Heat and Heat Transfer

PHYSICS II

Technological University of the Philippines-Taguig Electrical Engineering Department

What is HEAT?

• Form of energy and measured in

JOULES

• Particles move about more and take

up more room if heated – this is why

things expand if heated

• It is also why substances change

from: solids liquids gases

when heated

In physics, especially in calorimetry, and

in meteorology, the concepts of

latent

heat

and of

sensible heat

are used.

– Latent heat is associated with phase

changes, while

– Sensible heat is associated with

temperature change.

• The temperature of an object tells

us how HOT it is measured in

degrees Celsius - °C

• Heat is related to temperature but

the two are not the same.

Temperature is a measure of

the kinetic energy of the particles.

Temperature does not depend on

the mass of the substance. The amount

of heat energy which a substance has does depend on its mass.

• If an object has become hotter,

it means that it has gained heat energy. • If an object cools down, it means it has

lost energy

Figure Latent heat exchanges of energy involved with the phase changes of water.

-it is the energy needed to change a substance to a higher state of matter. This same energy is released from the substance when the change of state (or

phase) is reversed.

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),

m is the mass of the substance (in kg or in lb), and

L is the specific latent heat for a particular substance (kJ-kg_m−1 _{or in BTU-lb}

m−1), either Lf for fusion

(melting or freezing), or L_v for vaporization (boiling or condensing.

A specific latent heat (L) expresses the amount of energy in form of heat (Q) required to completely affect a phase change of a unit of mass (m),

usually 1kg, of a substance as an intensive property:

Intensive properties are material characteristics and are not dependent on the size or extent of the sample.

Sensible heat is heat exchanged by a body that has as its sole effect a change of temperature. The term is used in contrast to a latent heat, which is the

amount of heat exchanged that is hidden, meaning it occurs without change of temperature.

The sensible heat of a thermodynamic

process may be calculated as the product of the body's mass (m) with its specific heat

• The specific heat is the

amount of heat per unit mass required to raise the

temperature by one degree Celsius.

• The specific heat of water is 1 calorie/gram °C = 4.186

joule/gram °C which is higher than any other common

substance. As a result, water plays a very important role in temperature regulation.

Substance C (J/g

_C)

Illustrative samples:

• 1. Calculate the amount of heat needed to increase the temperature of 250g of water from 20o_{C to 46}o_C.

• 2. The temperature of a piece of Metal X with a mass of 95.4g increases from 25.0°C to

48.0°C as the metal absorbs 849 J of heat. What is the specific heat of Metal X?

Answer

q = m x C x ΔT = 250g x 4.18J/go_{C x 26}o_C q = 37,620J or 38kJ

Figure: Water transformation as temperature changes from -50 °C to 150 °C

HEAT TRANSFER

The transfer of heat is normally from a high temperature object to a lower temperature object. Heat transfer changes the internal

energy of both systems involved according to the First Law of Thermodynamics.

Heat

Transfer

Conduction

Convection

• Heat is transferred through a material

by being passed from one particle to

the next

• Particles at the warm end move faster

and this then causes the next particles

to move faster and so on.

• In this way heat in an object travels

from:

Occurs by the particles hitting each other and so energy is transferred. Can happen in solids, liquids and gases,

Happens best in solids-particles very close together Conduction does not occur very quickly in liquids or gases

• Conductors are substances that

transfer thermal energy well.

Materials that conduct heat quickly

are called conductors

Iron skillet

Cookie sheet Copper pipes

Coils on stove Curling iron

• Insulators are materials that

conduct heat slowly or poorly.

Flannel PJ’s

Fiberglass

Oven Mitt

Ceramic bowl

• For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:

Heat conduction Q/ Time = (Thermal conductivity) x (Area) x (Thot - Tcold)/Thickness

Convection is the transfer of thermal

energy by the movement of liquid or gas.

Water on the bottom of the pan is heated by conduction and

becomes less dense and therefore rises. At the surface it begins to cool and move closer together and sink again. This circular motion is called a convection current.

Two types of convective heat transfer may be distinguished:

• Free or natural convection: when fluid motion is caused by buoyancy forces that result from the density variations due to variations of temperature in the fluid. Familiar examples are the upward flow of air due to a fire or hot object and the circulation of water in a pot that is heated from below.

• Forced convection: when a fluid is forced to flow over the surface by an external source such as fans, by stirring, and pumps, creating an artificially induced convection current.

where

q = heat transferred per unit time (W)

A = heat transfer area of the surface (m

₎

h

= convective heat transfer coefficient of

the process (W/m

_{K or W/m}

_C)

ΔT = temperature difference between the

surface and the bulk fluid (K or

_C)

Medium Heat Transfer Coefficient h (W/m2_.K) Air (natural convection) 5-25 Air/superheated steam (forced convection) 20-300 Oil (forced convection) 60-1800 Water (forced convection) 300-6000 Water (boiling) 3000-60,000 Steam (condensing) 6000-120,000

Table: The following table shows some typical values for the convective heat transfer coefficient:

• Radiation is the transfer of energy through matter or space as electromagnetic waves, such as

visible light and infrared waves.

Heat radiation is also known as INFRA-RED RADIATION All objects that are hotter than their surroundings give out heat as infra-red radiation.

Heat transfer by radiation does not need particles to occur and is the only way energy can be transferred across empty space

The relationship governing radiation from hot objects is called

the Stefan-Boltzmann law:

The energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature and is given by:

• Hotter objects emit (give out) heat

• Different surfaces emit heat at different speeds • A dull black surfaces loses energy more quickly

– it is a good radiator

• A bright shiny or white surface is a poor radiator • Marathon runners need to keep warm at the

end of races, covering in shiny blankets reduces radiation and therefore heat loss.

Bright shiny can Poor radiator

Dull black can Good Radiator

Cooler objects absorb (take in) heat

Substances absorb heat at different speeds Dull, black surfaces absorb heat quickly

Bright, shiny surfaces absorb heat slowly

In hot countries, people wear bright white clothes and paint their houses white to reduce absorption of energy from the sun.

Petrol storage tanks sprayed silver to reflect sun’s rays

Absorbers

Shiny, bright can

Poor absorber

Dull black can Good absorber