1. Thermal Energy

2. Heat
Heat: transfer of thermal energy from one object to another because of temperature differences
Heat flows spontaneously from hot objects to cold objects
Why does a cold drink become
warm?

3. Temperature
Temperature: measure of how hot or cold an object is compared to a reference point
Reference points for Celsius are freezing and boiling points of water
Reference point for Kelvin is absolute zero which equals 0 K

4. Temperature
Temperature is related to average kinetic energy of particles
KE of particles is due to random motion through space
As an object heats up the particles move faster = higher KE of particles = higher temperature

5. Thermal Energy
Thermal energy = total potential and kinetic energy of all the particles in an object
Depends on the mass, temperature and phase (solid,liquid,gas) of an object

6. Thermal Energy Example
Cup of tea
Temp of 110°F
KE of particles 80 J
Thermal Energy lower
Tea pot full of tea
Temp of 110°F
KE of particles 80 J
Thermal energy higher

7. Thermal Contraction and Expansion
As temp decreases the particles move slower causing less collisions
Balloon outside on a cold day
Thermal Expansion
As temp increases the particles move faster and spread apart
Balloon brought in from cold into warm room
Gases expand more than liquids and liquids expand
more than solids
Thermal expansion is used in thermometers to
show temperature

8. Measuring Heat Changes
Calorimeter is used to measure changes in thermal energy
Uses the principle that heat flows from hotter to cooler objects until they reach the same temperature

9. Specific Heat
Specific Heat: Amount of heat needed to raise the temperature of 1 gram of material by 1 degrees Celsius
The lower the specific heat the more its temperature rises
Unit is joules per gram per degree Celsius
J/g°C

10. Specific Heat Q = m x c x T Q = heat absorbed by material (Joules)
m = mass of material (grams)
c = specific heat (J/g °C)
T = change in temperature

11. Specific Heat
An iron skillet has a mass of grams. The specific heat of iron is J/g °C. How much heat must be absorbed to raise the skillet’s temperature by 95 degrees Celsius?
Q = m x c x T
Q = (500) (0.449) (95)
Q = 21, J

12. Specific Heat
In setting up an aquarium, the heater transfers 1,200,000 joules of heat to 75,000 grams of water which has a specific heat of 4.18 J/g °C. What is the increase in the water’s temperature?
1,200,000 = (75,000) (4.18) (T)
Change in temp = 3.8 °C

13. Specific Heat
34,000 J of heat is absorbed by a 750 g iron skillet when it’s temperature rises from 25°C to 125°C. What is the specific heat of the iron skillet?
34,000 = (750) (c) (100)
34,000 = 75,000 (c)
0.45 J/g°C = c

14. Heat Transfers
Conduction
Convection
Radiation

15. Conduction
Conduction: Occurs within a material or between materials that are touching
Collisions between particles transfer thermal energy without any overall transfer of matter
Conduction is slower in gases than in liquids or solids because the particles are farther apart
Conduction is faster in metals because the electrons are free to move around

16. Conduction
Thermal conductors: material that conducts thermal energy well
Wire rack in oven
Pots and pans
Aluminum and copper
Not all thermal conductors are hot
Tile floors
Feel cold because they are transferring thermal energy rapidly away from your skin

17. Conduction
Thermal insulators: material that conducts thermal energy poorly
Air
Wooden spoon
Wool
Plastic foam cups
Use trapped air to slow down conduction

18. Convection
Convection: transfer of thermal energy when particles of a fluid move from one place to another
Fluids = liquids or gases
Convection currents: fluid circulates in a loop as it heats up and cools down

19. Radiation Radiation: transfer of energy by waves moving through space
All objects radiate energy. As the temperature increases, the rate at which it radiates energy increases

20. Thermodynamics
Study of conversions between thermal energy and other forms of energy
Zeroth law: two systems in thermal equilibrium with a third system are in thermal equilibrium with each other

21. Thermodynamics 1st law: Energy is conserved
2nd law: Thermal energy can flow from colder objects to hotter objects only if work is done on the system
3rd law: Absolute zero cannot be reached