Fundamentals of electronics - Ohm's law, Power, Electromotive force

Ohm's law unites in one single formula the quantities involved in a circuit: tension (V), current (I) and resistance (R). It involves three equivalent expressions that are derived from simple algebraic operations on the basic formula:

Let's take a look at a practical example to experience first-hand these quantities. If we apply a tension of 220 Volt to a 50 Ohm conductor we'll have a current of:

Equation 4.5. Calculation of current with Ohm's law 

In physics, power is equal to the work carried out by a power source when it produces movement within a time-unit. In other words, if we imagine that we pick up a weight and we move it a few metres, we have carried out work, which we'd measure as power. In electronics, power is calculated differently, but it's important to remember that in any given physical context in which we calculate power, all results are equivalent. To better understand this fact, let's take a concrete example: an amplifier piloting a loudspeaker. To move the loudspeaker's membrane (which in turn shall create air displacement) we'd have to carry out an amount of work that equals power. So, our amplifier will have to develop an electric power which equals the necessary physical power to move the membrane.

Ohm's law expresses itself in many ways other than the three we have just explored. One of these describes power, and defines it as follows: Power is the product of tension of a current and is measured in Watt:

Equation 4.6. Power 

If we substitute V or I with the expressions of Ohm's Law, we obtain:

Equation 4.7. Joule's Law 

This formula is called Joule's law

The best example of electromotive force is found in common household batteries. The latter supply to their extremities a constant potential difference until they run out. This can take place thanks to the combination of certain chemical elements which, when they contact each other, generate electrons. As the electrons become consumed (for example, if we put our batteries in a torch), the chemical components change and slowly lose their chemical proprieties. When these components are no longer able to supply electrons, the battery has run out.

In brief: an element (battery) which supplies an electromotive force results in constant tension at its extremities.