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

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

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. 1910 m/s E. 7100 m/s D 0.5mv^2 for nitrogen = 0.5mv^2 for hydrogen Solve for v for hydrogen General Physics

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

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

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

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

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 373.5 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 - 373.15 K is the freezing point of water. C changes significantly in this temp range. General Physics

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

General Physics

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

Potential Energy Diagrams Scan p 194 General Physics

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

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

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