Heat and Temperature.
Heat and modern technology are inseparable. These glowing steel slabs, at over 1,100OC (about 2,000OF), are cut by an automatic flame torch. The slab caster converts 300 tons of molten steel into slabs in about 45 minutes. The slabs are converted to sheet steel for use in the automotive, appliance and building industries.
(A) In a solid, molecules vibrate around a fixed equilibrium position and are held in place by strong molecular forces. (B) In a liquid, molecules can rotate and roll over each other because the molecular forces are not as strong. (C) In a gas, the molecules move rapidly in random free paths.
The basic forms of kinetic energy of molecules. (A) Translational motion is the motion of a molecule as a whole moving from place to place. (B) Rotational motion is the motion of a turning molecule. (C) Vibrational motion is the back-and-forth movement of a vibrating molecule.
The number of oxygen molecules with certain velocities that you might find a sample of air at room at temperature. Notice that a few are barely moving and some have velocities over 1,000 m/s at a given time, but the average velocity is somewhere around 500 m/s.
(A) A bimetallic strip is two different metals, such as iron and brass, bonded together as a single unit, shown here at room temperatures. (B) Since one metal expands more than the other, the strip will bend when it is heated. In this example, the brass expands more than the iron, so the bimetallic strip bends away from the brass.
This thermostat has a coiled bimetallic strip that expands and contracts with changes in the room temperature. The attached vial of mercury is tilted one way or the other, and the mercury completes or breaks an electric circuit that turns the heating or cooling system on or off.
One theory about how friction results in increased temperatures: Molecules on one moving surface will catch on another surface, stretching the molecular forces that are holding it. They are pulled back to their home position with a snap, resulting in a gain of vibrational kinetic energy.
External energy is the kinetic and potential energy that you can see. Internal energy is the total kinetic and potential energy of molecules. When you push a table across a floor, you do work against friction. Some of the external mechanical energy goes into internal kinetic and potential energy, and the bottom surface of the legs becomes warmer.
Heat and temperature are different concepts, as shown by a liter of water (1,000 mL) and a 250 mL cup of water, both at the same temperature. You know the liter of water contains more heat since it will require more ice cube to cool it, say, 25OC than will be required for the cup of water. In fact, you will have to remove 48,750 additional calories to cool the liter of water
The Calorie value of food is determined by measuring the heat released from burning the food. If there is 10.0 kg of water and the temperature increased from 10OC to 20OC the food contained 100 Calories (100,000 calories). The food illustrated here would release much more energy than this.
Joule worked with the English system of measurement used during his time. When a 100 lb object falls 7.78 ft, it can do 778 fl?lb of work. If the work is done against friction, as by stirring 1 lb of water, the heat produced by the wok raises the temperature 1OF.
Thermometers place in holes drilled in a metal rod will show that heat is conducted from a region of higher temperature to a region of lower temperature. The increased molecular activity is passed from molecule to molecule in the process of conduction.
(A) Two identical volumes of air are balanced, since they have the same number of molecules and the same mass. (B) Increased temperature causes one volume to expand from the increased kinetic energy of the gas molecules. (C) The same volume of the expanded air now contains fewer gas molecules and is less dense, and it is buoyed up by the cooler, more dense air.
Convection currents move warm air throughout a room as the air over the heater becomes warmed, expands, and is moved upwards by cooler air.
This graph shows three warming sequences and two phase changes with a constant input of heat. The ice warms to the melting point, then absorbs heat during the phase change as the temperature remains constant. When all the ice has melted, the now liquid water warms to the boiling point, where the temperature again remains constant as heat is absorbed during the second phase change from liquid to gas. After all the liquid has changed to gas, continued warming increases the temperature of the water vapor.
(A)Work is done against gravity to lift an object, giving the object more gravitational potential energy. (B) Work is done against intermolecular forces in separating a molecule from a solid, giving the molecule more potential energy.
Compare this graph to the one in Figure 5.20. This graph shows the relationships between the quantity of heat absorbed during warming and phase changes as water is warmed from ice at -20OC to water vapor at some temperature above 100OC. Note that the specific heat for ice, liquid water, and water vapor (steam) have different values.
Temperature is associated with the average energy of the molecules of a substance. These numbered circles represent arbitrary levels of molecular kinetic energy that, in turn, represent temperature. The two molecules with the higher kinetic values [25 in (A)] escape, which lowers the average value from 11.5 to 8.1 (B). Thus evaporation of water molecules with more kinetic energy contributes to the cooling effect of evaporation in addition to the absorption of latent heat.
The inside of this closed bottle is isolated from the environment so the space above the liquid becomes saturated. While it is saturated, the evaporation rate equals the condensation rate. When the bottle is cooled, condensation exceeds evaporation and droplets of liquid form on the inside surfaces.
A very simple heat engine. The air in (B) has been heated, increasing the molecular motion and thus the pressure. Some of the heat is transferred to the increased gravitational potential energy of the weight as it is converted to mechanical energy.
The heat supplied (QH) to a heat engine goes into the mechanical work (W) and the remainder is expelled in the exhaust (QL). The work accomplished us therefore the difference in the heat input and output (QH) - (QL), so the work accomplished represents the heat used, W = J(QH - QL)
A heat pump uses work (W) to move heat from a low temperature region (QL) to a high temperature region (QH). The heat moved (QL) requires work (W), so J QL = W.