Heat Transfer and Vaporization Notes

Heat Transfer

Conduction

• Ventilator heater example: A plate from the bottom of the heater touches the plate of a device, conducting heat to warm the water.

• Direct contact between hot and cold molecules.

• Example: Touching cold metal; heat transfers from your warm body to the metal, making it feel warmer.

• High thermal conductivity: Metal quickly draws heat away from the skin, creating a feeling of cold.

Convection

• Involves fluid molecules at different temperatures mixing.

• Infant incubator example: Air is warmed in one location and circulated to carry heat elsewhere.

• Forced air heating in houses: Mixing molecules in the air heat up the air, providing warmth.

• Convective oven: Heats molecules in the air, circulating heat to cook food.

Radiation

• Heat transfer without direct physical contact.

• Example: Feeling warmth from the sun.

• Radiant heat warmer: Used for babies in nurseries; heat is emitted from above to keep the baby warm.

• Kerosene heater, electric stove heater: Heat radiates outwards to provide warmth.

First Law of Thermodynamics

• Energy or heat must come from the surrounding environment.

Vaporization

• Bulk storage of liquid oxygen: Compressed liquid oxygen is exposed to ambient temperatures and vaporized for patient use.

• Evaporation: Heat is taken from the air surrounding the liquid, cooling the air.

  • Example: Sweating during exercise cools the skin through evaporation.

• Condensation: Heat is given back to the surrounding air, warming it.

• Refrigerator example: Refrigerant cools food, then vaporizes and expands, releasing heat into the atmosphere during condensation.

Heat Transfer: Additional Information

• Heat flows from hot objects to cold ones.

• Conduction: Transfer of heat between substances in direct contact.

  • Good conductors: Copper, silver, iron, and steel.

  • Poor conductors: Wood, styrofoam, paper, and air.

• Convection: Up and down movement of gases and liquids caused by heat transfer.

  • Warming and rising, cooling and falling creates a convection current.

  • Examples: Warm water on a cool surface, hot air balloon, cool evening breeze.

• Radiation: Electromagnetic waves traveling through space.

  • Electromagnetic waves transfer heat to objects.

  • Examples: Feeling warmth from a campfire, reheating pizza in a microwave, turning on a light.

Changes of State: Melting and Freezing

• Melting: Changing from a solid to a liquid; requires heat.

  • Latent heat of fusion: The number of calories required to change from a solid to a liquid without temperature change.

    • $Latent Heat of Fusion$$$Latent Heat of Fusion$$, is defined as the number of calories required to change from a solid to liquid without changes in temperature

    • Example: One gram of a solid into a liquid without changing temperature

• Freezing: Changing from a liquid to a solid; requires energy.

  • Requires a large amount of externally applied energy.

  • Freezing returns energy to its surroundings.

    • As it says here, requires a large amounts of external or externally applied energy, you would expect freezing to return energy to its surroundings, which it does.

• The process of freezing requires more energy than melting.

Sublimation

• The transition from a solid to a vapor without becoming a liquid.

• Example: Dry ice sublimates from its solid form into gaseous $CO_2$$$CO_2$$ without melting.

Vaporization: Boiling and Evaporation

• Vaporization: Changing a liquid to a vapor.

• Boiling: Liquid changes into a gas when vapor pressure exceeds atmospheric pressure.

  • Lower atmospheric pressure (higher altitude) affects boiling: liquids boil at lower temperatures.

  • Henry's Law: Pressure above a liquid affects solubility.

  • Dalton's Law: Atmospheric pressure affects partial pressures in the air and blood.

• Evaporation: Liquid changes into a gas at temperatures lower than its boiling point.

  • Example: Water evaporating into the atmosphere.

  • Requires heat, which comes from the air next to the water surface.

Vapor Pressure and Saturation

• In a closed container, water molecules move in and out of the water, increasing pressure above the water.

• Saturation: When the air can hold no more water (100% saturation).

• Equilibrium: When every molecule escaping into the air, another returns to the water reservoir.

• Temperature: Increasing temperature increases kinetic energy, allowing more molecules to escape the surface.

• Heating water increases kinetic energy, causing more molecules to escape the surface.

• Heated water covered: Air becomes saturated, containing more vapor molecules and exerting higher pressure.

• Temperature affects the capacity to hold molar water and water vapor pressure.

Humidity

• The discussion is now transitioning to humidity.