1. C10 – Thermal Energy C11 – Energy in Bonds General Physics
Get a spring
General Physics

2. Disappearing Energy
Contact friction interaction doesn’t change potential energy because the separation between the two objects doesn’t change
Kinetic energy of object decreases, but where does the energy go?
General Physics

3. Two Minute Problems – C10T.2
The root-mean-square (rms) speed of a molecule’s motion is the speed that the molecule would have if its actual kinetic energy were equal to its average kinetic energy Kavg. The rms speed of random motion of a nitrogen molecule in a container of air at room temperature is about 510 m/s. The mass of a hydrogen molecule is about 14 times smaller. What is its rms speed of random motion at the same temperature? Select the closest response.
A. 35 m/s
B. 140 m/s
C. 510 m/s
D m/s
E m/s D
0.5mv^2 for nitrogen = 0.5mv^2 for hydrogen
Solve for v for hydrogen
General Physics

4. Thermal Energy
All objects are constructed of a huge number of tiny molecules
These molecules are in ceaseless random motion
An object’s temperature is a measure of the intensity of motion
For gases and simple solids under everyday conditions, the temperature is directly proportional to the average kinetic energy per molecule of its molecules’ random motions in the substance.
General Physics

5. Friction and Thermal Energy
When a surface moves past another other, their surface molecules often become entangled.
The entangled molecules are stretched away from their normal positions
A small amount of kinetic energy is transformed to potential energy in the bonds between the molecules.
When the molecules snap back, they oscilate
The energy is transmitted to the rest of the molecules in the objects
General Physics

6. Two Minute Problems – C10T.5
Is the change in the following objects’ thermal energies due to a flow of heat (A) or work (B)?
A. A hot cup of tea on a table becomes cooler with time.
B. A meteorite entering the atmosphere glows white-hot.
C. Liquid nitrogen poured on a slab of ice boils furiously (Liquid nitrogen boils at 77 K).
D. A drill bit gets hot as it drills a hole in a metal slab.
A, B,A,B
General Physics

7. Heat and Work
Heat and work are defined to describe energy transfer across a system boundary
Heat is any energy that crosses the boundary between two objects because of the temperature difference between them
Work is any other kind of energy flowing across an object’s boundary
The change in internal energy of an object is the heat added to and the work flowing into the system
Work – stir a cup of water and it gets warm. I have done work on it.
General Physics

8. Two Minute Problems – C10T.6
The specific “heat” of water is roughly constant (within 0.2%) over a range from 5°C to 95°C, but changes significantly near its freezing and boiling points. Considering this, a change in temperature from 373 K to K will likely be “sufficiently small” that one can use equation C10.7 to calculate accurately the thermal energy change of a certain amount of water, T or F?
F K is the freezing point of water. C changes significantly in this temp range.
General Physics

9. Specific Heat
A small change in an object’s thermal energy is proportional to the mass, the specific heat, and the temperature change
dT must be small so that c doesn’t change over that temperature range
General Physics

10. General Physics

11. Example C10.3
Imagine that we place a 50-g block of aluminum with an initial temperature of 100°C into an insulating covered Styrofoam cup containing 250 g of water at 22°C. Heat will flow from the aluminum to the water until both are at the same temperature. What is that temperature?
General Physics

12. Potential Energy Diagrams
Scan p 194
General Physics

13. Bonds Typical potential energy curve for a bond.
When bonds form, energy is released
When bonds break, energy is absorbed
General Physics

14. Latent Heat For condensation or freezing For vaporization or melting
L is the latent heat, m is the mass
The latent heat is the energy a substance absorbs or
releases per kilogram during a change of phase.
General Physics

15. Group Problems
C10B.6
C10S.2
C11B.5
C11S.5
General Physics