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Heat Transfer Notes

Heat Transfer Types

  • Four types of heat transfer: conduction, convection, radiation, evaporation, and condensation.

  • Examples are very important.

Conduction

  • Transfer of energy directly from direct contact of hot and cold molecules.

  • Examples:

    • Pot on a stove: heat from the burner to the pot, then to the water.

    • Ventilator heater (hospital setting): plate of heater touches the plate of the device on top, transferring heat to warm the water.

    • Touching cold metal: body heat transfers to the metal, making it feel cold.

  • High thermal conductivity metals quickly draw heat away from the skin, creating a feeling of cold.

Convection

  • Involves fluid molecules at different temperatures mixing.

  • Examples:

    • Infant incubator: air is warmed in one location and circulated to carry heat.

    • Forced air heating in houses: air molecules mix and warm the air.

    • Convective oven: molecules heat up and circulate around the food.

Radiation

  • Heat transfer occurs without direct physical contact.

  • Examples:

    • Sunlight: radiant heat warms objects without direct contact.

    • Radiant heat warmer for babies: heat comes from above without direct contact.

    • Kerosene or electric heater: heat radiates across the room.

Vaporization

  • According to the first law of thermodynamics, energy must come from the surroundings.

  • Bulk storage of oxygen: Compressed liquid oxygen is exposed to ambient temperatures and vaporized into gaseous form.

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

  • Example: Sweating - liquid sweat evaporates and cools the skin.

Condensation
  • The opposite of evaporation; heat is given back to the surrounding air, warming it.

  • Example: Refrigerator - Food cools as energy is passed through the walls of the refrigerator into the pipes, warming the refrigerant which is carried into the atmosphere.

Heat Transfer in Our Environment

  • Heat always moves from hot objects to colder ones.

  • Materials impact heat transfer.

    • Conduction: Good conductors (copper, silver, iron, steel) vs poor conductors (wood, styrofoam, paper, air).

    • Convection: Up and down movement of gases and liquids; warmer rises, cooler falls, creating a convection current.

    • Radiation: Electromagnetic waves transfer heat to objects.

Melting and Freezing

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

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

    • 11$$1$$ gram of solid into a liquid without changing temperature.

  • Freezing: Changing from a liquid to a solid

  • Requires a large amounts of externally applied energy

  • Freezing returns energy to its surroundings.

Sublimation

  • Transition from a solid to a vapor without becoming a liquid.

  • Example: Dry ice - it sublimates from its solid form into a gas CO2CO_2$$CO_2$$ without first melting and becoming a liquid.

Vaporization (Liquid to Vapor)

  • Two forms:

    • Boiling

    • Evaporation

Boiling
  • A liquid can also change into a gas at temperatures lower than its boiling point and called evaporation.

  • Liquid is the temp at which the vapor pressure exceeds atmospheric pressure

    • Boiling occurs earlier with lower atmospheric pressure and at lower temperatures.

Evaporation
  • A liquid can also change into a gas at temperatures lower than its boiling point through a process called evaporation.

  • Water is a good example of that.

Humidity

  • The amount of water vapor in the atmosphere involving the kinetic energy of molecules in the air.

Absolute Humidity
  • The actual weight or amount or weight of the water vapor in the air.

  • Can be measured by weighing the water vapor extracted from air.

  • Fully saturated or partially saturated

  • Air is fully saturated with the water vapor at 37 degrees Celsius and 760 millimeters of mercury, which is 43.843.8$$43.8$$ milligrams per liter.

  • Percent saturated *$$*$$ water vapor content (43.843.8$$43.8$$).

Relative Humidity
  • Ratio of the actual water vapor content to its saturated capacity at a given temperature.

  • RH=ContentCapacity100RH = \frac{Content}{Capacity} * 100$$RH = \frac{Content}{Capacity} * 100$$


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Heat Transfer Notes

Heat Transfer Types

  • Four types of heat transfer: conduction, convection, radiation, evaporation, and condensation.

  • Examples are very important.

Conduction

  • Transfer of energy directly from direct contact of hot and cold molecules.

  • Examples:

    • Pot on a stove: heat from the burner to the pot, then to the water.

    • Ventilator heater (hospital setting): plate of heater touches the plate of the device on top, transferring heat to warm the water.

    • Touching cold metal: body heat transfers to the metal, making it feel cold.

  • High thermal conductivity metals quickly draw heat away from the skin, creating a feeling of cold.

Convection

  • Involves fluid molecules at different temperatures mixing.

  • Examples:

    • Infant incubator: air is warmed in one location and circulated to carry heat.

    • Forced air heating in houses: air molecules mix and warm the air.

    • Convective oven: molecules heat up and circulate around the food.

Radiation

  • Heat transfer occurs without direct physical contact.

  • Examples:

    • Sunlight: radiant heat warms objects without direct contact.

    • Radiant heat warmer for babies: heat comes from above without direct contact.

    • Kerosene or electric heater: heat radiates across the room.

Vaporization

  • According to the first law of thermodynamics, energy must come from the surroundings.

  • Bulk storage of oxygen: Compressed liquid oxygen is exposed to ambient temperatures and vaporized into gaseous form.

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

  • Example: Sweating - liquid sweat evaporates and cools the skin.

Condensation
  • The opposite of evaporation; heat is given back to the surrounding air, warming it.

  • Example: Refrigerator - Food cools as energy is passed through the walls of the refrigerator into the pipes, warming the refrigerant which is carried into the atmosphere.

Heat Transfer in Our Environment

  • Heat always moves from hot objects to colder ones.

  • Materials impact heat transfer.

    • Conduction: Good conductors (copper, silver, iron, steel) vs poor conductors (wood, styrofoam, paper, air).

    • Convection: Up and down movement of gases and liquids; warmer rises, cooler falls, creating a convection current.

    • Radiation: Electromagnetic waves transfer heat to objects.

Melting and Freezing

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

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

    • 11 gram of solid into a liquid without changing temperature.

  • Freezing: Changing from a liquid to a solid

  • Requires a large amounts of externally applied energy

  • Freezing returns energy to its surroundings.

Sublimation

  • Transition from a solid to a vapor without becoming a liquid.

  • Example: Dry ice - it sublimates from its solid form into a gas CO2CO_2 without first melting and becoming a liquid.

Vaporization (Liquid to Vapor)

  • Two forms:

    • Boiling

    • Evaporation

Boiling
  • A liquid can also change into a gas at temperatures lower than its boiling point and called evaporation.

  • Liquid is the temp at which the vapor pressure exceeds atmospheric pressure

    • Boiling occurs earlier with lower atmospheric pressure and at lower temperatures.

Evaporation
  • A liquid can also change into a gas at temperatures lower than its boiling point through a process called evaporation.

  • Water is a good example of that.

Humidity

  • The amount of water vapor in the atmosphere involving the kinetic energy of molecules in the air.

Absolute Humidity
  • The actual weight or amount or weight of the water vapor in the air.

  • Can be measured by weighing the water vapor extracted from air.

  • Fully saturated or partially saturated

  • Air is fully saturated with the water vapor at 37 degrees Celsius and 760 millimeters of mercury, which is 43.843.8 milligrams per liter.

  • Percent saturated * water vapor content (43.843.8).

Relative Humidity
  • Ratio of the actual water vapor content to its saturated capacity at a given temperature.

  • RH=ContentCapacity100RH = \frac{Content}{Capacity} * 100