Chapter 16: Thermal Chapter 16: Thermal Energy and HeatEnergy and Heat

Section 1: Thermal Energy and Section 1: Thermal Energy and MatterMatter

TemperatureTemperature

We think of temperature in terms of hot and cold, We think of temperature in terms of hot and cold, but what exactly is temperature?but what exactly is temperature?

TemperatureTemperature is a term that describes the is a term that describes the average kinetic energy of the atoms or average kinetic energy of the atoms or molecules that make up a substance.molecules that make up a substance.

As the atoms move faster, we perceive an object As the atoms move faster, we perceive an object to be hotter.to be hotter.

If atoms are moving more slowly, we perceive an If atoms are moving more slowly, we perceive an object to be cooler.object to be cooler.

Motion of the MoleculesMotion of the Molecules

Molecules that are moving very fast (we Molecules that are moving very fast (we think of these things as warm or hot) bump think of these things as warm or hot) bump into the molecules of objects that are into the molecules of objects that are moving more slowly.moving more slowly.

When a fast atom bumps into a slow atom, When a fast atom bumps into a slow atom, some of the energy is transferred to the some of the energy is transferred to the slower moving atom slower moving atom (remember the Law of (remember the Law of Conservation of Momentum?).Conservation of Momentum?).

Thermal Energy & TemperatureThermal Energy & Temperature

The The sumsum of all of the kinetic and potential of all of the kinetic and potential energy of all the molecules in an object is energy of all the molecules in an object is known as known as thermal energy.thermal energy.

As the As the temperaturetemperature of an object increases, of an object increases, the kinetic energy of the molecules in the the kinetic energy of the molecules in the object increase. As a result of this, the object increase. As a result of this, the total total thermal energythermal energy of the object of the object increased due to the increase in increased due to the increase in temperature.temperature.

Thermal Energy and MassThermal Energy and Mass

If two objects with different masses are at the If two objects with different masses are at the same temperature, the object with the larger same temperature, the object with the larger mass will have more thermal energy. mass will have more thermal energy.

This is the case because the larger object will be This is the case because the larger object will be composed of more molecules and the composed of more molecules and the sumsum of the of the thermal energy will be greater.thermal energy will be greater.

HeatHeat

Heat Heat is thermal energy that flows from is thermal energy that flows from something at a higher temperature to something at a higher temperature to something at a lower temperature.something at a lower temperature.

Heat always goes from HOT Heat always goes from HOT COLD COLDSo if your mom says ”Close the door, So if your mom says ”Close the door,

you‘re letting the cold in!”, is this a correct you‘re letting the cold in!”, is this a correct statement?statement?

NO, in fact you are letting the heat out!NO, in fact you are letting the heat out!

Specific HeatSpecific Heat

Specific heatSpecific heat is the amount of heat is the amount of heat required to heat a 1 kg object one degree required to heat a 1 kg object one degree Celsius.Celsius.

Specific heat is measured in J / kgSpecific heat is measured in J / kg..C.C.Water is a common substance that has a Water is a common substance that has a

very high specific heatvery high specific heat. It takes 4,184 . It takes 4,184 joules of energy to raise one kg of water joules of energy to raise one kg of water one degree C. Water can absorb a great one degree C. Water can absorb a great deal of heat without changing temperature.deal of heat without changing temperature.

Specific Heat valuesSpecific Heat values

The lower the specific heat value of a The lower the specific heat value of a substance, the less energy it takes to raise substance, the less energy it takes to raise it’s temperature.it’s temperature.

Metals in general have a low specific heat Metals in general have a low specific heat as they are good conductors of heat.as they are good conductors of heat.

Water has a relatively high specific heat as Water has a relatively high specific heat as it takes a lot of energy to change its it takes a lot of energy to change its temperature. temperature.

Some specific heat values (J/kg.C)Mercury 140Steel 450Water (at 15°C) 4184

Calculating Changes in Thermal Calculating Changes in Thermal EnergyEnergy

A formula?.........but, of courseA formula?.........but, of courseChange in thermal energy = mass x Change in thermal energy = mass x specific heat x change in temperature specific heat x change in temperature

***OR******OR***Q = m C Q = m C tt

if Q is positive, the temperature of the object increasesif Q is positive, the temperature of the object increases If Q is negative, the temperature of the object decreasesIf Q is negative, the temperature of the object decreases

Q = change in thermal energy (J), m = mass (kg),Q = change in thermal energy (J), m = mass (kg),

C = Specific Heat (J/g C = Specific Heat (J/g .. C), ∆T = (T C), ∆T = (Tf f - T- Tii))

Measuring Specific HeatMeasuring Specific Heat

Specific Heat can be measured using a Specific Heat can be measured using a device called a device called a calorimeter.calorimeter.

There is a picture and a description of a There is a picture and a description of a calorimeter in your book on page 478.calorimeter in your book on page 478.