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Basic Physics in Anesthesia
Davis & Kenny 5th edition Ch 10 Heat Capacity and Latent Heat
22
Medical
Graduate
02/24/2009

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Term
SPECIFIC HEAT CAPACITY
Definition

a physical property of a substance

 

the amount of heat required to raise the temperature of 1 kg of a substance by 1 K

 

SI unit J kg-1 K-1

Term
RELATIONSHIP BETWEEN HEAT AND TEMPERATURE
Definition

Equation:

Q = cm∆T

Q = Heat added

C = specific heat

M = mass

∆ = change in temp

Term
COMMON SPECIFIC HEATS
Definition

H2 O  4.18 KJ

 

Blood 3.6 KJ

Term
Calculate the amount of heat needed to increase the temperature of 250 g of water from 20 degrees C to 56 degrees C?
Definition

 

q = m x Cg x (Tf - Ti)
m = 250g
Cg = 4.18 J oC-1 g-1
Tf = 56oC
Ti = 20oC

 

q = 250 x 4.18 x (56 - 20)
q = 250 x 4.18 x 36
q = 37 620 J = 38 kJ

Term
LATENT HEAT
Definition
Amount of energy in the form of heat released or absorbed by  substance during a change of state (i.e. solid, liquid, or gas), or while in transition
Term
EQUATION FOR LATENT HEAT
Definition

Q = mL

Q =amount of energy released or absorbed during the change of phase of the substance (in joules

m =mass of the substance

L =specific latent heat for a particular substance (J kg-1 )

 

Specific latent heat is found when energy is divided by mass.

Term
LATENT HEAT OF FUSION
Definition

The amount of heat required to cause a change of phase from solid to liquid, or the heat released when the phase change is from liquid to solid

 

Temperature at which it occurs is called the melting point

 

The units of heat of fusion are usually expressed as:

* kilojoules per mole (the SI units)

* calories per gram 

* British thermal units per pound or BTU per pound-mole

Term
LATENT HEAT OF VAPORIZATION
Definition

The heat required to convert a substance from the liquid to the gas

 

The amount of heat needed to turn one gram of a liquid into a vapor, without a rise in temperature of the liquid

 

Often measured at the normal boiling point of a substance

 

Diminishes with increasing temperature and it vanishes completely at the critical temperature

 

Above the critical temperature the liquid and vapor phases don't coexist anymore

Term
HEAT CAPACITY
Definition

is defined as the amount of heat required to raise the temperature of a given object by 1 Kelvin

 

SI unit of heat capacity J K-1

 

Can be applied to an object or substance as a whole.

 

Is the product of the specific heat capacity and the mass.

Term
MEASURING THE TOTAL HEAT CAPACITY OF A PERSON
Definition

(Specific heat capacity of tissues =3.5 kj kg-1 ºC-1)

 

TOTAL HEAT CAPACITY = SPECIFIC HEAT CAPACITY x MASS

 

For a 70kg patient the toal heat capacity would by 245kJ ºC-1

Term
SPECIFIC HEAT OF AIR
Definition
1.01 kj kg-1 ºC-1 at constant pressure.
Term
SPECIFIC HEAT OF GASES
Definition

ARE LOW DUE TO GASES' LOW DENSITY

 

This is important in anesthesia.

 

The specific heat of gas is measured in terms of a volume, it is the heat required to raise 1 liter of gas through 1K (1C).

 

Extremely small quantity of heat is required or lost when the temperature changes in a small volume of a gas.

Term
TOTAL HEAT CAPACITY AND VAPORIZERS
Definition
Its total heat capacity can be calculated by adding the capacities of the components.
Term
TEMPERATURE OF A GAS FROM A VAPORIZER DELIVERED TO A PATIENT
Definition
The gases at source may be quite cold if supplied directly from a vaporizer. As they pass through the anesthetic breathing system, only a small quantity of heat is transferred, but this is sufficient to ensure that the gas is closer to ambient temp by the time it arrives at the patient.
Term
Specific latent heat
Definition

the heat required to convert 1 kg of a substance from one phase to another at a given temp

 

SI unit of specific latent heat J kg-1

 

The lower the temp the more latent heat is needed to vaporize a substance.

Term
LATENT HEAT OF VAPORIZATION FOR NITROUS OXIDE
Definition

The temperature at which the latent heat of vaporization becomes ZERO corresponds to its critical temperature of 36.5 ºC.

 

At this temp, N2O changes spontaneously from liquid to vapour without the supply of any external energy.

 

 

Above this critical temp, N2O cannot exist as a liquid

Term
CLINICAL EXAMPLE USING LATENT HEAT OF VAPORIZATION WITH EYTHYL CHLORIDE
Definition

Stored as a liquid under pressure in glass ampules with a tap so that a fine jet of the substance can be directed onto the skin.  

 

Vaporization causes pronounced cooling of the skin, thus impairing conduction in the sensory nerves and providing enough analgesia for minor surgical procedures.

Term
CLINICAL EXAMPLE USING LATENT HEAT OF VAPORIZATION WITH VOLATILE ANASTHETICS
Definition

This principle is important to the design of vaporizers.

 

As gas passes over liquid anesthetic in a vaporizer, the liquid vaporizes, taking latent heat of vaporization from the remaining fluid and from the surrounding vaporizer walls. Thus, the temp of the remaining anesthetic agent and of the vaporizer  walls falls.  But a fall in temperature of the anasthetic in the vaporizer renders it less volatile, lowering its saturated vapour pressure and so reducing the amount of anesthetic vaporized.

 

 

Term
CLINICAL EXAMPLE USING LATENT HEAT OF VAPORIZATION WITH NITROUS OXIDE
Definition

If a nitrous oxide cylinder (stored as a liquid) is allowed to empty rapidly. Latent heat is required to convert liquid to gas. Thus temp of the cylinder falls and water vapour from the air may condense or freeze on the outside of the cylinder.  Vapor pressure falls rapidly inside the cylinder, and pressure gauge gives a low reading.  It recovers after cylinder is turned off.

Term
CLINICAL EXAMPLE USING LATENT HEAT OF VAPORIZATION WITH CARBON DIOXIDE AND CYCLOPROPANE
Definition
These are stored in liquid form, but since the rate of use is slow, cooling of the cylinder is not noticed.
Term
CLINICAL EXAMPLE USING LATENT HEAT OF VAPORIZATION WITH OXYGEN
Definition

It is more economical to store oxygen as a liquid. Stored at -160 ºC

 

Critical temp is -119 ºC, it cannot exist as a liquid above this temp.

 

Stored at -160 C. So it has to pass through a superheated coil. If O2 flows at a fast rate, temp falls due to the removal of latent heat and its vapour pressure falls. Supplementary heat is needed and provided by a pressure raising vaporizer.  A control valve controls the flow of liquid O2 to the pressure-rasking vaporizor. In the vaporizer, the oxygen is warmed and vaporized to the pipeline pressure.

 

 

Term
LATENT HEAT AND HEAT LOSS FROM THE PATIENT
Definition

In anesthesia heat is lost by the dry, inspired gases.

 

This heat loss may contribute to the general problem of hypothermia, esp. in young children. 

 

Heat loss can be avoided by humidifying inspired gases and is reduced when an anesthetic circle system with soda lime is used.

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