Chapter 16 – Thermal Energy and Heat

1. Chapter 16 – Thermal Energy and Heat Jennie L. Borders Modified by Mrs. Rawls

2. Section 16.1 – Thermal Energy and Matter • In the 1700’s most scientists thought that heat was a fluid called caloric that flowed between objects. • Today’s calorie, a unit of heat energy, comes from this.

3. Count Rumford • Rumford discovered that the heat was a result of the motion of the drill, not a form of matter.

4. Work and Heat • Heat is the transfer of thermal energy from one object to another because of a temperature difference. • Heat flows spontaneously from hot objects to cold objects.

5. Temperature • Temperature is a measure of how hot or cold an object is compared to a reference point. • Temperature is related to the average kinetic energy of the particles in an object due to their random motions through space. • As an object heats up, its particles move faster, on average.

6. Thermal Energy • Thermal energy is the total potential and kinetic energy of the particles of an object. • Thermal energy depends on the mass, temperature, and phase (solid, liquid, or gas) of an object. • Thermal energy, unlike temperature depends on mass.

7. Thermal Contraction • Slower particles collide less often and exert less force, so pressure decreases and the object contracts.

8. Thermal Expansion • Thermal expansion is an increase in the volume of a material due to a temperature increase. • Gases expand more than liquids, and liquids usually expand more than solids.

9. Thermal Expansion • Thermal expansion is used in glass thermometers. • As temperature increases, the alcohol (or mercury) in the tube expands and its height increases.

10. Specific Heat • Specific heat is the amount of heat needed to rise the temperature of one gram of a material by one degree Celsius. • The lower the material’s specific heat, the more its temperature rises when a given amount of energy is absorbed by a given mass.

11. Specific Heat • Formula for Specific heat: Q = m x c x DT Q = heat (J) m = mass (g) c = specific heat (J/goC) DT = change in temperature final – initial (oC)

13. Sample Problem • An iron skillet has a mass of 500.0g. The specific heat of iron is 0.449 J/goC. How much heat must be absorbed to raise the skillet’s temperature by 95.0oC? Q = m x c x DT m = 500.0 g c = 0.449 J/goC DT = 95.0oC Q =(500.0g)(0.449 J/goC)(95.0oC) = 21,327.5J

14. Practice Problems • How much heat is needed to raise the temperature of 100.0g of water by 85.0oC? • How much heat in kJ is absorbed by a 750g iron skillet when its temperature rises from 25oC to 125oC? Q = (100.0g)(4.18J/goC)(85.0oC) = 35,530J DT = 125oC – 25oC = 100oC Q = (750g)(0.449J/goC)(100oC) = 33,675J K h d u d c m 33,675J = 33.7kJ   

15. Calorimeter • A calorimeter is an instrument used to measure changes in thermal energy. • A calorimeter uses the principle that heat flows from a hotter object to a colder object until both reach the same temperature. • According to the law of conservation of energy, the thermal energy released by a test sample is equal to the thermal energy absorbed by its surroundings.

16. Calorimeter

17. Section 16.2 – Heat and Thermodynamics • Conduction is the transfer of thermal energy with no overall transfer of matter. • Conduction occurs between materials that are touching. • Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often.

18. Conduction • In metals, conduction is faster because some electrons are free to move about.

19. Conductors • A thermal conductor is a material that conducts thermal energy well. • Examples: silver, copper, gold, aluminum, iron, steel, brass, bronze, mercury, graphite, dirty water, and concrete.

20. Insulators • A material that conducts thermal energy poorly is called a thermal insulator. • Examples: glass, rubber, oil, asphalt, fiberglass, porcelain, ceramic, quartz, cotton, paper, wood, plastic, air, diamond, and pure water.

21. Convection • Convection is the transfer of thermal energy when particles of a fluid move from one place to another. • A convection current occurs when a fluid circulates in a loop as it alternately heats up and cools down.

22. Convection • Convection currents are important in many natural cycles, such as ocean currents, weather systems, and movements of hot rock in Earth’s interior.

23. Radiation • Radiation is the transfer of energy by waves moving through space. • All objects radiate energy. As an object’s temperature increases, the rate at which it radiates energy increases.

26. Thermodynamics • The study of conversions between thermal energy and other forms of energy is called thermodynamics.

27. 1st Law of Thermodynamics • The first law of thermodynamics states that energy is conserved.

28. 2nd Law of Thermodynamics • The second law of thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system. (Disorder in the universe is always increasing.)

29. 3rd Law of Thermodynamics • The third law of thermodynamics states that absolute zero cannot be reached.