1. Thermal Energy
PS CH 5

2. Temperature
Measure of average kinetic energy of all of the particles in an object
How hot or cold and object is
Ex. 40 degrees
The S.I. Unit for temperature is Kelvin
Measured from absolute zero
The theoretical lowest temperature possible
Cannot actually be achieved
Does not include the word “degrees”
Zero Kelvin (0 K) is absolute zero
Most of the world, and science, uses Celsius
Based on water’s boiling and freezing points
Water freezes at 0 degrees C and boils at 100 degrees C
We use the Fahrenheit scale
Strange scale… Not sure how it was defined.
Water freezes at 32 degrees F and boils at 212 degrees F

3. Temperature Conversions
Not easily converted from one to another
Need 2 different equations and a bit of algebra
To get to Kelvin, easiest to first go to Celsius
Equations:
°F°C: °C=(°F−32)x 5 9
°CK: 𝐾=°C+273
How would we find the equations for K °C and °C °F?
What about from °F to K and vice versa?

4. Example
The average temperature for the human body is 98.6 °F. What is this in Celsius and Kelvin?
37°C
310 K

5. Problem #1
The lowest possible temperature in the universe is 0K, also called absolute zero.
What is this in °C and °F?
-273°C
-459°F
Worksheet w/8:00 partner.

6. Thermal Energy
Not the same as temperature, nor is it the same as heat, but related to both
The sum of the kinetic and potential energies of all of the particles that make up the object.
Recall that all matter is made up of atoms
Atoms move spontaneously (without input of energy)
The movement of the atoms (KE) determines the thermal energy of that object
Can be transferred to other objects or the environment
Cannot be created or destroyed
Can change forms
Measured in Joules (J)

7. Thermal Energy and Temperature
Temperature affects the thermal energy of an object or system
If the temperature is increased, the particles move faster and TE increases
If the temperature is decreased, the particles move slower and TE decreases

8. Heat
The transfer of TE from one place to another due to a temperature difference
Thermal energy moves spontaneously from higher temperature to lower temperature

9. Specific Heat
The specific heat (C) of a material is the amount of energy needed to raise the temperature of 1 kg of the substance by 1 °C.
Water has a very high specific heat
Requires a lot of energy to change the temperature
Makes it a good coolant for vehicles
Also results in ice forming on the surface, but little change lower

10. Changes in Thermal Energy
The change in thermal energy of an object depends on the object’s specific heat, the amount of temperature change, and the how much of the object there is.
Q=mΔTC
Where:
Q is the change (quantity) in thermal energy,
m is the mass of the object,
ΔT is the change in temperature, and
C is the specific heat of the object
Derive the units… Or just think about it…

11. Example
How much energy is needed to raise the temperature of 100 kg of water (C=4200 J/kg°C) from 10°C to 20°C?
4.2x106J

12. Practice #1
How much energy is needed to raise the temperature of 10 kg of copper (specific heat= 400J/kg°C) by 5°C?
20,000 J

13. Practice #2
8x104J of energy are added to 10 kg an unknown substance, and causes its temperature to increase 10°C. What is the substance?
Concrete

14. Practice #3 (Be Careful…)
What is the change in thermal energy that will cause 1 kg of steel (specific heat 500J/kg°C) initially at 30°C to go to a final temperature of 25°C?
-2500 J
Negative because the object cooled
Expect something related to this on the test…

15. Interesting
1 Calorie (as in the calories from food) is the amount of energy needed to raise the temperature of 1 kg of water by 1°C.
Calories were originally calculated by burning the foods and seeing how much they raised the temperature of 1 kg of water.
Now they use averages of known values based on previous measurements collected by this method
Saves time, but not exactly accurate
This method is called calorimetry
A calorimeter is a device used to measure the specific heat of objects
Once the researcher has that, they can calculate the change in thermal energy, and convert this to calories

16. 3Methods of Heat Transfer
Conduction-Transfer of energy due to collisions between particles that make up the substance
Convection-transfer of energy in a fluid (air or liquid) due to the density difference between cold and hot fluid
Radiation- Transfer of energy by electromagnetic waves

17. Conduction Most effective in thermal conductors Best in solids
Materials that heat moves easily through
Best in solids
In solids, atoms are close together
When heated, atoms move faster and bump into one another
Good thermal conductors
Metal spoon in hot soup
The spacing of atoms in liquids and gases make them poor conductors
Called thermal insulators
Objects that do not easily transfer energy by conduction

18. Convection More energetic particles move from one place to another
As an object is heated, the particles move around more
Increases the volume of the object, and thus decreases the density
Less dense materials rise, while more dense materials sink
Works best in liquids and gases, poorly in solids
Warm air rises (air near ceiling is warmest)
As it rises, it cools and then sinks
It is replaced by rising hot air
Same thing occurs in liquids (boiling water)
Creates convection currents

20. Convection currents and Weather
Convection in the atmosphere is responsible for wind, clouds, and tornadoes
As the cooler air rapidly sinks, it moves across the surface
As the warm air rises, it carries moisture, and forms clouds
If there are rapid convection currents, they can gain enough rotation to form a tornado
High and low pressure areas on weather map

21. Radiation The only transfer that does not require matter
Can move through space
Good for us, or wouldn’t be any “us”
Radiation can interact with particles
Due to particles in liquids and gases being far apart, radiation usually passes through
Because it interacts with more of the particles, it can transfer more energy
Best in gases, okay in liquids, poor in solids
Solids block a lot of the radiation, so only the surface is affected
Then moves by conduction

22. Thermal Insulators Insulators prevent rapid transfer of energy
Slows it down
Insulators are the opposite of conductors
If heat readily conducts, it is a poor insulator
Often, pockets of air are used as air is a very poor conductor
Thermoses, winter coats, home insulation

23. Thermodynamics
Thermodynamics is the study of the relationships between TE, heat, and work
1st Law of Thermodynamics-The total energy of a system is constant. Energy can be transferred, but cannot be created or destroyed.
Restatement of Law of Conservation of Energy
Energy can be transferred by heat (transfer of TE) or by doing work on the system

25. 2nd Law of Thermodynamics
Energy spontaneously moves from areas of greater concentration to areas of less concentration
Hot objects heat cool objects
The reverse cannot happen without doing work (inputting energy) on the system

26. Thermal Energy Transformations
As with any energy, it can be transformed, but not created or destroyed
Engines burn fuel to create mechanical energy (goal is KE)
Solar collectors gather radiation and convert it to potential energy
Read more in section 3, but don’t spend tons of time.