Chapter 21: Temperature, Heat and Expansion

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1 Chapter 21: Temperature, Heat and Expansion All matter solid, liquid and gas is made of atoms or molecules, which are continually jiggling. As this jiggling is a movement, all these particles must have a kinetic energy. This kinetic energy of all the particles of an object is our concept of temperature. When the particles of a substance have a high kinetic energy, they are all jiggling and bouncing around very fast. To us, the substance would feel hot.(?) Temperature On the Celsius scale, the freezing and boiling points of water were chosen to be 0 and 100 respectively. This is an arbitrary system used to give us a guide to the temperature of something. Consider a substance whose particles have no kinetic energy. Due to our definition of temperature, this would mean the substance couldn t be any colder. This temperature is 273 C and called absolute zero. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 21 Temperature, Heat and Expansion Page 1 of 7

2 The Kelvin temperature scale starts at absolute zero, while each degree is the same size as a Celsius degree. Thus 273 K is equal to 0 C. As it starts at absolute zero, the Kelvin scale can have no negative temperatures. The Kelvin is the S.I. unit of temperature. Heat Heat is the energy that transfers between two objects that have different temperatures. Once transferred, the energy ceases to be heat and is now know as internal energy. Matter does not contain heat, matter contains internal energy. When heat is flowing from one object to another it is in contact with, the two objects are said to be in thermal contact. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 21 Temperature, Heat and Expansion Page 2 of 7

3 The direction of heat flow is independent of the total molecular kinetic energy of the two objects, rather it is dependent on the temperature difference (or the average molecular kinetic energy per molecule). i.e. a large warm object may have a high total molecular kinetic energy because of the large number of molecules which it contains, but a smaller object which is hotter (but with a lower total molecular kinetic energy) will lose heat to it in thermal contact. Thermal Equilibrium When objects of different temperatures come into contact, heat will flow between them. After time there will be no temperature difference between to two objects so the heat will stop flowing. When this occurs the two objects are said to be in thermal equilibrium. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 21 Temperature, Heat and Expansion Page 3 of 7

4 Internal Energy The kinetic energy of jostling molecules is not the only energy contain by an object. There is also rotational kinetic energy of molecules, kinetic energy due to the movements of atoms within molecules and potential energy due to the forces between molecules. The total of all these energies is called the internal energy. A substance does not contain heat, it contains internal energy. Measurement of Heat We know that heat is energy transferred when there is a temperature difference. Therefore, we can calculate the amount of heat by measure the temperature change. The same amount of heat will have a greater effect on the temperature of a cup of water than it will have on a pot of soup. Therefore it is necessary Conceptual Physics 3 rd Edition Paul Hewitt Chapter 21 Temperature, Heat and Expansion Page 4 of 7

5 to take into account the mass, and kind, of substance the heat is acting on. As heat is a type of energy, it is measured in joules J of heat will raise the temperature of 1 gram of water by 1 K. Specific Heat Specific materials require a specific amount of heat to raise the temperature of a given mass of it by a specific number of degrees. Specific heat is the quantity of heat required to raise the temperature of a unit mass of a substance by one degree. The heat absorbed by materials is stored in different ways. It can be transformed into other types of internal energies that do not effect the temperature of the object. For example water molecules will transform much heat into rotations, internal vibrations and bond stretching. Whereas, Conceptual Physics 3 rd Edition Paul Hewitt Chapter 21 Temperature, Heat and Expansion Page 5 of 7

6 iron will transform almost all the heat into translational movement, which increases its temperature. We say that water has a higher specific heat capacity. The High Specific Heat Capacity of Water As water has a high specific heat it will absorb a large amount of energy without increasing in temperature much. This makes it useful as a cooling agent. Thermal Expansion When the temperature of a substance increases its molecules jostle faster and move farther apart. This means the substance needs to take up more room and results in an expansion of the material. In general, gases will expand more than liquids, and liquids more the solids. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 21 Temperature, Heat and Expansion Page 6 of 7

7 Expansion of Water Water has the unusual property in that when it is heated slightly from its freezing point, it will contract. Then at 4 C it will expand continually with an increasing temperature. This is due to the shape of its molecules and the forces acting between them. We know that ice is less dense than water (ice floats on water), so therefore the same amount of water would have a greater volume as ice than as a liquid. So when the water is being heated the ice is melting, causing the volume to decrease, then there is enough liquid for the expansion effects of heating to be seen. Conceptual Physics 3 rd Edition Paul Hewitt Chapter 21 Temperature, Heat and Expansion Page 7 of 7