2: Heat and Heat Transfer

HEAT

Defined as the transfer of energy between substances of different temperatures

HEAT

Any spontaneous flow of energy from one object to another, caused by a difference in temperature between two objects

HEAT

Also referred to as energy in transit from a high temperature object to a lower temperature object

HEAT

Added energy that causes substances to rise in temperature, fuse, evaporate, expand, or undergo any of various other related changes, that flows to a body by contact with or radiation from bodies at higher temperatures, and that can be produced in a body (as by compression

HEAT

The energy associated with the random motions of the molecules, atoms, or smaller structural units of which matter is composed

Heat by James Clerk Maxwell

It is “something which may be transferred from one body to another”, as per the second law of thermodynamics

Heat by James Clerk Maxwell

It can be spoken of as a “measurable quantity”, and this treated mathematically like other measurable quantities

Heat by James Clerk Maxwell

It “can not be treated as a substance”; for it may be transformed into something which is not a substance, e.g. mechanical work

• conduction

• convection

• radiation

Three modes of heat transfer

conduction

The transfer of heat by —— in solids or fluids at rest

convection

The transfer of heat by —— in liquids or gases in a state of motion, combining conduction with fluid flow

radiation

The transfer of heat by ——-, which takes place with no material carrier

Fourier, Biot

In 1816, ——— a French mathematician analytically studied the flow of heat in metal bars and measured by ———

Biot, Fourier

— formulated the laws of conduction in 1804, and —— published a mathematical description of this phenomenon in 1822

Fourier’s Law

This is the governing law for heat conduction. It states that the rate of heat conduction through a plane layer is proportional to the temperature gradient across the layer and the heat transfer area of the layer.

• Temperature gradient must exist

• Heat flows in the direction of decreasing temperature

The fundamental conditions for heat transfer by conduction within a solid

Second Law of Thermodynamics

It states that "Heat will flow naturally from one reservoir to another at a lower temperature but not in opposite direction without assistance."

Second Law of Thermodynamics

This law established the direction of energy transport

Law of Mass Conservation

This law is used to determine the parameters of flow

Newton’s Law of Motion

These laws are used to determine fluid flow parameters

warmer to cooler

Heat energy always flows from the —— object to the —— object

Conduction

This is known as the transfer of heat from the warmer to cooler particles of a medium or of two bodies in direct contact, occurring without perceptible displacement of the particles.

Conduction

This is the most significant means of heat transfer in a solid. On a microscopic scale, it occurs as hot, rapidly moving or vibrating atoms and molecules interact with neighboring atoms and molecules, transferring some of their energy (heat) to these neighboring atoms

Convection

The transfer of heat by circulatory motion of the heated parts of a liquid or gas owing to a variation in density and the action of gravity.

Convection

This is usually the dominant form of heat transfer in liquids and gases. This is a term used to characterize the combined effects of conduction and fluid flow.

Convection

transfer occurs by the movement of hot or cold portions of the fluid together with heat transfer by conduction.

Convection

when water is heated on a stove, hot water from the bottom of the pan rises, heating the water at the top of the pan.

Free convection

the condition where gravity and buoyancy forces drive the fluid movement

Forced convection

this is where a fan, stirrer, or other means is used to move the fluid

Radiation

The process in which energy in the form of waves or particles is emitted by one body, passed through an intervening medium or space, and absorbed by another body.

Radiation

Is the only form of heat transfer that can occur in the absence of any form of medium

Radiation

it is the only means of heat transfer through a vacuum.

Radiation

Heat transfer by —— is invisible to the eye and not affected by air flow

Thermal radiation

a direct result of the movements of atoms and molecules in a material.

electromagnetic radiation

Since moving atoms and molecules are composed of charged particles (protons and electrons), their movements result in the emission of ———, which carries energy away from the surface.

heat

When Earth absorbs the sun's energy (most of which arrives in the form of visible light), the energy changes into ——-.

height

Because the atmosphere is heated from below, the temperature in the troposphere decreases with ——.

conduction and convection

Heat energy is also spread throughout Earth's atmosphere through ?

Heat

energy in the process of being transferred from one substance (or object) to another. This process occurs when there is a temperature difference between the two substances. It is always transferred from a warmer object to a cooler one

Temperature

a measurement of the average speed of the atoms and molecules that make up a substance

Conduction

the direct spread of heat from a warmer substance (in this case, land or water) to a cooler substance (the atmosphere). The heat energy transfers when molecules collide with one another.

Conduction

as a heat transfer mechanism, occurs at Earth's surface where the air is in direct contact with the surface.

convection

Heat is transferred vertically in the troposphere by ?

convection

the spread of heat in a fluid, defined as a gas or liquid in which atoms and molecules are moving relatively freely. Consequently, it can occur in the atmosphere or in bodies of water.

Convection currents

form when there is unequal heating of the atmosphere or water. As air or water warms, it expands and becomes less dense than the air or water above, and it rises. As air or water cools, its density increases and it sinks.

sensible heating

Conduction and convection work together to transfer heat. We can sense the resulting change in temperature, so these heat transfer mechanisms are known as ——-.

absorbed

Heat is ——— when water changes from a liquid to a gas (water vapor).

evaporation

The energy that is absorbed gives the molecules the extra motion that is needed to escape the surface of the liquid to become a gas. This process is known as ——-

latent heat of evaporation

the absorption of heat

released

Heat is ——— when water changes from a gas (water vapor) to a liquid. This happens as warm and humid air rises through the atmosphere into cooler temperatures.

latent heat of condensation

Cooler air cannot hold as much moisture, so the water vapor condenses. The latent or hidden heat is then released, which is why this process is known as the ———-.

Conduction

heat traveling through a solid material

Radiation

heat traveling in the form of visible and non-visible light.

Infrared radiation

is the reason why the heat of a hot burner element on a stovetop, can be felt even from across the room.

Convection

is another means for the heat from walls and ceiling to reach the occupants. Hot air naturally rises, carrying heat away from the walls and causing it to circulate throughout the structure.