Presentation on the topic "thermal effect of current". Presentation on the topic "thermal effect of current" Presentation on the use of the thermal effect of electric current
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Completed by: Julia Bakovskaya, 9th grade student. Checked: Tsypenko L. V. Physics teacher. 2011
Thermal Action of Electric Current.
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The practical use of electricity is based on three fundamental actions that appear during the operation of an electric current: thermal, electromagnetic and chemical. Negative charged particles, which are usually called electrons, flowing through a certain substance, constantly have to collide with atoms, ions or molecules. After the collision, the electrons slow down, transferring the available energy to the elementary particles of the substance through which the electric current flows. The received energy contributes to an increase in the speed of movement of particles, the substance is heated.
Heat release.
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Increased heating of the conductor, as follows from the Lenz-Joule law, can occur not only due to the passage of a large current through it, but also due to an increase in the resistance of the conductor. With loose electrical contact and poor connection of conductors, the electrical resistance in these places increases greatly, and here there is an increased heat release. As a result, a loose connection of conductors will be a fire hazard, and significant heating can lead to complete burnout of poorly connected conductors.
Heating in transitional resistance.
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Functionality of the heat pump The total energy in the surrounding area provides about 75% of the heat energy of the heat pump. Using only 25% of external energy in the form of electricity, a thermal performance of 100% is achieved. Energy is obtained from the ambient air, ground or ground water through heat exchange systems. After that, the heat enters the heat pump cycle, where the temperature rises to values sufficient for heating.
Examples thermal action.
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Its internal tungsten filament has a high electrical resistance. Flowing along this filament (spiral), negative charged particles are transferred to tungsten ions a large number of energy. The tungsten filament of the lamp warms up to white - the electric light bulb shines. If the current strength is excessive, the energy that is transferred to the tungsten ions will be too large that the existing ions of the substance simply cannot be held in their original places. As a result, the tungsten filament will melt.
Incandescent lamp.
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In addition, the resistance of the conductor also depends on its thickness. The larger its cross section (thickness) of the wire, the better its conductivity, and the lower the electrical resistance. If we turn on any electrical device - a stove, an iron, an incandescent bulb, then the current strength in the existing electrical wiring at home is determined by the current voltage in the mains, the resistance of the electrical appliance and its wires. For example, the iron is turned on. The main role in this case is played by the electrical resistance of the iron, since the resistance of the supply wires is small, and the voltage of the electrical network is standard (an alternating voltage of 220 volts is used for everyday life).
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Electricity.
Electric current heats the conductor. It is explained by the fact that free electrons in metals, moving under the influence of an electric field, interact with ions or atoms of the conductor substance and transfer their energy to them. As a result of the work of the electric current, the speed of oscillations of ions and atoms increases and the internal energy of the conductor increases. Experiments show that in motionless metal conductors all the work of the current goes to increase their internal energy. The heated conductor gives the received energy to the surrounding bodies, but already by heat transfer. This means that the amount of heat released by the conductor through which the current flows is equal to the work of the current. We know that the work of the current is calculated by the formula: A \u003d U·I·t. Electric current in a conductor
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Ohm's law.
We denote the amount of heat by the letter Q. According to what was said above, Q = A, or Q = U·I·t. Using Ohm's law, it is possible to express the amount of heat released by a current-carrying conductor in terms of current strength, resistance of a circuit section and time. Knowing that U \u003d IR, we get: Q \u003d I R I t, i.e. Q \u003d I R t The amount of heat released by a current-carrying conductor is equal to the product of the square of the current strength, conductor resistance and time. The same conclusion, but on the basis of experiments, was first arrived at independently by the English scientist Joule and the Russian scientist Lenz. Therefore, the conclusion formulated above is called the Joule-Lenz law. Ohm's law for a circuit section
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Problem on Ohm's law for a chain section.
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Incandescent lamp device.
Consider the device of an incandescent lamp. The heated element in it is a thin tungsten filament coiled into a spiral 1. Tungsten was chosen for the manufacture of the filament because it is refractory and has a sufficiently high resistivity. The spiral with the help of special holders 2 is fixed inside a glass container filled with an inert gas, in the presence of which tungsten is not oxidized. The balloon is attached to the base 3, to which one end of the current-carrying wire is soldered at point 4. The second end of the wire is soldered to the lower contact through the insulating gasket 5. The lamp is screwed into the socket. It is a plastic case A, in which there is a metal sleeve B with a thread; one of the wires of the network is attached to it. The cartridge contacts base 3. The second wire from the mains is connected to pin B, which touches the lower contact of the lamp. Incandescent lamps are convenient, simple and reliable, but they are not economically viable. So, for example, in a 100 W lamp, only a small part of the electricity (4 W) is converted into visible light energy, and the rest of the energy is converted into invisible infrared radiation and transferred to the environment in the form of heat.
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Efficiency factor (COP).
To assess the effectiveness of a device in technology, a special value has been introduced - the coefficient of performance (COP). Efficiency is the ratio of energy usefully converted (work or power) to all energy consumed or expended (work or power):
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Efficiency is often expressed as a percentage (%). Let's calculate the efficiency of an electric incandescent lamp according to the data given above: h=4/100=0.04=4%; For comparison, we point out that the efficiency of a fluorescent lamp is approximately 15%, and that of outdoor sodium lamps is about 25%. Daylight lamp power supply circuit
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There are a large number of electric heating devices, such as electric stoves, irons, samovars, boilers, heaters, electric blankets, hair dryers, which use the thermal effect of current. The main heating element is a coil made of a material with high resistivity. It is placed in ceramic insulators with good thermal conductivity, which are made in the form of a kind of beads. In appliances designed to heat liquids, the insulated coil is placed in stainless steel tubes. Its conclusions are also carefully isolated from the metal parts of the devices. The temperature of the coil during the operation of the heating device remains constant. This is explained by the fact that very quickly a balance is established between the electricity consumed from the network and the amount of heat given off by heat exchange to the environment.
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Electric arc.
A very efficient converter of electrical energy, which gives a lot of heat and light, is an electric arc. It is widely used for electric welding of metals, as well as a powerful light source. To observe an electric arc, it is necessary to fix two carbon rods with wires attached to them in well-insulating holders, and then connect the rods to a current source that gives a low voltage (from 20 to 36 V) and is designed for high currents (up to 20 A). Consistently, the rods must turn on the rheostat. In no case should you connect coals to the city network (220 or 127 V), as this will lead to the burning of wires and a fire. Touching the coals to each other, you can see that at the point of contact they are very hot. If at this moment the coals are moved apart, a bright blinding flame appears between them, having the shape of an arc. This flame is harmful to the eyes. The flame of an electric arc has a high temperature at which the most refractory materials melt, so an electric arc is used in electric arc furnaces for melting metals. The arc flame is a very bright light source, so it is often used in spotlights, fixed movie projectors, etc.
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Electrical circuits.
Electrical circuits are always designed for a certain amount of current. If, for one reason or another, the current strength in the circuit becomes more than permissible, then the wires can heat up significantly, and the insulation covering them can ignite. The reason for a significant increase in the current strength in the network may be either the simultaneous inclusion of powerful current consumers, such as electric stoves, or a short circuit. A short circuit is the connection of the ends of a circuit section with a conductor whose resistance is very small compared to the resistance of the circuit section. A short circuit can occur, for example, when repairing live wiring or when bare wires accidentally come into contact. The resistance of the circuit during a short circuit is negligible, therefore, a great power current, the wires may become very hot and cause a fire. To avoid this, fuses are included in the network. The purpose of the fuses is to immediately turn off the line if the current strength suddenly turns out to be more than the permissible norm.
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Consider the arrangement of fuses used in residential wiring. The main part of the fuse shown in the figure is wire C made of low-melting metal (for example, lead), passing inside a porcelain cork P. The cork has a screw thread P and a central contact K. The thread is connected to the central contact with a lead wire. The cork is screwed into a cartridge inside a porcelain box. The lead wire is thus part of the overall chain. The thickness of the lead wires is designed so that they can withstand a certain current, for example 5, 10 A, etc. If the current exceeds the allowable value, then the lead wire will melt and the circuit will be open. Fuses with a consumable conductor are called fuses.
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Presentation on the topic: Thermal effect of current
slide number 1
Description of the slide:
slide number 2
Description of the slide:
Electricity. Electric current heats the conductor. It is explained by the fact that free electrons in metals, moving under the influence of an electric field, interact with ions or atoms of the conductor substance and transfer their energy to them. As a result of the work of the electric current, the speed of oscillations of ions and atoms increases and the internal energy of the conductor increases. Experiments show that in motionless metal conductors, all the work of the current goes to increase their internal energy. The heated conductor gives the received energy to the surrounding bodies, but already by heat transfer. This means that the amount of heat released by the conductor through which the current flows is equal to the work of the current. We know that the work of the current is calculated by the formula: A \u003d U·I·t. Electric current in a conductor
slide number 3
Description of the slide:
Ohm's law. We denote the amount of heat by the letter Q. According to what was said above, Q = A, or Q = U·I·t. Using Ohm's law, it is possible to express the amount of heat released by a current-carrying conductor in terms of current strength, resistance of a circuit section and time. Knowing that U \u003d IR, we get: Q \u003d I R I t, i.e. Q \u003d I R t The amount of heat released by a current-carrying conductor is equal to the product of the square of the current strength, conductor resistance and time. The same conclusion, but on the basis of experiments, was first arrived at independently by the English scientist Joule and the Russian scientist Lenz. Therefore, the conclusion formulated above is called the Joule-Lenz law. Ohm's law for a circuit section
slide number 4
Description of the slide:
slide number 5
Description of the slide:
slide number 6
Description of the slide:
slide number 7
Description of the slide:
slide number 8
Description of the slide:
Incandescent lamp device. Consider the device of an incandescent lamp. The heated element in it is a thin tungsten filament coiled into a spiral 1. Tungsten was chosen for the manufacture of the filament because it is refractory and has a sufficiently high resistivity. The spiral with the help of special holders 2 is fixed inside a glass container filled with an inert gas, in the presence of which tungsten is not oxidized. The balloon is attached to the base 3, to which one end of the current-carrying wire is soldered at point 4. The second end of the wire is soldered to the lower contact through the insulating gasket 5. The lamp is screwed into the socket. It is a plastic case A, in which there is a metal sleeve B with a thread; one of the wires of the network is attached to it. The cartridge contacts base 3. The second wire from the mains is connected to pin B, which touches the lower contact of the lamp. Incandescent lamps are convenient, simple and reliable, but they are not economically viable. So, for example, in a 100 W lamp, only a small part of the electricity (4 W) is converted into visible light energy, and the rest of the energy is converted into invisible infrared radiation and transferred to the environment in the form of heat.
slide number 9
Description of the slide:
Efficiency factor (COP). To assess the effectiveness of a device in technology, a special value has been introduced - the coefficient of performance (COP). Efficiency is the ratio of energy usefully converted (work or power) to all energy consumed or expended (work or power):
slide number 10
Description of the slide:
Efficiency is often expressed as a percentage (%). Let's calculate the efficiency of an electric incandescent lamp according to the data given above: h=4/100=0.04=4%; For comparison, we point out that the efficiency of a fluorescent lamp is approximately 15%, and that of outdoor sodium lamps is about 25%. Daylight lamp power supply circuit
slide number 11
Description of the slide:
There are a large number of electric heating devices, such as electric stoves, irons, samovars, boilers, heaters, electric blankets, hair dryers, which use the thermal effect of current. The main heating element is a coil made of a material with high resistivity. It is placed in ceramic insulators with good thermal conductivity, which are made in the form of a kind of beads. In appliances designed to heat liquids, the insulated coil is placed in stainless steel tubes. Its conclusions are also carefully isolated from the metal parts of the devices. The temperature of the coil during the operation of the heating device remains constant. This is explained by the fact that very quickly a balance is established between the electricity consumed from the network and the amount of heat given off by heat exchange to the environment.
slide number 12
Description of the slide:
Electric arc. A very efficient converter of electrical energy, which gives a lot of heat and light, is an electric arc. It is widely used for electric welding of metals, as well as a powerful light source. To observe an electric arc, it is necessary to fix two carbon rods with wires attached to them in well-insulating holders, and then connect the rods to a current source that gives a low voltage (from 20 to 36 V) and is designed for high currents (up to 20 A). Consistently, the rods must turn on the rheostat. In no case should you connect coals to the city network (220 or 127 V), as this will lead to the burning of wires and a fire. Touching the coals to each other, you can see that at the point of contact they are very hot. If at this moment the coals are moved apart, a bright blinding flame appears between them, having the shape of an arc. This flame is harmful to the eyes. The flame of an electric arc has a high temperature at which the most refractory materials melt, so an electric arc is used in electric arc furnaces for melting metals. The arc flame is a very bright light source, so it is often used in spotlights, fixed movie projectors, etc.
slide number 13
Description of the slide:
Electrical circuits. Electrical circuits are always designed for a certain amount of current. If, for one reason or another, the current strength in the circuit becomes more than permissible, then the wires can heat up significantly, and the insulation covering them can ignite. The reason for a significant increase in the current strength in the network may be either the simultaneous inclusion of powerful current consumers, such as electric stoves, or a short circuit. A short circuit is the connection of the ends of a circuit section with a conductor whose resistance is very small compared to the resistance of the circuit section. A short circuit can occur, for example, when repairing live wiring or when bare wires accidentally come into contact. The resistance of the circuit during a short circuit is negligible, so a large current is generated in the circuit, the wires can become very hot and cause a fire. To avoid this, fuses are included in the network. The purpose of the fuses is to immediately turn off the line if the current strength suddenly turns out to be more than the permissible norm.
slide number 14
Description of the slide:
Consider the arrangement of fuses used in residential wiring. The main part of the fuse shown in the figure is wire C made of low-melting metal (for example, lead), passing inside a porcelain cork P. The cork has a screw thread P and a central contact K. The thread is connected to the central contact with a lead wire. The cork is screwed into a cartridge inside a porcelain box. The lead wire is thus part of the overall chain. The thickness of the lead wires is designed so that they can withstand a certain current, for example 5, 10 A, etc. If the current exceeds the allowable value, then the lead wire will melt and the circuit will be open. Fuses with a consumable conductor are called fuses.