Today the most generally accepted scientific explanation of the origin of the universe is the Big Bang theory. According to this theory, our universe began between 12 and 14 billion years ago with a humongous explosion. Prior to that time all of the matter and energy in the universe were compacted into a single, inconceivably dense point. This is a concept that is difficult to grasp! The universe continues to expand and to cool. During the previous 12 to 14 billion years galaxies, stars and planets have formed from the gas clouds.
The nebular hypothesis argues that diffuse, slowly rotating clouds of hydrogen and helium contract under the force of gravity. Contraction accelerates the rotation of the particles. Matter drifts to the center of the cloud and a proto-Sun may form. Compressed under its own wieght, the proto-Sun becomes hotter and hotter until a temperature is reached where hydrogens fuse to form heliums and energy is released.
Einstein showed that Energy is equal to the square of the speed of light times the mass. A small mass can produce a large amound ot energy.
The following Flash animation shows the collapse of a rotating dust cloud to form a solar system with a central star and orbiting planets. The angular velocity vector is shown in yellow. Escape of light elements to the outer regions is shown immediately after the collapse phase. Show the Animation.
In our solar system, the composition of the planets is a function of distance to the Sun. The inner planets (Mercury, Venus, Earth and Mars) are small and made up rocks and metals. Forming close to the Sun, the low density gasses boiled away and were not retained in quantity.
The outer planets (Jupiter, Saturn, Uranus and Neptune (Pluto is thought to have been captured as a solid object) are sometimes referred to as the gas bags and are made up of ices and gases.
At some temperature and pressure the solid material may begin to melt. Rarely does the entire volume of rock melt. A mixture of melt and solids is produced by partial melting. The melt tends to rise (it is usually less dense than the material around it) and is called magma. If the magma is ejected onto the surface it is called lava.
A system will be defined as that portion of the universe in which there is an interest. Everything else is part of the surroundings. The system is separated from the surroundings by a boundary.
Physicists have found that they need to specify length, mass and time in order to describe and predict the motions of rigid bodies and fluids. These measures can be described as three fundamental variables.
Every particle of matter in a body (including you and I) is attracted by the Earth and the direction of the force on each particle is toward the center of the Earth. The weight of a body is the gravitational force exerted on it by the Earth. The moon is smaller than the Earth and its gravitational field is less than that of the Earth. You and I would each weigh less on the moon than on the Earth. Our mass, however, is the same on both the moon and the Earth.
Consider two objects of the same size. One is "heavier" than the other. We can compare the two objects by giving their density:
If mass is measured in grams (or pounds) and the volume is measured in cubic centimeters (or cubic feet) then density has the units of g/cc (pounds per cubic foot). If two objects have the same volume (imagine that both are cubes with sides equal to one centimeter), then the one with the greatest mass will have the greater density.
One method for determining the density of an object is to use the Principle of Archimedes. In this method the volume is determined by measuring the apparent loss of weight when a weighed object is immersed in a suitable liquid (we will use water with a density of 1.0 grams/cubic centimeter - g/cc). The object displaces an amount of liquid equal to its own volume and its weight is apparently diminished by the weight of the liquid displaced. If W1 is the weight of the object in air, W2 the weight of the object in water of density 1.0 g/cc, the density is:
Watch the Archimedes Animation and record the weight of the object in air and its weight in water.
Using  compute the density of the object.
Do you recall the story of Archimedes? He was asked to determine if a crown was made of pure gold without destroying the crown. Do you see how to apply the information in the animation to his problem?
Often we will need to be able to compare the densities of two objects of different size and shape. The specific gravity of an object is its density divided by the density of an equal volume of water. What are units of specific gravity? Note that the units cancel each other [(g/cc)/(g/cc)] so specific gravity is a dimensionless number.
The density of the continental crust is about 2.7 g/cc. Its specific gravity is 2.7 (since the density of water is about 1.000 g/cc). This means that a unit of continental crust is 2.7 times heavier than an equal unit of water.
One cubic foot of water weighs about 60 pounds. Therefore, one cubic foot of granite (typical continental crust) weighs about 160 pounds [2.7 times 60 pounds]. The specific gravity of gold is about 18. How much does a cubic foot of gold weigh? When I was a kid I saw a movie in which a stagecoach driver picked up a large sack of "gold" and threw it down on the ground. Bah Humbug.
The fourth fundamental variable is the temperature. The temperatures associated with two objects are equal only when the two objects are in thermal equilibrium. Crudely, the higher the temperature of an object the greater the vibration of its constituents atoms. As the temperature is lowered, the degree of vibration decreases. When two bodies at different temperature are brought into contact, heat flows from the warmer to the cooler until the two bodies have identical temperatures - a state of thermal equilibrium is reached.
Note that temperature is not a measure of the amount or quantity of heat. A burning match and a forest fire may have the same temperature but differ markedly in the quantity of heat each possesses. Suppose that two containers of water (a cup and a quart) are placed on identical burners (same temperature) and heated for 10 minutes. The temperature of the cup of water will be higher than the temperature of the quart of water.
Heat can be measured in several units. One calorie is the quantity of heat needed to cause a rise in temperature of 1 o C in one gram of water. Water has a high heat capacity. That is, it takes a lot of heat to raise the temperature of water by a given amount. Similarly, it takes a lot of heat loss to lower the temperature of water. Water, can "store" heat energy more efficiently than rock material.
Imagine a system consisting of a container of water which is hotter at the bottom than it is at the top -- perhaps the system is sitting on a burner on the top of a stove. The system will attempt to even out the variation in temperature so that the system attains thermal equilibrium.
Watch a Convecting Movie and read the first few paragraphs about heat flow.
In general, the density of a material (the ratio of its mass to its volume or M/V) decreases as the temperature increases. Increasing temperature has the effect of increasing the thermal vibration of the atom (or ions or molecules) that comprise the material and it expands (its volume increases). Warm material rises above cooler material of the same composition. Convection may occur when a column of the homogeneous (same composition throughout) material is heated at the bottom. If the material can flow, the warm, lower density material begins to rise to the top of the column, displacing the colder, higher density material. If the column is confined, the colder material sinks, is heated, and begins to rise. Given time, the convection cell works to even out the temperature distribution in the column. Watch a column of water boil. Note that there is a physical movement of material when convection is operative. At elevated temperature and pressure some solids are capable of sustaining this type of flow.
In contrast, heat energy can also be transmitted by conduction in which the temperature contrast is removed by increasing vibration of the atoms (or ions or molecules). Hold a metal knife on a heat source. As most metals are good conductors, it doesn't take long for the knife to become too hot to hold. In general, most rocks are poor conductors.
The Earth is warmed by radiation from the sun. Heat energy is carried by photons emitted from the star. The Earth receives about 10,000 times as much energy from the sun as is generated within the Earth.
We will encounter several situations in which processes are driven by an attempt to redistribute excess heat.