Created it, 05/10/15
Update it, 05/10/17
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ELECTRIC POWER AND HEAT “1st PART”
The concept of energy was suggested with the man by the observation of natural phenomena; while observing for example the volcanic wind, lightning or eruptions, it comes spontaneously to the idea that nature is not an inert thing but which it has an energy that the man then ingénié himself to use.
For that, it is however necessary to domesticate the manifestations of natural energy, but this is not always feasible and the man had artificially to reproduce these natural phenomena in the way most adapted for then using energy brought into play.
In these cases, one says commonly that energy “is consumed”, to obtain a work or heat from it. When we are opposite work or heat, products artificially by the man, we must remember that this work or this heat was obtained at the expense of a corresponding energy which was consumed. For example, the heating of the filament of a bulb, which becomes incandescent until producing light “consumes” energy, this energy is of electric nature. Actually, energy is not consumed but simply not transformed into another type of energy. It is thus more correct of saying than the electric power is transformed into mechanical energy (i.e. in work) or into thermal energy (heat or light).
We now will analyze the production of heat starting from the electric power, then we will analyze how from this electric power we can obtain work.
The heat produced thanks to the electric power is due to the heating effect of the current, which consists of the heating of a driver traversed by this current.
Let us see initially how a current, traversing a driver, can produce its heating. Like us it soap already, the bodies and thus the drivers consist of atoms which occupy the given positions.
When a current circulates in a driver, the passage of the electrons of this current is obstructed by the atoms of the driver which these electrons run up; the latter thus yield a share of their energy which heats the driver.
ELECTRIC POWERThe electric power is an electric quantity which can be quantified. That is important because this energy is very expensive. To see how we can measure the electric power, refer us with a very simple circuit such as that of figure 1.
This circuit consists of a battery connected to two equal resistances R assembled in series. For our explanation, we suppose that these two resistances belong to an electric radiator.
It is important to remember that all the loads constituting the electrical current circulating in our circuit are equal. Therefore, which is true for one of it is true for all the others. For our explanation, let us analyze what occurs on a load, for example an electron.
Figure 1, following the passage of the current in two resistances, it occurs a release of heat, the energy of the electron is thus consumed there.
At the boundaries of two resistances (between the points C and E), the tension is identical to that at the boundaries of the battery (points A and B) thus of 90 V (the voltage drop in the drivers being negligible). This tension of 90 V is divided into two equal parts of 45 V since resistances identical and are assembled in series. These two resistances thus provide each one half of the total heat produced by the radiator.
The electron which crosses these two resistances in turn loses a half of its energy in the first resistance and other half in the second resistance.
Now let us consider the resistance connected between the points C and D and see which values have the energy of the electron and the electric potential.
At the point C, the electron has all its energy, the point C thus a potential higher of 90 V than the point E.
At the point D, after having crossed the first resistance, the electron has nothing any more but half of its energy since this resistance in consumed a half to produce heat.
The point D has a potential of 45 V superior at the point E, i.e. half of 90 V present at the point C.
We note thus that to a reduction in energy undergone by the electron while crossing resistance a reduction similar in the potential corresponds at the boundaries of this same resistance.
The potential difference thus created corresponds to the energy yielded to resistance by the electrons of the electrical current, energy transformed into heat.
Nothing to omit in my explanation, it is necessary for us to specify that the energy had by the electric charge is provided by the battery following the chemical reactions which occur inside this one between its electrons and the electrolytic solution that it contains.
The deterioration of the electrodes and the phenomenon of polarization explained previously and which causes the exhaustion of the battery must precisely with the chemical reactions intern with the pile.
What occurs for an electron and obviously truth for all those composing the electrical current because each electron contributes its share of energy which it received from the pile.
So now, we wish to know the total energy consumed by the radiator to produce heat, it is enough to multiply the tension which is applied to him by the battery, by the number of loads i.e. the quantity to electricity which crossed resistances during the totality of the operating time.
As we will see it a little later, the tension is easily measurable, on the other hand, it is not the same for the quantity of electricity. However, we can also measure the intensity of the electrical current which, as us it soaps corresponds to the quantity of electricity, in other words, the number of coulombs which cross a circuit in one second.
In conclusion, if we multiply the tension applied to the radiator by the intensity of the electrical current which crosses it, we know the energy used in one second by the radiator to produce heat. This energy represents the electric output (symbol P) of the radiator. It should be retained that :
The electric output of an electrical appliance corresponds to the energy absorptive by this apparatus in one second: it is obtained by multiplying the tension applied on its terminals by the intensity of the current which crosses it :
The measuring unit of the electric output is Watt (symbol W) while the tension and the intensity are expressed respectively in volt and amp.
In the practical applications, you will have to meet very large powers or on the contrary very small: for the strong powers, one uses the kilowatt (symbol kw) which is worth thousand Watts. For the low powers, one uses the milliwatt (symbol MW) which is the thousandths left Watt.
To know the electric output of an electrical appliance is very important because this information immediately gives an idea of the power consumption by this apparatus. For this reason, the manufacturers indicate on their apparatuses the electric output of those.
Let us suppose for example, that on an electric radiator the power of 500 W is reproduced. That means that this radiator consumes an energy of 500 W at each second. If it functions one hour, it will consume an energy 3.600 times larger, since 3.600 seconds ago in one hour (60 x 60). we will be able to say that:
The power consumption by an electrical appliance maintained under operation during a determined time, is obtained by multiplying its power expressed in Watt per the time expressed in seconds.
Since to obtain energy, we multiply the power in Watt per time in second. It is obvious that this energy is measured in Watt a second (Ws). With this measuring unit of the electric power the name of joule (symbol J) was given.
The electricals appliance functioning in general during a time much higher than the second, it is not practical to calculate the energy thus consumed by multiplying the power in Watt per the operating time expressed in second.
For this reason, it is preferable to multiply the power in Watt per the time expressed in hour, energy is then expressed either in Watt a second, i.e. in joule, but in Watt per hour i.e. in watt-hour (Wh symbol) which is equivalent to 3 600 joules (1 hour = 3 600 seconds)
In practice, you will meet the kilowatt-hour (symbol kWh) which is worth 1 000 Wh. For example, the electricity meters installed in the dwellings measure the electric power consumed in kilowatt-hour.
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