Hello! To calculate a physical quantity called power, they use the formula where the physical quantity - work - is divided by the time during which this work was performed.
It looks like this:
P, W, N=A/t, (W=J/s).
Depending on the textbooks and sections of physics, power in the formula can be denoted by the letters P, W or N.
Most often, power is used in such branches of physics and science as mechanics, electrodynamics and electrical engineering. In each case, power has its own formula for calculation. It is also different for alternating and direct current. Wattmeters are used to measure power.
Now you know that power is measured in watts. In English, watt is watt, the international designation is W, the Russian abbreviation is W. This is important to remember because in all household appliances there is such a parameter.
Power - scalar quantity, it is not a vector, unlike a force, which can have a direction. In mechanics, the general form of the power formula can be written as follows:
P=F*s/t, where F=A*s,
From the formulas you can see how instead of A we substitute the force F multiplied by the path s. As a result, power in mechanics can be written as force multiplied by speed. For example, a car having a certain power is forced to reduce speed when driving uphill, since this requires more force.
The average human power is taken to be 70-80 W. Power of cars, planes, ships, rockets and industrial installations, often measured in horsepower. Horsepower was used long before watts were introduced. One horsepower is equal to 745.7 W. Moreover, in Russia it is accepted that l. With. equal to 735.5 W.
If you are suddenly asked by chance 20 years later in an interview among passers-by about power, and you remember that power is the ratio of work A performed per unit time t. If you can say so, pleasantly surprise the crowd. Indeed, in this definition, the main thing to remember is that the divisor here is work A, and the divider is time t. As a result, having work and time, and dividing the first by the second, we get the long-awaited power.
When choosing in stores, it is important to pay attention to the power of the device. The more powerful the kettle, the faster it will heat the water. The power of the air conditioner determines how large a space it can cool without putting extreme load on the engine. The greater the power of an electrical appliance, the more current it consumes, the more electricity it will consume, and the higher the electricity bill will be.
IN general case electrical power is determined by the formula:
where I is current, U is voltage
Sometimes it is even measured in volt-amperes, written as V*A. The total power is measured in volt-amperes, and to calculate the active power you need to multiply the total power by the coefficient of performance (efficiency) of the device, then we get the active power in watts.
Often appliances such as an air conditioner, refrigerator, or iron operate cyclically, turning on and off from the thermostat, and their average power for total time The work may be small.
In alternating current circuits, in addition to the concept instantaneous power, coinciding with the general physical one, there are active, reactive and total powers. Apparent power is equal to the sum of active and reactive power.
To measure power use electronic devices- Wattmeters. The unit of measurement, Watt, was named after the inventor of the improved steam engine, which revolutionized the world. power plants of that time. Thanks to this invention, the development of industrial society accelerated, trains, steamships, and factories appeared that used the power of the steam engine for movement and production of products.
We have all come across the concept of power many times. For example, different cars have different engine power. Also, electrical appliances can have different power levels, even if they have the same purpose.
Power is a physical quantity characterizing the speed of work.
Respectively, mechanical power is a physical quantity that characterizes the speed of mechanical work:
That is, power is work per unit of time.
Power in the SI system is measured in watts: [ N] = [W].
1 W is 1 J of work done in 1 s.
There are other units of power measurement, such as horsepower:
It is in horsepower The engine power of cars is most often measured.
Let's return to the formula for power: We know the formula by which work is calculated: 
Therefore we can rearrange the expression for power:
Then in the formula we form the ratio of the displacement modulus to the period of time. This is, as you know, the speed:
Just note that in the resulting formula we use the velocity module, since we divided not the movement itself, but its module by time. So, power is equal to the product of the force modulus, the velocity modulus and the cosine of the angle between their directions.
This is quite logical: say, the power of the piston can be increased by increasing the force of its action. By applying more force, it will do more work in the same time, that is, it will increase power. But even if we leave the force constant and make the piston move faster, it will undoubtedly increase the work done per unit time. Consequently, the power will increase.
Examples of problem solving.
Task 1. The motorcycle's power is 80 hp. Moving along a horizontal section, a motorcyclist reaches a speed of 150 km/h. At the same time, the engine operates at 75% of its maximum power. Determine the friction force acting on the motorcycle.
Task 2. The fighter, under the influence of a constant thrust force directed at an angle of 45° to the horizon, accelerates from 150 m/s to 570 m/s. At the same time, the vertical and horizontal speed of the fighter increases by the same amount at each moment of time. The mass of the fighter is 20 tons. If the fighter accelerates for one minute, then what is the power of its engine?
If you need to bring power units into one system, you will need our power conversion - online converter. And below you can read how power is measured.
Who is the faster person or crane will lift the entire load to a height? Which lifting mechanism has more power?
Power characterizes the speed at which work is done.
Power (N) is a physical quantity equal to the ratio of work A to the time period t during which this work was performed.
Power shows how much work is done per unit of time.
IN International system units (SI) The unit of power is called the Watt (W) in honor of the English inventor James Watt (Watt), who built the first steam engine.
[N] = W = J/s
1 W = 1 J/s
1 Watt equal to power force that does 1 J of work in 1 second or,
when a load weighing 100g is lifted to a height of 1m in 1 second.
James Watt himself (1736 - 1819) used a different unit of power - horsepower(1 hp), which he introduced in order to compare the performance of a steam engine and a horse.
1 hp = 735 W
However, in real life the average horse has about 1/2 hp, although of course different horses are different.
“Living engines” can briefly increase their power several times.
When running and jumping, a horse can increase its power up to ten times or more.
Making a jump to a height of 1 m, a horse weighing 500 kg develops a power equal to 5,000 W = 6.8 hp.
It is believed that the average power of a person during quiet walking is approximately 0.1 hp. i.e. 70 - 90W.
Like a horse, a person can develop power many times greater when running and jumping.
It turns out that the most powerful source of mechanical energy is a firearm!
Using a cannon, you can throw a cannonball weighing 900 kg at a speed of 500 m/s, developing about 110,000,000 J of work in 0.01 seconds. This work is equivalent to the work of lifting 75 tons of cargo to the top of the Cheops pyramid (height 150 m).
The power of the cannon shot will be 11,000,000,000 W = 15,000,000 hp.
The force of tension in a person's muscles is approximately equal to the force of gravity acting on him. When 2 people of equal weight climb the stairs to the same height, but with at different speeds, then which of them develops more power?
DON'T FORGET THAT
This formula is valid for uniform motion with constant speed and in the case of variable motion for average speed.
It follows that
From the above formulas it is clear that with constant engine power, the speed of movement is inversely proportional to the traction force and vice versa
This is the basis for the operating principle of the gearbox (gearbox) of various vehicles.
WHAT ARE YOU GOING WITH WITH THE “SUMMARY”?
Let's check it now!
1. Do the engines of a tram car develop the same power when it moves at the same speed without passengers and with passengers?
Answer: Pri nalitshii passashiriv sila tjashesti (ves) vagona bolshe, uvelitshivaetsja sila treniya, ravnaja v dannom slutshae sile tjagi, vosrastaet motshnost, uvelitshivaetsja rashod electroenergii.
2. Why does a ship with cargo move slower than without it? After all, the engine power in both cases is the same.
Answer: S uvelitsheniem nagruski korabl bolshe pogrushaetsja v wodu. eto uvelitshivaet silu soprotivlenija wodi dvisheniu korablja, tshto privodit k potere skorosti.
3. The tractor has three speeds: 3.08; 4.18 and 5.95 km/h. At what speed will it develop greater traction force on the hook with the same power?
Answer: 
If you figured it out on your own, then you are WELL DONE!
What if you looked at the answers? Maybe tired? Never mind, the holidays are coming!
Who will lift the entire load to a height faster, a person or a crane?
In order to drag 5 bags of potatoes from a garden located a couple of kilometers from home, you will need to run back and forth with a bucket all day. And if you take a cart, you can do it in two to three hours. What's the difference? The difference is in how quickly the work gets done.
Power characterizes the speed at which work is done.
Power (N) is a physical quantity equal to the ratio of work A to the time period t during which this work was performed.
power = work/time,
or
where N is power,
A – work,
t – time.
Power shows how much work is done per unit of time.
In the International System (SI), the unit of power is called the Watt (W) in honor of the English inventor James Watt (Watt), who built the first steam engine.
[N] = W = J/s
1 W = 1 J / 1s
1 Watt is equal to the power of a force that does 1 J of work in 1 second
or, when a load weighing 100g is lifted to a height of 1m in 1 second
James Watt himself (1736 - 1819) used another unit of power - horsepower (1 hp), which he introduced in order to compare the performance of a steam engine and a horse. 1hp = 735W. The measurement of power in horsepower is still used today, for example, when talking about power passenger car or truck
Application of power in physics
Power is the most important characteristic any engine. Different engines produce completely different power. This can be either hundredths of a kilowatt, for example, the engine of an electric razor, or millions of kilowatts, for example, the engine of a spacecraft launch vehicle.
Under different loads, a car engine produces different power to keep moving at the same speed. For example, as the mass of the load increases, the weight of the car increases, and accordingly, the friction force on the road surface increases, and to maintain the same speed as without the load, the engine will have to do more work. Accordingly, the power generated by the engine will increase. The engine will consume more fuel. This is well known to all drivers. However, on high speed The inertia of a moving vehicle also plays a significant role, which is greater the greater its mass. Experienced truck drivers find optimal combination speed with the gasoline consumed so that the car burns less fuel.
It turns out that the most powerful source of mechanical energy is a firearm!
Using a cannon, you can throw a cannonball weighing 900 kg at a speed of 500 m/s, developing about 110,000,000 J of work in 0.01 seconds. This work is equivalent to the work of lifting 75 tons of cargo to the top of the Cheops pyramid (height 150 m)
The power of the cannon shot will be 11,000,000,000 W = 15,000,000 hp.
Lesson objectives:
- Get acquainted with power as a new physical quantity;
- Develop the ability to derive formulas using the necessary knowledge from past lessons; develop logical thinking, ability to analyze and draw conclusions;
- Apply knowledge of physics in the world around you.
Lesson progress
“And eternal battle! We only dream of peace
Through blood and dust...
The steppe mare flies, flies
And the feather grass crumples...
And there is no end! Miles and steeps flash by...
Stop it! ...There is no peace! The steppe mare gallops!”A. Blok “On the Kulikovo Field” (June 1908). (Slide 1).
Today I want to start the lesson with questions for you. (Slide 2).
1. Do you think the horse has anything to do with physics?
2. What physical quantity is a horse associated with?
Power– that’s right, this is the topic of our lesson. Let's write it down in a notebook.
Indeed, the engine power of cars and vehicles is still measured in horsepower. Today in the lesson we will learn everything about power from the point of view of physics. Let's think together and determine what we should know about power as a physical quantity.
There is a plan for studying physical quantities: (Slide 3).
- Definition;
- Vector or scalar;
- Letter designation;
- Formula;
- Measuring device;
- Unit of magnitude.
This plan will be the goal of our lesson.
Let's start with a real-life example. You need to collect a barrel of water to water the plants. The water is in the well. You have a choice: collect using a bucket or using a pump. Let me remind you that in both cases the mechanical work done will be the same. Of course, most of you will choose the pump.
Question: What is the difference when doing the same job?
Answer: The pump will do this work faster, i.e. will take less time.
1) The physical quantity characterizing the speed of work is called power. (Slide 4).
2) Scalar, because has no direction.
5) [N] = [1 J/s] =
The name of this unit of power is given in honor of the English inventor of the steam engine (1784), James Watt. (Slide 5).
6) 1 W = power at which 1 J of work is done in 1 s (Slide 6).
Planes, cars, ships and others vehicles often move at a constant speed. For example, on highways a car can move at a speed of 100 km/h for quite a long time (Slide 7).
Question: what does the speed of movement of such bodies depend on?
It turns out that it directly depends on the power of the car engine.
Knowing the power formula, we will derive another one, but for this let’s remember the basic formula for mechanical work.
The student goes to the board to derive the formula. (Slide 8).
Let the force coincide in direction with the speed of the body. Let us write down the formula for the work of this force.
1.
2. At a constant speed of movement, the body travels a path determined by the formula
Substitute into the original power formula: 
, we get 
- power.
We have another formula for calculating power, which we will use when solving problems.
Power is always indicated in the passport technical device. And in modern technical passports cars there is a column:
Motor power: kW/hp
Therefore, there is a relationship between these units of power.
Question: Where did this unit of power come from? (Slide 11).
J. Watt came up with the idea of measuring mechanical power in “horsepower”. The unit of power he proposed was very popular, but in 1948, the General Conference of Weights and Measures introduced a new unit of power in the international system of units - the watt. (Slide 12).
1 hp = 735.5 W.
1 W = .00013596 hp
Examples of capacities modern cars. (Slide 13,14).
Different engines have different power ratings.
Textbook, page 134, table 5.
Question: What is the power of a person?
Textbook text, § 54. Human power at normal conditions work on average is 70-80 watts. When jumping or running up stairs, a person can develop power up to 730 W, and in some cases even more.
Question: How do “living engines” differ from mechanical ones? (Slide 15).
Answer: The fact that “living engines” can change their power several times.
Fixing the material.
1.Tell everything you know about power. Answer according to the plan for studying a physical quantity.
Answer: N ≈ 2.9 kW.
- § 54.
- Write power formulas in the formula table.
- Ex. 29 (2.5) – 1 level.
- Ex. 29 (1.3) – level 2.
- Ex. 29 (1.4) – 3rd level.
- Task 18 - on additional assessment(on pieces of paper).
Literature:
- A.V. Peryshkin “Physics textbook for 7th grade”, Bustard, Moscow, 2006.
- A. Blok “On the Kulikovo Field.”
- 1C: School Physics 7th grade
Before considering electrical power, it is necessary to determine what power is in general, as a physical concept. Usually, speaking about this value, a certain internal energy or the power possessed by some object. This could be the power of a device, such as a motor or an action (explosion). It should not be confused with strength, since these are different concepts, although they are in a certain relationship with each other. Any physical actions are carried out under the influence of force. With its help, a certain path is followed, that is, work is performed. In turn, work A done during a certain time t will amount to a power value expressed by the formula: N = A/t (W = J/s).
Another concept of power is related to the rate of energy conversion of a particular system. One of these transformations is power electric current, with the help of which the set is also fulfilled various works. First of all, it is associated with electric motors and other devices that perform useful actions.
What is electric power
Current power is associated with several physical quantities. Voltage (U) represents the work required to move 1 coulomb. The current strength (I) corresponds to the number of coulombs passing in 1 second. Thus, current multiplied by voltage (I x U) corresponds to full time job completed in 1 second. The resulting value will be the power of the electric current.
The given formula for current power shows that power is equally dependent on current and voltage. It follows that the same value of this parameter can be obtained due to high current and low voltage and, conversely, at high voltage and low current. This property makes it possible to transmit electricity over long distances from sources to consumers. During the transmission process, the current is converted using transformers installed at step-up and step-down substations.
There are two main types of electrical power -. In the first case, there is an irreversible transformation of the power of electric current into mechanical, light, thermal and other types of energy. The unit of measurement used is the watt. 1W = 1V x 1A. In production and in everyday life, larger values are used - kilowatts and megawatts.
TO reactive power refers to the electrical load that is created in devices due to inductive and capacitive oscillations of the energy of the electromagnetic field. IN alternating current this quantity is a product expressed by the following formula: Q = U x I x sin(angle). The sine of the angle means the phase shift between the operating current and the voltage drop. Q is reactive power, measured in Var - volt-ampere reactive. These calculations help to effectively solve the question of how to find the power of an electric current, and the formula that exists for this allows you to quickly perform calculations.
Both powers can be clearly seen in simple example. Any electrical device is equipped heating elements- heating elements and electric motor. For the manufacture of heating elements, a material with high resistance is used, therefore, when current passes through it, all electrical energy is converted into thermal energy. This example very accurately characterizes active electrical power.
As for the electric motor, there is a copper winding inside it that has inductance, which, in turn, has the effect of self-induction. Thanks to this effect, a partial return of electricity back to the network occurs. The returned energy is characterized by a slight shift in voltage and current parameters, causing negative impact on the electrical network in the form of additional overloads.
Capacitors have the same properties due to their electrical capacitance when the accumulated charge is given back. Here, the current and voltage values are also shifted, only in the opposite direction. This energy of inductance and capacitance, with a phase shift relative to the values of the existing electrical network, is precisely reactive electrical power. Due to the opposite effect of inductance and capacitance on the phase shift, it becomes possible to perform reactive power compensation, thereby increasing the efficiency and quality of power supply.
What formula is used to calculate the power of electric current?
The correct and accurate solution to the question of what the power of the electric current is equal to plays a decisive role in ensuring safe operation electrical wiring, fire prevention due to incorrectly selected cross-section of wires and cables. The current power in an active circuit depends on the current and voltage. To measure current strength there is a device - an ammeter. However, it is not always possible to use this device, especially when the building design is still being drawn up, and electrical circuit simply doesn't exist. For such cases it is provided special technique carrying out calculations. The current strength can be determined by the formula given the values of power, network voltage and the nature of the load.
There is a formula for current power in relation to constant values of current and voltage: P = U x I. If there is a phase shift between current and voltage, another formula is used for calculations: P = U x I x cos φ. In addition, the power can be determined in advance by summing the power of all devices that are scheduled for commissioning and connection to the network. This data is available in technical data sheets and operating manuals for devices and equipment.
Thus, the formula for determining the power of electric current allows you to calculate the current strength for single-phase network: I = P/(U x cos φ), where cos φ represents the power factor. If there is a three-phase electrical network the current strength is calculated using the same formula, only a phase coefficient of 1.73 is added to it: I = P/(1.73 x U x cos φ). The power factor depends entirely on the nature of the planned load. If you intend to use only lighting lamps or heating devices, then it will be one.
If there are reactive components in active loads, the power factor is already considered to be 0.95. This factor must be taken into account depending on what type of electrical wiring is used. If the devices and equipment have a sufficiently high power, then the coefficient will be 0.8. This concerns welding machines, electric motors and other similar devices.
For calculations subject to availability single-phase current The voltage value is assumed to be 220 volts. If present, design voltage will be 380 volts. However, in order to obtain the most accurate results, it is necessary to use in the calculations the actual voltage value measured with special instruments.
What does current power depend on?
The power of the current, various devices and equipment depends immediately on two main quantities - and. The higher the current, the more value power, accordingly, as the voltage increases, the power also increases. If the voltage and current increase simultaneously, then the power of the electric current will increase as the product of both quantities: N = I x U.
Very often the question arises, how is current power measured? The basic unit of measurement for this quantity is (W). Thus, 1 watt is the power of a device that draws 1 ampere of current at 1 volt. A light bulb from a regular flashlight, for example, has similar power.
The calculated power value allows you to accurately determine the consumption electrical energy. To do this, you need to take the product of power and time. The formula itself looks like this: W = IUt where W is electricity consumption, the product IU is power, and t is the amount of time worked. For example, the longer the work continues electric motor, those great job it is accomplished by him. Electricity consumption also increases accordingly.