Thermal Physics

Thermal Energy Physics I Chapter 21

Kinetic Theory What is the kinetic theory of matter? Kinetic theory explains the behavior of solids, liquids, and gases. ALL particles of matter are in constant random motion. We don’t observe the movement directly, but we can measure it indirectly Pressure and temperature (macroscopic) are indicators of particle movement (microscopic)

Kinetic Theory: “big indicates little” Macroscopic properties: big, easy to observe (like temperature and pressure) Microscopic properties: small, difficult to observe (like motion, velocity, and momentum of atoms) http://home.roadrunner.com/~enloephysics/macro_micro.gif

Temperature The measure of an object’s average kinetic energy. how hot or cold something is, with respect to a standard (temperature scale – ⁰F, ⁰C, or K)) Measured with a thermometer

Temperature and Molecular Kinetic Energy All the particles in a given object have different KE’s. Temperature measures the average KE of all the particles in an object. So… Higher KE = Higher Temperature Lower KE = Lower Temperature

Change of State (Phase) of Matter There are 3 states (phases) of matter: Solids, liquids and gases When the temperature of a solid is increased enough, it will become a liquid When temperature is increased even more, it becomes a gas.

Phase Change To change phase, we must add or remove ENERGY in the form of heat When adding heat energy… If phase DOES NOT change, temperature and KE increase If phase DOES change, temperature remains constant and PE increases

Phase Change and Temperature Heat transferred during a change of state doesn't change the temperature.

Temperature Scales and Water Celsius Scale - Freeze at 0⁰ Boil at 100⁰ Fahrenheit Scale – Freeze at 32⁰ Boil at at 212o Kelvin Scale – Freeze at 273 K Boil at 373 K Freezing is freezing is freezing… 0 ⁰C = 32 ⁰F = 273 K 100 ⁰C = 212 ⁰F = 373 K

Pressure A measure of the force of particle collisions over the surface of a container Pressure = Force/Area P = F/A (units: N/m2)

Pressure and Molecular Kinetic Energy Pressure increases when the average kinetic energy (temperature) of the particles increase More KE means… More velocity, and therefore more momentum Particles hit with more force and are moving faster

Observations… As the volume changes: Describe how pressure changes Describe how the motion of the molecules change

Discuss & write… Describe Use the kinetic theory of matter to describe the effect of cold winter weather on the air pressure inside a car tire. Describe Cake batter and bread dough can “rise” dramatically during baking. Use the kinetic theory of matter to describe one way in which the heat of an oven can help to produce this increase in size.

Heat Heat is the amount of energy transferred between two objects as a result of differences in temperature represented by “Q” Measured in Joules (J) or calories (c) Direction of energy flow is always from hot to cold +Q – heat has been absorbed (gained) - Q – heat has been lost

Thermal Equilibrium Thermal energy transfers between two objects until the reach the same temperature Thermal equilibrium occurs when the average kinetic energy of the atoms and molecules is the same

Specific Heat When heat flows into an object, thermal energy increases, increasing the temperature Increase depends on the material and its mass Different materials require different amounts of heat to change temperature

Specific Heat The amount of energy that must be added to a material to raise the temperature of a unit mass of the material by one unit temperature Measured in J/g●K or J/g●ºC Kelvin or Celsius, doesn’t matter… the magnitude of each degree is the same.

Calculating Heat Changes Q = mc∆T Q = heat gained(+) or lost(-) m = mass (g) c = specific heat of material (J/g●K or J/g●ºC) ∆T = temperature change (K or ºC) IMPORTANT: Notice that mass is measured in GRAMS!!!!!

Example 1 How much heat is absorbed by 60 g of copper (c = 0.386 J/gºC) when its temperature is raised from 20ºC to 80ºC ? ANS: 1389.6 J

Example 2 A child is given a 175 g silver spoon which she promptly puts in her mouth. The spoon was initially at room temperature (20°C) and the child’s mouth is 37°C. If 684 Joules of energy is gained by the spoon, what is the specific heat of silver?

Law of Heat Exchange Remember the Law of Conservation of Energy The sum of heat loss and heat gain in a closed system is zero. When 2 bodies of unequal temp. are mixed, the cold body absorbs heat from the warm body (loses heat) until an equilibrium temperature is reached. Qloss + Qgain = O

Example 3 If 30 grams of water (c = 1 cal/g°C) at 12⁰C is mixed with 80 grams of water at 88 ⁰C , what will the final temperature be?

Discuss and Write Describe Use the kinetic theory of matter to describe why the high specific heat of water makes it an ideal substance for thawing frozen food or for cooling down overheating machinery.

Happens in most solids, liquids & gases Thermal Expansion Increase in the size due to an increase in temperature If temp then size Happens in most solids, liquids & gases Water is an exception – it expands as it becomes a solid!

Thermal Expansion Materials expand when heated Materials contract when cooled Expansion joints Ring sizes

Heat Transfer & Thermodynamics Chapters 22 & 24

Heat Transfer Conduction – transfer of thermal energy due to particle collision Convection - transfer of thermal energy due to motion of fluid Radiation – transfer of thermal energy by electromagnetic waves

Heat Transfer

Heat Transfer

Absolute Zero A theoretical temperature at which no further thermal energy can be removed from an object Usually shown as -273ºC Kelvin Scale is based on Absolute zero

First Law of Thermodynamics Whenever heat flows into a system, the gain of thermal energy equals the amount of heat transferred. Whenever heat flows out of a system, the loss of thermal energy equals the amount of heat transferred. So, the net heat put into a system is equal to the change in internal energy of the system plus the work done BY the system. Simply, the Law of Conservation of Energy restated.

First Law of Thermodynamics Heat added = increase in internal energy + work done by the system

Second Law of Thermodynamics Natural processes go in a direction that maintains or increases the total entropy of the universe. Recall: Entropy – Measure of disorder; the more entropy, the higher the temperature. Simply, heat flows from high to low temperature