Electrostatic induction
Induction (Latin influence, rare Electrostatic induction called ) refers to the spatial displacement of electric charges by the action of an electric field. With a conductor, the movable charges almost always electrons move on the surface and change its place. This leads to position-dependent charge densities. The fixed atoms are not affected.
And in no charges on an insulator can be moved, but there are the atoms or molecules existing polarized.
- 2.1 Polarization
- 2.2 orientation polarization
- 5.1 Past and current applications
Influence on electrical conductors
Equal electrical charges ( / or - / -) repel each other, ungleichnamige ( / -) attract each other ( Coulomb's law ). Bring to a body with a conductive surface in an electric field, so it changes the charge density. This is because negative and positive charges are striving by the action of this field in other directions. In certain areas there is then more charge carriers than on others. The overall charge of the body remains constant.
The interior of a conductor, however, is always free of electrostatic fields, because the electrons can move in until all differences are balanced.
Induction for charge separation is in conjunction with the variable capacitors basic principle in Elektrophor, Influence, de Graaff generator and the signal voltage generation when electret electrostatic condenser microphone.
Electrically charged objects and generate charge carriers in their environment an electric field, which depends in the immediate area through its own form, at a greater distance also from the fields and potentials of the environment. The electron is the negative carrier, the proton of positive charge. Charges of the same polarity repel each other. In conductive solids, electrons are easily influenced by electric fields, while keeping protons usually their position. In conducting gases or liquids electrons are easily influenced by electrical fields and more mobile than protons, because they have considerably less mass. Therefore, often only electrons to the transport of electrical charges - the electricity - significantly.
Number of electrons involved
Very many electrons moved from their original places - By induction - measured in absolute terms. Relative to the total of available electrons on the surface of a sphere, but it is a very small fraction. An estimate is to show that:
The breakdown field strength Ekritisch in air is between 107 V / m in harsh and 109 V / m at smooth surfaces. In Influenzversuchen one has to values below Emax = 105 V / m limit, lest unwanted discharges influence the results. This allows the surface charge density σ estimate.
This contributes negatively charged any square centimeter, the excess charge of 1.8 x 10-10 As, which in turn corresponds to 1.1 × 109 electrons.
To estimate the number of unbound electrons at all included in this square centimeter, one must know the number of atoms. A copper atom to the atomic radius 200.10 -12 m occupies an area of 1.3 × 10-19 m2. So fill about 8.1014 atoms form a square centimeter. Copper is a very good conductor of electricity, each atom represents approximately a conduction electron to the electron gas of the metal available.
This leads to the estimate that the relative excess charge on a copper surface is approximately
Amounts. Among the approximately 700,000 " anyway " existing free electrons of the metal comes in strong negative electric charge to a single. In order to interpret the following pictures properly, you have to imagine that each blue point represents about 100,000 unbound electrons.
Example
In the picture above the symmetric charge distribution is shown on a spherical surface when other charged objects are very far away. Then contains every square millimeter of the surface of an equal number of positive and negative charges and the ball appears uncharged. D is the charge density on the entire surface to zero because there is nowhere in excess of charges. In the image, only the freely moving conduction electrons are drawn (about one per atom of copper ) and only one proton per atom, although each copper core contains 29 protons. The effect of the remaining 28 protons is compensated by the remaining 28 electrons in the atomic shell, therefore, waived their presentation.
The right image is shown, while the freely moving electrons " escape " if left another negative charge is approached. They would prefer to run all to the right side of the metal ball, because there the distance from the neighboring charge is maximum. The mutual repulsion prevents, however, that they crowd there on too narrow a space. Moreover, it then would be on the left hemisphere only positive ions withdraw some electrons to the left. Ultimately arises within nanoseconds, a compromise between a mutual dislike of the electrons escape from the neighboring charge and attract the positively become left hemisphere.
Each square millimeter of the right-hand hemisphere has more electrons than protons, and therefore the charge density is negative there. These electrons are missing on the left hemisphere, so D is a positive change. On a ring ( horizontal axis ) at right angles to the drawing plane, the center of which is located approximately at the center of the sphere, the charge density is compensated for ( D = 0), where each area element contains an equal number of positive and negative charges. In the drawing it can be drawn only to the upper and lower parts.
In the lower image is moved " disturbing " neighbor charge closer to it, " escape " as a result of the freely moving electrons even further to the right. The charge density on the far right is even more negative, far left even more positive than in the previous picture. The "neutral" ring with D = 0 is also indented to the right. The charge density also increases the field strength and if it exceeds a maximum value that depends on the curvature radius and the ambient gas is a corona discharge.
The accumulation of electrons on the far right should not be too taken literally. In reality, the electrons are indeed point-like and therefore can hardly mutually take up space. Only for graphic reasons, the electrons are drawn as circles voluminous.
Prerequisite for the induction is the availability of free and mobile charge carriers, either electrons or ions (which are atoms with electron deficiency or surplus ).
An electrically conductive body, for example a metal, with its large number of free electrons, or an insulating body housed in the vicinity of a negative charge, with its only a few free carriers, a small portion of the electrons is transferred to the charge opposite side. On the cargo side facing then a positive charge excess remains.
This displacement of charge carriers, that is electrons or ions, due to the action of an electric field is called induction.
Model concept
To illustrate, a conductive (metallic ) cuboid is represented in the image that comes to an electric field whose field lines are oriented from left to right. This field provides by induction that the electrons move very quickly to the left, because they are attracted to the positive pole there. The positive ions which constitute the material of the box, remain fixed in place.
The new distribution of the electrons in turn generates an electric field, but which is oriented from right to left ( red arrows). Inside of the box - no matter whether hollow or not - to compensate for both fields and therefore ends the electron migration. However, if the external field is enhanced, more electrons begin to migrate to the left. Until the rise of the external field is fully compensated in the inside of the box. If the external field is reversed, the electrons move to the right and all the signs are reversed.
At extremely high frequencies, the electrons can not sufficiently rapidly follow the change of the external field, the shielding effect can be therefore in the interior of the box by ( plasma oscillation ).
This redistribution of electrons also works with any other forms and always has the same effect: The inside of a closed conductive body is always free of electric field lines at sufficiently low frequencies. The surface charge density on the surface but can be very different from place to place.
Is located in the field of the conductive body is now separated approximately in the middle across the field, the charges remain separate. The left part of the body is negatively charged, and will remain so after removal or disconnection of the external field. The right part is positively charged - there is a potential difference between the parts. If they are separated, one requires energy derived from the mechanical working of the mutual - removal, but not from the energy of the outer ( influenzierenden ) field. This process, as an experiment with two metal plates carried out in an air capacitor array is a basic experiment to demonstrate the induction.
- Measurement of the charge transfer by induction
Charge displacement in the conductor by the external field
Charge separation by inserting an insulator
Charge measurement outside of the capacitor: two plates are loaded
Induction in insulators
In contrast to insulating electrical conductors can only be bad electrically charged, since few free charge carriers occur.
For metals, the charge transport to the forwarding of the displacement current is limited at the moment of field change in approach. When insulating bodies but it comes to the polarization, that is, it is also formed along the surface and inside an electric field. This leads to uneven distribution of the charge carriers at the surface of the body. Your polarity corresponds to the opposite side of the field to that of the influenzierenden charge. On the side facing it of opposite polarity to the charge influenzierenden.
In electrical conductors, the displacement of the charge carriers takes place more rapidly than on the surface or even inside of insulating bodies, because in the head freely moving electrons assume the charge transport. On and in an insulator is in addition to the polarization of the complete displacement ( current flow ) of charge carriers only to a small extent instead, since very few charge carriers are at defects of which they can be solved through a field. The process takes longer because less freely moving electrons or ions are present assume the charge transport.
Along the surface and inside of insulators can be constructed by polarization electric fields that have greater energy content than in empty space:
Polarization
The electrical induction does not in the form of a separation of electric charges by movement of electrons, but by means of the displacement of the polarization of an insulator. Here, the positive nucleus is pulled in one direction, the oppositely charged electron cloud to the other. The electron shell is not deformed! When an alternating field is applied, " swinging " the positive nucleus within the negative electron shell back and forth. This causes no thermal energy.
Orientation polarization
Induction acting on an electric dipole orientation by polarization. The dipole molecules are aligned with the electric field and polarized.
Force effect by influence
The charge separation causes always that the influenzierte body to an electric dipole. The distance between the focal points of unlike charges is always less than the distance of the like charges. This follows from the Coulomb law that the attractive force is always greater than the repulsive force. The difference ( net force ) is therefore attractive. In practice, the recoverable charges are relatively low, so this attraction is noticeable only at very low-mass objects such as paper pulp.
In the capacitance diode causes induction of different width of the space charge zone.
Demarcation
The effect of induction must be distinguished from mechanisms for separation of electric charges. Static electricity is based on the separation of electric charges through the triboelectric effect and is a special case of contact electricity. The transition of charges between two contacting bodies is used.
In the induction itself, however, no charge transport to or from the charged bodies takes place, it can be used only by charge transport ( making contact with the charged body ) for the production of electrical energy.
Induction can in connection with a variable capacitor to reduce the electrical voltage and also to increase extremely without producing useful electrical energy first. An object by contact with another body ( often referred to as static electricity ) is electrically charged, the body can, after they are separated from each other to accept a very high electric potential. This can lead to unwanted arcing. Similar to a charged plate capacitor whose plates are away from each other, the voltage rises because the capacitance C becomes smaller for the same charge Q. Thus, the mutual voltage U of the two bodies increases:
With
This is the operating principle of electrostatic generators, ie the influence machines, but also the electrophorus and the band generator and its development, the Pelletron or Laddertron.
Discovery
In 1754 John Canton discovered the change in the distribution of electrical charges on bodies of different material on the approach and explains this effect in 1758 at the same time Johan Carl Wilcke.
Alessandro Volta constructed on the basis of Cantons and Wilckes work the Elektrophor 1775 and the first electroscope. He coined the term induction.
Historical and current applications
The induction was used in Elektrophor for low-cost production of electricity. Abraham Bennet developed in 1787 Elektrophor the so-called Bennet doubler further, let the double electrostatic voltages. In the Wimshurst electrostatic induction is used in a cyclic process for the continuous production or increase of DC voltage. Wimshurstmaschine the combined charge separation by influence the principle of the Bennet - doubler.
In the electroscope associated with the induction mechanical forces to measure the electrical charge of goods or electroless plating voltage measurement are used.
The Kelvin - generator is also based on the effect of the induction.
Pelletron be used in a number of particle accelerators, as a DC voltage source. They also use induction to generate high voltages of up to 32 million volts.
The shielding effect of a Faraday cage relative to stationary electric fields is also based on the induction.