This Lesson is about heat. Heat is a form of energy and is a result of the vibrating movement of molecules or atoms within a substance.

Heat energy is always transported (transferred) from a hotter object to a colder object. This can happen in one of three ways -- by conduction, by convection or by radiation. We'll tell you more about heat transfer in a minute.

But first we need to tell you a little bit about energy.

Energy
In physics, energy is defined as the ability to do work . Energy is what makes things happen. Or, more correctly, it is the change from one kind of energy to another that makes things happen.

Some scientists believe that there are only two kinds of energy -- potential energy (PE) and kinetic energy (KE). Potential energy is energy waiting to happen. This is energy that is stored in our muscles, for example, or in batteries or fuel tanks. Kinetic energy is matter in motion. It's the energy contained in a moving mass of matter or in a particle, which is a tiny bit of matter. When something happens, it's because potential energy has been changed into kinetic energy.

When you have been sitting down and get up to dance, for example, your body converts potential energy into kinetic energy. You do the same thing when you use the muscles in your face to smile -- or when you turn on a battery-powered radio or start a car engine.

Different Kinds of Energy

As we said, some scientists believe that there are only two kinds of energy -- potential energy and kinetic energy. But others talk about a number of different kinds of energy, including:

• Radiation (sometimes called light energy, which includes infra-red and ultra-violet radiation)
• Chemical Energy (energy stored in food, fuel and batteries)
• Electrical Energy
• Atomic Energy (energy stored in the nuclei of atoms)
• Electromagnetic Energy
• Mechanical Energy
• Heat (Thermal) Energy
• Internal Energy (Heat Content)
• Solar Energy
• Sound Energy ,etc.

You get your energy from food. Your toaster and your washing machine get their energy from electricity.

Energy is measured in such units as the joule (J), erg, kilowatt-hour (kW-hr), kilocalorie (kcal), foot-pound (ft-lb.), electron-volt (ev), and British Thermal Unit (BTU).

Thermodynamics

Thermodynamics is the study of the inter-relation between heat, work and the internal energy of a system. The word "thermo" means either "heat" or "energy." The word "dynamics " means "movement" or "motion" or "change."

The First Law of Thermodynamics states that the change in internal energy of a system is equal to the heat added to the system minus the work done by that system.
This law is also called The Law of Conservation of Energy. This law suggests that energy can be transferred from one system to another in many forms -- although energy can neither be created nor destroyed. Thus, the total amount of energy in the Universe always remains the same. Einstein's famous equation describes the relationship between energy and matter:

E = MC²

In this equation, energy (E) is equal to matter (M) times the square of a constant (C) like the speed of light.ß Einstein suggested that energy and matter are interchangeable and that they exist in a fixed quantity in the Universe.

There are also two other Laws of Thermodynamics.

The Second Law of Thermodynamics states that heat can never pass spontaneously from a colder to a hotter body. As a result, natural energy- transfer processes always move in one direction (from hot to cold). This means that all natural processes are irreversible. This law also predicts that the disorder or randomness of any given system will increase with time. This means that both the energy and the matter in the Universe are becoming less useful as time goes on.

The Third Law of Thermodynamics states that if all the thermal motion of molecules (kinetic energy) could be removed, a state called absolute zero would occur. The Universe will attain absolute zero when all energy and matter is randomly distributed across space. Currently, the temperature of the Universe is 2.735 Kelvin. The Kelvin Scale is the way scientists measure temperature -- and this means that the Universe is 2.735 points above absolute zero.

Where Does Energy Come From?
 

Energy starts in the Sun. It is used for a while on Earth and then gets sent back out into space. Here's how energy moves from the Sun to you.

1. In the Sun, matter is changed into electromagnetic energy and sent out into space.
    
2. Some of this solar energy lands on Earth. Through a process called photosynthesis, plants change some  of this solar energy into chemical energy, which they
   store as carbohydrates in their cells.
    
3. You eat the plant or you eat an animal that ate the plant. The cells in your body change the chemical energy into mechanical energy. Your muscles contract, your legs push and you get up and move around in a more-or-less coordinated manner. If you're at a dance or in a club, this movement is called dancing (at which some people are much better than others).

Heat (Energy) Transfer
 

As we said, heat is always transferred from a hotter object to a colder object. There are three ways that this can happen.

Conduction

Conduction is the flow of heat energy through a material without any movement of the material itself. This is the most important type of heat transfer in solids. It occurs when two objects are in direct contact with one another. In conduction, heat energy is passed from atom to atom. The rate at which this happens depends on the temperature difference between the objects involved.

Certain substances, like metals, are said to be good "conductors" of heat. This means that heat passes through them easily.

The insulation materials we use in our homes are not good conductors -- which is why they keep the heat in. Substances such as Silicon and Germanium -- whose conductivity is improved by heat, light or voltage -- are known as semiconductors. These semiconductors are what made personal computers and other electronic advances possible. They were developed as commercial properties in the Industrial park near Stanford University just south of San Francisco, an area that has come to be known as Silicon Valley.

Some examples of conduction include holding an ice cube in your hand, drinking a cup of hot coffee, taking a hot shower or walking on a bed of hot coals (if you should ever be foolhardy enough to do it).

Convection
 

Convection is the transfer of heat energy by the mass movement of a fluid such as water -- or by the movement of air. That's why convection only happens in liquids and gasses.

Convection occurs above a hot surface because as hot air expands it becomes less dense and rises. This is the principle behind many home heating systems. Hot water is also less dense than cold water and rises, causing convection currents which also transport energy. Convection also leads to circulation in a liquid, such as in the heating of a pot of water on the stove.

Convection is thought to play a major role in transporting energy from the center of the Sun to its surface. It also is thought to propel the movement of hot magma beneath the surface of the Earth. A good example of convection is warming your hands over a fire.

Radiation

In radiation, radiant energy (in the form of either particles or electromagnetic waves) is given off by a hot object and directly absorbed by another, cooler object. This causes the cooler object to get warmer. Examples of radiation include heat, light, alpha particles and beta particles.

Electromagnetic waves are oscillating electric and magnetic fields that travel through space at nearly 186,000 miles per second. There is a limitless range of possible wavelengths (frequencies) in the electromagnetic spectrum. This spectrum includes radio waves, infrared radiation, visible light (light waves), ultraviolet radiation, X-rays and gamma rays.

Radio waves have the lowest frequency. TV's, microwave ovens, radar, heat-seeking missiles, ultraviolet light, X-rays and gamma rays have progressively
higher frequencies.

The Sun transmits energy to Earth by electromagnetic radiation.

Waves

In discussing radiation, we mentioned electromagnetic waves as an instrument of heat (energy) transfer. There are also other kinds of waves that are important in heat transfer. Scientists describe a "wave" as a disturbance that travels through a medium from one location (its source) to another -- without any movement of matter.

This medium can be any one of a number of different substances or materials. In the case of a water wave in the ocean, the medium through which the wave travels is the ocean water itself -- as you can see at the beach. In an earthquake, seismic waves travel through the Earth.

If you're at church and the choir is singing, sound waves travel from the choir to your ears through the medium of air. If you put a ringing alarm clock inside a jar with no air in it, you won't hear anything. That's because sound waves need air (or metal or water) to transport them.

One way to categorize waves is by the direction in which they move.

Transverse Waves

A transverse wave is a wave in which particles of the medium move in a direction perpendicular to the direction in which the wave is moving. Electromagnetic waves are transverse waves.

Longitudinal Waves

A longitudinal wave is a wave in which the particles of the medium move in a direction parallel to the direction in which the wave is moving. A sound wave is a longitudinal wave.

Waves traveling through a solid medium can be either transverse or longitudinal. But waves traveling through the bulk of a fluid (such as liquid or  gas) are always longitudinal.

Another way to categorize waves is on the basis of their ability to transmit energy through empty space (through a vacuum).

Electromagnetic and Mechanical Waves
 

Electromagnetic waves (like radio waves) can be transmitted through a vacuum, as when the Sun transmits energy through the emptiness of space. But a mechanical wave (like a sound wave, water wave or telephone cord wave) cannot.

A water wave requires water, a sound wave requires air, water or steel and a telephone chord wave requires a telephone chord.

We'll tell you more about heat, energy and fuel in our next Instruction.

Reading List From the California Dept. of Education

To read a description of each of these books from the California Dept. of Education, click on this address and follow the instructions:
http://www.cde.ca.gov/ci/sc/ll/ap/searchlist.asp

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