lec. 4

TRANSMISSION OF HEAT.

Heat energy can be transmitted by three methods

1. Conduction

2. Convection

3. Radiation.


1. CONDUCTION.

Heat is transmitted by conduction when it passes from the hotter to the colder parts of the medium material without any movement of the medium itself and all intermediate parts of the material being warmed in the process.
e.g. Metal rod held at one end in a fire after a period of time the top end becomes hot and all intermediate parts also are warm.




A metal contains some free (conduction ) electrons which are free to move through the material.
When an metal is heated these free electrons gain kinetic energy and their speed is increased.
The higher energy electrons drift towards the cooler parts of the material there by increasing the average kinetic energy and heating.

THERMAL CONDUCTIVITY.

Thermal conductivity is defined as the Heat energy flowing through a piece of material per second, which is 1m in length, 1 m2 in cross-sectional area and has a temperature difference of 1oC between its ends.

Symbol k




Where

Q = Heat energy flowing through the material

A = Area of the material through which
the heat flows

T2 = Temperature at face 2 i.e the higher temp

T1 = Temperature at face 1 the lower temp

t = time taken

x = length of the material through which
the heat flows

k = Thermal conductivity of the material
S.I. Units of k Rearranging the equation making k the subject gives



2. CONVECTION.

Convection in the most general terms refers to the movement of molecules within fluids (i.e. liquids, gases).

In fluids, convective heat transfer takes place through both diffusion – the random Brownian motion of individual particles in the fluid – and by advection, in which matter or heat is transported by the larger-scale motion of currents in the fluid.

Density is defined as mass per unit Volume. As the fluid is heated the Volume increases, the mass is constant therefore the density decreases. Hot fluids are less dense than cold fluids and will rise therefore convection currents (circular currents or movement within a fluid) due to different densities of the hotter and cooler parts are set up
When heat is transferred by the circulation of fluids due to buoyancy from the density changes induced by heating itself, then the process is known as free or natural convective heat transfer.

3. Radiation.

Thermal radiation is electromagnetic radiation emitted from the surface of an object which is due to the object s temperature. Infrared radiation from a common household radiator or electric heater is an example of thermal radiation. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation.
Any object that is hot gives off light known as Thermal Radiation (or sometimes Blackbody Radiation ). The hotter an object is, the more light it emits. And, as the temperature of the object increase, it emits most of its light at higher and higher energies. (Higher energy light means shorter wavelength light.) The relationship between the amount of light emitted, its wavelength and its temperature is an equation known as the Planck Law. For a hot object at a given temperature, T, the equation gives the amount of light emitted at each wavelength.
The rate at which a body radiates heat depends on
• Temperature of the body
• Surface area of the body
• Nature of the surface.

Stefan–Boltzmann law, states that the total energy radiated per unit surface area of a body in unit time is directly proportional to the fourth power of the body s temperature in Kelvin T (also called absolute temperature):


Where
R = Rate of energy emitted in Watts
? Stefans constant = 5.67x10-8 W/m2K4

? emissivity of the material

T = Temperature of the surface in Kelvin

A = Surface area in m2

A black body is an object that absorbs all electromagnetic radiation that falls on it.
The emissivity of a material (usually written ?) is a measure of a material s ability to radiate absorbed energy. A true black body would have an ? = 1 while any real object would have ? < 1. Emissivity is a dimensionless quantity (does not have units).
In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. Highly polished silver has an emissivity of about 0.02.


Biological Effects of Radiation:
Biological Effects of X Rays the responses of living systems to ionizing radiation are complex and subtle. X rays are only one component of the radiations to which people are exposed. The interaction of radiation with matter depends upon the absorption of energy, and the extent of biological damage depends upon the energy absorbed per gram in the organism.
Absorbed radiation produces physical changes in the cell. Some specific cellular changes that may occur are break-up of molecules, production of free radicals, inactivation of enzymes, change of deoxyribonucleic acid synthesis, and break-up of chromosomes. In addition gross physical changes may occur. Among these are increase of the viscosity of cellular fluids, increase of permeability of cellular membranes, swelling, and death.
In many cases the effects are known, but the exact steps that occur between the absorption of energy and final results are not known. Also the absorption of a constant amount of energy does not produce the same effect in all types of cells. As a sort of rule of thumb, cells that are growing rapidly are most susceptible to radiation. The most sensitive cells in the body are the bone marrow, lymphoid, and epithelial tissues. Cells of a fetus are very sensitive to radiation. The cells of bone, muscle, and blood vessels are less sensitive, and the nerve cells are the least sensitive, most resistant, to radiation.

Ionizing radiation produced gene mutation.
Only the mutations which are produced in a gene, cell can be transmitted to its progeny. These are known as genetic effects of radiation. The genetic changes in an offspring may be from minor changes to severe handicaps. The frequency of mutations induced by radiation depends upon both the dosage and the dose rate. It has apparently become noticeable with the dosage of the order of a 20-50 rads, and then increases linearly with total dosage. Also the frequency and type of mutation depends upon time of mating relative to the time of exposure.

Ionizing radiation in very high levels is known to increase the incidence of cancer, birth anomalies, erythema, and other problems.
In low levels, these effects are either very, very small compared to natural incidences or non-existent depending on the biological model used for estimating the potential risk. Regulatory agencies assume that radiation effects observed in people exposed to very high doses can be linearly extrapolated to background levels. This model is called the “linear no-threshold theory” because the modeled effects are linear with dose and no threshold is assumed. The linear model most likely over-estimates harmful biological effects because it does not fully account for the body’s ability to repair damage.
Medical Exposures. The value of 60 mrem/year from medical and dental exposures is a population average. The dose to individual patients varies depending on the medical exam. The following are typical doses from routine radiological procedures. The Entrance Skin Exposure is dose to the skin closest to the x-ray tube. The Marrow Dose is the average dose to the bone marrow, a primary organ of concern.
Typical Doses from Diagnostic X-ray Procedures

X-ray Exams Entrance Skin Exposure marrow Exposure
Chest 10 2
Skull 200 10
Lumbar spine 300 60
Abdomen 400 30
CT (pelvis) 4000 100


Ionizing radiation (e.g., beta particles, alpha particles, neutrons, gamma rays) has sufficient energy to ionize surrounding atoms. Ionization is a process of removing one or more electrons from an atom. The ionization process can lead to biological damage when DNA, the controlling molecule of a cell, is affected. When damaged, the DNA molecule can repair itself, lead to the cell death, or mutate. The most likely outcome for low doses is repair. Mutations can lead to cancer. Cancer caused by radiation is no different than that caused by other carcinogens. Radiation is not the only agent that can cause mutations in DNA. Some other mutagens are chemicals, heat, and ultraviolet light. One parameter of DNA damage is single strand breaks to the DNA molecule. In every cell, every day, about 150,000 single strand breaks naturally occur from chemical and physical processes, primarily related to oxygen and thermal effects. One rem of radiation would add an estimated 20 single strand breaks - an inconsequential amount.


DIRECT (33%) e- interacts directly with DNA
INDIRECT (67%) e- interacts with water creating radicals which interact chemically with DNA




Single strand break
– repairable from mirror half of DNA
• Double strand break
– less repairable
– chromosome aberration
– distant breaks repaired as if 2 single breaks