Physical potency: formulas, potency types and examples

The physical strength refers to the amount of work (or energy consumption) per unit time. Power is of scalar magnitude, its unit of measure in the International System of Units in July per second (J/s), known as watt in honor of James Watt.

Another very common unit of measurement is the traditional steam horse. In physics, different types of power are studied: mechanical power, sound power, thermal power, among others. In general, there is an intuitive idea of ​​the meaning of power. It is usually associated with greater power, greater consumption.

Thus, a light bulb consumes more electrical energy if its energy is greater; The same goes for a hair dryer, a radiator or a personal computer.

Therefore, it is necessary to understand well its meaning, the different types of existing powers and understand how it is calculated and what are the relationships between its most common measurement units.


By definition, the following expression is used to calculate the energy consumed or supplied over a period of time:

P = W / t

In this expression, P is power, W is work, and t is time.

If what you want is to calculate the instantaneous power, the following formula should be used:

Physical potency: formulas, potency types and examples 2

In this formula, it is the increase in time, F is the force, and v is the speed.


The single power in the International System of Units is July per second ( J/s ), known as the watt ( W ). It is also quite common in certain contexts to use other units, such as kilowatt (kW), power (CV), among others.

Of course, the kilowatt is equal to 1000 watts. In turn, the equivalence between the steam horse and the watt is as follows:

1 CV = 745.35 W

Another unit of energy, although its use is much less common, is erg per second (erg / s), equivalent to 10 -7 W.

Related:   10 examples of Newton’s second law in everyday life

It is important to distinguish the kilowatt of hour from the kilowatt (kWh), as the latter is a unit of energy or work and not energy.

types of power

Among the different types of power that exist, some of the most important are those that will be studied below.

mechanical power

The mechanical power exerted on a rigid solid is obtained by making the product between the total resulting force applied and the velocity transmitted to that body.

P = F ∙ v

This expression is equivalent to the expression: P = W / t, and in fact is obtained from it.

In case there is also a rotational movement of the rigid solid and, therefore, the forces exerted on it modify its angular velocity, resulting in angular acceleration, it is necessary:

P = F ∙ v + M ∙ ω

In this expression, M is the moment resulting from the applied forces and ω is the angular velocity of the body.

Electric power

The electrical energy supplied or consumed by an electrical component is the result of dividing the amount of electrical energy supplied or absorbed by that component and the time spent on it. It is calculated from the following expression:

P = V ∙ I

In this equation, V is the potential difference between the component and I is the intensity of the electrical current flowing through it.

In the specific case where the component is an electrical resistance, the following expressions can be used to calculate the power: P = R ∙ I 2 = V 2 / R, where R is the value of the electrical resistance of the component in question.

heat output

A component’s heat output is defined as the amount of energy dissipated or released as heat by that component in a unit of time. It is calculated from the following expression:

P = E / t

In the aforementioned expression E is the energy released in the form of heat.

sound power

Sound power is defined as the energy transported by a sound wave in a unit of time across a given surface.

Thus, the sound power depends on the intensity of the sound wave and the surface crossed by that wave and is calculated using the following integral:

S = I S I S ∙ d S

In this integral, Ps is the sound power of the wave, Is is the sound intensity of the wave and dS is the surface differential crossed by the wave.

Rated power and real power

Rated power is the maximum power that a machine or motor can offer under normal conditions of use; that is, the maximum power that the machine or engine can withstand or deliver.

The term nominal is used because this power is usually used to characterize the machine, to name it.

On the other hand, the actual or useful power – that is, the power actually used, generated or used by the machine or motor – is usually different from the nominal one, usually smaller.


first example

It is desirable to crane a 100 kg piano to the seventh floor at a height of 20 meters. The crane takes 4 seconds to lift the piano. Calculate the power of the crane.


The following expression is used to calculate the power:

P = W / t

However, it is first necessary to calculate the work performed by the crane.

W = F ∙ d ∙ cos α = 100 ∙ 9.8 ∙ 20 ∙ 1 = 19.600 N

Therefore, the power of the crane will be:

P = 19,600 / 4 = 4900 W

second example

Calculate the power dissipated by a resistor of 10., a current of 10 A is passed through.


In this case, it is necessary to calculate the electrical energy, for which the following formula is used:

P = R ∙ I 2 = 10 ∙ 10 2 = 1000 W

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