Energy Transfer

Chemistry in History Elements and Compounds Periodic Table Chemical Reactions     Energy Transfer Table of Contents


Lesson Objectives

The student will:

  • explain the difference between kinetic and potential energy.
  • state the law of conservation of matter and energy.
  • define heat.
  • define work.


chemical potential energy


kinetic energy

law of conservation of energy

law of conservation of matter and energy

potential energy



Just like matter, energy is a term that we are all familiar with and use on a daily basis. Before you go on a long hike, you eat an energybar; every month, the energy bill is paid; on TV, politicians argue about the energy crisis. But have you ever wondered what energy really is? If you stop to think about it, energy is very complicated. When you plug a lamp into an electric socket, you see energy in the form of light, but when you plug a heating pad into that same socket, you only feel warmth. Without energy, we couldn’t turn on lights, we couldn’t brush our teeth, we couldn’t make our lunch, and we couldn’t travel to school. In fact, without energy, we couldn’t even wake up because our bodies require energy to function. We use energy for every single thing that we do, whether we're awake or asleep. Although we all use energy, very few of us understand what it is.

Types of Energy: Kinetic and Potential

Energy is the ability to do work or cause change. Machines use energy, our bodies use energy, energy comes from the sun, energy causes forest fires, and energy helps us to grow food. With all these seemingly different types of energy, it’s hard to believe that there are really only two different forms of energy: kinetic energy and potential energy.

Kinetic energy is energy associated with motion. When an object is moving, it has kinetic energy, and when the object stops moving, it has no kinetic energy. Although all moving objects have kinetic energy, not all moving objects have the same amount of kinetic energy. The amount of kinetic energy possessed by an object is determined by its mass and its speed. The heavier an object is and the faster it is moving, the more kinetic energy it has.

Kinetic energy is very common and is easy to spot in the world around you. Sometimes we even capture kinetic energy and use it to power things like our home appliances. Have you ever seen windmills lining the slopes of a hill like the ones in Figure below? These windmills capture the kinetic energy of the wind to provide power that people can use in their homes and offices. As wind rushes along the hills, the kinetic energy of the blowing air particles turns the windmills, which convert the wind’s kinetic energy into electricity.


Tehachapi wind farm

This is a photograph of a wind farm in Southern California. Kinetic energy from the rushing air particles turns the windmills, allowing us to capture the wind


Capturing kinetic energy can be very effective, but you may already realize that there is a small problem: kinetic energy is only available when something is moving. When the wind stops blowing, there’s no kinetic energy available. Imagine what it would be like trying to power your television set using the wind’s kinetic energy. You could turn on the TV and watch your favorite program on a windy day, but every time the wind stopped blowing, your TV screen would flicker off because it would run out of energy.

You'd have noticed, however, that you can usually rely on your TV to stay on. This is largely because we don’t rely on kinetic energy alone for power. Instead, we primarily use energy in its other form as potential energy. Potential energy is stored energy that remains available until we choose to use it. Think of a battery in a flashlight. If you leave a flashlight on, the battery will run out of energy within a couple of hours. If, instead, you only use the flashlight when you need it and turn it off when you don’t, the battery will last for days or even months. Because the battery stores potential energy, you can choose to use the energy all at once, or you can save it and use a small amount at a time.

Any stored energy is potential energy and has the “potential” to be used at a later time. Unfortunately, there are a lot of different ways in which energy can be stored, making potential energy very difficult to recognize. Generally speaking, an object has potential energy due to its position relative to another object. For example, when you hold a rock above the earth, it has more potential energy than a rock on the ground. As long as you're holding the rock, the rock has potential energy stored. Once you drop the rock, though, the stored energy is released. This can confuse students because it doesn't seem like a falling rock is releasing energy. Remember, however, that energy is defined as the ability to do work or cause change.

For some examples of potential energy, though, it’s harder to see how “position” is involved. In chemistry, we are often interested in what is called chemical potential energy. Chemical potential energy is energy stored in the atoms, molecules, and chemical bonds that make up matter. How does this depend on position? As you learned earlier, the world and all of the chemicals in it are made up of atoms. These atoms store potential energy that is dependent on their positions relative to one another. Although we cannot see atoms, scientists know a lot about the ways in which atoms interact. This allows them to figure out how much potential energy is stored in a specific quantity of a particular chemical. Different chemicals have different amounts of potential energy because they are made up of different atoms, and those atoms have different positions relative to one another.

The image below represents two hydrogen atoms chemically joined to an oxygen atom to form a water molecule. Scientists use their knowledge of what the atoms and molecules look like and how they interact to determine the potential energy that can be stored in any particular chemical substance.

Since different chemicals have different amounts of potential energy, scientists will sometimes say potential energy depends on not only position but also composition. Composition affects potential energy because it determines which molecules and atoms end up next to each other. For example, the total potential energy in a cup of pure water is different than the total potential energy in a cup of apple juice because the cup of water and the cup of apple juice are composed of different amounts of different chemicals.

The Law of Conservation of Matter and Energy

While it’s important to understand the difference between kinetic energy and potential energy, the truth is energy is constantly changing. Kinetic energy is constantly being turned into potential energy, and potential energy is constantly being turned into kinetic energy. Even though energy can change form, it must still follow the fundamental law: energy cannot be created or destroyed, it can only be changed from one form to another. This law is known as the law of conservation of energy. In a lot of ways, energy is like money. You can exchange quarters for dollar bills and dollar bills for quarters, but no matter how often you convert between the two, you won’t end up with more or less money than you started with.

Think about what happens when you throw a ball into the air. When the ball leaves your hand, it has a lot of kinetic energy. At some point, the ball will stop momentarily in the air and then falls back down. When the ball stops, it no longer has any kinetic energy. According to the law of conservation of energy, the initial kinetic energy that the ball had does not just disappear. Instead, as the ball moves higher and higher into the sky, the kinetic energy is converted to potential energy. When the ball stops moving upward, all of the kinetic energy has been converted to potential energy. The ball then starts to fall back down, and the potential energy is once again changed into kinetic energy.

As it turns out, the law of conservation of energy isn’t completely accurate. Energy and matter are actually interchangeable. In other words, energy can be created (made out of matter) and destroyed (turned into matter). As a result, the law of conservation of energy has been changed into the law of conservation of matter and energy. This law states that: the total amount of mass and energy in the universe is conserved (does not change). This is one of the most important laws you will ever learn. Nevertheless, in chemistry we are rarely concerned with converting matter to energy or energy to matter. Instead, chemists deal primarily with converting one form of matter into another form of matter (through chemical reactions) and converting one form of energy into another form of energy.

Heat and Work

When we talk about using energy, we are really referring to transferring energy from one place to another. When you use energy to throw a ball, you transfer energy from your body to the ball, which causes the ball to fly through the air. When you use energy to warm your house, you transfer energy from the furnace to the air in your home, which causes the temperature in your house to rise. Although energy is used in many kinds of different situations, all of these uses rely on energy being transferred in one of two ways: as heat or as work. Unfortunately, both “heat” and “work” are used commonly in everyday speech, so you might think that you already know their meanings. In science, the words “heat” and “work” have very specific definitions that may be different from what you expect. Do not confuse the everyday meanings of the words “heat” and “work” with the scientific meanings.

When scientists speak of heat, they are referring to energy that is transferred from an object with a higher temperature to an object with a lower temperature as a result of the temperature difference. Heat will “flow” from the hot object to the cold object until both end up at the same temperature. When you cook with a metal pot, you witness energy being transferred in the form of heat. Initially, only the stove element is hot; the pot and the food inside the pot are cold. As a result, heat moves from the hot element to the cold pot, as illustrated in Figure below. After a while, enough heat is transferred from the element to the pot, raising the temperature of the pot and all of its contents.


Glass Saucepan on the Gas Stove

Energy is transferred as heat from the hot stove element to the cooler pot until the pot and its contents become just as hot as the element.


We’ve all observed heat moving from a hot object to a cold object, but you might wonder how the energy actually travels. Whenever an object is hot, the molecules within the object are shaking and vibrating vigorously. The hotter an object is, the more the molecules jiggle around. Anything that is moving has energy, and the more it’s moving, the more energy it has. Hot objects have a lot of energy, and it’s this energy that is transferred to the colder objects when the two come in contact.

The easiest way to visualize heat transfer is to imagine a domino effect. When the vibrating molecules of the hot object bump into the molecules of the colder object, they transfer some of their energy, causing the molecules in the colder object to start vibrating vigorously as well. In the image below, the red molecules are jiggling around and vibrating. As these molecules vibrate, they bump into their neighbors (the blue molecules) and transfer some of their energy. These colder molecules begin to heat up and begin to vibrate faster. Just like dominoes, the heat gets passed along the chain until the energy is spread equally between all of the molecules. At the end, all of the molecules will be at the same temperature.

Heat is only one way in which energy can be transferred. Energy can also be transferred as work. The scientific definition of work is force (any push or pull) applied over a distance. Whenever you push an object and cause it to move, you’ve done work and transferred some of your energy to the object. At this point, it is important to warn you of a common misconception. Sometimes we think that the amount of work done can be measured by the amount of effort put in. This may be true in everyday life, but this is not true in science. By definition, scientific work requires that force be applied over a distance. It doesn’t matter how hard you push or pull. If you haven’t moved the object, you haven’t done any work. For example, no matter how much you sweat, if you cannot lift a heavy object off the ground, you have not done any work.

Lesson Summary

  • Energy is the ability to do work or cause change.
  • The two forms of energy are kinetic energy and potential energy.
  • Kinetic energy is energy associated with motion.
  • Potential energy is stored energy.
  • Kinetic energy is constantly being turned into potential energy, and potential energy is constantly being turned into kinetic energy.
  • Even though energy can change form, it must still follow the law of conservation of energy.
  • The law of conservation of energy states that energy cannot be created or destroyed, it can only be changed from one form to another.
  • When scientists speak of heat, they are referring to energy that is transferred from an object with a higher temperature to an object with a lower temperature as a result of the temperature difference.
  • Heat will “flow” from the hot object to the cold object until both end up at the same temperature.
  • Energy can also be transferred as work.
  • Work is force (any push or pull) applied over a distance.

Further Reading / Supplemental Links

Summary of concepts of matter and energy and benchmark review.

Classroom videos about energy.

The Three States of Matter

Lesson Objectives

  • The student will describe molecular arrangement differences among solids, liquids, and gases.
  • The student will describe the basic characteristic differences among solids, liquids, and gases.


The Kinetic Molecular Theory allows us to explain the existence of the three phases of matter. In addition, it helps explain the physical characteristics of each phase and how phases change from one to another. The Kinetic Molecular Theory is essential for the explanations of gas pressure, compressibility, diffusion, and mixing. Our explanations for reaction rates and equilibrium also rest on the concepts of the Kinetic-Molecular Theory.

The Assumptions of the Kinetic Molecular Theory

According to the Kinetic Molecular Theory, all matter is composed of tiny particles that are in constant, random, straight-line motion. This motion is constantly interrupted by collisions between the particles and between the particles and surfaces. The rate of motion of the particles is related to their temperature. The velocity of the particles is greater at higher temperatures and lower at lower temperatures.

In our discussions of gases, we will be referring to what are called ideal gases. In real gases, there is some slight attraction between the gas molecules and the molecules themselves do take up a small amount of space. However, in an ideal gas, we assume there are no attractions between molecules and we assume the molecules themselves take up no space. Later in this chapter, real and ideal gases will be discussed again in more detail.

The Characteristics of Solids

In a solid, the molecules are held in a tightly packed pattern (see Figure above), in which molecules hold a set position in spite of random motion. Molecular motion is reduced to vibrating in place.

In a liquid, the molecules touch each other but are not held in a pattern. The liquid structure has holes in it, which allow molecules to pass each other and change position in the structure. In a gaseous substance, the molecules are completely separated from each other and move around independently. Most of the volume of a gas is empty space. The molecular arrangement in the three phases accounts for the various characteristics of the phases of matter.

In a liquid, the molecules touch each other but are not held in a pattern. The liquid structure has holes in it, which allow molecules to pass each other and change position in the structure. In a gaseous substance, the molecules are completely separated from each other and move around independently. Most of the volume of a gas is empty space. The molecular arrangement in the three phases accounts for the various characteristics of the phases of matter.

For example, mixing of the particles in solids is almost non-existent. This is because the molecules cannot pass by one another in the tightly packed pattern. Solids are essentially incompressible because when a substance is compressed, it is the spaces between molecules that are compressed, not the molecules themselves. Since solids have almost no empty space in their structure, they do not compress. Solids have their own shape and volume as shown in Figure below. A 25 \;\mathrm{mL} rectangular piece of copper has the same shape and volume when it is resting on the table top as it does inside a beaker. The volume and shape of a solid is maintained by the particle structure of the solid which is a tightly held pattern of atoms.

The Characteristics of Liquids

In liquids, mixing occurs more readily because there are spaces between the molecules that allow molecules to pass each other. The spaces between the molecules in liquids are small and so liquids have very little compressibility. Liquids maintain their own volume but take their shape from the shape of the container (Figure below).

25 \;\mathrm{mL} sample of liquid in a graduated cylinder has a volume of 25-\;\mathrm{mL} and has the shape of a cylinder. If the 25 \;\mathrm{mL} sample is placed in a beaker, it still has a volume of 25 \;\mathrm{mL} but now it has the shape of the beaker. The structure of the liquid keeps the particles in touch with each other so the volume does not change but the particles can slide by each other so they can flow to the shape of the container.

The Characteristics of Gases

Mixing in gases is almost instantaneous because there are no inhibitions for particles to pass one another. The volume of a gas is nearly all empty space and so particles move completely freely. Gases are highly compressible because there is a great deal of empty space in gaseous structures which allows the particles to be pushed closer together. Gases do not have either their own volume or their own shape. They take both volume and shape from their container.

Gases mix readily. (Source: Richard Parsons. CC-BY-SA)

Matter is examined in its three principle states – gases, liquids, and solids – relating the visible world to the submicroscopic in an Annenberg video at Video on Demand – The World of Chemistry – A Matter of State (

Phase Changes

Freezing or solidification is a phase transition in which a liquid turns into a solid when its temperature is lowered below its freezing point.

All known liquids, except liquid helium, freeze when the temperature is lowered enough. Liquid helium remains liquid at atmospheric pressure even at absolute zero, and can be solidified only under pressure

Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid. The internal energy of a substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to themelting point, at which the rigid ordering of molecular entities in the solid breaks down to a less-ordered state and the solid liquefies. An object that has melted completely is molten, though nowadays this often refers to melting points above the boiling point of water, such as lava (conversely, thawing is now often used for melting points below boiling for this reason). Substances in the molten state generally have reduced viscosity with elevated temperature; an exception to this maxim is the element sulfur, whose viscosity increases with higher temperatures in its molten state.[1]

Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization isboiling, which, instead, occurs within the entire mass of the liquid.

Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse ofvaporization

Deposition is a process in which gas transforms into solid (also known as desublimation). The reverse of deposition is sublimation.

One example of deposition is the process by which, in sub-freezing airwater vapor changes directly to ice without first becoming a liquid. This is how snow forms in clouds, as well as frost and hoar frost on the ground. Another example is when frost forms on a leaf. For deposition to occur thermal energy must be removed from a gas. When the leaf becomes cold enough, water vapor in the air surrounding the leaf loses enough thermal energy to change into a solid. Deposition in water vapor occurs due to the pureness of the water vapor. The vapor has no foreign particles, and is therefore able to lose large amounts of energy before forming around something. When the leaf is introduced, the supercooled water vapor immediately begins to condensate, but by this point is already past the freezing point. This causes the water vapor to change directly into a solid.

Sublimation is the process of transformation directly from the solid phase to the gaseous phase without passing through an intermediate liquid phase. Sublimation is an endothermic phase transition that occurs at temperatures and pressures below a substance's triple point in its phase diagram.


[edit]Carbon dioxide

Solid carbon dioxide (dry ice) sublimes readily at atmospheric pressure at -78.5°C (197.5 K, −104.2 °F), whereas liquid CO2 can be obtained at pressures and temperatures above the triple point (5.2 atm, -56.4°C).


Snow and ice sublime, although more slowly, below the melting point temperature.[1] This allows a wet cloth to be hung outdoors in freezing weather and retrieved later in a dry state. In freeze-drying, the material to be dehydrated is frozen and its water is allowed to sublime under reduced pressure or vacuum. The loss of snow from a snowfield during a cold spell is often caused by sunshine acting directly on the upper layers of the snow. Ablation is a process that includes sublimation and erosive wear ofglacier ice.

Lesson Summary

  • All matter is composed of tiny particles called atoms or molecules.
  • These particles are in constant random motion at all temperatures above absolute zero.
  • In the solid phase, the molecules are held in a highly organized, tightly-packed pattern.
  • Due to the tightly-packed pattern of molecules in a solid, solids maintain their own shape and volume and do not mix readily.
  • In the liquid phase, molecules are in touch with each other but they loosely packed and may move past each other easily.
  • Due to the loosely packed structure of a liquid, liquids maintain their own volume but take the shape of their container and they are able to mix readily.
  • In the gaseous phase, molecule are completely separate from each other.
  • The volume of a gas is mostly empty space.
  • Due to the structure of gases, they take both the volume and the shape of their container and they mix almost instantaneously.

The Big Ideas

Heat is a form of energy transfer. Materials do not contain heat. They contain internal energy that can be transferred (i.e. heat) from one body to another. Internal energy is the vibrating and rotating and general jostling of atoms and/or molecules that make up the ‘thing,’ whether it is wood, steel, water. One way to measure the heat of an object is to measure its temperature. This is really a statement about its internal kinetic energy.

Heat expansion is a result of the increase of kinetic energy of the molecules. As their movement increases, they bump into each other more and the material slightly expands as a result. Most materials expand with heat. Water is a particularly interesting substance in that it contracts as its temperature increases from 0^\circ \mathrm{C} to 4^\circ \mathrm{C} (and then expands from 4^\circ\mathrm{C} to 100^\circ \mathrm{C}).

Heat can be transferred in three ways, through conduction, convection, and radiation. Conduction is the transfer of heat by physical contact. Heat flows form the hotter object to the cooler object. Convection is heat transfer by an intermediate substance (for example air or water). Your oven (often properly called the ‘convection oven’) works by heating up the air and then the air heats up your food. Radiation is the release of heat (and thus the lowering of its internal energy) by releasing electromagnetic waves. The hotter the object the higher the frequency of the light emitted. When you look at a fire the blue flames our hotter than the red flames because blue has a higher frequency than red.

Entropy is a measure of disorder, or the variety of ways in which a system can organize itself with the same total energy. The entropy of any isolated system always tends to disorder (i.e. entropy is always increasing). In the universe, the entropy of a subset (like evolution on Earth) can decrease (i.e. more order), but the total entropy of the universe is decreasing (i.e. more disorder).

Heat can be transferred from one place to another by three methods: conduction in solids, convection of fluids (liquids or gases), and radiation through anything that will allow radiation to pass. The method used to transfer heat is usually the one that is the most efficient. If there is a temperature difference in a system, heat will always move from higher to lower temperatures.

A thermal infrared image of a coffee cup filled with a hot liquid. Notice the rings of color showing heat traveling from the hot liquid through the metal cup. You can see this in the metal spoon as well. This is a good example of conduction.

Conduction occurs when two object at different temperatures are in contact with each other. Heat flows from the warmer to the cooler object until they are both at the same temperature. Conduction is the movement of heat through a substance by the collision of molecules. At the place where the two object touch, the faster-moving molecules of the warmer object collide with the slower moving molecules of the cooler object. As they collide, the faster molecules give up some of their energy to the slower molecules. The slower molecules gain more thermal energy and collide with other molecules in the cooler object. This process continues until heat energy from the warmer object spreads throughout the cooler object. Some substances conduct heat more easily than others. Solids are better conductor than liquids and liquids are better conductor than gases. Metals are very good conductors of heat, while air is very poor conductor of heat. You experience heat transfer by conduction whenever you touch something that is hotter or colder than your skin e.g. when you wash your hands in warm or cold water.


In liquids and gases, convection is usually the most efficient way to transfer heat. Convection occurs when warmer areas of a liquid or gas rise to cooler areas in the liquid or gas. As this happens, cooler liquid or gas takes the place of the warmer areas which have risen higher. This cycle results in a continous circulation pattern and heat is transfered to cooler areas. You see convection when you boil water in a pan. The bubbles of water that rise are the hotter parts of the water rising to the cooler area of water at the top of the pan. You have probably heard the expression "Hot air rises and cool air falls to take its place" - this is a description of convection in our atmosphere. Heat energy is transfered by the circulation of the air.

This thermal infrared image shows hot oil boiling in a pan. The oil is transfering heat out of the pan by convection. Notice the hot (yellow) centers of rising hot oil and the cooler outlines of the sinking oil. Image courtesy of K.-P. Möllmann and M. Vollmer, University of Applied Sciences Brandenburg/Germany.
A thermal infrared image of the center of our galaxy. This heat from numerous stars and interstellar clouds traveled about 24,000 light years (about 150,000,000,000,000,000 miles!) through space by radiation to reach our infrared telescopes.

Both conduction and convection require matter to transfer heat. Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object. For example, we feel heat from the sun even though we are not touching it. Heat can be transmitted though empty space by thermal radiation. Thermal radiation (often called infrared radiation) is a type electromagnetic radiation (or light). Radiation is a form of energy transport consisting of electromagnetic waves traveling at the speed of light. No mass is exchanged and no medium is required.

Objects emit radiation when high energy electrons in a higher atomic level fall down to lower energy levels. The energy lost is emitted as light or electromagnetic radiation. Energy that is absorbed by an atom causes its electrons to "jump" up to higher energy levels. All objects absorb and emit radiation. ( Here is a java applet showing how an atom absorbs and emits radiation) When the absorption of energy balances the emission of energy, the temperature of an object stays constant. If the absorption of energy is greater than the emission of energy, the temperature of an object rises. If the absorption of energy is less than the emission of energy, the temperature of an object falls.

Key Concepts

  • When an object feels cold to the touch, it is because heat is flowing from you to the object.
  • When an object feels hot to the touch, it is because heat is flowing from the object to you.
  • Some objects (like metals) conduct heat better than others (like wood). Thus if you stick a metal rod in the fireplace and hold the other end, the heat is conducted well and you get burned. On the other hand, if you place a wood stick in the fire and hold the other end you’ll be OK.
  • The temperature of a gas is a measure of the amount of average kinetic energy that the atoms in the gas possess.
  • If you heat something, you increase its internal energy, so you increase the movement of molecules that make up this thing, thus it expands. This is called heat expansion; most everything expands as heated and contracts as cooled.
  • Most materials expand as they are heated. This can cause bridges to collapse if they are not designed to have a place to expand in the summer months (like the placing of metal ‘teeth’ at intervals on the Golden Gate Bridge).
  • Water contracts from 0^\circ \mathrm{C} to 4^\circ \mathrm{C} and then expands from 4^\circ \mathrm{C} to 100^\circ \mathrm{C}. Remembering that density is mass divided by volume explains why water at 4^\circ \mathrm{C} is more dense than water below and above 4^\circ \mathrm{C}. This also explains why lakes freeze on the top first and not throughout. As the water on the top of the lake drops below 4^\circ \mathrm{C}, it is now more dense than the water below it, thus it sinks to the bottom, allowing the warmer water to rise up to the top and cool down in the winter weather. Only when the entirety of the lake is at 4^\circ \mathrm{C}, then the lake can start to freeze. It freezes from the top down, because water below 4^\circ \mathrm{C} is less dense than water at 4^\circ \mathrm{C}.

  • There are 3 different temperature scales you should know -the Kelvin scale, the Celsius scale and the Fahrenheit scale.
  • The Kelvin scale (\mathrm{K}) is the one used in most scientific equations and has its zero value set at absolute zero (the theoretical point at which all motion stops).
  • The Celsius scale (^\circ \mathrm{C}) is the standard SI temperature scale. It is equal to the Kelvin scale if you minus 273 from the Celsius reading. Water has a boiling point of 100^\circ \mathrm{C} and a freezing point of 0^\circ \mathrm{C}.
  • The Fahrenheit scale (^\circ \mathrm{F}) is the English system and the one we are familiar with.
  • Newtons’ Law of Cooling: The rate of heat transfer is proportional to the difference in temperature between the two objects. For example, hot liquid that is put in the freezer will cool much faster than a room temperature liquid that is put in the same freezer.
  • Heat capacity is the amount of internal energy that the substance can store. A large heat capacitance means the substance can store a lot of internal energy and thus the temperature changes slowly. Aluminum foil has a small heat capacitance and water has a large one.
  • The amount of heat capacitance (and thus its specific heat value) is related to something called ‘degrees of freedom,’ which basically says how free is the object to move in different ways (and thus how much kinetic energy can it store inside itself without breaking apart). For example, solids have a more fixed structure, so they cannot rotate and jostle as much, so they can’t store as much internal energy so they have lower heat capacitance then liquids.
  • Specific heat is similar to heat capacitance, but is a specific number. The specific heat tells you how much energy one must put in per unit mass in order to raise the temperature 1^\circ \mathrm{C}.
  • Phase changes: it takes energy to changes phases from a solid to a liquid and from a liquid to a gas. The substance releases energy when changing phase from gas to liquid or from liquid to solid. How much energy per unit mass depends on the substance in question. When you get out of the shower you often feel cold. This is because the water on you is evaporating, and heat is flowing from you to the water droplets in order for them to change phase from water to gas. You are losing heat and thus feel cold.

Key Application

  • The human body radiates heat in the range of infrared light. Night goggles work by ‘seeing’ the infrared light emitted by our bodies.

  • The thermostat works with a bi-metallic strip. A bi-metallic strip is a flat rectangular object with two different metal strips glued together back to back. In the example above (courtesy of ‘hyperphysics’) we have brass and steel. Brass has a larger heat expansion than steel. When the temperature gets Hotter, the brass strip will expand more than the steel one and thus the strip will bend downward, triggering the thermostat to turn on the air conditioner. When the Temperature goes below the set value, then the brass one contracts more than the steel one and it bends upward. This triggers the heating to be turned on.
  • A calorie is a unit of energy. The food Calorie, with a capital C, is actually 1,000\;\mathrm{calories} (a kcal). Thus, for example, a snicker bar labeled with 200\;\mathrm{Cal} is actually 200,000\;\mathrm{cal}.
  • Food calories are determined by burning the food and measuring the heat released.
  • The human body is at a temperature that radiates away heat in the form of infrared wavelengths. Night goggles work by ‘seeing’ the infrared light and then converting it into visible light (the green screen you usually see in the movies is because the conversion is usually done with a phosphorous screen).
  • For electronics in space, one must use very large heat sinks to radiate away the heat from components and chips like the processor. Since there is no air in space, a fan will not do anything. The processor chip cannot release heat built up by convection, so it must radiate it away over a large heat sink.
  • More than 50\% of the water rise expected from global warming is due to the thermal expansion of water (more on greenhouse effect below).

The Greenhouse Effect

Greenhouse Effect: The solar energy reaching the surface of the Earth is concentrated in short wavelengths, which can easily penetrate the greenhouse gases, such as carbon dioxide and methane. The Earth, however, is cooler than the sun and it radiates its heat in the form of energy in the far infrared range. These longer wavelengths are partially absorbed by the greenhouse gases and some of the solar heat is returned to Earth. At a certain temperature these processes are in equilibrium and the surface temperature of the Earth is stable. However, if more greenhouse gases are put in the atmosphere the amount of trapped terrestrial radiation increases, leading to an increase in global temperature.