Chapter 16: Thermal Energy and Heat Section 1: Thermal Energy and Matter

Temperature We think of temperature in terms of hot and cold, but what exactly is temperature? We think of temperature in terms of hot and cold, but what exactly is temperature? Temperature is a term that describes the average kinetic energy of the atoms or molecules that make up a substance. Temperature is a term that describes the average kinetic energy of the atoms or molecules that make up a substance. As the atoms move faster, we perceive an object to be hotter. As the atoms move faster, we perceive an object to be hotter. If atoms are moving more slowly, we perceive an object to be cooler. If atoms are moving more slowly, we perceive an object to be cooler.

Motion of the Molecules Molecules that are moving very fast (we think of these things as warm or hot) bump into the molecules of objects that are moving more slowly. Molecules that are moving very fast (we think of these things as warm or hot) bump into the molecules of objects that are moving more slowly. When a fast atom bumps into a slow atom, some of the energy is transferred to the slower moving atom (remember the Law of Conservation of Momentum?). When a fast atom bumps into a slow atom, some of the energy is transferred to the slower moving atom (remember the Law of Conservation of Momentum?).

Thermal Energy & Temperature The sum of all of the kinetic and potential energy of all the molecules in an object is known as thermal energy. The sum of all of the kinetic and potential energy of all the molecules in an object is known as thermal energy. As the temperature of an object increases, the kinetic energy of the molecules in the object increase. As a result of this, the total thermal energy of the object increased due to the increase in temperature. As the temperature of an object increases, the kinetic energy of the molecules in the object increase. As a result of this, the total thermal energy of the object increased due to the increase in temperature.

Thermal Energy and Mass If two objects with different masses are at the same temperature, the object with the larger mass will have more thermal energy. If two objects with different masses are at the same temperature, the object with the larger mass will have more thermal energy. This is the case because the larger object will be composed of more molecules and the sum of the thermal energy will be greater. This is the case because the larger object will be composed of more molecules and the sum of the thermal energy will be greater.

Heat Heat is thermal energy that flows from something at a higher temperature to something at a lower temperature. Heat is thermal energy that flows from something at a higher temperature to something at a lower temperature. Heat always goes from HOT  COLD Heat always goes from HOT  COLD So if your mom says ”Close the door, you‘re letting the cold in!”, is this a correct statement? So if your mom says ”Close the door, you‘re letting the cold in!”, is this a correct statement? NO, in fact you are letting the heat out! NO, in fact you are letting the heat out!

Specific Heat Specific heat is the amount of heat required to heat a 1 kg object one degree Celsius. Specific heat is the amount of heat required to heat a 1 kg object one degree Celsius. Specific heat is measured in J / kg. C. Specific heat is measured in J / kg. C. Water is a common substance that has a very high specific heat. It takes 4,184 joules of energy to raise one kg of water one degree C. Water can absorb a great deal of heat without changing temperature. Water is a common substance that has a very high specific heat. It takes 4,184 joules of energy to raise one kg of water one degree C. Water can absorb a great deal of heat without changing temperature.

Specific Heat values The lower the specific heat value of a substance, the less energy it takes to raise it’s temperature. The lower the specific heat value of a substance, the less energy it takes to raise it’s temperature. Metals in general have a low specific heat as they are good conductors of heat. Metals in general have a low specific heat as they are good conductors of heat. Water has a relatively high specific heat as it takes a lot of energy to change its temperature. Water has a relatively high specific heat as it takes a lot of energy to change its temperature. Some specific heat values (J/kg. C) Mercury 140 Steel 450 Water (at 15°C) 4184

Calculating Changes in Thermal Energy A formula? but, of course A formula? but, of course Change in thermal energy = mass x specific heat x change in temperature ***OR*** Q = m C  t if Q is positive, the temperature of the object increases if Q is positive, the temperature of the object increases If Q is negative, the temperature of the object decreases If Q is negative, the temperature of the object decreases Q = change in thermal energy (J), m = mass (kg), C = Specific Heat (J/g. C), ∆T = (T f - T i )

Measuring Specific Heat Specific Heat can be measured using a device called a calorimeter. Specific Heat can be measured using a device called a calorimeter. There is a picture and a description of a calorimeter in your book on page 478. There is a picture and a description of a calorimeter in your book on page 478.