How does a rheostat work in an electrical circuit? Using resistors and rheostats to regulate current in an electrical circuit

The lesson discusses a device called a rheostat, the resistance of which can be changed. The design of the rheostat and the principle of its operation are discussed in detail. The designation of the rheostat is shown on the diagrams, and possible options for including the rheostat in the electrical circuit. Examples of the use of a rheostat in everyday life are given.

Topic: Electromagnetic phenomena

Lesson: Rheostats

In previous lessons, we said that there are not only consumers and sources of electric current, but also so-called control elements. One of the important control elements is the rheostat or any other device based on its action. The rheostat uses a conductor made of a previously known material with a certain length and cross-section, which means we can find out its resistance. The principle of operation of a rheostat is based on the fact that we can change this resistance, therefore, we can regulate the current and voltage in electrical circuits.

Rice. 1. Rheostat device

Figure 1 shows a rheostat without a shell. This is done so that you can view all its parts. A wire (2) is wound on a ceramic pipe (1). Its ends are connected to two contacts (3a). There is also a rod at the end of which there is a contact (3b). A sliding contact (4), the so-called “slider,” moves along this rod.

If you place the sliding contact in the middle (Fig. 2a), then only half of the conductor will be used. If you move this sliding contact further (Fig. 2b), then more turns of wire will be used, therefore, its length will increase, the resistance will increase, and the current will decrease. If you move the “slider” in the other direction (Fig. 2c), then, on the contrary, the resistance will decrease and the current strength in the circuit will increase.

Rice. 2. Rheostat

The rheostat is hollow inside. This is necessary because when current flows, the rheostat heats up, and this cavity provides rapid cooling.

When we draw a diagram (a drawing of an electrical circuit), each element is indicated by a specific symbol. The rheostat is designated as follows (Fig. 3):

Rice. 3. Image of a rheostat

The red rectangle corresponds to the resistance, the blue contact is the wire leading to the rheostat, the green is the sliding contact. With this designation, it is easy to understand that when the slider moves to the left, the resistance of the rheostat will decrease, and when moved to the right, it will increase. The following image of the rheostat can also be used (Fig. 4):

Rice. 4. Another image of the rheostat

The rectangle indicates resistance, and the arrow indicates that it can be changed.

The rheostat is connected in series to the electrical circuit. Below is one of the connection diagrams (Fig. 5):

Rice. 5. Connecting a rheostat in a circuit with an incandescent lamp

Terminals 1 and 2 are connected to a current source (this can be a galvanic cell or a connection to a socket). It is worth noting that the second contact must be connected to the moving part of the rheostat, which allows you to change the resistance. If you increase the resistance of the rheostat, the intensity of the light bulb (3) will decrease, which means that the current in the circuit will also decrease. And, conversely, as the rheostat resistance decreases, the light bulb will burn brighter. This method is often used in switches to adjust light intensity.

A rheostat can also be used to regulate voltage. Below are two diagrams (Fig. 6):

Rice. 6. Connecting a resistor in a circuit with a voltmeter

In the case of using two resistances (Fig. 6a), we remove a certain voltage from the second resistor (a device that is based on the resistance of the conductor), and thus, as it were, regulate the voltage. In this case, you need to know exactly all the parameters of the conductor to correctly regulate the voltage. In the case of a rheostat (Fig. 6b), the situation is noticeably simplified, since we can continuously adjust its resistance, and therefore change the voltage removed.

A rheostat is a fairly universal device. In addition to adjusting current and voltage, it can also be used in various household appliances. For example, on televisions the volume is adjusted using rheostats, and switching channels on a TV is also in some way connected with the use of rheostats. It is also worth noting that for safety it is better to use rheostats equipped with a protective casing (Fig. 7).

Rice. 7. Rheostat in a protective casing

In this lesson we looked at the structure and use of a control element such as a rheostat. In the following lessons, problems related to conductors, rheostats and Ohm's law will be solved.

Bibliography

Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. Physics 8 / Ed. Orlova V.A., Roizena I.I. - M.: Mnemosyne.
Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.

Education Center "Teaching Technologies" ().
School demonstration physics experiment ().
Electrical Engineering ().

Homework

Page 108-110: questions No. 1-5. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
How can you regulate the intensity of a lamp using a rheostat?
Will the resistance always decrease when moving the rheostat slider to the right?
What is the reason for using a ceramic pipe in a rheostat?

In many electronic devices, you need to change the current to control the volume of the sound. Let's consider a device (rheostats) with which you can change the current and voltage. The current strength depends on the voltage at the ends of the circuit section and on the resistance of the conductor: I=U/R. If you change the conductor resistance R, then the current strength will change.

Resistance depends on length L, from the cross-sectional area S and on the conductor material - resistivity. In order to change the resistance of a conductor, you need to change the length, thickness or material. It is very convenient to change the length of the conductor.

Let's analyze a circuit consisting of a current source, a switch, an ammeter and a conductor in the form of an AC resistor made of wire with a high resistivity.

By moving contact C along this wire, you can change the length of the conductor that is involved in the circuit, thereby changing the resistance, and therefore the current strength. Therefore, it is possible to create a device with variable resistance, with which you can change the current strength. Such devices are called rheostats.

A rheostat is a device with variable resistance that serves to regulate current and voltage.

Rheostat device

A metal conductor, which is made of a material with high resistivity, is wound around a cylinder made of ceramic. This was done so that with a small change in length the resistance would change significantly. This metal wire is called winding. It is so called because it is wound on a ceramic cylinder.

The ends of the winding are brought out to clamps, which are called terminals. At the top of the rheostat there is a metal rod, which also ends with terminals. A sliding contact called a slider can move along the metal rod and along the winding. Since the sliding contact has this name, such a rheostat is called a slider rheostat.

Operating principle

The slider rheostat is connected to the circuit through two terminals: the lower one from the winding and the upper terminal, where the metal rod is. When it is connected to a circuit, the current through the lower terminal passes along the turns of the winding, and not across the turns. Next, the current passes through the sliding contact, then along the metal rod, and again into the circuit.

Thus, only part of the rheostat winding is used in the circuit. When the slider moves, the resistance of that part of the rheostat winding that is in the circuit changes. The length of the winding, resistance and current in the circuit change.

It is necessary to note that the current in the part of the rheostat through which it passes flows along each turn of the winding, and not across them. This is achieved by the fact that the winding turns are insulated with each other by a thin layer of insulating material. Let's figure out how contact is made between the turns of the winding and the slider.

When moving along the winding, the slider moves along its top layer, which has a stripped section of insulation in the path of the slider. This is how contact is made between the slider and the winding turn. The turns are insulated from each other.

The diagram shows a circuit with a current source, a switch, an ammeter and a slider rheostat. When you move the rheostat slider, its resistance and the current in the circuit change.

The slider rheostat can be connected to the circuit using two terminals: upper and lower. But rheostats are connected in a different way.

The rheostat can be connected via three terminals. The two bottom terminals are connected to the ends of the winding, and one wire from the top terminal. Voltage is applied to the entire winding, and only part of the winding is removed. The slider divides the rheostat into two resistors, which are connected in series.

The total voltage is equal to the sum of the voltages of each resistor. Therefore, the output voltage is less than the input value. The output voltage is less than the input voltage as many times as the resistance of part of the winding is less than the resistance of the entire winding. That is, a rheostat divides the voltage and is called a voltage divider or potentiometer.

Types and features of rheostats

Rheostat in the form of a torus

The two outer clamps are the ends of the winding, and the middle clamp is connected to the slider. By rotating the slider along the winding, you can change the resistance and current in the circuit.

Lever rheostats

They got this name because at its lower part there is a switch - a lever. Using it you can turn on different parts of the resistor spiral. The figure shows the operating principle of a lever rheostat.

A lever rheostat changes the current strength stepwise, while a slider rheostat changes the current strength smoothly. If there is a resistor in the circuit, then when you move the slider on the slider rheostat or when you switch the lever of the lever rheostat, the current strength and voltage at the ends of the resistor will change.

Plug

Such devices consist of a resistance store.

This is a set of different resistances. They are called spiral resistors. Using a plug, you can turn on or off different spiral resistors. When the plug is in a jumper, more current flows through the jumper rather than through the resistor. Thus, the resistor is turned off. Using a plug, you can get different resistances.

Materials and Cooling

The main element in the rheostat design is the manufacturing material, according to the type of which rheostats are divided into several types:

Coal.
Metal.
Liquid.
Ceramic.

The electric current in the resistances is converted into thermal energy, which must somehow be removed from them. Therefore, rheostats are also divided according to the type of cooling:

Airborne.
Liquid.

Liquid rheostats are divided into water and oil. The air type is used in any device design. Liquid cooling is used only for metal rheostats; their resistances are washed by the liquid or completely immersed in it. We must not forget that the coolant must also be cooled.

Metal rheostats

This is an air cooled rheostat design. Such models have gained popularity because they are easily suitable for various operating conditions with their electrical and thermal characteristics, as well as their design shape. They come with a continuous or step type of resistance adjustment.

The device has a movable contact that slides along fixed contacts located in the same plane. Fixed contacts are made in the form of flat-head screws, plates or bars. The moving contact is called a brush. It can be bridge or lever.

These types of rheostats are divided into self-aligning and non-self-aligning. The latter type has a simple design, but is unreliable in use, since the contact is often broken.

Oily

Oil-cooled devices increase heat capacity and heat-up time due to the good thermal conductivity of the oil. This makes it possible to increase the load for a short time, reduces the consumption of resistance material and the dimensions of the rheostat housing.

Parts immersed in oil must have a significant surface area for good heat transfer. In oil, the contacts' ability to disconnect increases. This is an advantage of this type of rheostat. Thanks to lubrication, increased forces can be applied to the contacts. Disadvantages include the risk of fire and contamination of the installation site.

In practice, it is often necessary to change the current strength in the circuit, making it either more or less. So, by changing the current in the speaker of the radio, we adjust the volume of the sound. By changing the current in the sewing machine motor, you can regulate its rotation speed.

In many cases, special devices - rheostats - are used to regulate the current in a circuit.

The simplest rheostat can be a wire made of a material with high resistivity, for example, nickel or nichrome. By connecting such a wire into the circuit of an electric current source through contacts A and C and moving the movable contact C, you can reduce or increase the length of the AC section included in the circuit. In this case, the resistance of the circuit will change, and, consequently, the current strength in it, this will be shown by the ammeter.

Rheostats used in practice are given a more convenient and compact shape. For this purpose, wire with high resistivity is used, and so that the long wire does not interfere with it, it is wound in a spiral.

One of the rheostats (slider rheostat) is shown in Figure a), and its symbol in the diagrams is in Figure b).

In this rheostat, nickel wire is wound on a ceramic cylinder. Above the winding there is a metal rod along which the slider can move. With its contacts it is pressed against the turns of the winding.

The electric current in the circuit passes from the turns of the wire to the slider, and through it into the rod, which has a clamp 1 at the end. Using this clamp and clamp 2, connected to one of the ends of the winding and located on the rheostat body, the rheostat is connected to the circuit.

The arrows indicate how electric current flows through the rheostat

By moving the slider along the rod, you can increase or decrease the resistance of the rheostat included in the circuit. That is, we increase or decrease the number of turns through which electric current flows (the more turns, the greater the resistance).

Each rheostat is designed for a certain resistance (the more wire is wound, the greater the resistance such a rheostat can provide) and for the highest permissible current, which should not be exceeded, since the rheostat winding becomes hot and can burn out. The rheostat resistance and the maximum permissible current value are indicated on the rheostat ( see figure a).

[The values of 6Ω and 3 A mean that this rheostat is capable of changing its resistance from 0 to 6 Ohms, and a current with a force of more than 3 Amps should not be passed through it.]

Now is the time to move from theory to practice!

Part 1. Adjusting the current in the light bulb.

The video shows how by moving the rheostat slider to the right and left, the light bulb burns brighter or dimmer.

You can understand the principle of the experiment by looking at the diagram (see Figure 4).

The figure shows the circuit diagram that we assembled in the video. The total resistance of the circuit consists of the resistance R l of the light bulb and the resistance of the part of the wire included in the circuit (shaded in the figure) of the rheostat. The unshaded part of the wire is not included in the circuit. If you change the position of the slider, the length of the part of the wire connected to the circuit will change, which will lead to a change in the current strength.

So, if you move the slider to the extreme right position (point C), then the entire wire will be included in the circuit, the resistance of the circuit will be greatest, and the current strength will be the smallest, so the light bulb filament will burn dimly or will not burn at all (since the electric current such a force cannot heat the coil of a light bulb until it glows).

If you move the rheostat slider to position A, then the electric current will not flow through the rheostat wire at all and, therefore, the resistance of the rheostat will be zero. All current will be spent on burning the lamp, and it will shine as brightly as possible.

Part 2. Connecting the light bulb from a flashlight to a 220 V network.

Attention! Do not repeat this experience yourself. We remind you that electric shock from the lighting network can lead to death.

What happens if you plug a flashlight bulb into a 220 V lighting network? It is clear that a light bulb designed to operate on batteries with a total voltage of 3.5 Volts (3 AA batteries) is not able to withstand a voltage 63 times higher - it will immediately burn out (it may even explode).

How then to do this? A device already known to us - a rheostat - will come to the rescue.

We need a rheostat that could hold back the rapid flow of electric current coming from the lighting network and turn it into a thin stream of electricity that will power our fragile light bulb without harming it.

We took a rheostat with a resistance of 1000 (Ohm). This means that if email. If the current passes through the entire wire of this rheostat, then the output from it will be a current with a force of only 0.22 Amperes.

I=U/R=220 V / 1000 (Ohm) = 0.22 A

To power our light bulb, we need even stronger electricity (0.28 A). That is, the rheostat will not pass enough current to light our small light bulb.

This is what we see in the second part of the video, where in the extreme position of the slider the light does not light up, and when moving it to the right, the light begins to light up brighter and brighter (by moving the slider we release more and more current).

At a certain moment (at a certain position of the rheostat slider), the light bulb burns out, because the rheostat (at a given position of the slider) passed too much electricity, which burned out the filament of the light bulb.

So is it possible to include a low-voltage light bulb in a lighting network? Can! You just need to hold back all the excess electricity with a rheostat with a sufficiently high resistance.

Part 3. Connecting a 3.5 V lamp together with a 60 W lamp to a 220 V network.

We took a 60 W lamp rated at 220 V and a 3.5 V flashlight bulb with a current of 0.28 A.

What happens if you connect these light bulbs to a 220 V lighting network? It is clear that a 60-watt light bulb will light normally (that’s what it’s designed for), but a light bulb from a flashlight will immediately burn out when it is plugged in (since it is designed to operate on only 3.5 Volt batteries).

But experience shows how when connecting light bulbs one after another (in series) and plugging them into a 220 V network, both lamps burn at normal intensity and don’t even think about burning out. Even when the rheostat slider is in its extreme position (i.e. it does not create any resistance to current), the small light bulb does not burn out.

Why is that? Why does the lamp not burn out even when the rheostat is turned off (with zero resistance)? What prevents her from burning out under such high voltage? And is the voltage on a small light bulb really that high? Will a small lamp work if I replace a 60 W lamp with a 100 W lamp (100 W)?

You will already be able to answer most of the questions if you carefully followed the discussion in the previous part of the article. In this experiment, a small light bulb is prevented from burning out by a large light bulb. It acts as a rheostat with high resistance and takes on almost the entire load.

Let's try to figure out how this can happen, that a small light bulb does not burn out thanks to a 60 W light bulb, and prove by calculation that the same current strength is needed for both light bulbs to glow normally.

Physics, and specifically its section electricity (studied in 8th grade), will come to our aid in solving this issue.

Rheostat (from Greek rhéos - flow, stream and statós - standing, motionless)

an electrical apparatus (device) for regulating and limiting current or voltage in an electrical circuit, the main part of which is a conductive element (PE) with variable electrical resistance. The PE resistance value can change smoothly or stepwise. If it is necessary to change the current or voltage within small limits, the radios are included in the electrical circuit in series (for example, when limiting the starting current in electrical machines). To regulate current or voltage over a wide range (from zero to maximum value), potentiometric switching of R is used, which in this case is an adjustable voltage divider (See Voltage divider).

In accordance with the purpose of R., they are divided into starting, starting-control, load and R. excitations. Based on the method of heat removal, radiators are classified into air-, oil-, and water-cooled ones. Depending on the material from which PE is made, R. are divided into metal (the most common), liquid and carbon. The simplest metal radios are sliders, in which the resistance is changed by moving a contact slider directly along turns of wire made of a material with high resistivity (manganin, constantan, nichrome, fechral, steel), wound on a cylinder of electrical insulating material (See Electrical insulating materials) (porcelain , soapstone). Liquid R. consists of a vessel filled with an electrolyte (10-15% solution of Na 2 CO 3 or K 2 CO 3 in water), with electrodes lowered into it. Its resistance is regulated by changing the distance between the electrodes or the depth of their immersion in the liquid. Coal R. is made in the form of columns made of thin coal washers. Its resistance is regulated by changing the pressure applied to the columns.

Lit.: Chunikhin A. A., Electrical devices, M., 1975.

T. N. Dildina.

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

Synonyms:

See what “Rheostat” is in other dictionaries:

- (from the Greek rheos current flow and ... stat), a device for regulating voltage and current in an electrical circuit, the main part of which is a conductive element with active electrical resistance, the value of which can be changed smoothly or ... ... Big Encyclopedic Dictionary

RHEOSTAT, a variable RESISTOR for regulating ELECTRIC CURRENT. The resistive element can be a metal wire, a carbon electrode, or an electrically conductive liquid, depending on the application. Rheostats are used to regulate... Scientific and technical encyclopedic dictionary

RHEOSTAT, rheostat, male. (from Greek rheos flow and lat. status motionless position, standing) (physical). A device with which one or another resistance is introduced into an electrical circuit in order to change the current strength. Ushakov's explanatory dictionary. D.N.... ... Ushakov's Explanatory Dictionary

REOSTAT, ah, husband. (specialist.). A device for regulating current and voltage. | adj. rheostat, oh, oh. Ozhegov's explanatory dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 … Ozhegov's Explanatory Dictionary

- (Rheostat) a device with a resistance that is introduced into an electrical circuit to change the voltage or current in it. Depending on their purpose, there are control and starting valves, and by design they can be wire, lamp, liquid, or carbon. Samoilov K.I.... ...Marine Dictionary

A device used to regulate electrical resistance. circuits to change current or voltage. R. have a wide variety of devices. R. for adjusting the voltage of machines, starting motors, etc. are usually carried out in the form ... ... Technical railway dictionary

Exist., number of synonyms: 1 agometer (1) Dictionary of synonyms ASIS. V.N. Trishin. 2013… Synonym dictionary

rheostat- EN rheostat resistor the resistance of which can be adjusted without interruption of electric current FR rhéostat, m résistance dont la valeur peut être réglée sans… … Technical Translator's Guide

RHEOSTAT- an electrical apparatus (device) included in an electrical target for regulation (smoothly or in steps) and limitation of current or voltage. The relay consists of an active (ohmic) resistance and a moving contact (step switch) ... Big Polytechnic Encyclopedia

AGOMETER OR RHEOSTAT is a device for measuring the force of resistance introduced into a galvanic circuit and for maintaining the current at the same degree of voltage. Dictionary of foreign words included in the Russian language. Pavlenkov F., 1907. AGOMETER, ... ... Dictionary of foreign words of the Russian language

Powerful toroidal rheostat Rheostat (potentiometer, variable resistance, variable resistor; from other Greek ... Wikipedia

Usually, rarely does anyone think about how the sound level is regulated in various devices. In many electrical appliances, sound volume is adjusted by changing the current. For this purpose, a special device, developed by Johann Christian Poggendorff, is most often used, which regulates the current strength and voltage of the electrical network; it is called a rheostat.

So, a rheostat is a device whose main task is to regulate voltage and current. This element of the electrical network is very common; it is used in physics, radio engineering, and electronics.

Rheostat device

The rheostat device does not pose any difficulties for an experienced physicist and is a ceramic hollow cylinder with a metal winding, the ends of which are connected to special contacts, called terminals, located on both sides of the ceramic cylinder. A material with high resistivity is used as a winding, due to which even a small change in length reflects a change in resistance. Along the cylinder there is a metal hose on which a moving contact is attached, which is called a slider.

The ceramic cylinder inside is empty so that the device cools when electricity passes through it. For safety, a number of devices have a special casing that hides all the insides of the mechanism.

Principle of operation

Regardless of the type of rheostat, the principle of operation is approximately the same for all. For example, a slider rheostat works as follows:

Connection to the network occurs through terminals located on both sides of the cylinder;
The current passes along the entire length, depending on the location of the slider. So, if the slider is in the center of the device, then the current passes only to the middle; if the slider is located at the end of the device, then the entire current passes, therefore the voltage is maximum.

Most often, only part of the device is involved in operation, i.e. The slider does not reach the edge of the rheostat. The change in the location of the slider is directly proportional to the change in current strength. The rheostat is connected to the electrical network in series.

Types of rheostats

The type of rheostats depends on their main purpose:

Starting rheostats are designed to start electric motors with direct or alternating current;
Starter rheostats are not only designed to start DC motors, but also to regulate the current;
Ballast rheostats, also called load rheostats, absorb energy that is necessary to regulate the load on electric generators, i.e. create the required resistance in the electrical network;
Excitation rheostats are used in electrical machines to regulate direct and alternating current; they absorb excess energy;
A special group includes rheostats designed to divide voltage; they are called potentiometers. They allow you to use different voltages in one device without using additional devices such as transformers and power supplies. In this case, the rheostat has 3 terminals, where the lower terminals are used for current input, and the upper and one lower terminals are used as output. The voltage is adjusted by moving the slider.

Thanks to the use of rheostats in electrical appliances and machines, there is a reduction in electrical current surges and motor overloads, this, in turn, increases the service life of electrical appliances.

The rheostat on the electrical diagram has its own special designation.

Types of rheostats according to the material of their manufacture

The main element that determines the operating principle of a rheostat is the material from which it is made. In addition, when current passes through the device, it must be cooled: air or liquid. Air cooling occurs thanks to a hollow cylinder and is applicable in all devices. Liquid cooling is used only for rheostats made of metal. Cooling occurs due to complete immersion in liquid or individual parts of the device. Liquid rheostats can be water or oil.

The following rheostats can be distinguished based on the material of manufacture:

Metal rheostats with air cooling are the most common, since they are applicable in various fields and for various devices; the resistance in them can be constant or stepped. The advantages of such structures are their compact size, fairly simple design, and affordable price. Metal liquid rheostats are a vessel filled with liquid. Steel, cast iron, chromium, nickel, iron, etc. can be used as manufacturing materials;
Liquid rheostats are used to regulate current strength;
Ceramic – applicable for relatively light loads;
Coal ones are currently used only in the industrial sector and are a series of coal washers compressed together using springs. The resistance of this type of rheostat changes by changing the compression force of the springs.

When wondering why this device is needed in everyday life, you can get a banal answer: not a single modern TV can do without a rheostat. Thanks to this device, the volume level is adjusted, and it is also associated with the ability to switch channels.

As you can see, this is a truly universal and indispensable component. It is worth emphasizing that there are many types of rheostats, depending on their main purpose. Today, the rheostat is used in industry, in the automotive industry, and in modern electronic technology. It is widely used in radio engineering and various types of electric motors. Failure of the rheostat can damage the entire electrical system.