LATENT HEAT

Structure of atom:

1)- Nucleus: composed of proton (positive charge) and neutron (neutral). → atomic number = nb of proton → atomic mass = nb of protons + nb of neutrons2)- Outer structure: electrons orbits (shells) 7 shells at least: K → 2 electrons. L → 8 electrons. M → 18 electrons. …. etc

Number of electrons in shell = 2N2 ( N = the nb of shell )

STATES OF MATTER: Matter can be solid, liquid or gaseous, e.g. water ( ice, water or stream)The molecule of substance are usually influenced by at least two forces a cohesive force which attract the molecules of the substance to one another, and a kinetic force the force of mvt of the molecules which is dependent on the thermal energy contained by the mass of molecules.In the solid state there is a strong cohesion force holding the molecules in a rigid lattice formation, the kinetic energy produces only a vibration of the molecules about a mean position.As more energy (heat) is added to the solid the kinetic energy ↗and the mvt of molecules ↗, the rigid structure collaps so that the liquid state is reach ( the molecule are in contact but can move freely past one another ), it maintains its volume but takes on the shape of it’s container. If even more heat is applied→ the kinetic energy became so much > the cohesive force that the molecules fly apart to form a gas. The gas molecules are continually colliding with one another and with the walls of the container, so that the gas exerts pressure, which increases with any further rise in temperature.

Latent heat:Def.:The quantity of heat absorbed or released by a substance undergoing a change of state. Also called heat of transformation. In the case of water, 1 gram of ice at 0⁰C requires 336 joules of energy to convert it to 1g of water at 0⁰C (latent heat of fusion). As matter change from higher kinetic energy to lower one (e.g. stream to water) this latent energy is released.The concept of latent energy has practical applications:Ice milting on the skin takes considerable energy(heat) from the skin, thus cooling it , whereas paraffin wax solidifying on the skin gives out considerable heat to the skin, thus warming it.

TRANSMISION OF HEAT1) Conduction: if one end of a solid metal is heated, the

energy added cause an increase vibration of molecules. This vibration is transmitted to adjacent molecules and in this way heat is conducted along the bar, some materials are good conductors of heat (metal) others are not (wood).

2) Convection: it take place on a liquid or a gas. If one part of the fluid is heated, the kinetic energy of the molecules in that part is ↗ they move further apart and this part becomes less dens → it rises, displacing the more dense fluid above, which descends to take it’s place. the current so produced called convection current.

TRANSMISION OF HEAT .. cont

3) Radiation: heat may be transmitted by infra-red electromagnetic radiation, the heating of certain atoms cause an electron to move a higher electron shell. As it returns to its normal shell, the energy is released as a pulse of infra-red electromagnetic energy.

PHYSICAL EFFECTS OF HEAT:

1) Expansion: is a result of increased kinetic energy producing a greater vibration of molecules, which thus move further apart.

2) Change of state: solid ↔ liquid ↔ gas.3) Acceleration of chemical action: Van’t Hoff’s law says

that any chemical action capable of being accelerated is accelerated by a rise in temperature. The converse is also true.

4) Production of a potential difference: if the junction of dissimilar metals ( bismuth & antimony) is heated, a potential difference is produced between their free ends(the thermocouple principle).

PHYSICAL EFFECTS OF HEAT .. cont:5) Production of electromagnetic waves: when energy is

added to the atom→ an e- move out to a higher shell ( exited stage ) when this e- returns to its normal level energy is released as a pulse of electromagnetic energy ( a photon).

6) Thermionic emission: the heating of molecules of some materials, e.g. tungsten, may cause such molecular agitation that some e- leave their atoms and may even break free of the surface of the metal. This leave a positive charge which tend to attract the negative e- back. However a point is reached where the rate of loss of electrons equals the rate of return and a cloud of electrons then exists as a space charge around the object. The process is called thermionic emission and is the principle upon which electric valves work.

7) Reduce viscosity of fluids: heat increase the kinetic mvt of molecules & reduces their cohesive mutual attraction.

Methods of heating the tissues:o ELECTRIC HEATING PADS: (non latent heat method) There are various sizes, the temperature may be regulated by a series of resistors to the required level, the heating of the tissues is by conduction so that the effect is merely superficial, but this method is easy & comfortable for the pt. o PARAFFIN WAX (latent heat):Wax baths are available in many variation of size and shape. The milted wax needs to be maintained at 40 - 44⁰C , so thermostatic control is essential. The temperature of the wax must be checked before ttt is given. this method is the most convenient way of applying conducted heat to the extremities as the wax solidifies from its molten state it release its energy of latent heat

o PARAFFIN WAX:Method: the part to be treated must be clean and free from cuts, rashes or other damages. Position the pt. According to the part to be treated and type of wax bath selected, and instruct the pt. To dip the part in and out of the bath until a thick cat of wax sets on the skin ( 4 to 6 immersion. The wax gives off heat slowly due to its low thermal conductivity, In order to retain the wax wrap the part in a layer of plastic sheet or greaseproof paper.The ttt is usually given for about 20 minutes. After this time remove the towel and the wax glove, inspect and dry the part.

Effect & Indications:During application of wax there is marked increase in the T⁰ of the skin, and in lesser degree that of the other superficial tissues. the T⁰ obviously drops rapidly after the remove of the wax. Circulatory effects: there is stimulation of superficial capillaries and arterioles, causing local hyperemia and reflex vasodilatation may be due to the action of a vasodilator (bradykinin) formed as the result of sweet gland activity.Effect on sensory nerves: mild heating appears to have a sedative effect on the sensory nerve endings. As wax can be molded round the contours of the hands and feet, it is of value in treating rheumatoid arthritis or degenerative joint disease reducing pain and ms. Spasm.Effect on the skin: the skin is moist & pliable following wax application, which can therefore help to soften adhesions and scars in the skin prior to mobilizing and stretching procedures.

ContraIndications:a) Impaired skin sensation.b)Skin conditions.c) Allergic rash.d)Open wounds.e)Defective arterial blood supply

(including DVT & varicose veins).