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

Let's take a look at a practical example to gain some first-hand experience of these quantities. If we apply a voltage 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 were to pick up a weight and move it a few metres, we will 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. Let's take a concrete example: an amplifier driving 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 needed to move the membrane.

Ohm's law can be expressed in many ways other than the three we have just mentioned. One of these identifies power: power is the product of voltage multiplied by 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 a constant potential difference at their far ends, until they run out. This takes place thanks to the combination of certain chemical elements which generate electrons when put into contact. As the electrons are consumed (for example, if we put our batteries in a torch), the chemical components change and slowly lose their electrical properties. When these components are no longer able to supply electrons, the battery runs out.

In a nutshell, electromotive force can be define as: an element (battery) which supplies an electromotive force resulting in a constant voltage at its far ends.