WORK AND POWER: BASIC CONCEPTS ABOUT PHYSICAL QUANTITIES
The Work  Velocity and power  Types of energy  Efficiency  Measure units  I S  Dimensional analysis


These notes are intended to enable all persons whith poor physics knowledge  and who are going to read the notes on the laser LLLB (Low Level Laser Beam) applied to medical treatments LLLT (Low Level Laser Treatment)  to acquire a minimum notion of the physical quantities involved in the process. It is therefore good to speak advisely of thermal power, light output, duration of LLLT treatment and dose. NOTION OF 'WORK' WORK: IN THIS CASE LET'S DEFINE 'WORK' [L], THAT IS THE NECESSARY ENERGY, AS PRODUCT OF THE USED FORCE [F] AND DISPLACEMENT [S] AGAINST GRAVITY L = F x S In summary, so far have defined the following variables: 

VELOCITY (Speed): V = S/T If for example we went by car from Milan to Rome in six hours along 700 Km, our average speed was:Vm = 700/6 that is about 117 km / h POWER W = 200 Kgm/s (2000/10) with an average lifting speed V = 2/10 = 0.2 meters per second. W = L/T The power required is the ratio between the needed energy L and the time T spent, thus can also be written as a product of the force F for the lifting speed: W = (F x S) / T in in fact W = F x (S / T) and therefore W = F x V THEREFORE WE HAVE DEFINED THE FOLLOWING QUANTITIES




KINDS OF ENERGY The energy thus presents itself in different major forms. Depending on how it is manifested as MECHANICAL, ELECTRICAL, THERMAL, CHEMICAL, ENERGY, and the twentieth century also atomic energy.
POTENTIAL ENERGY More generally  regardless of the physical definitions of orthodoxy  one can think the potential energy as the ability or capacity of an object (or system) to transform its own energy into another form of energy. 

PERFORMANCE OR ENERGY EFFICIENCY ENERGY in every processing is not all converted to useful energy to the purpose for which the transformation occurs, but a part of it is lost forever and is not useful. In each transformation we can measure therefore the TRANSFORMATION PERFORMANCE or the EFFICIENCY. Taking the example of the winch in the picture, let's put a few numbers:
In this case we have  very optmistically indeed  assumed that the energy lost due to friction in the linkage, the efficiency of the motor, and other factors is 11% of the input and is lost as heat. In such case, the YIELD (or efficiency) is the ratio of useful energy and total supplied energy. In this case is 2000/2250 = 0.89 (in percentage 89%) EXAMPLE: a solid state laser module receives incoming electrical energy and emits a beam of electromagnetic polarized energy (polarized light). Even In this case, the polarized light's power of the ray is less than the input electrical power, and then a part of the energy goes to heat the module itself and is lost in the environment .


MEASUREMENT UNITS OF QUANTITIES The units of measurement of quantities must be such as to make consistent and measurable all types of energy transformation. The basic of the system are the measurement units of the fundamental variables, including those used to measure space, time and strength, from which all the others derive. A system of units of physical quantities must be consistent, that is, its derived quantities must be obtained by processing of fundamental physical quantities. There are various measuring systems. For example, in continebtal Europe we are accustomed to use the metric system and is commonly know that the force is measured in Kg, the space in meters, and the time in seconds, minutes and hours; in the anglosaxon countries for the time the measure is the same, while to the rest it's still in use instead the imperial system, ie lbs, feet, inches. Is aroused then the need to have a unified reference units system, and with time it has come to the INTERNATIONAL SYSTEM, which is used mainly for scientific and technical purposes.


INTERNATIONAL SYSTEM Today the SI is based on seven fundamental physical quantities and the corresponding units with which are defined the derived quantities and the corresponding measurement units. Also it defines the prefixes to be added to the measurement units to identify multiples and submultiples. The international system is consistent as its derived quantities are obtained as a product and relationship of physical fundamentals. Each physical quantity and the unit of measure is a combination of two or more physical units of measurement and the corresponding basic or the reciprocal one. All units are defined by measuring natural phenomena except kilogram. In addition, the kilogram is the only unit of basic measurement containing the prefix K because the gram is a unit of measure too small for most applications. The physical fundamentals of the international system involved in the calculations for the example of Figure 1 are to date the following:
We write, for example l = 10 m to indicate a length of 10 meters
For complete information on all the units of measure you can click and open this link http://it.wikipedia.org/wiki/Sistema_internazionale_di_unit%C3%A0_di_misura APPLICATION EXAMPLE OF dimensional analysis: when we write a formula or any relationship with certain symbols, to check if it is correct we have to make the dimensional analysis. For example, let's calculate the mean velocity V of the flow of an incompressible liquid in a tube with diameter D, knowing the mass flow Q and the specific weight of the fluid itself. We have to verify immediately that our relationship for calculating the velocity V give the result in meters per second
To check whether the formula is correct let's put the following dimensions for the involved variables and then use the dimensional analysis Flow in weight Q = [kg/s] (Kilograms per second) Substituting in the previous formulas the symbols of measurement's units to those of magnitudes, and neglecting of course the numbers 4 and π which do not have size, we will then get
The symbols kg in the numerator and the denominator cancel out, while the ratio of m cubed and m squared gives m, then the formula, if the dimensionless constants is attributed to the correct value, is exact. It seems clear that if the result was not [m / s] the formula would be wrong or if the unit of measurement of the variables were not homogeneous the result of the calculation would be wrong too. Lino Bertuzzi  novembre 2014 
