Power: what is measured, how it is applied, calculation formulas

What is power and power? In what measure this indicator is measured, what devices are used in this case, and how the named physical quantities are applied in practice, we will consider further in the article.

Force

In the world, all the bodies of physical nature begin to move due to strength. When it is exposed, with the associated or opposite direction of the body moving, work is done. Thus, any force acts on the body.

So, the bike starts to move due to the strength of the person's legs, and the train pulls the power of the electric locomotive. Such an impact happens with any movement. The work of force is the magnitude in which the strength module is multiplied, the module for moving the point of its application and the cosine of the angle between the vectors of these indicators. The formula in this case is as follows:

A = F · s · cos (F, s)

If the angle between these vectors is not zero, then the work is always done. In this case, it can have both a positive and a negative value. The force will not act on the body at an angle equal to 90 °.

Consider for example the cart, which pulls the muscular strength of the horse. In other words, the work is performed by the traction force in the direction of the cart's motion. But the force of gravity, directed downwards or perpendicularly, does not work (by the way, horsepower is what the engine's power is measured ).

The work of force is a scalar quantity and is measured in joules. She may be:

• Resultant (under the influence of several forces);
• (Then the computation is performed with the integral).

Power

In what way is this value measured? First, let's see what she is like. It is clear that the movement of the body begins at the expense of the force that performs mechanical work. However, in practice, in addition, it is necessary to know exactly how it is done.

The work can be completed in different terms. For example, one or the same action can be performed by a small motor or a large electric motor. The only question is, for how long it will be produced. The size responsible for this task is power. In what is measured, it becomes clear from the definition - is the ratio of work for a particular time to its magnitude:

N = A / Δt.

By logical actions we arrive at the following formula:

N = F · v,

That is, the product of the force vectors by the speed of motion - and is the power. How is it measured? According to the international SI system, the unit of measurement for this value is 1 Watt.

Watt and other power units

Watt means the power, where in one second the work is done in one joule. The last unit was named so in honor of the Englishman J. Watt, who invented and built the first steam engine. But he at the same time used a different amount - the horsepower, which is still used today. One horsepower is approximately 735.5 watts.

Thus, in addition to Watts, power is measured in metric horsepower. And with a very small value, Erg is also used, equal to ten minus the seventh power of Watts. It is also possible to measure in one unit of mass / force / meters per second, which is 9.81 watts.

Engine power

The named value is one of the most important in any motor, which happens to be of a very different power. For example, an electric razor has hundredths of a kilowatt, and the rocket of a spacecraft has millions.

Different loads require different power to maintain a certain speed. For example, a car will become heavier if more cargo is placed in it. Then the friction force on the road will increase. Therefore, to maintain the same speed as in an unloaded state, more power will be required. Accordingly, the engine will eat more fuel. This fact is known to all drivers.

But at high speed, the inertia of the machine is also important, which is directly proportional to its mass. Experienced drivers who know about this fact find a better combination of fuel and speed when driving, so that gasoline goes less.

Current Power

What is the current measurement? In the same SI unit. It can be measured directly or indirectly.

The first method is realized by means of a wattmeter, which consumes substantial energy and strongly loads the current source. With its help it is measured from ten watts and more. Indirect method is used if necessary to measure small values. Instruments for this are an ammeter and a voltmeter connected to the consumer. The formula in this case will look like this:

P = U · I.

With the known resistance of the load, we measure the current flowing through it and find the power as follows:

P = I ² ∙ R _n.

According to the formula P = I ² / R _{n, the} current power can also be calculated.

What is measured in a three-phase current network is also not a secret. To do this, use the already familiar device - a wattmeter. And to solve a problem, than the electric power is measured , it is possible by means of one, two or even three devices. For example, a four-wire installation requires three devices. And for three-wire with an asymmetric load - two.