2.1: Energy and Heat

Energy is the ability to do work on matter. The work can be manifested in a variety of ways. Matter can be pushed, pulled, or lifted over a distance when energy is applied. In other words, work done on matter implies a change of position or movement. Potential energy is the energy of position. A block of rock attached to a high cliff face has substantial potential energy due to its position above the ground. When it breaks away from the cliff and falls to the surface potential energy is converted into kinetic energy of motion. When the rock hits the ground, kinetic energy is converted into work when it dislodges surface material or breaks the rock.

Figure \(\PageIndex{1}\): Forms of Energy

Heat or, thermal energy is the total energy associated with random atomic and molecular motions of a substance. Heat is transferred in four ways. Radiation (1) is the transfer of energy via electromagnetic waves. Radiation does not need an intervening medium to pass heat energy from the emitter to the absorber: it can pass through vacuum. When radiation from the Sun is absorbed by the Earth it does work by setting molecules in motion and raising their kinetic energy level, and therefore, the heat of the substance that contains the molecules. In a solid, the molecules may vibrate more rapidly and collide with one another and transfer heat from warmer to colder portions of the mass by conduction (2). Though conduction is typically thought of occurring within a solid, it can occur between a solid and a fluid. When air, a fluid, comes in contact with the ground, a solid, heat can be transferred through molecular collisions. In fluids like air and water, heat is transferred by the circulation of molecules via the process of convection (3). Convection implies a vertical transfer of heat, like that which occurs in a heated pot of water. As water warms it circulates to the surface. The same is true for air. When air is heated by the earth's surface it too circulates upward. While convection is applied to vertical transfer of heat, advection (4) is a term that is applied to the horizontal transfer of heat by the wind.

Don't confuse temperature and heat. They are not the same thing. Temperature is a measure of the average kinetic energy level of a substance, in other words, the degree of hotness or coldness. Heat is the total energy associated with the motion of molecules while temperature is the average energy level. A boiling pan of water has a higher temperature than a tepid bathtub of water, but the tub contains more heat because there is more mass.

Energy is commonly expressed in terms of joules. One joule is the equivalent of one watt of power radiated or dissipated for one second. Another unit of energy, used in the context of heat energy, is the calorie. A calorie is the amount of heat required to raise the temperature of one gram of water by 1^oC. Since it requires about 4.186 joules of energy to achieve that temperature increase of one gram of water, one calorie equals 4.186 joules.

Specific heat is the heat required to raise the temperature of one unit substance (e.g., gram) through a particular temperature interval (1^oC, for example). The specific heat of water is 1 calorie/gram °C = 4.186 joule/gram °C which is higher than any other common substance on Earth. This is one reason why large bodies of water play such an important role in the heat budget of the Earth system. Air, in contrast, has a specific heat of only 0.718 joule/gram °C (at constant volume). Radiation intensity is often measured in watts per meter² .

• 2.1.1: The Nature of Electromagnetic Radiation

  Unlike convection or conduction, heat transfer by electromagnetic radiation requires no intervening medium to transmit it. Electromagnetic radiation travels through space in the form of waves. It's hard to imagine radiation moving as waves through empty space without a medium to transfer the wave form. The waves created when you drop a rock into a pool require molecules of water to propagate them, but not so for radiation.

• 2.1.2: Selective Absorption by the Atmosphere