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 (kJkgm−1 or in BTU-lbm−1), either Lf for fusion (melting or freezing), or Lv 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.

Table: Specific latent heat of different substances

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 capacity (c) and the change in temperature (T):

Substance

C (J/g oC)

Air

1.01

Aluminum

0.902

Copper

0.385

Gold Iron

0.129 0.450

Mercury

0.140

NaCl

0.864

Ice

2.03

Water

4.18

• 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.

Illustrative samples: • 1. Calculate the amount of heat needed to increase the temperature of 250g of water from 20oC to 46oC. Answer q = m x C x ΔT = 250g x 4.18J/goC x 26oC q = 37,620J or 38kJ • 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: 849 J /(95.4g x 23.0°C)0.387 J/g°C

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

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

Heat Transfer Radiation

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: the HOT end

the cold end

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

All metals are good conductors of heat

Curling iron

Iron skillet

• Conductors are substances that transfer thermal energy well. Materials that conduct heat quickly are called conductors Coils on stove

Cookie sheet

Copper pipes

Oven Mitt Flannel PJ’s •

Insulators are materials that conduct heat slowly or poorly. Plastic spatula

Ceramic bowl Fiberglass

• 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 (mo) hc= convective heat transfer coefficient of the process (W/m2K or W/m2oC) ΔT = temperature difference between the surface and the bulk fluid (K or oC)

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

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

• 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 infrared 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 Dull,

absorb heat at different speeds

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