Temperature Energy and Heat CHAPTER 3 Temperature Energy and Heat 3.2 Heat and Thermal Energy

We know now that heat is not the same thing as temperature.

We know now that heat is not the same thing as temperature. Measured in oF, oC, K

We know now that heat is not the same thing as temperature. Measured in oF, oC, K Measured in… ?

Heat can be measured in joules (J). The joule is the fundamental SI unit of energy and heat.

Heat can be measured in calories. It takes 1 calorie to raise 1 g of water by 1oC. 1 Calorie = 1 kilocalorie = 1,000 calories

Heat can be measured in British thermal units (BTU).

British thermal units (BTU) joules (J) calories British thermal units (BTU) Heat can be measured in 1 calorie = 4.184 joules 1 BTU = 1,055 joules

second law of thermodynamics: energy (heat) spontaneously flows from higher temperature to lower temperature.

SURROUNDING

first law of thermodynamics: energy can neither be created nor destroyed.

first law of thermodynamics: first law of thermodynamics: energy can neither be created nor destroyed. The energy inside an isolated system is constant. The energy lost by a system must be gained by the surroundings or another system.

What happens when hot and cold water are not allowed to mix but are allowed to exchange energy? Does one side stay hot and one side stays cold? According to the second law of thermodynamics, the energy will flow from higher temperature to lower temperature. Eventually, both sides will have the same temperature (thermal equilibrium, discussed in the next slide).

Thermal equilibrium

Same energy input Different temperature change

Specific heat of water: specific heat: the quantity of energy it takes per gram of a certain material to raise the temperature by one degree Celsius. Specific heat of water: 4.184 J/(g·oC) Specific heat of gold: 0.129 J/(g·oC)

Why do different metals have different specific heats? One reason is:

A metal-working process needs to heat steel from room temperature (20oC) to 2,000oC. If the mass of steel is 100 g, how much heat is required?

A metal-working process needs to heat steel from room temperature (20oC) to 2,000oC. If the mass of steel is 100 g, how much heat is required? Asked: Quantity of heat Given: 100 g of steel, temperature difference is 2,000oC – 20oC Relationships:

A metal-working process needs to heat steel from room temperature (20oC) to 2,000oC. If the mass of steel is 100 g, how much heat is required? Asked: Quantity of heat Given: 100 g of steel, temperature difference is 2,000oC – 20oC Relationships: Solve: Answer: It takes 93,060 joules to raise the temperature of 100 g of steel to 2,000oC, assuming no heat gets lost during the process (which is not a very good assumption!).

A mass of 300 grams of water at 80oC cools down to 20oC A mass of 300 grams of water at 80oC cools down to 20oC. Assume all the heat from the water is absorbed by 100 m3 of air (a small room) with a mass of 100,000 g. What is the temperature change in the air?

A mass of 300 grams of water at 80oC cools down to 20oC A mass of 300 grams of water at 80oC cools down to 20oC. Assume all the heat from the water is absorbed by 100 m3 of air (a small room) with a mass of 100,000 g. What is the temperature change in the air? Asked: Temperature change in oC Given: 300 g of water [cp = 4.184 J/(g·oC)], change of 60oC (80oC – 20oC), and 100,000 g of air [cp = 1.006 J/(g·oC)] Relationships:

A mass of 300 grams of water at 80oC cools down to 20oC A mass of 300 grams of water at 80oC cools down to 20oC. Assume all the heat from the water is absorbed by 100 m3 of air (a small room) with a mass of 100,000 g. What is the temperature change in the air? Asked: Temperature change in oC Given: 300 g of water [cp = 4.184 J/(g·oC)], change of 60oC (80oC – 20oC), and 100,000 g of air [cp = 1.006 J/(g·oC)] Relationships: Solve:

A mass of 300 grams of water at 80oC cools down to 20oC A mass of 300 grams of water at 80oC cools down to 20oC. Assume all the heat from the water is absorbed by 100 m3 of air (a small room) with a mass of 100,000 g. What is the temperature change in the air? Asked: Temperature change in oC Given: 300 g of water [cp = 4.184 J/(g·oC)], change of 60oC (80oC – 20oC), and 100,000 g of air [cp = 1.006 J/(g·oC)] Relationships: Solve: Answer: The air in the room gets warmer by about 0.75oC.

conduction: the flow of heat energy through the direct contact of matter.

Thermal equilibrium was reached (60oC both inside and outside the test tube). Because the test tube allowed heat to flow, it is a thermal conductor. Would you describe the glass of the test tube as a thermal conductor or a thermal insulator?

A styrofoam cup is an example of a thermal insulator because it resists the flow of heat. Would you describe the styrofoam cup of the test tube as a thermal conductor or a thermal insulator?

Temperature is measured in: oF, oC, kelvin Heat is measured in: joules (J), calories, British thermal units (BTU) first law of thermodynamics: energy can neither be created nor destroyed. second law of thermodynamics: energy (heat) spontaneously flows from higher temperature to lower temperature.