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Investice do rozvoje vzdělávání

Tento projekt je spolufinancován Evropským sociálním fondem a státním rozpočtem ČR.

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Mechanická práce, energie, účinnost

Datum zveřejnění : 11. 6. 2010Tisk
Kategorie : Český jazyk
Autoři :  Jugasová , Neznámý, talhoferova
Škola : Jaroška,
Pod vedením : Jugasová
In our project we would like to explain to you principles of mechanical work, energy and efficiency.

Mechanical work, energy, efficiency

Hello, we are David Janča, Filip Zelníček and Igor Džama from 3.D and we would like to tell you something about our project  Mechanical work, energy and efficiency. What does it mean? These terms are physical effects on which advanced branches as energetics, engeneering are based. They are very important in modern world, like in the usual questions about resources of energy or higher effectiveness of machines.
We chose it, because we are interested in physics and this effects are a base of modern life. Without them we could not have electricity, most of machines and more important things.


First, we would like to talk about mechanic work. Mechanic work is a process, when the force affects on a thing and it moves with it on specific trajectory. For example if a box or a cart is pushed on the floor or if something is lifted up, it is mechanical work. In physics, mechanical work is the amount of energy transferred by a force acting through a distance. We can count it as multiplication of affecting force  in newtons and trajectory in meters. If the affecting force is not parallel with trajectory, we must count it as a multiplication of force, trajectory and cosinus alfa. Alfa is an angle between out of parallel force and trajectory.

Mechanical work is not proceeding when: A thing is not moving, when on moving thing is not acting any force and when a force is perpendicular on direction of moving.

That is all important about mechanical work.

Next we want to explain the most important things about energy.

Generally the energy:

We need energy to do a work

Work is the force on a path (kicking the ball, lifting weights, stretching or compression springs, ...)

Living organisms needs energy

There are 2 types of energy:

- Potential - Hidden, positional Ep = m*g*h

- Kinetic - energy of the moving body Ek = ½ *m*v²

Units of energy:

The unit for energy and power according to the International System of Units SI is the joule (J). But this unit complies only with physicians. Energetics prefer to calculate energy with kilowatt-hour (kWh).

Energy of our planet:

Our planet has these kinds of energy:

- Kinetic energy of rotation - helps generate tides and thus inhibit its rotation.

- Translational kinetic energy - the energy the sun and planets around the galaxy's movement.

- Geothermal energy - comes from the slow decay of radioactive substances on Earth, sometimes manifested by volcanic activity and the outpouring of thermal water and vapo.

- Nuclear energy - stored in the form of fissile materials (uranium, thorium) or  in light substances (hydrogen, deuterium).

On Earth, the sun's rays fall. The total power emitted by the Sun affects our Earth, a mere two-billionth.However, it is 174 000 TW.

Distribution of this energy (174,000 TW = 100%)

- 31% - a reflection from the surface of the earth's atmosphere, no benefit
- 17.4% - absorb the atmosphere, the heat goes into space
- 4.2% - this will be reflected from the surface of the Earth
- 14.4% - falls on the surface of the continent
- 33% - the impact on the ocean surface waters and all

Generation of energy:

Energy can be generated from wind, sun, waves, but you replace "dirty" power plants with fossil fuels. What is Earth from the Sun take, it must come back. If not, begins to warm the atmosphere and then will show the so-called greenhouse effect. Simply heat the average surface of only 1° C to alter the speed of life processes in the biosphere (that could have disastrous consequences for life itself).


At last we will tell you something about efficiency.
Efficiency is in physical concept a variable used for assessing  efficiency of machines.
So, how to define efficiency. The definition is: “Efficiency is the proportion of useful work done by the machine actually performed, and the work which we supplied.” We sign efficiency with Greek letter η(éta). Efficiency does not have unit. About that can us, persuade this fraction J/J=1.


Because it does not have unit, we sign it mostly in per cents. So it looks this way (η=80%).


And how to calculate efficiency? It is: “éta” equals work done by the machine, divided the energy, we delivered the machine.


And here is another way to calculate efficiency. For this calculation we use input and useful output. Both input and output should be constant. Otherwise our result can be inaccurate. The formula is this: éta equals output divided input.


Let´s talk about reached efficiency. The machines reach higher efficiency in good conditions, in bad conditions machines reach worse efficiency, for example if there is sand in transmission, it will not go fluently and will overheat a lot.
Efficiency can be affected by a lot of things. For example, how well  frictional parts are lubricated. Efficiency is limited by its max physical value. The max value can not be overcome. On this picture you can see the  graph of max efficiency by Carnot cycle. Temperature of  the cooler is in this case °C. Efficiency we can reach is under its line. Here is a similar graph but the cooler has a temperature 100°C, you can see that maximal efficiency is lower.


But there are other factors, which affect efficiency, for example the wear of the machine. Another enemy of high efficiency is friction. We want a lot of times to  minimise friction. So we make parts with friction from materials which reduce friction. We also lubricate machines and clean machines.


Now I would like to say something about efficiencies of machines. Here you can see the table with efficiencies of different machines.


Some machines as you can see have high efficiency, for example water turbines are having it up to 90 percent, big electric motors, electric heater others have it lower, like: Photosynthesis; Muscle; Incandescent bulbs etc.
And that is all, I wanted say about efficiency.


Multimédia

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