This circuit consists of a battery connected to two equal resistors R connected in series. For our explanation, we assume that these two resistors belong to an electric radiator.

It is important to remember that all the charges constituting the electric current flowing in our circuit are equal. So, what is true for one of them is true for all others. For our explanation, let's analyze what happens on a load, for example an electron.

Figure 1, following the passage of the current in the two resistances, there is a release of heat, the energy of the electron is therefore consumed.

At the terminals of the two resistors (between the points C and E), the voltage is identical to that at the terminals of the battery (points A and B) therefore of 90 V (the voltage drop in the conductors being negligible). This voltage of 90 V is divided into two equal parts of 45 V since the resistors are identical and connected in series. These two resistors each provide half of the overall heat produced by the radiator.

The electron that passes through these two resistances in turn loses half of its energy in the first resistance and the other half in the second resistance.

Consider now the resistance connected between the points C and D and see which values have the energy of the electron and the electric potential.

At point C, the electron has all its energy, point C therefore has a potential greater than 90 V at point E.

At point D, after passing through the first resistance, the electron has only half of its energy since this resistance consumed one half to produce heat.

Point D has a potential of 45 V higher than point E, that is to say half of the 90 V present at point C.

We thus note that a decrease in energy experienced by the electron, through the resistance corresponds to a similar decrease in potential across the same resistance.

The potential difference thus created corresponds to the energy given over to resistance by the electrons of the electric current, energy transformed into heat.

To omit nothing in my explanation, we must specify that the energy possessed by the electric charge is supplied by the battery following the chemical reactions that take place inside it between its electrons and the electrolytic solution that it contains.

The deterioration of the electrodes and the polarization phenomenon explained above and which causes the battery to be depleted are precisely due to the internal chemical reactions in the battery.

What happens for an electron and obviously true for all those component electric current because each of the electrons brings its contribution of energy that it received from the battery.

If now, we want to know the total energy consumed by the radiator to produce heat, it suffices to multiply the voltage applied to it by the battery, by the number of charges that is to say that the quantity electricity that has passed through the resistors for the entire operating time.

As we will see a little later, the voltage 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 electric current which, as we know, corresponds to the amount of electricity, in other words, the number of coulombs that pass through a circuit in one second.

In conclusion, if we multiply the voltage applied to the radiator by the intensity of the electric current passing through it, we will know the energy used in one second by the radiator to produce heat. This energy represents the electrical power (symbol P) of the radiator. It must be remembered that :

The electrical power of an electrical appliance corresponds to the energy absorbed by this appliance in one second : it is obtained by multiplying the voltage applied to its terminals by the intensity of the current flowing through it :

P = V x I

The unit of measure of the electric power is the watt (symbol W) while the tension and the intensity are expressed respectively in volt and in ampere.

In practical applications, you will be required to meet very large or very small powers : for high power, we use the kilowatt (kW symbol) which is worth a thousand watts. For low power, we use the milliwatt (symbol mW) which is the thousandth part of the watt.

Knowing the electrical power of an electrical device is very important because this information immediately gives an idea of the energy consumed by this device. For this reason, manufacturers indicate on their devices the electrical power of these devices.

Suppose, for example, that an electric heater has the power of 500 W. This means that this heater consumes 500 W of energy every second. If it runs for one hour, it will consume 3 600 times more power, since there are 3 600 seconds in one hour (60 x 60). We can say that :

The energy consumed by an electrical device maintained in operation for a given time is obtained by multiplying its power expressed in watts by the time expressed in seconds.

W = P x t

Since to obtain energy, we multiply the power in watts by the time in seconds. It is obvious that this energy is measured in watts per second (Ws). To this unit of measure of electric energy has been given the name of Joule (symbol J).

As electrical appliances generally operate for a time much greater than one second, it is not practical to calculate the energy thus consumed by multiplying the power in watts by the operating time expressed in seconds.

For this reason, it is preferable to multiply the power in watts by the time expressed in hours, the energy is then expressed not more in watt per second, that is to say in joule, but in watt per hour. that is to say watt-hour (Wh symbol) which equals 3 600 joules (1 hour = 3 600 seconds).

In practice, you will encounter the kilowatt hour (symbol kWh) which is 1000 Wh. For example, electricity meters installed in homes measure the electrical energy consumed in kilowatt hours.