Thermal Energy

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Common phase transitions are

Solid  liquid (melting)

Liquid  gas (boiling)

Phase transitions involve a change in the internal energy, but no change in temperature

Kinetic energy of molecules (which is related to temperature) is not changing, but their potential energy changes as work is done to change their positions

Energy required to change the phase of a given mass m of a pure substance is:

L = latent heat – depends on substance and nature of phase transition

+ (–) sign used if energy is added (removed)

Slide 12

Phase Transitions

All phase changes can go in either direction

Heat flowing into a substance can cause melting (solid to liquid) or boiling (liquid to gas)

Heat flowing out of a substance can cause freezing (liquid to solid) or condensation (gas to liquid)

Latent heat of fusion Lf is used for melting or freezing

Latent heat of vaporization Lv is used for boiling or condensing (somewhat larger for lower pressures)

Table 11.2 gives the latent heats for various substances

Large Lf of water is partly why spraying fruit trees with water can protect the buds from freezing

In process of freezing, water gives up a large amount of energy and keeps bud temperature from going below 0°C

Slide 13

T vs. Q for Transition from Ice to Steam

Part A: Temperature of ice changes from –30°C to 0°C

Q = mcice DT = (1.00  10–3 kg)(2090 J/kg°C)(30.0°C) = 62.7 J

Part B: Ice melts to water at 0°C

Q = mLf = (1.00  10–3 kg)(3.33  105 J/kg) = 333 J

Part C: Temperature of water changes from 0°C to 100°C

Q = mcwater DT = (1.00  10–3 kg)(4.19  103 J/kg°C)(100°C) = 419 J

Part D: Water changes to steam at 100°C

Q = mLv = (1.00  10–3 kg)(2.26  106 J/kg) = 2.26  103 J

Part E: Temperature of steam changes from 100°C to 120°C

Q = mcsteam DT = (1.00  10–3 kg)(2.01  103 J/kg°C)(20°C) = 40.2 J

Initial state: 1 g of ice at –30°C

Final state: 1 g of steam at 120°C

Qtot = 3.11  103 J

Slide 14

Evaporation and Condensation

The previous example shows why a burn caused by 100°C steam is much more severe than a burn caused by 100°C water

Steam releases large amount of energy through heat as it condenses to form water on the skin

Much more energy is transferred to the skin than would be the case for same amount of water at 100°C

Evaporation is similar to boiling

Molecular bonds are being broken by the most energetic molecules

Average kinetic energy is lowered as a result, which is why evaporation is a cooling process

Approximately the same latent heat of vaporization applies