HEAT TRANSFER AND CHANGE OF STATE

By: Krissia Mae Doniza

HEAT TRANSFER

Heat Transfer, in physics, process by which energy in the form of heat is exchanged between bodies or parts of the same body at different temperatures. Heat is generally transferred by convection, radiation, or conduction.

Conduction

Occurs when two object touch If the temperature at one end of a

metal rod is raised by heating, heat is conducted to the colder end.

Materials such as gold, silver, and copper have high thermal conductivities and conduct heat readily, this are called conductors

materials such as glass and asbestos have values of thermal conductivity hundreds and thousands of times smaller, conduct heat poorly, are called insulator.

Conduction

Convection

Occurs when the heated substances moves itself.

If a liquid or gas is heated, its mass per unit volume generally decreases. If the liquid or gas is in a gravitational field, the hotter, lighter fluid rises while the colder, heavier fluid sinks. This kind of motion, due solely to no uniformity of fluid temperature in the presence of a gravitational field is an example of nature convection.

Radiation

Occur in empty space, such as in outer space

Any energy transferred by this method is radiant energy

When the atoms and molecules are no longer closely associated with one another, all bonds have been broken and the liquid has become a gas. In gaseous state, each atom or molecule is no longer surrounded by several neighbors but occasionally bumps into another atoms or molecule.

CHANGE OF STATE

Previous heat equation:

Q = mcΔT Covers the temperature changes,

but we also need to be able to compute the heat needed to change from one state to another.

CHANGE OF STATE

Qf=m Hf

In melting, it takes amount of heat to change from solid to a liquid

Qf = amount of heat in kcal,

m = mass of solid in kg,

Hf = the heat of fusion in kcal/,

for water Hf is 80 kcal/kg

Melting and Heat of Fusion

Qv=mHv

In evaporating, it takes an amount of heat

To change from a liquid to a gas

Qv=amount of liquid in kg

Hv=the heat of vaporazation in kcal/kg.for water is 540 kcal/kg

Evaporation and Heat of Vaporization

The change of solid into gas The reason that solid carbon dioxide,

CO2, is used to keep ice cream frozen while being shipped.

Ex. Dry ice solid CO2 change directly to gas.

Sublimation

The amount of heat Qtotal needed to turn a mass, m of ice at some initial temperature Ti into gas at a final temperature Tf can be computed by the following equation:

Qtotal = mcice (Tm-Ti) + mHf + mcwater (Tv-Tm) + mHv + mcwv (Tf-Tv)

Tm= melting temperature of ice(o°C)

cice, cwater and cwv=are especifec heats of ice, water and water vapor, in kcal/kg°C

Tv=boiling temperature of water,100°C

Equation for Change of State

Or

Qtotal = mcice (Tm-Ti) + mHf + mcwater (Tv-Tm) + mHv + mcsteam (Tf-Tv)

This equation is used to determine the amount of heat needed to convert a material from one state to another.

Increasing temperature Qtotal is beat gained

Decreasing temperature Qtotal of heat lost

Equation for Change of State