electric current in a conductor arises under the influence of an electric field causes the particles to come loose charged in the direction of travel.Creating a particle current - a serious problem.To build such a device, which will maintain the potential difference of the field for a long time in the same state - a task that proved the power of humanity until the end of the XVIII century.
first attempts
first attempts to "save electricity" to further his research and use were made in the Netherlands.German Ewald Jurgen von Kleist and the Dutchman Pieter van Musschenbroek who conducted their research in the town of Leiden, created the world's first capacitor, later called "Leyden jar".
accumulation of electrical charge has passed under the influence of mechanical friction.Use discharge through the conductor can be for a short enough period of time.
victory of human intelligence on such ephemeral substance, such as electricity, proved revolutionary.
Unfortunately discharge (electric current generated by a capacitor) lasted so short that create a DC could not.In addition, the stress given by the capacitor is gradually reduced, leaving no opportunity to receive continuous current.
We had to find another way.
first source
Italian Galvani experiments to study "animal electricity" was an original attempt to find a natural source of power in nature.The prepared frogs legs hanging on metal hooks iron bars, he drew attention to the typical reaction of the nerve endings.
However, the conclusions Galvani refuted another Italian - Alessandro Volta.Intrigued by the possibility of generating electricity from the animal, he conducted a series of experiments with frogs.But the conclusion was the complete opposite of his previous hypotheses.
Volta noticed that a living organism is only an indicator of the electrical discharge.When the current legs muscles are reduced, pointing to the potential difference.The source of the electric field turned out to be the contact of dissimilar metals.The farther apart they are in the range of chemical elements, the greater the effect.
plates of dissimilar metals laid paper discs impregnated with the electrolyte solution, creating a long time needed potential difference.And let it was low (1.1 V), but an electric current can be investigated for a long time.The main thing is that the voltage remained unchanged as long.
What happens
Why sources, called "electrochemical cells", called this effect?
Two metal electrode placed into the insulator, play different roles.One supplies the electrons, the other takes them.The process of oxidation-reduction reaction leads to an excess of electrons at one electrode, which is called the negative pole, and the lack of a second, we denote it as a positive pole of the source.
In the most simple galvanic cell oxidation reactions occur at one electrode, restoration - at the other.The electrons come to the electrodes from the outer part of the chain.The electrolyte is a conductor of ions inside the current source.The strength of the resistance led by a lengthy process.
copper-zinc element
The principle of operation of electrochemical cells is interesting to consider the example of a copper-zinc galvanic cell, the effect of which goes to the energy of zinc and copper sulfate.This source of copper plate is placed in a copper sulfate solution and a zinc electrode immersed in the zinc sulphate solution.The solution was separated porous lining to avoid mixing, but always touching.
If the circuit is closed, the surface layer of zinc is oxidized.During the interaction with the liquid zinc atoms, becoming ions appear in the solution.On the electrode, electrons are released, which can take part in the formation of the current.
Getting on the copper electrode, the electrons participating in the reduction reactions.From the solution to the surface layer arrive copper ions in the recovery process, they are converted into atoms of copper deposited on the copper plate.
summarize what is happening: the work process of the cell is accompanied by the transfer of electrons to the oxidant of a reducing agent on the outer part of the chain.Reactions occur at the two electrodes.Inside source ionic current flows.
complexity of using
In principle, any of the possible redox reactions can be used in batteries.But the substances capable to work in the securities of technical elements, not so much.Moreover, many reactions require expensive materials costs.
modern batteries have a simple structure.Two electrodes are placed in one electrolyte, the vessel is filled - a battery housing.Such design features simplify the structure and reduce the price of batteries.
Any electrochemical cell is capable of producing a constant current.
current resistance does not allow all the ions simultaneously turn on the electrodes, so the unit operates for a long time.The chemical reaction to form ions eventually cease, the element is discharged.
internal resistance of the power source is of great importance.
little bit of resistance
Using electric current, no doubt, has brought scientific and technological progress to a new level, gave him a huge boost.But the strength of the resistance to current flow gets in the way of this development.
On the one hand, the electric current has the priceless properties that are used in everyday life and technology, on the other - there is a significant resistance.Physics as a science of nature is trying to strike a balance, to harmonize these circumstances.
current resistance arises from the interaction of electrically charged particles with matter, by which they move.To exclude this process under normal temperature conditions is impossible.
Resistance
internal resistance of the current source and the resistance to the outer part of the chain have several different nature, but equally in these processes is the commission for the movement of the charge.
work itself depends only on the properties of the source and its content: an electrode and electrolyte, as well as for the external circuit parts whose resistance depends on the geometry and chemical characteristics of the material.For example, the resistance of the metal wire increases with its length and decreases with the expanding cross-sectional area.When solving the problem of how to reduce the resistance, Physics recommends the use of specialized materials.
current job
In accordance with the law of Joule in conductors allocated quantity of heat proportional to the resistance.If the amount of heat to designate Qvnut., The current strength I, during his course of t, we obtain:
- Qvnut.= I2 · r · t,
where r - the internal resistance of the power supply.
in the whole chain including both internal and external parts, select the total amount of heat, the formula has the form:
- Qpolnoe = I2 · r · t + I2 · R · t = I2 · (r + R)· t,
known as indicated by the resistance in physics: the external circuit (all elements except the source) has a resistance R.
Ohm's Law for the complete chain
We consider that the basic work is done outside forces inside the power supply.Its value is the product of the charge carried by the field, and electromotive force source:
- q · E = I2 · (r + R) · t.
realizing that charge is equal to the product of the current at the time of its occurrence, we have:
- E = I · (r + R).
In accordance with the cause-and-effect relationships, Ohm's law is given by:
- I = E: (r + R).
The current in the closed circuit EMF is directly proportional to the power source and inversely proportional to the general (total) resistance of the circuit.
Based on this pattern, it is possible to determine the internal resistance of the power supply.
discharge capacity source
The main characteristics of the sources can be attributed to the discharge capacity.The maximum quantity of electricity obtained when operating under certain conditions, depending on the discharge current.
Ideally, when certain approximation, the discharge capacity can be considered constant.
For example, the standard battery potential difference of 1.5 V has a discharge capacity of 0.5 Ah.If the discharge current of 100 mA, then operated for 5 hours.
Methods charging
battery life leads to their discharge.Recovery battery charging small elements is performed using a current value of the force does not exceed one-tenth the capacity of the source.
The following charging methods:
- use of constant current for a predetermined time (about 16 hours of the current 0.1-capacity battery);
- lowering charge current to a predetermined value of the potential difference;
- use of asymmetric currents;
- consistent application of short pulses of charge and discharge, in which the first is greater than the second.
Lab
proposed task: to determine the internal resistance of the current source and the EMF.
for its implementation need to be reserved by the current source, an ammeter, voltmeter, a slide rheostat, a key set of conductors.
use Ohm's law for closed circuit will determine the internal resistance of the power supply.To do this, you must know the EMF, the resistance value of the rheostat.
resistance calculation formula current in the external part of the circuit can be determined from Ohm's law for subcircuit:
- I = U: R,
where I - current in the outer circuit, is measured by an ammeter;U - the voltage at the external resistance.
To improve the accuracy of measurements are made at least 5 times.What is it for?Measured in the experiment, voltage, resistance, current (or rather, the current) are used on.
To determine the emf current source, use the fact that the voltage across its terminals at the open key is almost equal to the EMF.
Putting a chain of series-connected batteries, rheostat, ammeter, key.The terminals of the current source connect a voltmeter.Opening the key, remove his testimony.
internal resistance, the formula of which is derived from Ohm's law for the complete chain, define mathematical calculations:
- I = E: (r + R).
- r = E: I - U: I.
measurements show that the internal resistance is significantly smaller than the external.
practical function of batteries and battery is widely used.The undisputed ecological safety of electric motors can be no doubt, but to create a capacious battery, ergonomic - the problem of modern physics.Its decision will lead to a new round of development of automotive technology.
Compact, lightweight, high-capacity batteries are also urgently needed in mobile electronic devices.The stock of energy used in them are directly connected with the operation of the device.
