Moving in any conductor, an electric current transfers some energy to it, which causes the conductor to heat up. Energy transfer occurs at the molecular level: as a result of the interaction of current electrons with ions or atoms of the conductor, part of the energy remains with the latter.
The thermal effect of current leads to more fast movement conductor particles. Then it increases and transforms into heat.
Calculation formula and its elements
The thermal effect of current can be confirmed by various experiments, where the work of the current is converted into internal conductive energy. At the same time, the latter increases. Then the conductor gives it to surrounding bodies, that is, heat transfer occurs with heating of the conductor.
The formula for calculation in this case is as follows: A=U*I*t.
The amount of heat can be denoted by Q. Then Q=A or Q=U*I*t. Knowing that U=IR, it turns out Q=I 2 *R*t, which was formulated in the Joule-Lenz law.
The law of thermal action of current - Joule-Lenz law
The conductor where it flows has been studied by many scientists. However, the most noticeable results were achieved from England and Emilius Christianovich Lentz from Russia. Both scientists worked separately and drew conclusions from the results of the experiments independently of each other.
They derived a law that allows one to estimate the heat resulting from the action of current on a conductor. It was called the Joule-Lenz law.
Let us consider in practice the thermal effect of current. Let's take the following examples:
- An ordinary light bulb.
- Heating devices.
- Fuse in the apartment.
- Electric arc.
Incandescent light bulb
The thermal effect of current and the discovery of the law contributed to the development of electrical engineering and an increase in the possibilities for using electricity. How research results are applied can be seen using the example of an ordinary incandescent light bulb.
It is designed in such a way that a thread made of tungsten wire. This metal is refractory with high resistivity. When passing through a light bulb, a thermal effect occurs electric current.
The energy of the conductor is transformed into heat, the spiral heats up and begins to glow. The disadvantage of a light bulb is its large energy losses, since it begins to glow only with a small part of the energy. The main part simply heats up.
To understand this better, it is introduced which demonstrates the efficiency of operation and conversion to electricity. Efficiency and thermal effect of current are used in different areas, since there are many devices manufactured based on this principle. IN to a greater extent These are heating devices, electric stoves, boilers and other similar devices.
Design of heating devices
Typically, the design of all heating devices has a metal spiral, the function of which is heating. If water is heated, the spiral is installed in isolation, and such devices ensure that a balance is maintained between energy from the network and heat exchange.
Scientists are constantly tasked with reducing energy losses and finding better ways and the most effective schemes for their implementation to reduce the thermal effect of current. For example, a method is used to increase the voltage while reducing the current. But this method, at the same time, reduces the safety of the operation of power lines.
Another research area is wire selection. After all, heat loss and other indicators depend on their properties. In addition, when heating devices operate, a large release of energy occurs. Therefore, spirals are made from materials specially designed for these purposes and capable of withstanding high loads.
Apartment fuses
To improve protection and secure electrical circuits, special fuses are used. The main part is a wire made of fusible metal. It runs in a porcelain plug, has a screw thread and a contact in the center. The plug is inserted into a cartridge located in a porcelain box.
The lead wire is part of the overall circuit. If the thermal effect of the electric current increases sharply, the cross-section of the conductor will not withstand it, and it will begin to melt. As a result of this, the network will open and there will be no current overloads.
Electric arc
The electric arc is a fairly efficient converter electrical energy. It is used in welding metal structures and also serves as a powerful light source.
The device is based on the following. Take two carbon rods, connect the wires and attach them in insulating holders. After this, the rods are connected to a current source, which gives a low voltage, but is designed for high current. Connect the rheostat. It is prohibited to include coals in the city network, as this may cause a fire. If you touch one coal to another, you will notice how hot they become. It is better not to look at this flame, because it is harmful to your eyesight. An electric arc is used in furnaces for melting metal, as well as in such powerful lighting devices as spotlights, film projectors, etc.
It's pretty hard to imagine life modern man without electricity. It has become one of the main and most valuable attributes of modern existence. In fact, anyone who has ever worked with electricity knows that when current passes through wires, they tend to heat up. Why does this depend?
What is current
Current is the ordered movement of charged particles called electrons. And if current flows through a conductor, then different things begin to happen in it. physical processes, namely, electrons collide with molecules.
Molecules are neutral or those that have lost their negatively charged particle. As a result of collisions, either electrons can become neutral molecules, or an electron is knocked out of another similar molecule, forming a positively charged ion. During these collisions, the kinetic energy of the charged particles is consumed. It is this energy that becomes heat.
Thermal heating of the conductor can also be affected by resistance. For example, you can take certain body and drag it along the ground. The earth in this case is resistance. What will happen to him? That's right, a frictional force will occur between the body and the surface, which, in turn, heats the body. The current behaves exactly the same in this case.
Addiction
And, taking into account all of the above, scientists were able to determine this relationship between current strength, resistance and amount of heat. This dependence is called the Joule-Lenz law, the formula of which is known to all physicists. In 1832-1833, the Russian physicist Emilius Lentz discovered that when metal conductors were exposed to heat, their conductivity changed dramatically. This actually complicated the scientist’s work and made it difficult to calculate electrical circuits.
At the same time, the young scientist came up with the idea that perhaps there was some kind of relationship between the current strength and the temperature of the conductor. But what to do? At that time there were no exact electrical devices, allowing to measure current strength, resistance, there was not even a source of stable EMF. This did not stop Lenz; he decided to conduct an experiment.
Experiments of a Russian physicist
The essence of this experiment was so simple, like everything ingenious, that even a schoolboy could repeat it. The scientist designed special device, which served to measure the amount of heat generated by the conductor. This device turned out to be an ordinary vessel, into which Lenz poured a solution of diluted alcohol and placed a conductor - a platinum wire, to which an electric current was supplied.
After the device was created, the scientist began conducting experiments. He measured the exact amount of time required for the alcohol in the vessel to be heated to 10 o C. Many not only months, but also years were spent on this. And in 1843, 10 years later, a law was published, the essence of which was that the heating of a conductor by current is proportional to the square of the current used for heating.
Joule and Lenz
But it was not there! It turns out that several years ago English physicist James Prescott Joule conducted similar experiments and has already published his observations. What should I do? Lenz did not give up and carefully studied Joule's work and came to the conclusion that, even though they performed the same experiments, Lenz's experiments were much more accurate. In connection with this science community added Lenz amendments to Joule's work and this law became known as the Joule-Lenz law. The mathematical formulation of the law looks like this:
Q = I *U*t, where:
- I - current strength, A;
- U - voltage, V;
- t is the time it takes the current to pass through the conductor, s.
The law itself sounds like this: the amount of thermal energy released in a conductor through which an electric current flows is equal to the product of the current strength, the voltage and the time the current passes through the conductor.
Ohm's law
However, will this statement always be true? You can try to derive it using Ohm's law. Judging by it, U = I*R, where R is resistance, Ohm.
Taking into account Ohm's law, you can substitute the value into the formula Q = I*U*t = I 2 *R*t. From this we can conclude that the amount of heat directly depends on the resistance of the conductor. Also for the Joule-Lenz law this statement will be true: I = Q = I*U*t.
All three formulas will be correct, but Q = I 2 *R*t will be true for all situations. The other two are also correct, but under certain circumstances.
Conductors
Now about the conductors. Initially, in their experiments, Joule and Lenz used platinum wires, as mentioned above. In all similar experiments, scientists of that time used mainly metal conductors, since they were quite inexpensive and stable. It is not surprising, because until now metal conductors are the main type of conductors, and therefore it was initially believed that the Joule-Lenz law was applicable only to them. However, a little later it was discovered that this law applies not only to metal conductors. It is true for any of them. The conductors themselves according to classification can be divided into:
- Metal (copper, iron, silver, etc.). The main role in them is played by negatively charged particles (electrons) that flow through the conductor.
- Liquid. In them, ions are responsible for the movement of charges - these are atoms in which there are either too many or too few electrons.
- Gaseous. Unlike their counterparts, in such conductors the current is determined by the movement of both ions and electrons.
And despite the differences, in any case, as the current or resistance increases, the amount of heat will also increase.
Application of the law by other physicists
The discovery of the Joule-Lenz law held great promise. After all, in fact, this law made it possible to create various kinds of electric heating devices and elements. For example, a little later after the discovery of the law, scientists noticed that when certain elements are heated, they begin to glow. They wanted to experiment with them using different conductors, and in 1874, Russian engineer Alexander Nikolaevich Lodygin invented the modern incandescent lamp, the filament of which was made of tungsten.
The Joule-Lenz law is also applied in electrical engineering - for example, when creating fuses. A fuse is a certain element of an electrical circuit, the design of which is made in such a way that when a current flows through it above the permissible value (for example, during a short circuit), it overheats, melts and opens the power circuit. Even an ordinary electric kettle or microwave oven, which virtually everyone has, works according to this law.
Conclusion
It is quite difficult to determine the contribution of these scientists to modern electronics and electrical engineering, but one thing is for sure - the emergence of the Joule-Lenz law changed people’s understanding of electricity and gave more specific knowledge of what the electric field is in a current-carrying conductor.
Without a doubt, the law discovered by these great physicists became a defining step in all science, and it was thanks to this discovery that other more or less grandiose achievements of other scientists were subsequently made. All science is a close interweaving of discoveries, some solved and unresolved problems. The law discussed in this article in a certain way influenced many studies and left an indelible and quite distinct mark on science.
Content:The famous Russian physicist Lenz and the English physicist Joule, conducting experiments to study the thermal effects of electric current, independently derived the Joule-Lenz law. This law reflects the relationship between the amount of heat generated in a conductor and the electric current passing through this conductor over a certain period of time.
Properties of electric current
When electric current passes through a metal conductor, its electrons constantly collide with various foreign particles. These can be ordinary neutral molecules or molecules that have lost electrons. In the process of moving, an electron can split off another electron from a neutral molecule. As a result, his kinetic energy is lost, and instead of a molecule, a positive ion is formed. In other cases, an electron, on the contrary, combines with a positive ion and forms a neutral molecule.
In the process of collisions of electrons and molecules, energy is consumed, which is subsequently converted into heat. The expenditure of a certain amount of energy is associated with all movements during which resistance has to be overcome. At this time, the work spent on overcoming friction resistance is converted into thermal energy.
Joule Lenz law formula and definition
According to Lenz's Joule law, an electric current passing through a conductor is accompanied by an amount of heat directly proportional to the square of the current and the resistance, as well as the time of flow of this current through the conductor.
In the form of a formula, the Joule-Lenz law is expressed in the following way: Q = I 2 Rt, in which Q displays the amount of heat released, I - , R - conductor resistance, t - time period. The value "k" represents the thermal equivalent of work and is used in cases where the amount of heat is measured in calories, current in , resistance in Ohms, and time in seconds. Numerical value the value of k is 0.24, which corresponds to a current of 1 ampere, which, with a conductor resistance of 1 ohm, releases an amount of heat equal to 0.24 kcal within 1 second. Therefore, to calculate the amount of heat released in calories, the formula Q = 0.24I 2 Rt is used.
When using the SI system of units, the amount of heat is measured in joules, so the value of “k”, in relation to the Joule-Lenz law, will be equal to 1, and the formula will look like: Q = I 2 Rt. According to I = U/R. If this current value is substituted into the basic formula, it will acquire next view: Q = (U 2 /R)t.
Basic formula Q = I 2 Rt is very convenient to use when calculating the amount of heat that is released in the case of a series connection. The current strength in all conductors will be the same. When several conductors are connected in series at once, each of them will release so much heat that will be proportional to the resistance of the conductor. If three identical wires made of copper, iron and nickel are connected in series, then maximum amount heat will be released last. This is due to the highest resistivity of nickel and the stronger heating of this wire.
At parallel connection the same conductors, the value of the electric current in each of them will be different, and the voltage at the ends will be the same. In this case, the formula Q = (U 2 /R)t is more suitable for calculations. The amount of heat generated by a conductor will be inversely proportional to its conductivity. Thus, the Joule-Lenz law is widely used to calculate installations electric lighting, various heating and heating devices, as well as other devices related to the conversion of electrical energy into heat.
Joule-Lenz law. Work and power of electric current
The Joule-Lenz law determines the amount of heat released in a conductor with resistance during a time t when an electric current passes through it.
Q = a*I*2R*t, where
Q - amount of heat released (in Joules)
a - proportionality coefficient
I - current strength (in Amperes)
R - Conductor resistance (in Ohms)
t - Travel time (in seconds)
The Joule-Lenz law explains that electric current is a charge that moves under the influence of electric field. In this case, the field does work, and the current has power and energy is released. When this energy passes through a stationary metal conductor, it becomes thermal energy, since it is aimed at heating the conductor.
In differential form, the Joule-Lenz law is expressed as bulk density The thermal power of the current in the conductor will be equal to the product of the specific electrical conductivity and the square of the electric field strength.
Application of the Joule-Lenz law
Incandescent lamps were invented in 1873 by the Russian engineer Lodygin. In incandescent lamps, as in electric heating devices, the Joule-Lenz law applies. They use a heating element, which is a high-resistance conductor. Due to this element, it is possible to achieve localized heat release in the area. Heat generation will appear with increasing resistance, increasing the length of the conductor, or choosing a specific alloy.One area of application of the Joule-Lenz law is to reduce energy losses.
The thermal effect of current leads to energy loss. When transmitting electricity, the transmitted power depends linearly on voltage and current, and the heating power depends on the current quadratically, so if you increase the voltage while lowering the current before supplying electricity, it will be more profitable. But an increase in voltage leads to a decrease in electrical safety. To increase the level of electrical safety, the load resistance is increased according to the increase in voltage in the network.
Also, the Joule-Lenz law affects the choice of wires for circuits. If the wires are selected incorrectly, the conductor, as well as it, can become very hot. This occurs when the current exceeds the maximum valid values and too much energy is released. With the correct selection of wires, it is worth following regulatory documents.
Sources:
There is a directly proportional relationship between current and voltage, described by Ohm's law. This law determines the relationship between current, voltage and resistance in a section of an electrical circuit.
Instructions
Remember current and voltage.
- Electric current is an ordered flow of charged particles (electrons). For quantification the quantity I used is called current strength.
- Voltage U is the potential difference at the ends of a section of an electrical circuit. It is this difference that causes the electrons to move, like a fluid flowing.
Current strength is measured in amperes. In electrical circuits, the current strength is determined by an ammeter. The unit of voltage is , you can measure the voltage in a circuit using a voltmeter. Collect the simplest electrical circuit from a current source, resistor, ammeter and voltmeter.
When a circuit is closed and current flows through it, record the instrument readings. Change the voltage at the ends of the resistance. You will see that the ammeter reading will increase as the voltage increases and vice versa. This experience directly demonstrates proportional dependence between current and voltage.
Let's look at the Joule-Lenz Law and its application.
When electric current passes through a conductor, it heats up. This happens because free electrons in metals and ions in electrolyte solutions moving under the influence of an electric field collide with molecules or atoms of conductors and transfer their energy to them. Thus, when work is performed by current increases internal energy conductor , it releases a certain amount of heat, equal to work current, and the conductor heats up: Q = A or Q = IUt .
Considering that U = IR , as a result we get the formula:
Q = I 2 Rt, Where
Q
- amount of heat released (in Joules)
I
- current strength (in Amperes)
R
— conductor resistance (in Ohms)
t
— travel time (in seconds)
Joule–Lenz law : the amount of heat generated by a current-carrying conductor is equal to the product of the square of the current, the resistance of the conductor and the time the current travels.
Where does the Joule-Lenz law apply?
1. For example, in incandescent lamps and in electric heating devices the Joule-Lenz law applies. They use a heating element, which is a high-resistance conductor. Due to this element, it is possible to achieve localized heat release in a certain area. Heat generation will appear with increasing resistance, increasing the length of the conductor, or choosing a specific alloy.
2. One of the areas of application of the Joule-Lenz law is reduction of energy losses . The thermal effect of current leads to energy loss. When transmitting electricity, the transmitted power depends linearly on voltage and current, and the heating power depends on the current quadratically, so if you increase the voltage while lowering the current before supplying electricity, it will be more profitable. But an increase in voltage leads to a decrease in electrical safety. To increase the level of electrical safety, the load resistance is increased according to the increase in voltage in the network.
3. Also, the Joule-Lenz law affects selection of wires for circuits . Because if the wires are selected incorrectly, the conductor may become very hot and may catch fire. This occurs when the current exceeds the maximum permissible values and too much energy is released.