PHYSICS 197 Section 1 Chapter C13 Other Forms of Internal Energy October 2, 2017

Review of Last Class Example of internal energy: Thermal energy T is measured in Kelvin (0 K = −273.150C)

Heat, Work and Energy Transfer Heat (Q) is the energy that crosses the boundary between two objects because of a temperature difference between them. Work (W) is the energy flowing across a system boundary due to external interactions that exert a force. Heat and work both describe energy transfer across a boundary. Different from internal energy (including thermal energy) which is inside the object’s boundary. Both heat and work can contribute to change in internal energy.

Question C12T.3 Is the specified change in the following objects’ thermal energies due to a flow of heat (Q), work (W), or some other flow of energy (E)? Rubbing your palms together make them warmer. Answer: W. Because of frictional work you are doing on them.

Question C12T.4 Is the specified change in the following objects’ thermal energies due to a flow of heat (Q), work (W), or some other flow of energy (E)? A hot cup of coffee on a table becomes cooler with time. Answer: Q. The cup loses energy due to heat flow because it is hotter than its surroundings.

Question C12T.4 Is the specified change in the following objects’ thermal energies due to a flow of heat (Q), work (W), or some other flow of energy (E)? Infrared laser light falling on a metal slab makes it hot. Answer: E. This is energy transfer carried by light quanta. Not because the laser is hotter or exerts a force on the slab.

Conservation of Energy Requires that a multi-object system’s total change in energy must be equal to the heat, work and other energy transfers that flow into it: For a single object at rest, this reduces to (also known as the first law of thermodynamics)

Specific Heat Temperature and thermal energy are related, but not equivalent (like depth and volume of water in a jar). In a sufficiently small temperature range dT, can assume a linear dependence: c is the specific heat of the substance that expresses the change in thermal energy per unit mass per unit temperature.

Other “Hidden” Forms of Energy Outline of C13 Other “Hidden” Forms of Energy Chemical Bonds Latent Heat Chemical and Nuclear Energy Heat Transfer

Interactions Between Two Atoms (See Chapter C9) Nuclei repel each other Electron clouds repel each other Settle into a new arrangement with lower potential energy

Hill and Valley Forms a bond in the valley

How to Form a Bond To form a bond, two things must happen: Interacting particles must have enough energy to overcome the potential hill They must lose energy in the valley to their surroundings so that their energy falls below the outer barrier’s height. How does an atom-pair lose energy? By emitting a photon. By colliding with a third atom. How to break a bond? -- By supplying energy from the surroundings to increase its energy over the barrier Photon irradiation Bombardment by energetic nuclei

Example: Gasoline Engine Show demo for compressed air Needs a spark to ignite the air-fuel mixture (therefore called spark ignition engine)

Latent Heat When a substance undergoes a phase change (forming/breaking of bonds), it releases/absorbs thermal energy, without changing its temperature. Latent heat: energy absorbed/released per kg. Condensation/Freezing: Atom-atom pairs release energy to be confined in the potential valley (new bonds form)  latent energy decreases (converted to thermal energy). Vaporization/Melting: Atom-atom pairs gain energy to climb over the potential hill (bonds break)  latent energy increases (at the expense of thermal energy) Highlight water’s

Clicker Question C13T.2 Suppose we put a 0.3-kg hunk of ice at 00C into an insulating thermos bottle filled with 1kg of water at 200C. What will be the system’s final temperature? Below 00C Almost exactly 00C Somewhere between 00C and 100C Somewhere between 100C and 200C Almost exactly 200C Hint: Latent heat of water is 333 kJ/kg and its specific heat is 4.186 kJ/(kg K).

Answer C13T.2 Suppose we put a 0.3-kg hunk of ice at 00C into an insulating thermos bottle filled with 1kg of water at 200C. What will be the system’s final temperature? Below 00C Almost exactly 00C Somewhere between 00C and 100C Somewhere between 100C and 200C Almost exactly 200C Explanation:

Heat Transfer Three ways: Conduction (across a boundary between objects in contact) Convection (where some medium carries energy away) Radiation (through the emission of light) Demo on conduction (ask students to touch various objects and feel the difference), convection (hair dryer), radiation (infrared lamp)

Power Emitted by a Radiating Object Stefan-Boltzmann Law: Wien’s Law:

Practice Problem C13R.2 Suppose your car has a mileage of 35 mpg when traveling on a level road at 60 mph. Estimate your gas mileage when traveling at the same speed up an incline that gains 3 m of elevation for every 100 m along the slope (a 3% incline). Assume that your car and passengers have a total mass of 1500 kg. Also 1 gal = 3.79 liter and gasoline has a density of about 720 kg/m3.

Solution On a level road, the car uses (chemical) energy at a rate of Since the power is related to the inverse of mileage, the mileage when climbing the incline will be