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Wednesday, August 24, 2011

Work, Enegy and Power









WORK, ENERGY AND POWER

ENERGY
The terms force  and energy were not always clearly defined. Before the mid-nineteenth  century, they were often used interchangeably. However, progress in  mechanics and thermal physics helped clarify these ideas and the  distinction between them. In 1807,  the English scientist  Thomas Young  (1773 – 1829) introduced the word energy to denote the quantity of work  that a system can do.   Later, the Scottish engineer and thermal  physicist W. M. J. Rankine coined the terms potential energy and  conservation of energy. As is often the case in science, this  classification of terms and definitions led to greater insights and  understanding of natural laws and their consequences. Today the  principle of conservation of energy is part of the framework of physical  theory. Our faith in this principle is based on years of our  experience.  
Energy is a vital part of our daily lives. The food  we eat gives our bodies energy for movement; electrical energy lights  our homes and streets; oil and gas propel our cars and keep us warm.   The use of energy is growing at an incredible rate. In the U.S. alone,  energy demands have increased more than 250%  since 1940. The increase  maybe even more rapid in the years ahead. The energy demands are growing  more rapidly than population itself. In other words, the use of energy  per person is also rising.. The increase is largely a result of the  improvement in the standard of living. The switch from human power to  machine power has increased productivity and provided us with more goods  and leisure time.  Unfortunately, the use of labor-saving devices  places serious demands on our available energy supplies.
The wise  use of existing energy resources is one answer to this problem. By  avoiding unnecessary use of energy for heating, lighting, air  conditioning and transportation, serious energy shortages can be  prevented.
Conservation of energy is only a partial answer,  however; new energy sources must be found and developed in order to  achieve and maintain a desirable living standard for the growing  population of the world.
Actually, there is no shortage of energy  at all. The sun floods the earth with enough radiant energy everyday to  supply the whole world’s needs many times over. In fact, it has been  calculated that present worldwide demands could be met if we could  completely convert into electrical power the solar energy falling on a  plot of ground  near the equator at a mere 125 miles square ( 201.1 km  square ). It has also been estimated that the Gulf Stream in the  Atlantic Ocean transports enough warm water to generate the electric  energy needs of the U. S. many times over.
The shortage lies in  our knowledge and our means to convert the ample energy supplies in and  around our planet into usable forms.
Many sources of energy on  the earth come directly from energy radiated from the sun. Coal and oil,  the fossil fuels, were formed from the sun millions of years ago.  Hydroelectric power depends upon dammed-up rain water that was  evaporated from the sea by solar energy. Winds and ocean currents,  potential sources of additional energy, are caused by solar energy.
Some  energy sources cannot be attributed to the sun. Nuclear energy comes  from  changing matter into energy.  Geothermal energy ( heat from the  earth ) uses heat locked in rocks since the Earth was formed as a molten  mass. The Earth’s rotation and the gravitational attraction between the  Earth and the moon moves large masses of water on the earth. These  movements of water are called tides. In some places such as the Rance  river in France, tides are usefully harnessed.
The amount of  radiant energy we receive from the sun each day is limited. Also, the  amount of fossil  fuel is limited. Research efforts to supply more  energy have aimed at increasing our supply of solar energy, using our  solar resources more efficiently, and increasing energy production from  non-solar sources. 

ENERGY is the capacity or ability  to do work.
 POTENTIAL ENERGY is an energy due to its shape or  position.  A body is said to have potential energy if by virtue of its  state or position is able to do work. Water at the top of a hydraulic  dam has energy due to its position.. As the water runs downhill through a  turbine, the potential energy of the water is converted to electrical  energy.
 ELASTIC POTENTIAL ENERGY is an energy possess by a  compressed or stretched spring.
 KINETIC ENERGY is an energy of  motion. The kinetic energy possess by a moving body is defined as the  energy possess by the body by virtue of its motion.  
Sources of  energy :
1. Sun   
2. Water  
3. Nuclear 
4.  Wind  
5. Geothermal 
6. Fossil fuels ( coal, oil natural  gas)    

Forms of energy :
1. Chemical energy          
2. Nuclear energy
3. Electric energy         
4. Light energy (solar/ Radiant  energy)
5. Heat energy / Thermal  (internal energy)  
6. Atomic / molecular energy  
7. Mechanical energy
[ Forms :  a) kinetic and  b) potential energies  ]    

WORK
The word work means many different  things to us in our daily lives. We say that we work when we sweep the  yard, buy groceries or drive a car. We also work if we drag or push an  object across the floor. How much work we do depends on how hard we push  and how far we move the object. In a layman’s point of view, work is  the expenditure on one’s stored up bodily energy.  In the physical  sciences, work is more precise and restricted than in everyday usage.  Work is defined as the product of force and the displacement through  which the force acts as the object moves.
Factors to be considered  in measuring work :
1.  There must be an applied force.
2.   The force must act through a displacement, S. 
3.  The force  must have a component Fx  parallel to the direction of the displacement.  
If an applied force is not along the direction of motion, we  can resolve it into components parallel to and perpendicular to the  displacement. Only the component of the force that is parallel to the  displacement contributes to the work.  

UNITS OF WORK
1.   Joule ( J ) – one joule is the work done by a force of one Newton in  moving an object through a parallel distance of one meter.    1 J = 1 N   m
2.  Erg – one erg is the work done by a force of one dyne in  moving  an object through a distance of one centimeter.   1 erg = 1 dyne  . cm
3.  Foot–pound (ft-lb) – one foot pound is the work done by  a force of one pound in moving an object through a parallel distance of  one foot.

W =FS cos θ , if the force and the  displacement are oblique with each other
W = FS  , if the force  and the displacement are in the same direction.
When a mass m is  lifted to a height h, the force exerted is equal to the weight of the  mass, the work done against gravity approaches the potential energy and   S  h. 
Ep = mgh , potential energy [ Work done against gravity ]           
If the mass is release from rest the speed of the mass  is given by v = √ 2gh  ,  v2 = 2gh  and  h = v2/  2g.
The potential energy, Ep undergoes transformation to kinetic  energy, Ek.  
Ep =  Ek = mg ( v2/ 2g ) =  ½ mv2  ,  kinetic energy
Ep = ½ ky2, elastic potential energy  ; k = spring constant,  y  = elongation or deformation


CONVERSION  OF MASS TO ENERGY
In his special theory of relativity ( 1905 )  Einstein concluded that mass and energy are interchangeable. The  quantitative mass-energy relationship is given in his equation, 

E  = mc2  ,  where  m = mass   and   c = speed of light = 3  x   108 m/ s.

Mass is converted into energy in  nuclear rectors and nuclear weapons. As well as in the sun and other  stars.  For nuclear reactors using  U235 as fuel, about 1/1000 of the  mass of each fissioning atom is converted into other forms of energy.   Although the fraction of our energy needs supplied by nuclear reactors   on earth is relatively small, it is increasing rapidly as energy from  oil and natural gas becomes less plentiful and more expensive. 

THERMAL  ENERGY OR INTERNAL ENERGY is associated with the random kinetic  energies of the atoms and molecules in the object.


FOODS  AND OTHER FUELS
Many of the most common energy sources are  chemical in nature such as food, gasoline and natural gas. The energy  content in foods are given in units of kilocalories. ( 1 kcal = 4186  joule ). For foods and fuels the process by which stored chemical energy  is released is by oxidation. In machines, the oxidation process  produces thermal energy which is partially converted to work and other  forms of energy. In animals, the oxidation process is complex which also  results both in thermal energy and work being performed by the animal.  If the animal consumes more food than it needs, it will convert the  excess to fat, which is another form of chemical energy. The chemical  energy stored in fats is used if and when there is a food deficit.
Dieting  to lose weight would mean reducing of the food energy intake. Exercise  aids dieting partially because more food energy is converted to work.

CALORIC CONTENT ( kcal/g ) OF COMMON FOODS AND FUELS 

COMMON FOODS    kcal/g  Eggs           1.63    Sirloin,lean 1.66
Apples          0.58    Grapes         0.69    Sugar        4.00
Avocado         1.67    Ham, cooked    2.23    Tomato       0.22
Baby formula    0.67    Hamburger,lean 1.63    Tuna,in oil  1.97
Beans, kidney   1.18   Ice cream,chocolate 2.22   Wine      0.85
Beer            0.42    Lard ( fat )    9.30    COMMON  FUELS
Butter          7.20    Lobster, raw    0.91   Coal          8.00
Carrots         0.42    Milk, whole     0.64   Gasoline     11.4
Celery          0.14    Milk, low-fat    0.42  Furnace oil  10.5
Cheese, cheddar 4.00    Oranges          0.49  Methanol     5.20
Cheese,cottage  1.06    Peanuts, roasted 5.73  Natural gas  13.00
Chicken,roasted 1.60    Peas             0.71  Wood (average)4.00
Chocolate       5.28    Potato, baked    0.93  
coffee, black   0.008   Raisins          2.90  Average carbohydrates 4.10
Cola,carbonated 0.36    Rice,white,cooked 1.09 Average protein       4.10
Corn flakes     3.93   Shrimps,snails,raw 0.91 Average fat           9.30


ENERGY  CONSUMPTION  RATE for VARIOUS ACTIVITIES

ACTIVITY            RATE(kcal/min)  Playing tennis             6.30
Sleeping                1.20        Swimming breaststroke      6.80
Sitting at rest         1.70        Ice skating(14.5 km/hr)    7.80
Standing, relaxed       1.80        Climbing stairs(116/min)   9.80 
Sitting in class        3.00        Cycling (21 km/hr)        10.00
Walking slowly(4.8km/hr)0.80        Playing basketball        11.40 
Cycling (13–18 km/hr)   5.70       Cycling,professional racer 26.50 




ELECTRIC ENERGY
Capacitor is a device which  stores pure electric energy. Many electronic instruments,  such as the  heart defibrillators use capacitors to store energy. Fibrillation is a  potentially fatal malfunction of the beating of the heart. The electric  energy stored in the large capacitor of the defibrillator is used to  cause an electric current to pass through the patient’s heart to stop  fibrillation – that is, to defibrillate the heart. Ironically, electric  current through the heart can also cause fibrillation, depending on the  amount of current, it may even cause electric shock. Currents as low as  20 mA may cause difficulty in breathing, and at 75 mA breathing may stop  completely. Currents between 100 and 200 mA results in ventricular  fibrillation of the heart, which means an uncoordinated and uncontrolled  twitching of the heart muscles. The resulting loss in pumping action   is fatal. The defibrillator used in medical emergencies apply a large  momentary voltage to the body to stop the heart and facilitate the  restoration of the normal heart rhythm. 

LAW OF  CONSERVATION OF ENERGY.  
Energy can never be created or  destroyed, it maybe transformed from one form to another, but the total  amount of energy never changes ( remains constant ). 
Total  energy is the sum of all forms of energy in a system : kinetic, heat,  potential chemical, etc. Experiments have shown that the total energy in  a closed system is always conserved.  Energy can be transferred from  one system to another if one system does work on the other.  The  conservation of energy can be written in the form :

Ek  + Ep + Eo = constant    Eki  +  Epi  + Eoi  =  Ekf  +  Epf  +  Eof

where  the subscript i  and f  denote initial and final energies. Ek  represents kinetic energy, Ep represents potential energy and  Eo  represents all other forms of energy.  In the treatment of the equation  if  the Eo is constant, then Eoi = Eof and the equation can be reduced  to Eki  +  Epi =  Ekf  +  Epf. The law of conservation of energy  principle is very useful in solving problems. It can be applied to any  closed system, where only the initial and final conditions need to be  considered.

POWER AND EFFICIENCY
Power is the  time rate of doing work. It is the rate at which energy is used or  expended, since work done results  in energy being transferred from one  system to another.  The  SI unit  for power is the  joule per second  (  J/s ), which is called the watt in honor of James Watt ( 1736 – 1819 ).     1 J / s  = 1 watt.  
Watt is a very familiar unit. All light  bulbs and other electric devices are rated in watts. 
The  horsepower was defined by Watt as a unit for power. He was interested in  describing the rate at which his steam engine could do work and defined  his unit in terms of the common source of power, the horse. He found  out that on the average, the horse was doing about 550 ft-lbs of work  per second. He called this unit one horsepower and measured the rate at  which his steam engine could work and rated them in horsepower. 

P  = W / t 

1 horsepower ( hp ) = 550 ft-lbs / sec =  33,000 ft-lbs / min = 746 watts = 0.746 kW

1. A 60 kg  man climbs a flight of stairs 3.25 m high in 8 seconds. Determine the  power generated in watts and horsepower. What is the power developed if  the man is running up the stairs in 3 seconds ?
Note :  It is no  wonder that running upstairs is so stressful and causes the body to  utilize its available energy
very quickly. People with heart  problems are warned that climbing stairs is one of the most stressful
acts  that they can perform.

EFFICIENCY is the ratio of the  output work to the input work express in percentage.

Eff  = Wout / Win  = Pout / Pin = Eout /  Ein

EFFICIENCY in  humans
Body processes in humans requires energy not only in doing  physical work but also in blood circulation, digestion sleeping,  thinking. The body can be thought of as an energy converter, with the  source of energy being stored chemical energy ( food ) that the body  converts into mechanical work and thermal energy and back into stored  chemical energy ( fat ). 
Metabolic rate – is the rate of  conversion of food energy to some other forms of energy.
Basal  Metabolic Rate ( BMR ) – is the total energy conversion rate of a person  at rest which is divided among various systems of the body : (a) the  liver and the spleen, (b) brain, (c) skeletal muscles, (d) kidney, (e)  heart and (e) other organs – smooth muscles, intestines, bone marrow,  lymphatic systems. 
The BMR of an individual is related to  thyroid activity. An overactive thyroid results in a high BMR and  hyperacidity while an underactive thyroid leads to a low BMR and  lethargy.
Energy consumption is directly proportional  to oxygen  consumption, since the digestive process is basically one of oxidizing  food. Roughly, 4.9 kcal of energy are produced for each  liter of oxygen  consumed, independent of the type of food. The digestive process is  quiet effective in metabolizing food, only about 5 % of the caloric  value of foods is excreted in the feces and urine without being used by  the body. The body stores excess food energy by producing fatty tissue.  During rest, most of the energy consumption of the body ends up as  thermal energy. Work done by the heart on the blood is converted to  thermal energy by friction in the circulatory system, and most skeletal   muscles activity is in the form of small motions during rest.  
Basal  Metabolic rate and oxygen Consumption rate

Organ          Power Consumed    Oxygen     Per Cent  Kcal/min   
                  at Rest      Consumption  of BMR   
                   w            mLO2/min 
Liver and Spleen  0.33           23          67        27
Brain             0.23           16          47        19
Skeletal muscles  0.22           15          45        18
Kidney            0.13            9          26        10
Heart             0.08            6          17         7
Others            0.23           16          48        19

TOTALS            1.22           85         250       100



 



 



 



 



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