Heat

Georgia Perimeter College

Objectives

  1. Explain the difference between temperature and heat.

  2. List three scales used to measure temperature.
  3. Demonstrate an understanding of the three methods of heat transfer.
  4. Demonstrate an understanding of the units which are used to measure heat in various settings.
  5. Explain how a thermometer works.
  6. Explain how a thermostat works.
  7. Demonstrate an understanding of thermal expansion.
  8. Explain how water expands and contracts in relation to changing temperature and phase.
This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S8P2.  Students will be familiar with the forms and transformations of energy.
    c.  Compare and contrast the different forms of energy (heat, light, electricity, mechanical motion, sound) and their characteristics.
    d. Describe how heat can be transferred through matter by the collisions of atoms (conduction) or through space (radiation).  In a liquid or gas, currents will facilitate the transfer of heat (convection).

This section addresses, in whole or in part, the following Benchmarks for Science Literacy:
  • Heat can be transferred through materials by the collisions of atoms or across space by radiation. If the material is fluid, currents will be set up in it that aid the transfer of heat.
  • Energy appears in different forms. Heat energy is in the disorderly motion of molecules; chemical energy is in the arrangement of atoms; mechanical energy is in moving bodies or in elastically distorted shapes; gravitational energy is in the separation of mutually attracting masses.
  • Transformations of energy usually produce some energy in the form of heat, which spreads around by radiation or conduction into cooler places. Although just as much total energy remains, its being spread out more evenly means less can be done with it.
  • When warmer things are put with cooler ones, the warm ones lose heat and the cool ones gain it until they are all at the same temperature. A warmer object can warm a cooler one by contact or at a distance.
  • Some materials conduct heat much better than others. Poor conductors can reduce heat loss.

This section addresses, in whole or in part, the following National Science Education Standards:
  • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways.
  • Heat moves in predictable ways, flowing from warmer objects to cooler ones, until both reach the same temperature.
  • Heat consists of random motion and the vibrations of atoms, molecules, and ions. The higher the temperature, the greater the atomic or molecular motion.
  • Everything tends to become less organized and less orderly over time. Thus, in all energy transfers, the overall effect is that the energy is spread out uniformly. Examples are the transfer of energy from hotter to cooler objects by conduction, radiation, or convection and the warming of our surroundings when we burn fuels.

Vocabulary

heat
temperature
thermal energy
conduction
convection
radiation
joules
calorie
Btu
Fahrenheit
Celsius
Kelvin
absolute zero
thermometer
thermostat
thermal equilibrium
thermal expansion


What is heat?

Heat is a form of energy that raises the temperature of an object.  Heat is the amount of thermal energy entering or leaving an object due to a difference in temperature.  Heat increases the movement of atoms and molecules in a substance.

What is temperature?

Temperature is a measure of how hot or cold something is, with respect to some standard scale as measured using a thermometer

Temperature is related to the the average kinetic energy (or the energy of motion) of the individual atoms (or molecules) in a substance as they move and collide with one another.  The hotter something is, the faster the atoms are moving, and the higher their kinetic energy.

Thermometers can measure in various temperature scales, such as Fahrenheit or Celsius.  The Celsius scale is more widely used around the world, and is typically used in science.  Sometimes the Kelvin temperature scale is also used.  The Kelvin scale is a measure of absolute temperature.  Absolute zero (the coldest possible temperature) is the temperature at which molecular and atomic motion would theoretically cease.  It may not actually be possible to cool something to this temperature.

The Celsius scale is based on physical changes that occur at the same temperature each time.  In this case, it is based on the temperature at which water freezes (0oC) and the temperature at which water boils and is converted to steam (100oC).

 

What is the difference between heat and temperature?

We can get at the difference between heat and temperature by referring to very hot gases in space, such as the corona of the Sun.  The temperature of the Sun's corona is extremely HIGH, but its heat content is LOW.  This is because there are very few atoms in a given volume of space, but those atoms that are there, are moving very quickly.

There is twice as much thermal energy (or kinetic energy) in 2 liters of boiling water as there is in 1 liter of boiling water, but the temperatures are the same.  So temperature is not the same as thermal or kinetic energy. 

If you have a very tiny container of very hot water, and a large container of cool water, which has the most thermal energy?  A large container of cool water has more thermal energy than a thimbleful of very hot water because there are so many more water molecules in the large container.

Heat Transfer

If an object is surrounded by something at the same temperature, then it will remain at that temperature.

On the other hand, if the object is surrounded by something at a lower temperature, then it will transfer some of its energy to the cooler material surrounding it, and the object will grow colder.

If an object is surrounded by something at a higher temperature, some of the energy from its surroundings will be transferred to the object, and the surroundings will grow colder (unless that energy is replaced).

The hotter, fast-moving atoms come into contact with the slow-moving atoms of the cooler substance.  In these collisions, slower particles will tend to be sped up; faster particles will tend to be slowed down. As a result, the average energy of the particles in the cooler object will increase, raising its temperature, whereas the average energy of the particles in the hotter object will decrease, lowering its temperature.

Heat (or thermal energy) flows from a hotter material to a cooler one.  Heat flow continues until both objects reach the same temperature.

Eventually, the temperature of the two objects will equalize, and there will be no more net flow of energy from one to the other.  This situation is called thermal equilibrium.

There are three ways in which heat can be transferred:

  1. Conduction
  2. Convection
  3. Radiation

Conduction is thermal energy (or heat) moving within a material. If you touch a hot frying pan, its thermal energy will be transferred to your hand.  You will feel the heat.

Some materials conduct heat better than others.  Materials that are poor conductors of heat may be used as insulation, to reduce heat loss.

Convection is heat transport by material moving from one place to another.  Hot material rises and cooler material sinks.  Heating a pan of water on a stove or hotplate makes the water in the bottom of the pan hotter.  The heated water expands.  Hot water is less dense than cold water.  So the hot water rises to the top of the pan, and the cooler, denser water at the top sinks to the bottom. This transfers heat from the bottom throughout the pan of water.  (A lava lamp also illustrates convection or hot material rising, and cool material sinking.)




Convection.
Image courtesy of the U.S. Geological Survey.

Link to convection video

Radiation is heat energy carried by electromagnetic waves. Heat from the Sun is transferred by radiation through space to other objects in the Solar System, including the Earth.  Objects in space (where there is no air) are heated by radiation on the side facing the Sun.  Then, if there is no atmosphere (like on the Moon, or on asteroids), most of that heat is reflected and re-radiated back into space.

The Earth is heated by radiation from the Sun during the day. Some of that heat radiates back into space at night.  However, the Earth's atmosphere acts like a "blanket" and keeps much of the heat from escaping during the night. Clouds are a better "blanket" than air because clouds (which are made of droplets of liquid water) absorb more of the heat radiating from the Earth's surface into space.

The greenhouse effect illustrates the three types of heat transfer. A common misconception is that light from outside the greenhouse passes through the window, and changes into heat. Proper conception requires the understanding that different substances are relatively more transparent (or opaque) to different types of radiant energy. For example, the atmosphere and glass are transparent to visible light. Air is also transparent to heat, but glass is relatively opaque to heat. Light passes through the glass, but heat does not (at least not very well).  Sunlight travels through space (by radiation), through the atmosphere, through glass, through the air inside the glass, and finally hits some solid material.  The light interacts with the solid and changes into heat. This raises the temperature of the solid, which heats up the air around it by conduction. The air inside a greenhouse undergoes convection and hot air rises, carrying the heat upwards, where it is unable to escape through the glass.  As a result, the air inside the greenhouse is hotter than the air outside.

Measuring heat

Heat is usually abbreviated Q.  Heat is a measured in energy units called joules in the SI (metric) system. It takes about 4.2 joules of heat to raise the temperature of 1 gram of water by 1 degree Celsius.

An alternate unit of heat that you may have heard of, is the calorieA calorie is the amount of heat needed to raise the temperature of 1 gram of water by 1 degree Celsius.  If you need to convert between these units, the conversion factor is:  1 calorie = 4.186 joules.

The energy rating of foods (and fuels) is measured in calories.  This is a measure of the energy released when the food or fuel is burned.  (Note that foods are "burned" through metabolism.)  Actually, food is really labeled in kilocalories (or 1000 calories).  This is the amount of heat needed to raise 1 kg (or 1000 g) of water by 1 degree Celsius.  Kilocalories are abbreviated as Calorie (calorie with a capital C).  So, 1 Calorie is 1000 calories.

(These terms dates from the early 1800's, when it was thought that heat was the flow of a hypothetical substance called the caloric fluid.  In 1843 Robert Joule showed experimentally that one calorie of heat is equivalent to 4.186 joules of energy, establishing that heat is a form of energy flow.)

Another unit of heat is the Btu (or British thermal unit).  This is the measure of heat used when discussing household heating and air conditioning.  The Btu is defined as the amount of heat needed to raise the temperature of 1 pound of water by 1 degree Fahrenheit.

Remember, temperature is measured in degrees, but heat is measured in joules.

How does a thermometer work?

A thermometer works when the atoms of a fluid (such as air or a liquid) bounce around and impinge on the thermometer.  The moving atoms transfer some of their energy to the glass and liquid in the thermometer.  (If red, the liquid is alcohol; if silver, the liquid is mercury.)  If the outside fluid is hotter than the thermometer, that makes the atoms in the mercury in the thermometer move faster, and the mercury gets hotter.  The faster the atoms in the mercury move, the more they push on their surroundings.  The mercury expands into the open space in the tube of the thermometer, and rises up the tube as the temperature increases. 

When the temperature drops, the atoms move more slowly and the liquid contracts and moves down the tube.

Solids and liquids and gases expand when heated and contract when cooled.  This is called thermal expansion.

One exception is water, which expands when it changes phase and converts from a liquid to solid ice.  This is because the crystal structure of ice takes up more room than an equivalent amount of liquid water.  So ice is less dense than water.  As a result, ice will float on water.  This is why ice will form on the surface of a lake or a container of water, rather than sinking down to the bottom.

How does an expansion joint work?

 

How does a thermostat work?

Different materials expand and contract different amounts in response to a given change in temperature.  This is called thermal expansion.

Household thermostats (like for your furnace and air conditioner) contain something called a bimetal (or bimetallic) strip, which consists of thin sheets of two different metals bonded together flat.  As the temperature rises, one metal expands more than the other, and so the bimetal strip bends or changes shape, which triggers a switch that turns your air conditioner on (or your furnace off). 

If you look inside a thermostat, you will see a coiled metallic band.  That coil is the bimetal strip.  As the temperature changes, the difference in expansion and contraction between the two metals in the strip causes the coil to wind (or unwind).  This triggers the switch, which is usually a tube containing mercury, and causes it to turn your furnace or air conditioner on and off.  (Newer digital thermostats use a different type of control.)

 

Mechanism of a household thermostat. 
Image courtesy of Wikipedia.

Heat capacity

Different materials have different capacities for storing thermal energy.  In addition, it takes longer to heat some materials up, compared with other materials.

For example, it takes longer to heat water than it does metal (such as a frying pan).  But water also holds it heat longer than does a metal frying pan.  Water takes longer to cool than does metal. 

This characteristic is known as the specific heat capacity (or specific heat).  Specific heat capacity is sort of like a "thermal inertia".  It is the resistance of a material to a change in temperature.

Water has a higher specific heat capacity than metal (or rock).   This characteristic affects world climate.  For example, the high specific heat of water keeps Europe's climate milder than might be expected at that latitude.  This is due to the Gulf Stream, which carries warm water to the northeast in the Atlantic Ocean.  This is also the reason that Bermuda has a tropical climate although it is at the same latitude as North Carolina.

Heat capacity also affects coastal winds, creating land and sea breezes.  The land heats and cools more rapidly than does the sea. Water retains heat longer than land, and also takes longer to heat and cool.  This causes temperature differences between the land and the sea, which leads to a thermal circulation (or wind based on temperature differences).  During the day, the land gets hotter faster, and the hot air rises, creating an area of lower pressure. Wind blows from the sea to the land. This is a sea breeze. At night, the land cools off faster than the sea. Cooler air descends creating an area of higher pressure. Wind blows from the land to the sea. This is a land breeze.

 


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Content provided by Pamela J. W. Gore, Georgia Perimeter College,
Georgia Perimeter College Dunwoody Physics Lab Coursepack,
NASA Explores http://www.nasaexplores.com/show_58_teacher_st.php?id=040625124459
NASA Cosmicopia http://helios.gsfc.nasa.gov/qa_sp_ht.html

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created May 19, 2007
Modified May 28, 2007
Modified February 18, 2009