Maquina termica

Páginas: 15 (3640 palabras) Publicado: 13 de enero de 2012

The development of machines has been one of the key factors in the establishment of modern developed societies.

In this teaching unit we want you to understand the role that machines have played throughout history: first as simple human energy savers; then as devices able to take advantage of natural forces like wind or water; finally as sophisticated machinery able toconvert some forms of energy into others, always for our benefit. You will also be able to understand how Nature imposes some limitations on these transformations.

Click next to see our objectives in detail.

Objectives of the teaching unit

To understand the concept of a simple machine as an instrument which saves effort but not work.

To understand the way in which energy fromwind or water is converted into the mechanical energy of a circular movement.

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To assimilate the equivalence between mechanical energy and heat. To formulate the first and second law of thermodynamics in a simple way.

To learn to use the mathematical formulae for these laws to make simple predictions about the working of engines and machines.

© Proyecto Newton. MEC. José L. SanEmeterio

1.1 A simple machine Machines are devices aimed at reducing the effort necessary to carry out arduous tasks. In the illustration you can see a lever, the simplest machine found in history. It enables us to lift a heavy weight with a relatively small force.

In this machine we can already see a very generalized characteristic in the world of machines: the use of the characteristics ofcircular movement (the whole lever turns around the fulcrum).

In the following visual we will study another simple machine to define the usefulness of this type of device and to understand its limitations.

A pulley worked by a crank. See activity A.1

© Proyecto Newton. MEC. José L. San Emeterio

A1: The machine cannot lift the weight initially because it needs a greater torque. Can youremember the value of this magnitude? Increase the length of the crank until the movement is possible. Click on play and note the aumount of work done. Click on init. Repeat the experiment, but this time increase the force. What can you see regarding the work done either way? A2: A section of the axis has a radius of 0.25 m. Find the smallest value of length of the crank and force that allow theweight to be lifted by trial and error. Calculate the torque of the force in relation to the axis and the torque of the weight in relation to the axis. What can you see? Why is this machine useful? A3: Note the work done with different combinations of force and length for different values of the weight. Draw a table in your notebook with the different values, writing down the work done in eachcase. For each value of the weight, what can you see regarding the work carried out? Does this machine save energy if we compare it with the amount of energy that we would need to lift the weight with our own hands, without any help?

1.2 An energy saving machine

We have already seen that simple machines reduce the force necessary to carry out a task, but not the work we need to do to completeit. Throughout history we have often used the work of animals to replace human work. This, for example is the case of the noria or water wheel, traditionally moved by a mule or an ox.

A better alternative consists of using the energy from natural phenomena to transform it into useful mechanical work. You won't find it hard to identify what type of energy transformation can be carried out by thewindmill in the illustration. In the following visual you can study in detail another example of the transformation of a natural energy into mechanical work.

© Proyecto Newton. MEC. José L. San Emeterio

A1: Turn on the machine with the velocity of the water set to a value greater than zero. Explain what transformation of energy makes the turbine rotate. Calculate the work done in a minute...
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