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1st law of thermodynamics

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1st law of thermodynamics

The first law of thermodynamics is shared with most of science; it is one of the fundamental principals that have shaped our understanding of the working world.

        TOTAL ENERGY OF THE SYSTEM AND IT’S SURROUNDINGS IS CONSTANT or ENERGY IS CONSERVED, Brings back that long established idea that nothing can be created or destroyed. How do we know this? This is an empirical law, which means that we know that energy is conserved because of many repeated experiments by scientists. It's been observed that you can't get any more energy out of a system than you put into it.

Latent heat

Latent Heat is defined as the heat which flows to or from a material without a change to temperature. The heat will only change the structure or phase of the material. E.g. melting or boiling of pure materials.

        One very good illustration of latent heat in action is observed when we reduce ice to water. If we imagine a bucket of ice on the floor in an average temperature room (about 30 degrees Celsius) .The ice doesn’t instantly liquidize, nor does the room instantly freeze. Instead the temperature of the ice rises until it reaches zero degrees Celsius whereupon it begins to melt.

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T= Temperature rise.


         Enthalpy is a measure of heat and energy in the system. Scientists figure out the mass of a substance when it is under a constant pressure. Once they figure out the mass, they measure the internal energy of the system. All together, that energy is the enthalpy. They use the formula "H = U + PV." H is the enthalpy value, U is the amount of internal energy, and P and V are Pressure and Volume of the system. This system works really well for gases.
        There are things that affect the level of enthalpy in a system. The enthalpy is directly proportional to the amount of substance you have. Chances are if you have more of a substance, you have more energy. More energy means higher enthalpy.
        Another thing to remember is that the value for H (enthalpy) changes sign when the reactions or values are reversed. When a reaction moves in one direction, the sign is positive. When a reaction is moves in the opposite direction, the value is negative.
        Finally we have Hess's law. If a process happens in stages or steps, then the enthalpic change for the overall system can be determined by adding the changes in enthalpy for each step.



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2) times the velocity of the water (cm / sec.) we get Cubic centimeters per second, volume per unit time, a rate of flow. It makes sense both ways. Does it matter what cross sections we use? If our principle that the quantity of matter flowing in, is the same as the amount flowing out then it must also be true everywhere in the pipe.

        Therefore we can say that  AV = K

Where A is cross sectional area at a point.

V is the average velocity for this point.

And K is a constant ‘the rate of flow in the pipe’.

Since the equation is true for any two points in the pipe we can say that :

A¹V¹ = A²V²

Where A¹V¹ is the area and velocity at one point in the pipe and A²V² is the area and velocity at another point.

This is called the continuity equation and it shows us that the velocity of water at a point in a full pipe is inversely proportional to the cross sectional area of the pipe  

at that point. So if a pipe gets bigger the velocity in the pipe will decrease and vice versa.


          Area at X is 10cm² and velocity is 1cm per second

        Area at Y is 1cm² and velocity is 10 cm per second


Stephen Amos HND Engineering YR1

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