# Chapter 22 Heat Transfer.

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Chapter 22 Heat Transfer

Heat transfer Thermal energy always flows from hot to cold until an equilibrium is reached. The amount of heat energy that matter contains (joules) can be represented with the equation: Q = m Δ T CP The rate at which heat is transferred between materials depends on the ΔT (temperature difference). The greater the ΔT the greater the rate of heater transfer. Another factor that affects heat transfer is thermal conductivity. Materials that conduct heat well are known as conductors. Materials that do not conduct heat well are insulators.

Have you ever noticed that a carpet feels warmer to bare feet than a tile floor, in the same room at the same temperature? The answer is because the tile is a much better conductor of heat than carpet. This causes heat to be transferred more rapidly from your feet to the tile. This transfer of heat occurs by conduction.

Conduction: Heat transfer that occurs between materials that are in direct contact is called conduction.

Why use insulation in houses?
In the summer, air outside a house is hot relative to the air inside. Insulation slows the rate at which the air inside heats up, keeping your house cool in the summer. In the winter, air outside a house is colder relative to the air inside the house. Insulation again slows the rate at which heat is lost from inside a house to the outside.

Convection: Heat transfer within fluids is called convection. Both gases and liquids are considered fluids because the molecules of both are able to freely move past one another. When you heat water or air, convective currents form which move the fluid and transfer heat.

Radiation: Any energy than is transmitted via electromagnetic waves is called radiant energy. This includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.

Newton’s Law of Cooling:
The rate of cooling of an object depends on how much hotter the object is than the surroundings. Rate of cooling ~ Δ T

Run-away greenhouse effect
Venus: Surface Temperature ~ 494 C Surface air Pressure ~ 90X that of earth