TEMPERATURE AND HEAT

1. Lecture N13 TEMPERATURE AND HEAT

2. Content • Temperature. • Heat. • Specific heat. • Calorimetry. • Change of phase

3. Temperature Temperature is a measure of the hotness or coldness of a body Scottish physicist, William Thomson (1824-1907),(Baron Kelvin or Lord Kelvin): I often say that when you can measure what you speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meager and satisfactory kind.

4. The thermometer Anders Celsius (1742) Gabriel Fahrenheit (1714) Lord Kelvin (1848)

5. The thermometer

6. Example. Celsius to Fahrenheit. A temperature of is equivalent to what Fahrenheit temperature?

7. Microscopic definition of temperature There are balanced forces from right and left and in and out. The molecule is therefore in equilibrium. When heat is applied to a solid body, the added energy causes a molecule to vibrate around its equilibrium position. The higher the temperature of the solid, the larger is the vibrational motion of its molecules. The lower the temperature the smaller is the vibrational motion of its molecules. The temperature of a body is really a measure of the mean or average kinetic energy of the vibrating molecules of the body. Temperature is not a measure of the total kinetic energy of the particles within a substance. For example, there is twice as much molecular kinetic energy in 2 liters of boiling water as in 1 liter of boiling water – but the temperatures of both amounts of water are the same because the average kinetic energy per molecule in each is the same

8. Heat The sum of the potential energy and kinetic energy of all these molecules is called the internal energy of the body. Heat is thus the amount of energy flowing from a body at a higher temperature to a body at a lower temperature. Whenever two bodies at different temperatures are brought into contact, thermal energy always flows from the hotter body to the cooler body until they are both at the same temperature. When it occurs we say the two bodies are in thermal equilibrium. The kilocalorie is defined as the quantity of heat required to raise the temperature of 1 kg of water 10C. The unit of heat in the British engineering system is the British thermal unit (Btu). One Btu is the heat required to raise the temperature of 1 lb of water 10F 1Btu=0.252kcal 1kcal=1000calories=4186J 1Btu=1055J (Benjamin Thompson, James Joule )

9. Specific heat Tyndall`s demonstration Different bodies absorb a different quantity of thermal energy even when subjected to the same temperature change.

10. Specific heat The specific heat c of a body is defined as the amount of thermal energy Q required to raise the temperature of a unit mass of the material 10C. In terms of the traditional unit of kilocalories, the specific heat c of a body is defined as the number of kilocalories Q required to raise the temperature of 1kg of the material 10C.

11. Specific heat

12. Large bodies of water, such as lakes or oceans, tend to moderate fluctuations of the air temperature nearby because the bodies of water can absorb or release large quantities of thermal energy while undergoing only very small changes in temperature.

13. Calorimetry Calorimetry is defined as the measurement of heat. These measurements are performed in a device called a calorimeter. The basic principle underlying the calorimeter is the conservation of energy. The thermal energy lost by those bodies that lose thermal energy is equal to the thermal energy gained by those bodies that gain thermal energy

14. Calorimetry Thermal energy lost=thermal energy gained ms is the mass of the sample mw is the mass of the water mc is the mass of the calorimeter cup Cs is the specific heat of the unknown sample Cw is the specific heat of the water Cc is the specific heat of the calorimeter cup

15. Change of phase • Fusion (liquid to solid) • Melting (solid to liquid) • Vaporization (liquid to gas) • Condensation (gas to liquid) • Sublimation (solid to gas) • The change of a solid from one crystalline form to another

16. Change of phase

17. Change of phase The latent heat of fusion is the amount of heat necessary to convert 1kg of the solid to 1 kg of the liquid. The total heat of fusion The heat that is necessary to convert 1 kg of the liquid to 1 kg of the gas is called the latent heat of vaporization The total heat of vaporization For water

18. Example. Converting ice to steam. Let us compute the thermal energy that is necessary to convert 5.00 kg of ice at – to superheated team at is the energy needed to heat the ice up to is the energy needed to melt the ice is the energy needed to heat the water to is the energy needed to boil the water is the energy needed to heat the steam to

19. Example. Mixing ice and water. If 10.0 g of ice, at , are mixed with 50.0 g of water at , what is the final temperature of the mixture? Thermal energy gained=thermal energy lost

20. Example. Mixing ice and water. If 10.0 g of ice, at , are mixed with 50.0 g of water at , what is the final temperature of the mixture? Only 62.5% of the ice melted and the final temperature is