The Nature Of Heat (1953)

[Music] so [Music] heat enough to melt steel cold enough to freeze a gas into a solid block explosions generating temperatures running to millions of degrees hotter than the sun itself these are some aspects of our modern knowledge of heat and its transfer [Music] the modern study of heat begins with count rumford in 1798 his studies on the boring of canon led to the knowledge that the mechanical energy of a turning drill is transformed through friction into heat and that heat is thus another form of energy here if we touch a bit of phosphorus to the metal we see that enough heat has been produced to ignite the phosphorus to understand this we need to think of all matter as made up of moving particles called molecules the more heat these molecules acquire the faster they move thus heat is the energy of the motion of molecules heat is transferred by three methods conduction convection and radiation heat in the metal plate is transferred to the phosphorus to set it ablaze at one end of a wire the heated faster moving molecules at this end of the wire pass their motion or their heat to nearby molecules thus heat is transferred along the wire in another experiment we can see how heat from the flame of the burner is transferred along a wire passing from molecule to molecule to melt a ball of wax this process is called conduction we can compare how readily various materials conduct heat by fitting them with thermometers our test materials are copper glass wood and no material at all or a vacuum all the thermometers read 29 degrees centigrade at the start of the experiment the bottom of each test material will receive the same amount of concentrated heat 30 minutes later when we compare the thermometer readings what can we learn copper and metals in general conducts heat very readily the top of the copper block is quite hot glass and earth materials conduct heat only moderately well wood like other vegetable and animal fibers conducts very little heat and the vacuum in the light bulb conducts even less heat a perfect vacuum would conduct no heat at all there are large and important variations in the capacity of various materials to conduct heat when we make the same test with various building materials we find that glass is a fairly efficient conductor of heat brick conducts nearly as well plaster is less efficient as a heat conductor and fiberboard is the least efficient understanding the different rates at which various materials conduct heat can you explain some of their practical uses let's remember that heat is the energy of the motion of molecules solid materials conduct heat by passing increased motion along from molecule to molecule do liquids and gases also conduct heat here is a beaker of water with ice floating on top a second beaker contains a barrier of steel wool itself an excellent conductor of heat and also some ice we apply heat to the bottoms of both beakers by speeding up the action in the camera we can see that heat is readily conveyed to the top of one beaker but much less rapidly in the second how can we explain this difference let's refer again to the molecular theory of matter liquids too are made up of molecules in motion but here they circulate freely within the container now what happens if we heat some of these molecules the heated molecules move faster they tend to spread apart the heated water expands making this part of the water less dense than the rest it rises and heavier cooler water comes in contact with the source of heat to be warmed and expanded so a current is set up within the liquid this process of heat transfer is called convection since heated material is convected or conveyed from one place to another adding visible particles to the water we can see the convection currents as they distribute heat through the liquid but here the convection currents are turned aside by the barrier of steel wool and less heat reaches the top of the beaker in liquids and gases it is convection currents rather than conduction which transfers the heat understanding the convection of heat can you explain why a chimney on an outdoor fireplace helps create a draft for the fire can you explain the workings of this type of hot water heater and its storage tank do you understand why a frozen food cabinet can be left open at the top liquids and gases transfer heat by convection rapidly moving molecules cause the heated portions of a fluid to expand and rise setting up convection currents how does heat reach us from the sun traveling through empty space it travels as radiation which we can easily focus with the lens and so build up intense heat but can we distinguish between visible light and radiated heat using the same lamp and lens but placing over the lens of filter which absorbs almost all the visible light we find that there is no apparent sign of radiation reaching the paper yet enough invisible heat radiation reaches the paper to ignite it using the same lens we can focus radiation from an electric iron on the bulb of a thermometer and see that all warm bodies radiate heat heat travels by radiation radiant heat like the radiations we see as light can be focused it can be reflected certain materials like glass allow radiant heat to pass through others like plants and the earth absorb it and are warmed by it in a radiant heated room how is it possible to feel comfortably warm while the temperature of the air remains invigoratingly cool we have seen that heat is the energy of the motion of molecules it is transferred from place to place by three methods conduction convection and radiation from the beginnings made by count rumford we have developed our knowledge of the nature of heat and its transfer to help solve our practical problems of the control and use of heat how can the tremendous quantities of heat generated by changes within the nuclei of atoms be controlled and put to practical use [Music] you