Chapter 16 Thermal Energy and Heat Jennie L

Chapter 16 – Thermal Energy and Heat Jennie L. Borders

Section 16. 1 – Thermal Energy and Matter n In the 1700’s most scientists thought that heat was a fluid called caloric that flowed between objects.

Count Rumford n Rumford studied the process of drilling holes in the barrels of cannons. n The process occurred in water so that the metal would not melt due to the heat produced. n Rumford discovered that the heat was a result of the motion of the drill, not a form of matter.

Work and Heat n Some of the work done by the drill does useful work, but some energy is lost due to friction. n Heat is the transfer of thermal energy from one object to another because of a temperature difference. n Heat flows spontaneously from hot objects to cold objects.

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

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

Thermal Contraction n Slower particles collide less often and exert less force, so pressure decreases and the object contracts. n This is called thermal contraction.

Thermal Expansion n Thermal expansion is an increase in the volume of a material due to a temperature increase. n Thermal expansion occurs when particles of matter move farther apart as temperature increases. n Gases expand more than liquids, and liquids usually expand more than solids.

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

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

Specific Heat n Formula for Specific heat: Q = m x c x DT Q = heat (J) m = mass (g) c = specific heat (J/go. C) DT = change in temperature final – initial (o. C)

Sample Problem n An iron skillet has a mass of 500. 0 g. The specific heat of iron is 0. 449 J/go. C. How much heat must be absorbed to raise the skillet’s temperature by 95. 0 o. C? Q = m x c x DT m = 500. 0 g c = 0. 449 J/go. C DT = 95. 0 o. C Q =(500. 0 g)(0. 449 J/go. C)(95. 0 o. C) = 21, 327. 5 J

Practice Problems n How much heat is needed to raise the temperature of 100. 0 g of water by 85. 0 o. C? o. C)(85. 0 o. C) = 35, 530 J Q = (100. 0 g)(4. 18 J/g n How much heat in k. J is absorbed by a 750 g iron skillet when its temperature rises from 25 o. C to 125 o. C? DT = 125 o. C – 25 o. C = 100 o. C Q = (750 g)(0. 449 J/go. C)(100 o. C) = 33, 675 J K h d u d c m 33, 675 J = 33. 7 k. J

Practice Problems n In setting up an aquarium, the heater transfers 1200 k. J of heat to 75, 000 g of water. What is the increase in the water’s temperature? Q = m x c x DT DT = Q/(m x c) 1200 k. J = 1, 200, 000 J DT = 1, 200, 000 J/(75, 000 g x 4. 18 J/go. C) = 3. 8 o. C n What mass of water will change its temperature by 3. 0 o. C when 525 J of heat is added to it? Q = m x c x DT m = Q/(c x DT) m = 525 J/(4. 18 J/go. C x 3. 0 o. C) = 41. 9 g

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

Calorimeter

Section 16. 1 Section Assessment n In what direction does heat flow on its own spontaneously? n How is the temperature of an object related to the average kinetic energy of its particles? n Name two variables that affect thermal energy of an object. n What causes thermal expansion of an object when it is heated?

Section 16. 1 Section Assessment n How do the temperature increases of different materials depend on their specific heats? n What principle explains how a calorimeter is used to measure the specific heat of a sample material? n Why is it necessary to have regularly spaced gaps between sections of a concrete sidewalk?

Section 16. 1 Section Assessment n The specific heat of copper is 0. 39 J/go. C. How much heat is needed to raise the temperature of 1000. 0 g of copper from 25. 0 o. C to 45. 0 o. C? DT = 45 o. C – 25 o. C = 20 o. C Q = (1000. 0 g)(0. 39 J/go. C)(20 o. C) = 7800 J n. A peanut burned in a calorimeter transfers 18, 200 J to 100. 0 g of water. What is the rise in the water’s temperature? Q = m x c x DT DT = Q/(m x c) DT = 18, 200 J/(100. 0 g x 4. 18 J/go. C) = 43. 5 o. C

Section 16. 2 – Heat and Thermodynamics n Conduction is the transfer of thermal energy with no overall transfer of matter. n Conduction occurs between materials that are touching. n Conduction in gases is slower than in liquids and solids because the particles in a gas collide less often.

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

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

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

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

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

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

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

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

2 nd Law of Thermodynamics n 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. )

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

Section 16. 2 Section Assessment n Why is conduction in gases slower than conduction in liquids or solids? n Give three examples of convection currents that occur in natural cycles. n What happens to radiation from an object as its temperature increases? n State the first law of thermodynamics.

Section 16. 2 Section Assessment n What is the second law of thermodynamics? n State third law of thermodynamics. n If you bedroom is cold, you might feel warmer with several thin blankets than one thick one. Explain why.