EMK, electromotive force

EMK, electromotive force

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Electromotive Force - Important Considerations and Definitions

ϕ is defined as the galvanic potential of a phase Δϕ0 is the difference between galvanic potentials (the equilibrium galvanic voltage) and E.0(EMK, ΔΔϕ0) the difference between the galvanic voltages of two half-cells. If an electrode is related to the NHE electrode, this arrangement can still be seen formally as a half-cell and is designated with Δϕ0 (X, NHE).
Anode / cathode:
The terms anode and cathode have nothing to do with the + or - of the electrode, they refer to its potential. The electrode that reduces a substance is called the cathode (καϑοδος = cathode = descent, reduction of the oxidation number, electrons are given off by the electrode and enter the solution), the electrode that oxidizes a substance is called the anode (ανοδος = anodos = ascent, the electrons rise in the electrode to the "power source", increase of the oxidation number). In general: the anions migrate to the anode, the cations migrate to the cathode.
Direction of the reaction:
If you combine electrodes to form cells, you write the half-cell on the right in which the reduction takes place. The electrode on which reaction products are deposited (reduction of metal ions) is therefore generally on the right.
Tab. 2
Abbreviation for redox reactions
Cuz ++2eCuOx+neRed

The direction of the reaction is set in such a way that ΔG must be negative, E.0 so positive.

Electrolysis and galvanic element:
If electrochemical reactions are forced by an externally applied voltage, one speaks of electrolysis and describes the electrochemical cell as an electrolytic cell or electrolytic cell, whereby the anode is by definition the electrode on which the oxidation takes place. In the case of the galvanic element, the cathode is "positively" charged; in the case of electrolysis, electrons flow through the outer wire into the cathode (which is negatively charged) and further over the phase boundary into the boundary layer of the solution. As a result, positively charged ions are deposited on the cathode.

Minus times minus gives plus, that makes up the main part.

a) formal: You only need 2 Ag⁺ if you also have 2 e⁻

b) logical: calculate the potential against standard hydrogen for both electrodes, and only then calculate the voltage difference.

I only looked briefly - count on 3 Ag again, you need that for 1 Al

EMK, electromotive force - chemistry and physics

(See Leisi, Classical Physics II [Lei98, pp. 85]) (See Tipler, Physik [TM04, pp. 756])

An electric field inside a conductor creates a current. If this electric field is generated by charges, the resulting current balances this charge. Due to influence, the surface charges are rearranged in such a way that the current decreases and finally disappears.

Let us assume that in stationary operation there is a voltage between the ball and the base of the van de Graaff generator. The electric field along the tape is then, as a first approximation,

The work of bringing a unit of charge to the hemisphere against this electric field is 8

The power of the motor, which acts as a voltage source here, is

The electric field does the work in the resistor on the other hand in time

This is the power of the field

The energy of the electrical current is converted into Joule heat in the resistor, so the power of the heat source is also

With an ohmic conductor we get

If we slowly test a charge around the circuit, the work done is greater than zero. We call this work the electromotive force of the power source. So we define

This electromotive force 9 is the work done when a small charge is carried around by the power source. In the case of the van de Graaff generator, this work consists of two parts:

    At every point on the belt, the force of the electrostatic field is compensated by the force of the motor. On this branch the work is zero.

EMK, electromotive force - chemistry and physics

II. Nernst equation

Walther Nernst (1864-1941) has one equation derived how the potential depends on the content of the ionic solution.

For the single potential E applies:

Standard or normal potential

Activity (effective molar concentration)

For practical calculations, the temperature 25 ° C is mostly used and the natural logarithm is replaced by the decadic logarithm ("logarithm of ten"). With the numerical values ​​for R and F and the conversion factor & quotln & quot into & quotlog & quot, we get the

Equation mostly used in practice:

The relationship between the activity (effective, effective concentration) and the substance concentration is a = f c.

This equation also shows what EO does that mean Standard potential is the single potential for one Solution with a = 1. This EO is tabulated - and we used it as E in the previous section. With the Nernst equation you can obviously do the right thing Calculate E for any concentration!

This equation can be applied to a general redox equilibrium, not just the case of metal Metal ion, expand.

the EMK ("Electromotive force") is the difference in the E values ​​of the pairs in a redox reaction. Usually one chooses the difference larger E - small E. (In electrochemistry one learns that this EMF can be measured as a voltage if the redox reaction is investigated in a suitable experimental setup.)
For the copper-zinc pair the EMF = +0.342 - (-0.763) = 1.105 V for the copper-silver pair the EMF = + 0.800 - 0.342 = 0.458 V.

Further technical language : A redox couple is also called a "half-cell" and the overall combination is called a "cell" or a "chain".

In connection with the EMK there was also an abbreviation symbolic notation introduced:

Generally the pair in which the oxidation takes place is written on the left and the pair in which the reduction takes place on the right.
|| separates the two parts and | the different oxidation states of a half-cell.

For our Copper-zinc example:
The correct spelling shows that the parts were arranged in such a way that the partial reactions taking place can already be read from the symbol!

EMK, electromotive force - chemistry and physics

3.3 & # x00A0 & # x00A0 Electromotive force and Joule heat

(See Leisi, Classical Physics II [Lei98, pp. 85]) (See Tipler, Physik [TM04, pp. 756])

An electric field inside a conductor creates a current. If this electric field is generated by charges, the resulting current balances this charge. Due to influence, the surface charges are rearranged in such a way that the current decreases and finally disappears.

Charge transport in a van de Graaff generator short-circuited with a resistor R.

Let us assume that in stationary operation there is a voltage U between the ball and the base of the van de Graaff generator. The electric field along the band is then, as a first approximation,


The work of bringing a unit of charge dQ to the hemisphere against this electric field is 3


The power of the motor, which acts as a voltage source here, is


The electric field does the work in the resistor on the other hand in the time dt



This is the power of the E field


The energy of the electric current is converted into Joule heat in the resistor, so the power of the heat source is also


With an ohmic conductor we get


If we do a test charge q 0 slowly around the circuit, the work done is greater than zero. We call this work the electromotive force of the power source. So we define


This electromotive force 4 is the work that occurs when a small charge q 0 is provided by the power source. In the case of the van de Graaff generator, this work consists of two parts:

  • At every point on the belt, the force of the electrostatic field is compensated by the force of the motor. On this branch the work is zero.
  • The work that is converted into Joule heat in the resistor.

Experiment for the lecture: EMF of the Daniell element (experiment card TH-44)

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Difference between electromotive force (EMF) and potential difference Difference between 2021

Electromagnetism is an integral part of physics. There are terms and units that are very closely related and have a very fine line that distinguishes the two from one another. "Potential difference" and "EMF" are two such terms.

electromotive force (EMF)

The electromotive force, or EMF, is better described as the total voltage in an electrical circuit produced by the source or battery. Emf is not a physical force. It is basically the energy required to move a positive unit charge from the negative terminal of the battery to the positive terminal when the circuit is open. Emf is the underlying voltage that occurs across a line or circuit due to the fluctuating magnetic field. Formally, it was also defined as the force required to separate two charges (one positive and one negative) from each other.

It is measured in volts. The electromotive force is often denoted by the symbol 'ℰ' (epsilon).
If we define emf we get:
Where ℰ is the EMF and ECS the generated electrostatic field.

In simple terms, the electromotive force can be given as the maximum voltage that can be achieved by a particular circuit.

Potential difference

The potential difference is the work done per charge to move a charge between the negative and the positive pole of the battery. When the battery is in use or the circuit is closed, a small portion of the EMF is consumed to overcome the battery's internal resistance. This energy per unit of charge is called the potential difference.

If 'ℰ' is the emf of the battery used in the circuit and 'r' is the internal resistance of the specific battery and the external resistance of the circuit 'R' in a circuit of 'I' then is actual
ℰ = Ir + IR
Here ℰ - Ir is regarded as the potential difference between the connections of the battery, which is also referred to as the terminal voltage.
The emf can be measured with a voltmeter and is represented by the symbol 'V' (volt).
The term "potential difference" is also used in relation to magnetic and gravitational fields. Their units are different, but the concept is similar.

1. Emf is the total voltage in the battery, while the potential difference is the work done to move a charge against the electric field between two specific points in the circuit.
2. Emf is always greater than the potential difference.
3. The concept of EMF is only applicable to an electric field, while the potential difference is applicable to magnetic, gravitational and electric fields.

EMK, electromotive force - chemistry and physics

EMK = the electrical potential of a galvanic cell (measured in volts), depending on the concentration, type of substance and temperature

Standard EMF ΔE⁰ : All reactants and products are in standard states:

Example: Standard EMF: Daniell element = 1.10 V

2 e⁻ + Zn²⁺ ⇌ Zn E⁰ = - 0.76 V
2 e⁻ + 2 H⁺ ⇌ H₂ E⁰ = 0 V
2 e⁻ + Cu²⁺ ⇌ Cu E⁰ = + 0.33V

Cathode: 2 e⁻ + Cu²⁺ ⇌ Cu E⁰ = + 0.33V
Anode: Zn ⇌ 2 e⁻ + Zn²⁺ E⁰ = - 0.76 V

EMF = E ° (cathode) - (E ° (anode) = 1.10V

ΔE ° = E ° (cathode, acceptor half-cell) - E ° (anode donor half-cell)
ΔE ° = E ° (positive pole) - E ° (negative pole)

No electricity should flow

  • otherwise there will be changes in concentration
  • due to the internal resistance in the cell

⇨ lower voltage is measured

a) Potentiometer: applies a voltage opposite to the galvanic element (you don't have to know).
b) high resistance

Free enthalpy of reaction and EMF

Note: Only for the four-hour course:
ΔG is a measure of the maximum work that can be obtained (at constant temperature and constant pressure).

n = number of electrons that are converted during the reaction in mol!
F = Faraday constant = 96,485 C / mol
[Coulomb is defined as the electrical charge that is transported within one second through the cross section of a wire in which an electrical current of the strength of one ampere flows: 1 C = 1 A / s]

Electromotive force

If you can split a reaction into two spatially separate partial reactions, namely oxidation and reduction, you can split them up in the form of a electrochemical reaction in a so-called electrochemical cell carry out. An electrochemical cell consists of two Electrodes (metallic or non-metallic conductor), which are electrically connected to each other, and an electrolyte solution in which the electrodes are immersed.

Figure not included in this excerpt

There are two types of electrochemical cell: the galvanic cell and the Electrolytic cell. The best-known example of a galvanic cell is the Daniell element. Here a zinc and a copper electrode are immersed in their salt solutions, which are separated by a semipermeable membrane (diaphragm). The small anions cannot pass through this porous partition

however, the large cations migrate and thus ensure a charge balance. Since copper is more noble than zinc, zinc is oxidized and goes into solution as Zn2 +, through the

remaining electrons, the zinc electrode is negative. The copper is reduced, so solid copper is deposited on the electrode from the copper solution, which consumes electrons and the

The copper electrode is therefore positive compared to the zinc electrode. By definition, the electrode on which the oxidation takes place is the anode, the one on which the reduction takes place is the cathode. Compared to the anode, the cathode has a positive charge, which corresponds to a higher potential.

Instead of a diaphragm, a salt bridge (also called an electric key) is often used. This is usually a bent tube that is filled with a concentrated salt solution in a gel and thus conductively connects the two electrolysis cells.

In the electrolysis cell, there is always only one electrode space. A voltage that is higher than the potential difference of the cell is applied to the electrodes. Work is done on the system, the endergonic reaction takes place. If the external voltage is lower than the emf of the cell, the exergonic reaction takes place and the system does work. Electrons flow in the external circuit, in the electrolyte the charge is transported with the help of the ions.

Electrodes are removed from the anode, so that the electrode material is oxidized and cations go into solution, while electrons are added to the cathode, which reduce the cations from the solution.

There are several different types of electrodes:

A metal or a non-metal in the solid phase with activity 1 is in contact with a solution of its salt. The electrode potential is only dependent on the activity of the electrolyte ion, since the activity of the solid phase in the Nernst equation is by definition 1.

A metal M is surrounded by a porous layer of an insoluble salt MX and is immersed in a solution that contains X - ions, e.g. a silver / silver chloride electrode. The silver ions are precipitated by the chloride and only a small amount of cations remains in solution. The amount of dissolved silver is determined by the solubility product (constant under the respective test conditions) and the activity of the anions.

Figure not included in this excerpt

With the solubility product L = c

Figure not included in this excerpt

Another example is the calomel electrode. When chloride is added, mercury falls as calomel Hg2Cl2.

A gas is in equilibrium with a solution of its ions. A metal serves as a catalyst and electron conductor. the Normal hydrogen electrode (NWE) consists of a platinum-coated platinum electrode surrounded by H2, which is immersed in a hydrochloric acid solution of a defined concentration (1,153 molar, then the activity is equal to 1 with f ± = 0.867). It serves as a Reference electrode for potential measurements, which is why their individual potential was arbitrarily set to zero. If one determines the potentials of different half-cells with the help of the NWE, then they are mutually comparable and allow one to be set up Voltage series.

An element exists in a solution in two oxidation states. An inert metallic conductor makes electrical contact with the solution.

If the EMF is to be measured, no current must flow and no reaction must take place, so the cell must be in electrochemical equilibrium. Thus, the condition for reversible process management is given. Under ideal conditions, the chemical energy is completely converted into electrical work, which is why, due to the conservation of energy, the following applies:

∆G = free reaction enthalpy of the cell

Wel = maximum usable, reversible electrical work and from this:

n = number of electrons transported per particle

From the concentration and temperature dependence of the free enthalpy it follows:

Figure not included in this excerpt

from which by division with (- n × F) the Nernst equation receives:

Figure not included in this excerpt

Here a is the activity and ν is the stoichiometric factor. The product operator P is equivalent to the summation sign S, except that the factors are multiplied. The mean activity is related to the concentration as follows:

The activity coefficient can be determined with the help of the Debye-Hückel limit law to calculate:

Figure not included in this excerpt

With z + and z- as the valency of the cations and anions of the ion i and I as the ionic strength, which is defined as follows:

Figure not included in this excerpt

Figure not included in this excerpt

Since the limit law only applies exactly to infinite dilution, the determination of ∆E0 must be carried out with the aid of an extrapolation.

Figure not included in this excerpt

Using the Nernst equation, the activities and concentrations can also be determined if the other variables are known.

All measurement methods must meet the above-mentioned condition that no current may flow. This requirement can be met with the Poggendorff's compensation method realize. The cell is given a voltage Ua

switched in the opposite direction so that no more current flows. It follows that the emf of the cell must be equal to the applied voltage.

With the help of this method, relatively error-free EMF values ​​can be determined.

The use of a high-resistance voltmeter is also suitable for this problem. The following equations are relevant here:

with Ra = internal resistance of the voltmeter, external resistance of the cell, Ri = internal resistance of the cell.

A new method to increase the electromotive force of a galvanic current in the indefinite

In the course of an investigation, the results of which I intend to publish in the near future, I came across a principle for the amplification of the intensity or tension of electrical currents, which seems to me to be new in its application, and which, apart from the main subject, ought to be of sufficient interest to be to be communicated here.

It is known that when two homogeneous metal plates placed in a conductive liquid, e.g. B. two platinum plates are connected with a voltaic chain, they almost instantly experience the so-called polarization, as a result of which they weaken the current of the chain very significantly, and, if you separate them from this, a current in a metal wire connecting them in the opposite direction, of short duration, but always of considerable strength. It is also known that in this way a whole grater of such pairs of plates, as they are called, can be polarized. It is the column of charge Knight’S that caused so much attention in its time, which decomposes water, acts on an electrometer, gives off sparks and shocks.

For the activation of such charge columns, which are in themselves ineffective, no other means has been known than Volta’s columns with a large number of pairs of plates, that is, with a high intensity of the current, and the secondary current obtained in this way possessed [569] never a greater or even just as great an electromotive force as the primary one that produced it.

A closer examination of the some time ago by Mr. Grove The constructed gas column, which is nothing more than a column of charge, and indeed, as I intend to show, only an imperfect one, has now led me to the fact that for charging such secondary columns it is not necessary to have a primary of as many or more pairs of plates as those must contain, but can do it perfectly as well by means of a simple, voltaic chain, however large the number of plates of the secondary column may be, and that in it one possesses a means of increasing the electromotive force of a galvanic current into the indefinite.

My procedure now consists in first of all mixing all the H < displaystyle H> with the zinc, and all the O < displaystyle O> with the platinum of a simple one GroveAs a result of this, all these plates are polarized or charged, because the H < displaystyle H> clad themselves with hydrogen and the O < displaystyle O> clad with oxygen, and all of them are equally strong, just as strong as if only one a single pair of such plates, of the same size as they are in total, with the [570] primary chain. After this connection has existed for a certain time, I quickly break it off and connect the pairs of plates, which are now charged, according to the principle of the column, and at the same time with a voltameter, if the intention is to observe the chemical action of the secondary current.

The secondary current obtained in this way possesses an electromotive force which, in general, exceeds that of the primary of the simple chain all the more as the number of pairs of plates in the column of charge is greater, and even if it does not grow indefinitely with this number, because the plates the more weakly polarized, the more there are, the longer it will grow as the resistance in the primary chain is smaller.

The secondary current, however, has only a short duration, and this duration decreases as one increases its electromotive force. This is easily overlooked if one considers that the secondary column, when charged, receives the components of an equivalent of water broken down in it for every equivalent of water which is decomposed in the primary chain, but that these components are distributed to all the cells, hence every cell, if its n < displaystyle n> is present, only the parts of 1 n < displaystyle < tfrac <1>>> Receives equivalent, d. H. the less, the greater the number n < displaystyle n> of cells. The reunification of the components of this 1 n < displaystyle < tfrac <1>>> Equivalents which cause the secondary current will, if the resistance in the secondary column and the primary cell are equal, evidently take place in one of the times which the primary chain required for the decomposition of a full equivalent.

From this it follows that if one turns the secondary current into something other than a momentary [571] If you want to use concussion, you have to repeat the above operation very often. That would now be quite impossible work with the free hand, since even the simple operation, accomplished in this way, costs so much time that by far the greatest part of the effect is lost during the operation.

By means of a small mechanical device, a seesaw, of a construction similar to that used earlier for simpler interchanging of the locks, the charges and discharges of the secondary column can be carried out very easily and quickly. In order to avoid the tiresome waiting for the Mechanicus, I made such a seesaw myself out of a pair of pieces of wood, some mercury and a few copper wires Minute can perform [2]. One thus obtains a current that is always intermittent, but that works for any length of time, and which can now be used for various purposes.

Physicists who have great resources at their disposal will easily produce very striking effects if they wish to transfer the principle indicated to batteries of a few hundred platinum plates of considerable size. As for me, I have been limited to a much more modest number. I could only use four pairs of plates, each plate roughly 2 1 2 < displaystyle 2 < tfrac <1> <2> >> square inches on one side. The effects of this little pillar could of course only be moderate. Nevertheless, I believe that I have observed enough in them that the correctness of the principle and its applicability to larger batteries is beyond doubt.

As the primary chain I used a simple one from [572] Grove’Great construction. It is well known that such a chain only insignificantly decomposes the water in a voltameter with platinum plates. Basically it is just the plates that are covered with gas bubbles, and very few of them rise. If one now connects the rocker with this chain and the secondary column, in the circle of which the same voltameter is connected, and then sets the former in motion, one immediately obtains a very lively decomposition of water, as an obvious proof that the electromotive force of the secondary current is considerable is stronger than that of the primary that caused it.

With a voltameter, the plates of which, calculated on one side, offered about 3 square inches of area to the water mixed with sulfuric acid, I received 5 to 6 cubic centimeters of oxyhydrogen per minute if I let the seesaw move back and forth about 80 times in the same time.

Before attempting this, I expected a greater effect. After more careful consideration of the matter, it seems to me that the effect obtained must in a certain respect be called a striking one. Because while those 6 C.C. Oxyhydrogen accumulated in the voltameter had to be, according to well-known principles, which have been confirmed by more recent experiments by Grove have been confirmed in each of the four cells of the charge column 6 C.C. combine this gas mixture to form water, and this amount of gas had previously been released from the water by the action of the primary chains. So the primary chain, which without the play of the seesaw and the depolarization of the platinum plates caused by it, might not have to be 0.1 C.C. Delivering oxyhydrogen per minute, with this tool 6 × 4 < displaystyle 6 times 4>, i.e. H. 24 C.C. evolved from the water at the same time. And yet this was not yet half the effect, since the pauses and the closing moments [573] of the column of charge necessarily filled more than half the minute.

The decomposition of water takes place even if only two cells of the charge column are allowed to act under the above circumstances. It is then only weaker. I got 1.5 C.C. Oxyhydrogen per minute. It also depends, of course, on the play of the seesaw, the faster it is moved, the greater the amount of decomposed water, at least within the limits I have examined.

With a smaller voltameter, the four cells gave the column of charge when I passed them through a simple Grove’S chain suggested that four C.C. Oxyhydrogen per minute. When I said, instead of one, two GroveI received almost 8 C.C. Gas in the same time.

The effect of the secondary battery thus increases with the intensity of the current of the primary. That is quite in order, but it also shows that when one has increased the intensity of the primary current, one must at the same time increase the number of pairs of plates in the column of charge, as well as their conductivity, if otherwise one has an intensification of the effect want.

In the case just mentioned, there was even a weakening. Because the primary column, directly connected to the voltameter, would be about 20 C.C. Oxyhydrogen delivered oxyhydrogen per minute, while the column of charge excited by them gave only eight.

This is true, however, only of the useful effect, of the effect of the column of charge in the voltameter. The effect of the primary column is always reinforced by the play of the seesaw, however few cells the column of charge may contain, if these only offer no greater resistance than the voltameter. In the case mentioned, the primary column, instead of that 20 C.C., provided at least 8 × 4 < displaystyle 8 times 4>, i.e. H. 32 C.C. Gas in [574] the secondary, while this is only 1 × 8 < displaystyle 1 times 8> C.C. gave birth in the voltameter.

When using a primary column of two GroveBy the way, one has the opportunity to observe that the lively water decomposition which it causes in the cells of the column of charge as soon as the seesaw takes the position in which it closes the primary column ceases to exist for the eye, as well as the seesaw is set in rapid motion. It is quite clear that the small amount of gas which is now released in the brief moments of the alternating action of the primary current on the plates, is transferred to these [575] remains attached, without assuming the shape of a bubble, until it is destroyed or converted back into water in the subsequent moments of the closure of the column of charge.

If the chemical action of the secondary current is to be increased, then, as just shown, the chemical action of the primary must of necessity also be increased. Such a gain is no longer necessary if one merely demands an increase in the electromotive force. This grows almost like the number of cells in the column of charge, and you have the power to increase it at will. In order to have a powerful effect, it is of course good to make this force as strong as possible in every single cell.

The polarization or counter electromotive force, which arises from the accumulation of the gaseous constituents of the water on the plates of the column of charge, has, as only recently, been carried out Lenz, by Wheatstone and Daniell has been shown to be a maximum, and this maximum is likely to have a column of two Grove’Chains are pretty much reached. Eine solche Säule würde also hinreichen, jede beliebige elektromotorische Kraft, folglich auch Funken und Erschütterungsschläge in jedem beliebigen Maaße, hervorzubringen, sobald man nur dem entsprechend die Zahl der Plattenpaare in der Ladungssäule vermehrt, und zugleich den Widerstand der primären Kette verringert, oder, wenn dieß die Umstände nicht in hinreichendem Grade erlauben sollten, die Dauer der Wirkung des primären Stroms verlängert.

Leider konnte ich diese Seite des Phänomens aus angegebenem Grunde nicht hinreichend verfolgen. Ich habe mich indeß überzeugt, daß die Ladungssäule, welche durch eine einfache Grove’sche Kette angeregt worden ist, Funken giebt, sie mag aus four, aus three, aus two, ja selbst nur aus one Plattenpaare bestehen. Im letzteren Falle sind die Funken freilich nur schwach, [576] aber doch unverkennbar. Es ist wohl das erste Mal, daß man mit einem einzigen Paar polarisirter Platinplatten elektrische Funken erhalten hat.

Alle diese Funken, mit Ausnahme der vom einzigen Plattenpaar erhaltenen, erschienen, auffallend genug, nur bei Schließung der Ladungssäule, nicht beim Oeffnen derselben. Sie erschienen immer nach den Funken, die auf der anderen Seite der Wippe bei vorausgegangener Oeffnung der primären Kette zum Vorschein kamen.

Bei einem einzigen Paare Platinplatten von größeren Dimensionen, jede Platte nämlich von beinahe 29 Quadratzoll Par. Maaß Fläche auf einer Seite, erhielt ich indeß die Funken regelmäßig beim Oeffnen, selbst mehre Male hinter einander, ohne daß die Platten zuvor wieder durch die primäre Kette geladen worden wären. Nur zuweilen erschien auch beim Schließen ein Funke doch will ich nicht gerade behaupten, daß dieß ein wahrer Schließungsfunke gewesen sey, da bekanntlich das Quecksilber hiebei zu Täuschungen Anlaß geben kann.

Zwei Rücksichten sind es, derentwegen mir der Gegenstand dieser Mittheilung noch ein besonderes Interesse zu besitzen scheint.

Für’s erste in Bezug auf die Frage, ob ein elektrischer Strom durch Wasser gehen könne, ohne dasselbe zu zersetzen. Die Meinungen darüber sind bekanntlich verschieden, und ich selbst bin dieserhalb in eine Discussion gerathen mit Hrn. Martens [3] . Ich habe die Ansicht vertheidigt, daß ein elektrischer Strom, wie schwach er auch sey, das Wasser nicht ohne Zersetzung durchlaufen könne, und in dieser Ansicht bin ich durch das, was ich bis jetzt beobachtet habe, nur bestärkt worden.

One Daniell’sche Kette zersetzt bekanntlich das Wasser zwischen Platinplatten sichtbar gar nicht dennoch wird ihr Strom durch ein solches Plattenpaar nicht [577] ganz auf Null gebracht, sondern es bleibt ein an einem empfindlichen Galvanometer recht merkbarer Rest, von dem es sich nun fragen kann, ob er bloß geleitet oder zersetzend durch das Wasser gehe. Mir scheint nicht zweifelhaft, daß, wenigstens vorher, Wasser zersetzt worden seyn muß denn wenn man die Wippe mit der Daniell’schen Kette und der Ladungssäule verbindet, erhält man in dem Voltameter der letzteren, eine verhältnißmäßig ganz ansehnliche Wasserzersetzung, 1,5 C.C. Gas in 5 Minuten. Diese Wasserzersetzung kann aber offenbar nicht anders erfolgen, als dadurch, daß die Daniell’sche Kette zuvor die Bestandtheile des Wassers an den Platten der Ladungssäule ausgeschieden hat.

Das Zweite, was der hier beschriebenen Ladungsweise einer secundären Säule Interesse verleiht, ist die ziemlich nahe liegende Frage, ob sich nicht die Natur bei den elektrischen Fischen eines ähnlichen Processes bediene? Bekanntlich besitzt der Gymnotus ein der Volta’schen Säule analog geformtes Organ. Könnte dieß nicht bloß eine Ladungssäule seyn, bestimmt die elektromotorische Kraft eines Stroms zu erhöhen, der ihr aus einer im Gehirn des Thieres liegenden Elektricitätsquelle von verhältnißmäßig sehr niederer Spannung zugeführt würde? – Ich begnüge mich diese Fragen, anzuregen mögen Andere sie zur Entscheidung bringen.

Bei allen zuvor angeführten Versuchen war immer nur von den Wirkungen des secundären Stroms die Rede, da die Wippe eine solche Einrichtung hat, daß, wann dieser Strom wirkt, der primäre unterbrochen ist. Es hat indeß gar keine Schwierigkeit, mit derselben Wippe auch die vereinte Wirkung beider Ströme zu erhalten, falls dieß in Absicht läge es bedarf dazu nur einer anderen Drahtverbindung. Ich habe den Versuch mit einer einfachen Grove’schen Kette und einem einzigen Paare Platinplatten (den großen) angestellt, wo er eigentlich nur von Interesse ist, und dabei eine sehr lebhafte [578] Wasserzersetzung erhalten. Man hat hier Gelegenheit recht deutlich zu beobachten, wie sehr die Dauer der Ladung von Einfluß ist. Läßt man die Wippe eine Minute lang in der Lage, daß nur die primäre Kette ladend wirkt, und schlägt sie dann um, wodurch diese Kette und die secundäre ihre Wirkung vereint im Voltameter ausüben müssen, so erhält man eine lebhafte Wasserzersetzung, die anderthalb Minuten und länger anhält. Geschah dagegen die Ladung momentan, so ist auch die Wasserzersetzung von kurzer Dauer. Dasselbe zeigt sich auch in den Fällen, wo, wie früher, die Wirkung des secundären Stroms von der des primären Stroms geschieden wird, überhaupt immer dann, wenn der primäre Strom relativ schwach, und die Oberfläche, der zu ladenden Platten groß wird.

Differenz zwischen elektromotorischer Kraft (EMK) und Potentialdifferenz Differenz zwischen 2021

Electromagnetism is an integral part of physics. There are terms and units that are very closely related and have a very fine line that distinguishes the two from one another. "Potentielle Differenz" und "EMF" sind zwei solche Begriffe.

elektromotorische Kraft (EMK)

Die elektromotorische Kraft oder EMK wird besser als die Gesamtspannung in einem elektrischen Schaltkreis beschrieben, die von der Quelle oder der Batterie erzeugt wird. Emf is not a physical force. It is basically the energy required to move a positive unit charge from the negative terminal of the battery to the positive terminal when the circuit is open. Emf ist die zugrundeliegende Spannung, die durch das schwankende Magnetfeld über eine Leitung oder einen Stromkreis auftritt. Formal wurde es auch definiert als die Kraft, die benötigt wird, um zwei Ladungen (eine positive und eine negative) voneinander zu trennen.

Es wird in Volt gemessen. Die elektromotorische Kraft wird oft mit dem Symbol 'ℰ' (Epsilon) bezeichnet.
Wenn wir emf definieren, erhalten wir:
Wobei ℰ die EMK und ECS das erzeugte elektrostatische Feld ist.

Mit einfachen Worten ausgedrückt, kann als elektromotorische Kraft die maximale Spannung angegeben werden, die von einer bestimmten Schaltung erreicht werden kann.

Potential difference

Die Potentialdifferenz ist die pro Ladung durchgeführte Arbeit, um eine Ladung zwischen dem negativen und dem positiven Pol der Batterie zu verschieben. Wenn die Batterie in Gebrauch ist oder der Stromkreis geschlossen ist, wird ein kleiner Teil der EMK verbraucht, um den Innenwiderstand der Batterie zu überwinden. Diese Energie pro Ladungseinheit wird als Potentialdifferenz bezeichnet.

Wenn 'ℰ' die EMK der im Schaltkreis verwendeten Batterie ist und 'r' der Innenwiderstand der spezifischen Batterie und der externe Widerstand des Stromkreises 'R' in einem Stromkreis von 'Ich' bin dann aktuell
ℰ = Ir + IR
Dabei wird ℰ - Ir als die Potentialdifferenz zwischen den Anschlüssen der Batterie angesehen, die auch als Klemmenspannung bezeichnet wird.
Die EMK kann mit einem Voltmeter gemessen werden und wird durch das Symbol 'V' (Volt) dargestellt.
Der Begriff "Potentialdifferenz" wird auch in Bezug auf Magnet- und Gravitationsfelder verwendet. Ihre Einheiten sind unterschiedlich, aber das Konzept ist ähnlich.

1. Emf ist die Gesamtspannung in der Batterie, während die Potentialdifferenz die Arbeit ist, die ausgeführt wird, um eine Ladung gegen das elektrische Feld zwischen zwei spezifischen Punkten in der Schaltung zu bewegen.
2. Emf ist immer größer als die Potentialdifferenz.
3. Das Konzept der EMK ist nur auf ein elektrisches Feld anwendbar, während die Potentialdifferenz auf magnetische, gravitative und elektrische Felder anwendbar ist.

Video: Αξιολόγηση μπούμερανγκ: Και οι εξατομικευμένες εκθέσεις δείχνουν το Υπουργείο Παιδείας 706ο Δελτίο (July 2022).


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  3. Merwyn

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  4. Thurstan

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