How to find a job through resistance. How is the work of current measured?
How to calculate work electric current? We already know that the voltage at the ends of a section of the circuit is numerically equal to the work that is done when passing through this section electric charge in 1 Class. When an electric charge passing through the same area is not 1 C, but, for example, 5 C, the work done will be 5 times greater. Thus, in order to determine the work of electric current on any section of the circuit, it is necessary to multiply the voltage at the ends of this section of the circuit by the electric charge (amount of electricity) passing through it:
where A is work, U is voltage, q is electric charge. The electric charge passing through a section of the circuit can be determined by measuring the strength of the current and the time it passes:
Using this relationship, we obtain a formula for the work of electric current, which is convenient to use in calculations:
The work of an electric current on a section of a circuit is equal to the product of the voltage at the ends of this section by the current strength and the time during which the work was performed.
Work is measured in joules, voltage in volts, current in amperes and time in seconds, so we can write:
1 joule = 1 volt x 1 ampere x 1 second,
1 J = 1 VA s.
It turns out that to measure the work of electric current, three instruments are needed: a voltmeter, an ammeter and a clock. In practice, the work of electric current is measured with special instruments - counters. The design of the meter seems to combine the three devices mentioned above. Electricity meters can now be seen in almost every apartment.
Example. How much work does the electric motor do in 1 hour if the current in the electric motor circuit is 5 A and the voltage at its terminals is 220 V? Engine efficiency 80%.
Let's write down the conditions of the problem and solve it.
Questions
- What is equal to electrical voltage on the chain section?
- How can we express the work of electric current in this section through voltage and electric charge passing through a section of a circuit?
- How to express the work of current in terms of voltage, current and time?
- What instruments measure the work of electric current?
Exercise 34
- How much work is done by the electric current in the electric motor in 30 minutes if the current in the circuit is 0.5 A and the voltage at the motor terminals is 12 V?
- The voltage on the spiral of a light bulb from a flashlight is 3.5 V, the resistance of the spiral is 14 Ohms. How much work does the current do in the light bulb in 5 minutes?
- Two conductors, each with a resistance of 5 ohms, are connected first in series and then in parallel, and in both cases they are connected to a voltage of 4.5 V. In which case will the work done by the current be greater for the same time and by how many times?
It is known from a physics course that one of the characteristics of any body is its ability to do work, since the latter is nothing more than the transformation of one type of energy into another (for example, potential into kinetic). In this case, one should take into account the famous law of conservation of energy, formulated back in the 18th century by M.V. Lomonosov, according to whom energy never disappears anywhere, it only changes, takes on a different form. All of the above applies equally not only to solids, but also to other types of matter, including electric current.
As has long been proven, moving along a certain section of the circuit, these particles form an electric field that produces a current - this is the amount of energy that must be expended to transfer a charge along a given circuit. However, not all the work of the current is useful and effective. A fairly significant part of the energy is spent so that the electric charge overcomes the resistance of the elementary particles located in the conductor and in the source of the circuit.
The work of electric current, the formula of which, as follows from the above text, A = U.Q, is the most important characteristic this special type of matter. In this formula, U represents a section of the chain, and Q is a quantitative expression of the charge carried along a given section.
However, the work of electric current itself would not be of particular interest if a pattern had not been found that connected this work and the amount released. This pattern was almost simultaneously discovered by two famous physicists - Lenz and Joey Prescott, which is why the law in the scientific community received the name "Joule-Lenz law". According to this law, it turns out that the amount (or power) of heat that is released in a certain volume when charged particles flow through it is directly dependent on the product of the field strength and the density of the electric current flowing through a given area. This law is of great importance for calculating electricity losses during its transmission through wires to long distances.
The work of electric current is most directly related to another important quantity - power. In physics we mean the quantitative characteristics of the conversion and transmission rate of electrical energy. Power is measured in kilowatt-hours, while the work done by electric current is measured in joules.
To obtain maximum current power from a particular source, it is necessary to take into account the characteristics of this source, as well as the fact that the external circuit must be comparable with each other, otherwise all the work done will be spent on overcoming the difference in resistance.
The work of electric current is the most important physical characteristic that must be taken into account in almost all industries, as well as in the production and transmission of energy over long distances.
Content:Electric current is generated in order to be used in the future for certain purposes, to perform some kind of work. Thanks to electricity, all devices, devices and equipment function. The work itself represents a certain effort applied to move an electric charge over a set distance. Conventionally, such work within a section of the circuit will be equal to the numerical value of the voltage in this section.
To perform the necessary calculations, you need to know how the work of the current is measured. All calculations are carried out on the basis of initial data obtained using measuring instruments. The greater the charge, the more effort required to move it, the more work will be done.
What is the work of current called?
Electric current, as a physical quantity, in itself does not have practical significance. Most important factor is the action of the current, characterized by the work it performs. The work itself represents certain actions during which one type of energy is transformed into another. For example, electrical energy is converted into mechanical energy by rotating the motor shaft. The work of electric current itself is the movement of charges in a conductor under the influence of an electric field. In fact, all the work of moving charged particles is done by the electric field.
In order to perform calculations, a formula for the operation of electric current must be derived. To compile formulas, you will need parameters such as current strength and. Since the work done by an electric current and the work done by an electric field are the same thing, it will be expressed as the product of the voltage and the charge flowing in the conductor. That is: A = Uq. This formula was derived from the relationship that determines the voltage in the conductor: U = A/q. It follows that voltage represents the work done by the electric field A to transport a charged particle q.
The charged particle or charge itself is displayed as the product of the current strength and the time spent on the movement of this charge along the conductor: q = It. In this formula, the relation for the current strength in the conductor was used: I = q/t. That is, it is the ratio of the charge to the period of time during which the charge passes through the cross section of the conductor. In its final form, the formula for the work of electric current will look like the product of known quantities: A = UIt.
In what units is the work of electric current measured?
Before directly addressing the question of how the work of electric current is measured, it is necessary to collect the units of measurement of all physical quantities with which this parameter is calculated. Any work, therefore, the unit of measurement of this quantity will be 1 Joule (1 J). Voltage is measured in volts, current is measured in amperes, and time is measured in seconds. This means the unit of measurement will look like this: 1 J = 1V x 1A x 1s.
Based on the obtained units of measurement, the work of electric current will be determined as the product of the current strength in a section of the circuit, the voltage at the ends of the section and the period of time during which the current flows through the conductor.
Measurements are carried out using a voltmeter and a clock. These devices allow you to effectively solve the problem of how to find the exact value this parameter. When connecting an ammeter and a voltmeter to a circuit, it is necessary to monitor their readings for a specified period of time. The obtained data is inserted into the formula, after which the final result is displayed.
The functions of all three devices are combined in electric meters that take into account the energy consumed, and in fact the work done by electric current. Here another unit is used - 1 kW x h, which also means how much work was done during a unit of time.
Work of electric current
Attached to the circuit shown in Figure 1 is constant pressure U.
U = φ A – φ B
During t amount of electricity flows through the circuit Q. The forces of the electric field acting along the conductor transferred the charge during this time Q from point A exactly B. The work of electric field forces or, what is the same, the work of electric current can be calculated using the formula:
A = Q × ( φ A – φ B) = Q × U,
Because Q = I × t, then finally:
A= U × I × t,
Where A– work in joules; I– current in amperes; t– time in seconds; U– voltage in volts.
According to Ohm's law U = I × r. Therefore, the work formula can be written like this:
A = I 2× r × t.
Electric current power
The work done per unit time is called power and is denoted by the letter P.
From this formula we have:
A = P × t.
Power unit:
1 (J/sec) is otherwise called a watt (W). Substituting the expression for the work of electric current into the power formula, we have:
P = U × I(W).
The formula for electric current power can also be expressed in terms of current consumption and consumer resistance:
In addition to the watt, larger units of measurement of electrical power are used in practice. Electrical power is measured in:
100 W = 1 hectowatt (gW);
1000 W = 1 kilowatt (kW);
1,000,000 W = 1 megawatt (MW).
Electrical power is measured by a special device - a wattmeter. The wattmeter has two windings (coils): series and parallel. The series coil is a current coil and is connected in series with the load in the section of the circuit where measurements are made, and the parallel coil is a voltage coil, and accordingly it is connected in parallel to this load. The operating principle of the wattmeter is based on the interaction of two magnetic fluxes created by current, flowing through the winding of the moving coil (current coil), and the current passing through the fixed coil (voltage coil). When the measured current passes through the windings of the moving and stationary coils, two magnetic fields are formed, during the interaction of which the moving coil tends to position itself so that the direction of its magnetic field coincides with the direction of the magnetic field of the stationary coil. The torque is counteracted by the torque created by the helical springs, through which the measured current is conducted into the moving coil. The counteracting moment of the springs is directly proportional to the angle of rotation of the coil. An arrow mounted on a moving coil indicates the value of the measured quantity. The wattmeter connection diagram is shown in Figure 2.
If you decide to measure the power consumption of any load you have, and you do not have a wattmeter, you can “make” a wattmeter with your own hands. From the formula P = I × U It can be seen that the power consumed in the network can be determined by multiplying the current by the voltage. Therefore, to determine the power consumed from the network, two instruments should be used, a voltmeter and an ammeter. Having measured the current consumption with an ammeter and the voltage of the supply network with a voltmeter, it is necessary to multiply the ammeter reading by the voltmeter reading.
So, for example, the power consumed by the resistance r, with an ammeter reading of 3 A and a voltmeter of 220 V, it will be:
P = I × U= 3 × 220 = 660 W.
For practical measurements of electrical work (energy), the joule is too small a unit.
If time t substitute not in seconds, but in hours, we get larger units of electrical energy:
1 J = 1 W × sec;
1 W × h = 3600 watts × seconds = 3600 J;
100 W × h = 1 hectowatt × hour (gW × h);
1000 W × h = 1 kilowatt × hour (kW × h).
Electrical energy is measured by electrical energy meters.
Video 1. Operation and power of electric current
Video 2. A little more about power
Example 1. Determine the power consumed by the electric motor if the current in the circuit is 8 A and the motor is connected to a 220 V network.
P = I × U= 8 × 220 = 1760 W = 17.6 GW = 1.76 kW.
Example 2. What is the power consumed by an electric stove if the stove draws a current of 5 A from the network and the resistance of the stove's coil is 24 ohms?
P = I 2× r= 25 × 24 = 600 W = 6 gW = 0.6 kW.
When converting mechanical power into electrical power and vice versa, it must be remembered that
1 horsepower (hp) = 736 W;
1 kilowatt (kW) = 1.36 l. With.
Example 3. Determine the energy consumed by a 600 W electric stove over 5 hours.
A = P × t= 600 × 5 = 3000 W × h = 30 gW × h = 3 kW × h
Example 4. Determine the cost of burning twelve electric lamps within a month (30 days), if four of them, 60 W each, burn for 6 hours a day, and the remaining eight lamps, 25 W each, burn for 4 hours a day. Energy price (tariff) 2.5 rubles per 1 kW × h.
Power of four lamps 60 W each.
P= 60 × 4 = 240 W.
t= 6 × 30 = 180 hours.
A = P × t= 240 × 180 = 43200 W × h = 43.2 kW × h.
The power of the remaining eight lamps is 25 W each.
P= 25 × 8 = 200 W.
Number of hours of burning of these lamps per month:
t= 4 × 30 = 120 hours.
Energy consumed by these lamps:
A = P × t= 200 × 120 = 24000 W × h = 24 kW × h.
Total amount of energy consumed:
43.2 + 24 = 67.2 kW × h
Cost of all energy consumed:
67.2 × 2.5 = 168 rubles.
Every body is capable of producing work, this is called body energy. The simplest example is a body raised to a certain height. It has potential energy; if the body is released, it will begin to release energy, converting it into kinetic energy, at which point the body will do work.
Accordingly, the higher the height of the body, the greater its energy will be. Energy never disappears without a trace, it is only transformed into another form - this is one of the main laws of physics.
The same is true with electrical energy, it can be converted into another type of energy - thermal, kinetic, mechanical, chemical, etc.
Therefore, electricity has become so widely used. This type of energy, unlike any other, can be transmitted over long distances and stored with virtually no losses, and it can be obtained quite simply.
Work of electric current
When current flows through a certain section of the electrical circuit, the electric field does a certain amount of work. This is called the work of electric current. To transfer a charge of energy along this circuit, you need to expend a certain amount of energy. It is communicated to the receiver, and part of the energy is spent on overcoming the resistance of wires and sources in the electrical circuit.
This suggests that not all of the energy expended is distributed efficiently and not all of it is useful. Consequently, the work done is also not completely effective. IN in this case the formula will look like this: A = UQ.
U is the voltage at the receiver terminals, and Q- This is the charge transferred along a section of the circuit. In this case, you need to take into account Ohm's law for a circuit section , then the formula will look like this: R I2 Δt = U I Δt = ΔA.
Using this formula, you can trace the effect of the law of conservation of energy, which applies to a homogeneous section of the chain.
In 1850, the English physicist Joule Prescott, who made a significant contribution to the study of electricity, discovered a new law. Its essence was to determine the ways in which the work of electric current is converted into thermal energy. At the same time, another physicist, Lenz, was able to make a similar discovery and prove the law, so it was called the “Joule-Lenz law”, in honor of both outstanding physicists of that time.
Electric current power
Power is another characteristic used to determine the operation of electric current. This is a certain physical quantity that characterizes the transformation and speed of energy transfer.
When determining the power of an electric current, it is necessary to take into account such an indicator as instantaneous power. It represents the ratio of instantaneous values of such indicators as current and voltage in the form of a product. This ratio applies to a specific section of the circuit.
Indicators such as work and electric current power are taken into account when creating any electrical circuits. Along with other laws, they are fundamental; failure to comply with them will lead to serious violations.
In order to receive the greatest electric power, you need to take into account the characteristics of the generator, i.e. the resistance in the external circuit should be no more and no less internal resistance generator
Only in this case will the operating efficiency be maximum, because otherwise all the energy of the generator will be spent on overcoming the resistance, and all the work will be uneconomical. Naturally, such an operating scheme can negatively affect the efficiency of the entire electrical circuit.