may not have greater
total kinetic energy. Suppose you were to compare a milliliter of water near the boiling point with a bathtub full of
water at room temperature. The bathtub contains a billion times as many water molecules, and therefore has a higher
total kinetic energy and more heat. Nonetheless, we would consider the bathtub colder because its average kinetic
energy, or temperature, is lower.
Temperature Scales: Celsius and Kelvin
A thermometer is a device used to measure temperature. It is placed in contact with an object and allowed to
reach thermal equilibrium with the object (they will have the same temperature). The operation of a thermometer is
based on some property, such as volume, that varies with temperature. The most common thermometers contain
liquid mercury, or some other liquid, inside a sealed glass tube. The liquid expands and contracts faster than
the glass tube. Therefore, when the temperature of the thermometer increases, the liquid volume expands faster
than the glass volume, allowing the liquid to rise in the tube. The positions of the liquid in the tube can then
be calibrated for accurate temperature readings. Other properties that change with temperature can also be used to
make thermometers; liquid crystal colors and electrical conductivity change with temperature, and are also relatively
common thermometers.
The most commonly used temperature scale in the United States is the Farenheit scale. However, this scale is rarely
used throughout the world; the metric temperature scale is Celsius. This scale, based on the properties of water, was
devised by the Swedish physicist, Anders Celsius (1704 – 1744). The freezing point of water is 0◦ C and the boiling
point of water was assigned to be 100◦ C. The kinetic energies between these two points was divided evenly into
100 “degrees Celsius”.
The Kelvin or “Absolute” temperature scale is the scale often used by chemists and physicists. It is based on
the temperature at which all molecular motion ceases; this temperature is called absolute zero and is 0 K. This
temperature corresponds to -273.15◦ C. Since absolute zero is the coldest possible temperature, there are no negative
values on the Kelvin temperature scale. Conveniently, the Kelvin and Celsius scales have the same definition of a
degree, which makes it very easy to convert from one scale to the other. The relationship between Celsius and Kelvin
temperature scales is given by:
K = ◦ C + 273.15
On the Kelvin scale, water freezes at 273 K and boils at 373 K.
Example Problem: Convert 25◦ C to Kelvin.
Solution: K = ◦ C + 273 = 25◦ C + 273 = 298 K
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Chapter 1. Heat, Temperature, and Thermal Energy Transfer
Blender, the Next Stove?
This MIT video examines the phenomenon of Joule heating through the perspective of a blender, reproducing the
experiment of the English physicist James Prescott Joule. See the video at http://techtv.mit.edu/videos/14887-blend
er-the-next-stove-the-joule-experiment .
Summary
• The thermal energy, or heat, of an object is obtained by adding up the kinetic energy of all the molecules
within it.
• Temperature is the average kinetic energy of the molecules.
• Absolute zero is the temperature where molecular motion stops and is the lowest possible temperature.
• Zero on the Celsius scale is the freezing point of water and 100◦ C is the boiling point of water.
• The relationship between Celsius and Kelvin temperature scales is given by K = ◦ C + 273.15.
Practice
Questions
The following video demonstrates how some materials conduct heat better than others. Use this resource to answer
the questions that follow.
http://www.youtube.com/watch?v=FmG1Sc0AS3s
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/82540
1. Which material was a better conductor of heat?
2. Explain why metals feel cold even when thay are at room temperature.
Heat and temperature practice problems and questions:
http://www.education.com/study-help/article/telllperature-heat_answer/
Review
Questions
1. Convert 4.22 K to ◦ C.
2. Convert 37◦ C to K.
3. If you had beeswax attached to one end of a metal skewer and you placed the other end of the skewer in a
flame, what would happen after a few minutes?
4. Which contains more heat, a coffee cup of boiling water or a bathtub of room temperature water?
• thermal energy: The total energy of a substance particles due to their translational movement or vibrations.
• heat: energy transferred from one body to another by thermal interactions.
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• temperature: A measurement of the average kinetic energy of the molecules in an object or system and can
be measured with a thermometer or a calorimeter.
• conduction: The transfer of thermal energy by the movement of particles that are in contact with each other.
• absolute zero: The lowest possible temperature, at which point the atoms of a substance transmit no thermal
energy - they are completely at rest. It is 0 degrees on the Kelvin scale, which translates to -273.15 degrees
Celsius (or -459.67 degrees Fahrenheit).
References
1. Courtesy of the National Parks Service. http://commons.wikimedia.org/wiki/File:Volcano_q.jpg .
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