Ohm's law for the full chain, the definition of the value electric current In real circuits, is depending on the current source and from the load resistance. This law is also called another name - the law of Oma for closed chains. The principle of action of this law is as follows.

As the simplest example, the electrical lamp, which is an electric current consumer, together with the current source, is nothing but closed. This electrical circuit is visually shown in the figure.

Electrotock, passing through the light bulb, also passes through the current source itself. Thus, during the passage of the chain, the current will test the resistance of not only the conductor, but also resistance, directly, the source itself. In the source, the resistance is created by an electrolyte between plates and border layers of plates and electrolyte. It follows that in a closed chain, its overall resistance will consist of the summation of the bulb resistance and the current source.

External and internal resistance

Load resistance, in this case, the light bulb connected to the current source is called external resistance. Direct resistance of the current source is called internal resistance. For a more visual image of the process, all values \u200b\u200bmust be specified. I -, R - external resistance, R - internal resistance. When the current flows through the electrical circuit, then in order to maintain it, the potential difference is between the ends of the external circuit, which has the IHR value. However, current flow is observed in the inner circuit. So, in order to maintain the electric strokes in the inner circuit, the difference in potentials at the ends of the resistance r is also necessary. The value of this potential difference is IHR.

Accumulator electromotive power

The battery must have the next value of the electromotive force capable of maintaining the required current in the chain: e \u003d ihr + ihr. It can be seen from the formula that the electromotive power of the battery is the amount of external and internal. The value of the current must be taken out of the brackets: E \u003d i (R + R). Otherwise, you can submit: i \u003d e / (R + R). The two last formulas is expressed by the Ohm law for the total chain, the definition of which sounds as follows: in the closed circuit, the current strength is directly proportional to the electromotive strength and is inversely proportional to the sum of the resistance of this chain.

The source is a device that converts mechanical, chemical, thermal and some other forms of energy into electric. In other words, the source is an active network element designed to generate electricity. different types Sources available in the power grid are voltage sources and current sources. These two concepts in electronics differ from each other.

Source of constant voltage

The voltage source is a device with two poles, its voltage at any time is constant, and the current passing through it does not affect it. Such a source will be ideal having zero internal resistance. IN practical conditions It cannot be obtained.

In the negative pole of the voltage source accumulates excess electrons, the positive pole is their deficit. Pole states are supported by processes inside the source.

Battery.

Batteries store chemical energy inside and capable of transforming it into electric. Batteries cannot be recharged, which is their disadvantage.

Batteries

Batteries are rechargeable batteries. When charging electric Energy persisted inside in the form of chemical. During unloading, the chemical process proceeds in the opposite direction, and the electrical energy is released.

Examples:

1. Child-acid battery element. It is made from lead electrodes and electrolytic fluid in the form of a diluted distilled water of sulfuric acid. A cell voltage - about 2 V. In car batteries, six cells are usually connected into a sequential circuit, on the terminals of the exit resulting voltage - 12 V;

1. Nickel-cadmium batteries, cell voltage - 1.2 V.

Important! With small battery currents and batteries, you can consider as a good approximation to ideal voltage sources.

Source of alternating voltage

Electricity is performed at electrical stations using generators and after regulating the voltage is transmitted to the consumer. A variable voltage of the home network 220 V in the power blocks of various electronic devices is easily converted to a lower indicator when using transformers.

Tok Source

By analogy, as the perfect voltage source creates a constant output voltage, the task of the current source is to give a constant current value, controlling the required voltage automatically. Examples are current transformers (secondary winding), photocells, collector currents of transistors.

Calculation of internal resistance of the voltage source

Real voltage sources have their own electrical resistance, which is called "internal resistance". The load attached to the output conclusions is denoted by the "external resistance" - R.

Battery batteries generates EMF:

ε \u003d E / Q, where:

• E - Energy (J);
• Q - charge (CL).

The total EMF of the battery member is the voltage of its open circuit in the absence of a load. It can be monitored with good accurate digital multimeter. The potential difference measured on the output contacts of the battery when it is on the load resistor, will be a smaller value than its voltage with an open circuit, due to current flow through the load external and through the internal resistance of the source, this leads to the dispersion of the energy in it as thermal radiation .

The internal resistance of the battery with the chemical principle of action is between the fraction of Oma and several ohms and is mainly due to the resistance of electrolytic materials used in the manufacture of the battery.

If the resistance resistor R is connected to the battery, the current in the circuit I \u003d ε / (R + R).

Internal resistance is not constant value. It is influenced by the bodge of the battery (alkaline, lead-acid, etc.), it changes depending on the load value, temperature and battery life. For example, at one-time batteries, internal resistance increases during use, and the voltage in connection with this falls before coming to a state is unsuitable for further operation.

If the Source EMF is a predetermined value, the internal resistance of the source is determined by measuring the current flowing through the load resistance.

1. Since the inner and external resistance in an approximate scheme is included in series, the laws of Oma and Kirchoff can be used to use the formula:

1. From this expression R \u003d ε / I - R.

Example. The battery with a known EMF ε \u003d 1.5 V and is connected consistently with a light bulb. The voltage drop on the light is 1.2 V. Consequently, the internal resistance of the element creates a voltage drop: 1.5 - 1.2 \u003d 0.3 B. The resistance of wires in the chain is considered negligible, the lamp resistance is not known. The measured current passing through the chain: i \u003d 0.3 A. It is necessary to determine the internal resistance of the battery.

1. According to the law Ohm, the resistance of the light bulb R \u003d U / i \u003d 1.2 / 0.3 \u003d 4 ohms;
2. Now according to the formula for calculating the internal resistance R \u003d ε / i - R \u003d 1.5 / 0.3 - 4 \u003d 1 Ohm.

In the case of a short circuit, the external resistance drops almost to zero. Current can limit its value only by the small resistance of the source. The current current arising in such a situation is so high that the voltage source may be damaged by thermal exposure of the current, there is a risk of fire. The risk of fire is prevented by installing fuses, for example, in automotive batteries.

Internal resistance of the voltage source - important factorwhen the question is solved how to transfer the most effective power connected electrical apparel.

Important! The maximum power transmission occurs when the internal resistance of the source is equal to the load resistance.

However, on this condition, remembering the formula p \u003d I² X R, the identical amount of energy is given to the load and dissipates in the source itself, and its efficiency is only 50%.

Load requirements must be carefully considered to make a decision on best use Source. For example, a lead-acid car battery should provide high currents at a relatively low voltage of 12 V. Its low internal resistance allows it to do it.

In some cases, high voltage power supplies should have extremely large internal resistance to limit the current to. H.

Features of the internal resistance of the current source

At the perfect current source, infinite resistance, and for genuine sources you can imagine an approximate option. Equivalent electrical power is resistance connected to the source parallel, and external resistance.

The current output from the current source is distributed like this: partially current flows through the highest internal resistance and through low load resistance.

The output current will be from the sum of currents on the inner resistance and loading Io \u003d IH + IVN.

It turns out:

IH \u003d IO - IVN \u003d IO - UON / R.

This dependence shows that when the internal resistance of the current source is growing, the more the current on it is reduced, and the load resistor receives most of the current. Interestingly, the voltage will not influence the current magnitude.

Real source output voltage:

Up \u003d i x (R x R) / (R + R) \u003d I x R / (1 + R / R). Rate article:

Purpose of work: examine the method of measuring the EMF and the internal resistance of the current source using an ammeter and a voltmeter.

Equipment: metal tablet, current source, ammeter, voltmeter, resistor, key, clamps, connecting wires.

To measure the EMF and internal resistance of the current source, the electrical circuit is collected, the diagram of which is shown in Figure 1.

The current source is connected to an ammeter, resistance and key connected sequentially. In addition, a voltmeter is also connected directly to the output sockets of the source.

EMF is measured by the voltmeter reading when the key is open. This technique of determining the EMF is based on a consequence of the Ohm law for the full chain, according to which, with an infinitely large resistance of the external circuit, the voltage on the source clips is equal to its EMF. (See paragraph "Ohma law for a complete chain" Textbook "Physics 10").

To determine the internal resistance of the source, the K key is closed. At the same time, two sections can be selected in the chain: the external (one that is connected to the source) and the inner (one that is inside the current source). Since the source's emf is equal to the amount of voltage drops in the internal and external sections of the chain:

ε = U. R.+ U. R.T. U. R. = ε -U. R. (1)

According to the law of Ohm for the section of the chain U R \u003d I · r (2). Substituting the equality (2) in (1) get:

I.· r. = ε - U. R. , from R. = (ε - U. R.)/ J.

Therefore, to learn the internal resistance of the current source, it is necessary to pre-define its EMF, then closer the key and measure the voltage drop on the external resistance, as well as the current strength in it.

Progress

1. Prepare a table to record measurement results and calculations:

Draw in the notebook a scheme for measuring EMF and internal source resistance.

After checking the scheme, collect the electrical chain. Clean the key.

Measure the value of the emf of the source.

Click the key and determine the readings of the ammeter and voltmeter.

Calculate the internal resistance of the source.

1. Definition of EMF and internal resistance of the current source graphic method

Purpose of work: to study the measurements of the EMF, the internal resistance and current of the short circuit current of the current source based on the analysis of the circuit of the voltage at the output of the source from the current in the circuit.

Equipment: galvanic element, ammeter, voltmeter, resistor R. 1 , variable resistor, key, clips, metal tablet, connecting wires.

From the Ohm law, for the full chain, it follows that the voltage at the output of the current source depends directly proportional to the strength of the current in the chain:

since I \u003d E / (R + R), then IR + IR \u003d E, but Ir \u003d U, from where U + Ir \u003d E or U \u003d E - IR (1).

If you construct a graph of the dependence U from i, then at its intersection points with the axes of coordinates, it is possible to determine the E, I K.Z. - The short-circuit current (current flows in the source circuit when the outer resistance R becomes zero).

EMF is determined by the point of intersection of the graph with the axis of stresses. This point of the graph corresponds to the state of the chain, in which the current in it is missing and, therefore, U \u003d E.

The short circuit current is determined by the point of intersection of the graph with the axis of the currents. In this case, the external resistance r \u003d 0 and, therefore, the voltage at the output of the source U \u003d 0.

The internal resistance of the source is found along the tangent angle of the chart with respect to the axis of currents. (Compare formula (1) with a mathematical function of the form y \u003d ah + in and remember the meaning of the coefficient at x).

Progress

To record measurement results, prepare the table:

1. After checking the scheme, the teacher assemble the electrical chain. Set the variable resistor slider to the position at which the chain resistance connected to the current source will be maximal.

Determine the value of the current in the chain and voltage at the source clips at the maximum resistance of the variable resistor. Data measurements bring to the table.

Repeat several times the current and voltage force measurement, reducing the amount of variable resistance every time so that the voltage on the source clips has decreased by 0.1V. Measurements stop when the current in the circuit reaches the value in 1a.

Apply the points received in the experiment on the schedule. Sleep the voltage along the vertical axis, and the current strength is horizontal. Spend a straight line at points.

Continue the schedule to the intersection with the axes of the coordinates and determine the values \u200b\u200bof E and, I k.z.

Measure the emf of the source by connecting the voltmeter to its outputs when the external chain is open. Match the EMF values \u200b\u200bobtained in two ways, and indicate the reason for the possible discrepancy of the results.

Determine the internal resistance of the current source. To do this, calculate the tangent angle of inclination of the built schedule to the current axis. Since the tangent of the angle in a rectangular triangle is equal to the attitude of the opposite catech to the adjacent, then practically this can be done, finding the relation E / I k.z

The electric current in the explorer occurs under the influence of an electric field forcing free charged particles to come to directional movement. Creating a particle current is a serious problem. To build such a device that will maintain the difference in the potentials of the field for a long time in one state - the task, the solution of which was under the power of humanity only by the end of the XVIII century.

First attempt

The first attempts to "accumulate electricity" for its further research and use were undertaken in Holland. German Evald Jürgen von Magnist and Dutchman Peter Van Mushchenbrook, who conducted his research in the town of Leiden, created the world's first capacitor, named after the "Leiden Bank".

The accumulation of electrical charge has already passed under the action of mechanical friction. It was possible to use a discharge through the conductor for some, rather short, time interval.

The victory of the human mind over such an ephemeral substance, as electricity, turned out to be revolutionary.

Unfortunately, the discharge (electric current created by the capacitor) lasted so briefly that could not create. In addition, the voltage given by the condenser gradually decreases, which does not leave the ability to receive a long current.

It was necessary to look for a different way.

First source

Experiments of the Italian Galvani on the study of "Animal Electricity" were an original attempt to find a natural current source in nature. Having cut the paws of the prepared frogs on the metal hooks of the iron lattice, he drew attention to the characteristic reaction of nerve endings.

However, the findings of the galvana denied another Italian - Alessandro Volta. Inteb not to receive electricity from animal organisms, he conducted a series of experiments with frogs. But it turned out to be the complete opposite of the previous hypotheses.

Volta noticed that a living organism is only an electrical discharge indicator. When the power of the muscles, the legs are reduced, indicating the difference in potentials. The source of the electric field was the contact of heterogeneous metals. The farther from each other they are in a row chemical elementsThe more significant effect.

Plates made of heterogeneous metals laid by paper discs impregnated with a solution of electrolyte, created a long time of the potential difference. And let it be low (1.1 V), but the electric current could be explored for a long time. The main thing is that the tension remained unchanged just for a long time.

What's happening

Why in the sources that were called "galvanic elements", is such an effect?

Two metal electrodes placed in a dielectric play different roles. One supplies electrons, the other accepts them. The process of the oxidation reaction reaction leads to the appearance of an excess of electrons on one electrode, which is called a negative pole, and a disadvantage of the second, we denote it as a positive source pole.

In the most simple galvanic elements, oxidative reactions occur on one electrode, restorative - on the other. Electrons come to electrodes from the outer part of the chain. The electrolyte is a conductor of the ion current inside the source. Resistance strength manages the duration of the process.

Copper-zinc element

The principle of operation of electroplating elements is interesting to consider on the example of a copper-zinc electroplating element, the action of which goes to the energy of zinc and copper sulfate. In this source, the pressure plate is placed in a solution and a zinc electrode is immersed in a solution of zinc sulfate. The solutions are separated by a porous gasket in order to avoid mixing, but necessarily contact.

If the chain is closed, the surface layer of zinc is oxidized. In the process of interaction with the liquid of the zinc atoms, turning into ions appear in the solution. Electrographs are released on the electrode that can take part in the current formation.

Finding on the copper electrode, electrons take part in a rehabilitation reaction. Copper ions are received from the solution to the surface layer, during the reduction process, they turn into copper atoms, precipitated on the copper plate.

We summarize what is happening: the process of operation of the galvanic element is accompanied by the transition of the electrons of the reducing agent to the oxidizer along the outer part of the chain. Reactions go on both electrodes. An ion current flows inside the source.

Difficulties of use

In principle, any of the possible redox reactions can be used in batteries. But substances capable of working in valuable technically elements are not so much. Moreover, many reactions require costs of expensive substances.

Modern rechargeable batteries Have a simpler structure. Two electrodes placed in one electrolyte fill the vessel - the battery case. Such constructive features Simplify the structure and reduce the batteries.

Any galvanic element is able to create a permanent current.

The current resistance does not allow all ions at the same time to be on the electrodes, so the element works long enough. Chemical reactions The formation of ions is early or later terminated, the element is discharged.

The current source is of great importance.

A little about the resistance

The use of electric current is undoubtedly brought out scientific and technological progress to a new stage, gave him a giant push. But the strength of the current flow becomes on the way of such development.

On the one hand, the electric current has invaluable properties used in everyday life and technique, on the other - there is a significant opposition. Physics as a science of nature is trying to establish a balance, bring these circumstances in compliance.

The current resistance occurs due to the interaction of electrically charged particles with the substance by which they move. Eliminate this process in normal temperature conditions It is impossible.

Resistance

Current source and countering the outer part of the chain have several different nature, but the same in these processes is to perform work on the movement of charge.

The work itself depends only on the properties of the source and its filling: the qualities of the electrodes and electrolyte, as well as for the external parts of the chain, the resistance of which depends on the geometric parameters and chemical characteristics material. For example, the resistance of the metal wire increases with an increase in its length and decreases when expanding the cross-sectional area. When solving the problem, how to reduce resistance, physics recommends using specialized materials.

TKA operation

In accordance with the law of Jowle-Lenz, the amount of heat is allocated proportional to the resistance. If the amount of heat indicate q inside. , current strength i, time of its flow T, then we get:

• Q inside. \u003d I 2 · r · t,

where R is the internal resistance of the current source.

In the entire chain, including both the inner and the external parts, the total amount of heat, the formula of which has the form:

• Q complete \u003d i 2 · r · t + i 2 · r · t \u003d i 2 · (r + r) · t,

It is known how the resistance in physics is indicated: the outer chain (all elements, except the source) has resistance R.

Ohm law for full chain

We take into account that the main work makes third-party forces within the current source. Its value is equal to the work of the charge carried by the field, and the electromotive force of the source:

• q · E \u003d i 2 · (R + R) · t.

understanding that the charge is equal to the product of the current for its flow, we have:

• E \u003d i · (R + R).

In accordance with causal relations, the Ohm Law has the form:

• I \u003d E: (R + R).

In a closed circuit, directly proportional to the EMF of the current source and is inversely proportional to the total (full) chain resistance.

Relying on this pattern, you can determine the internal resistance of the current source.

The discharge capacity of the source

The main characteristics of sources include the discharge capacity. The maximum amount of electricity obtained during operation under certain conditions depends on the current of the discharge current.

In the ideal case, when certain approximations are performed, the discharge capacity can be considered constant.

For example, the standard 1-B potential difference battery has a discharge capacity of 0.5 A · h. If the discharge current is 100 mA, then runs for 5 hours.

Ways to charge batteries

Operation of batteries leads to their discharge. Charging the small-sized elements is carried out using current, the value of the strength of which does not exceed one tenth source capacity.

The following ways are offered:

• the use of unchanged current for a specified time (about 16 hours of current 0.1 battery capacity);
• charging downward current to a given potential difference value;
• using asymmetric currents;
• the sequential use of short charge and discharge pulses, at which the time is first higher than the second time.

Practical work

A task is proposed: to determine the internal resistance of the current source and EMF.

To perform it, it is necessary to stock up the source of the current, an ammeter, a voltmeter, a slide rheostom, a key, a set of conductors.

Use will determine the internal resistance of the current source. To do this, it is necessary to know its EMF, the amount of resistance of the rheostat.

The estimated current resistance formula in the outer part of the chain can be determined from the Ohm law for the chain section:

• I \u003d U: R,

where I is the current of the current in the outer part of the chain, is measured by an ammeter; U - voltage at external resistance.

To increase measurement accuracy, not less than 5 times are made. What is it for? The voltage measured during the experiment, resistance, current (or rather, the current strength) is used further.

To determine the emf of the current source, we use the fact that the voltage on its terminals with an open key is almost equal to the EMF.

We collect the chain of consistently included batteries, a row, ammeter, key. Connect a voltmeter to the current source terminals. Opening the key, remove its testimony.

Internal resistance, the formula of which is obtained from the Ohm law for the total chain, we define mathematical calculations:

• I \u003d E: (R + R).
• r \u003d E: I - U: I.

Measurements show that internal resistance is significantly less external.

The practical function of batteries and batteries finds wide application. The indisputable ecological safety of electric motors is not doubtful, but to create a capacious, ergonomic battery is the problem of modern physics. Its solution will lead to a new turn of the development of automotive equipment.

Small-sized, lightweight, capacious batteries are also extremely necessary in mobile electronic devices. The stock of the energy used in them is directly related to the performance of devices.

We concluded that to maintain direct current in a closed circuit, it needs to include the current source. We emphasize that the task of the source is not to supply charges into an electrical chain (in the conductors of these charges enough), but to force them to move, make work to move charges against the power of the electric field. The main characteristic of the source is electrical force 1 (EMF) - work performed by third-party forces to move a single positive charge

The unit of measurement of the EMF in the system of SI units is volt. EMF of the source is 1 volt, if it makes a job 1 joule when charging charge 1 pendant

To designate current sources on electrical circuits A special designation is used (Fig. 397).

fig. 397.
Electrostatic field performs positive work By moving a positive charge in the direction of reducing the potential of the field. The current source conducts the separation of electrical charges - positive charges accumulate on one pole, on the other of the negative. The electric field strength in the source is directed from the positive pole to the negative, so the operation of the electric field to move the positive charge will be positive when it moves from the "plus" to "minus". The work of third-party forces, on the contrary, is positive if the positive charges move from the negative pole to the positive, that is, from the "minus" to the "plus".
This is the fundamental difference between the concepts of the difference in potentials and the EMF, which should always be remembered.
Thus, the electromotive power of the source can be considered an algebraic value, the sign of which ("plus" or "minus") depends on the direction of the current. In the diagram shown in Fig. 398,

fig. 398.
Outside the source (in the outer chain) current flows 2 from the "plus" of the source to "minus", within the source from the "minus" to the "plus". In this case, both third-party source forces and electrostatic forces in the external chain are positive.
If on some section of the electrical circuit, in addition to electrostatic, third-party forces act, then over the movement of charges "work" both electrostatic and third-party forces. The total work of electrostatic and third-party forces on the movement of a single positive charge is called electrical voltage on the chain area

In the case when there are no third-party forces, the electrical voltage coincides with the difference in the potentials of the electric field.
Let us explain the determination of the voltage and the EMF sign on simple example. Let the circuit under which the electric current flows, there is a source of third-party strength and a resistor (Fig. 399).

fig. 399.
For certainty we assume that φ O\u003e Φ 1, that is, the electric current is directed from the point 0 To point 1 . When the source is connected, as shown in Fig. 399 A, third-party source forces make positive work, so the relation (2) in this case can be recorded as

With the opposite of the source (Fig. 399 b), charges are moving against third-party strength, so the work of the latter is negative. In fact, the forces of the external electric field overcome third-party power. Therefore, in this case, the considered relation (2) has the form

For the flow of electric current in the area of \u200b\u200bthe chain, which has electrical resistance, it is necessary to perform work, to overcome the resistance forces. For a single positive charge, this work, according to the law of Ohm, is equal to the work IR \u003d U. which naturally coincides with the voltage in this area.
Charged particles (both electrons and ions) inside the source move in some environmentTherefore, on the part of Wednesday, there are also braking forces that also need to be overcome. Charged particles overcome resistance strength due to the action of third-party forces (if the current in the source is directed from the "plus" to "minus") or through electrostatic forces (if the current is directed from the "minus" to the "plus"). Obviously, work to overcome these forces does not depend on the direction of movement, since the resistance strengths are always directed towards the opposite velocity of the particle movement. Since the resistance forces are proportional to the average velocity of the particle movement, then the work to overcome them is proportional to the speed of movement, therefore, strength strength. Thus, we can introduce another source characteristic - it internally resistance r., similar to the usual electrical resistance. Work on overcoming resistance forces when moving a single positive charge between the source poles is equal to A / Q \u003d IR. We emphasize once again, this work does not depend on the direction of the current in the source.

1 The name of this physical quantity is unsuccessful - so the electromotive force is the work, and not by force in the usual mechanical understanding. But this term was so established that it was not "in our power." By the way, the strength of the current is not mechanical strength! Not to mention such the concepts of the "power of the Spirit", the "power of will", "Divine Power", etc.
2 Recall, for the direction of electric current movement, the direction of the movement of positive charges is taken.