Heat Transfer and Vaporization Notes

Heat Transfer

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.

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.

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.

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 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.

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, 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.

The transition from a solid to a vapor without becoming a liquid.
Example: Dry ice sublimates from its solid form into gaseous CO_2 without melting.

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.

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.