<text><span class="style10">hermodynamics (4 of 6)</span><span class="style7">Alternatively, a body may have </span><span class="style26">potential energy</span><span class="style7">. In contrast to kinetic energy, which is dependent upon velocity, potential energy is dependent upon position. The gravitational potential energy of a body of mass </span><span class="style26">m</span><span class="style7"> at a height </span><span class="style26">h</span><span class="style7"> above the ground is </span><span class="style26">mgh</span><span class="style7">, where </span><span class="style26">g</span><span class="style7"> is the acceleration due to gravity. This gravitational potential energy is equal to the work that the Earth's gravitational field will do on the body as it moves to ground level.Potential energy can be converted into kinetic energy or it can be used to do work. It acts as a store of energy. If a body moves upward against the gravitational force, work is done on it and there is an increase in gravitational potential energy.</span><span class="style10">Temperature</span><span class="style7">Temperature is a measure of the internal energy or `hotness' of a body, not the heat of the body. Thermometers are used to measure temperature. They may be based on the change in volume of a liquid (as in a mercury thermometer), the change in length of a strip of metal (as used in many thermometers), or the change in electrical resistance of a conductor. Other parameters may also be involved in measuring temperature.The </span><span class="style26">thermodynamic temperature scale</span><span class="style7"> - also known as the </span><span class="style26">kelvin scale</span><span class="style7"> or the </span><span class="style26">ideal gas scale</span><span class="style7"> - is based on a unit called the </span><span class="style26">kelvin</span><span class="style7"> (K); the scale is used in both practical and theoretical physics. The scale is named after the Scottish physicist William Thomson, later Lord Kelvin (1824-1907), who did important work in thermodynamics and electricity. An </span><span class="style26">ideal gas</span><span class="style7"> is one that would obey the ideal gas law perfectly. In fact no gas is ideal, but most behave sufficiently closely that the ideal gas law can be used in calculations. At ordinary temperatures and pressures, dry air can be considered as a very good approximation to an ideal gas. On the kelvin scale the freezing point of water is 273.15 K (0 deg C or 32 deg F) and its boiling point is 373.15 K (100 deg C or 212 deg F): one degree kelvin is equal in magnitude to one degree on the Celsius scale. The temperature of 0 (zero) K is known as </span><span class="style26">absolute zero</span><span class="style7">. At this temperature, for an ideal gas, the volume would be infinitely large and the pressure zero.</span><span class="style10">Heat and internal energy</span><span class="style7">The molecular energy (kinetic and potential) within a body is called </span><span class="style26">internal energy</span><span class="style7">. When this energy is transferred from a place of high energy to one of lower energy, it is described as a flow of heat.If two bodies of different temperatures are placed in thermal contact with each other, after a time they are found both to be at the same temperature. Energy is transferred from the warmer to the colder body, until both are at a new </span><span class="style26">equilibrium temperature</span><span class="style7">. Heat is a form of energy, and heat flow is a transfer of energy resulting from differences in temperature.The unit of internal energy and heat is the </span><span class="style26">joule</span><span class="style7">, as defined above. Units used previously include the </span><span class="style26">calorie</span><span class="style7">, which is equivalent to 4.2 joules and is defined as the heat required to raise the temperature of 1 gram of water from 14.5 deg C (58.1 deg F) to 15.5 deg C (59.9 deg F). The unit used by nutritionists is actually the </span><span class="style26">kilocalorie</span><span class="style7">, or </span><span class="style26">Calorie</span><span class="style7"> (= 1000 calories).</span></text>
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<text><span class="style10">. Potential energy.</span><span class="style7"> The potential energy of a body is the product of its mass, its height above the ground, and the acceleration due to gravity: </span><span class="style26">Ep = mgh</span><span class="style7">.</span></text>
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<text>ΓÇó MOTION AND FORCEΓÇó FORCES AFFECTING SOLIDS AND FLUIDSΓÇó THE HISTORY OF SCIENCEΓÇó ENERGYΓÇó ENGINES</text>
