What is an electric field?
We hang a charged cartridge case on a thread and bring an electrified glass rod to it. Even in the absence of direct contact, the sleeve on the thread deviates from the vertical position, being attracted to the stick (Fig. 13).
Charged bodies, as we see, are able to interact with each other at a distance. How is the action transmitted from one of these bodies to another? Maybe it's all about the air between them? Let's find out by experience.
Let us place a charged electroscope (with the glasses removed) under the bell of the air pump, after which we will pump air out from under it. We will see that in airless space the leaves of the electroscope will still repel each other (Fig. 14). This means that air does not participate in the transmission of electrical interaction. Then by means of what all the same interaction of the charged bodies is carried out? The answer to this question was given in their works by the English scientists M. Faraday (1791-1867) and J. Maxwell (1831-1879).
According to the teachings of Faraday and Maxwell, the space surrounding a charged body differs from the space around non-electrified bodies. There is an electric field around charged bodies. With the help of this field, electrical interaction is carried out.
Electrical field is a special kind of matter that differs from matter and exists around any charged bodies.
It is impossible to see or touch it. The existence of an electric field can only be judged by its actions.
Basic properties of the electric field
Simple experiments make it possible to establish basic properties of the electric field.
1. The electric field of a charged body acts with some force on any other charged body that is in this field..
This is evidenced by all experiments on the interaction of charged bodies. So, for example, a charged sleeve, which was in the electric field of an electrified stick (see Fig. 13), was subjected to the action of an attractive force to it.
2. Near charged bodies, the field they create is stronger, and far weaker.
To verify this, let us again turn to the experiment with a charged cartridge case (see Fig. 13). Let's start bringing the stand with the cartridge case closer to the charged wand. We will see that as the sleeve approaches the stick, the angle of deviation of the thread from the vertical will become larger and larger (Fig. 15). An increase in this angle indicates that the closer the sleeve is to the source of the electric field (an electrified stick), the more force this field acts on it. This means that near a charged body, the field created by it is stronger than far away.
It should be borne in mind that not only the charged stick with its electric field acts on the charged sleeve, but the sleeve, in turn, acts on the stick with its electric field. In such a mutual action on each other and manifests itself electrical interaction charged bodies.
The electric field also manifests itself in experiments with dielectrics. When a dielectric is placed in an electric field, the positively charged parts of its molecules (atomic nuclei) are displaced in one direction under the action of the field, and the negatively charged parts (electrons) are displaced in the other direction. This phenomenon is called dielectric polarization. It is polarization that explains the simplest experiments on the attraction of light pieces of paper by an electrified body. These pieces are generally neutral. However, in the electric field of an electrified body (for example, a glass rod), they are polarized. On the surface of the piece that is closer to the stick, a charge appears opposite in sign to the charge of the stick. Interaction with it leads to the attraction of pieces of paper to the electrified body.
electrical force
The force with which an electric field acts on a charged body (or particle) is called electrical force:
Fel- electric force.
Under the action of this force, a particle in an electric field acquires an acceleration a, which can be determined using Newton's second law:
where m is the mass of the given particle.
Since the time of Faraday, for a graphical representation of an electric field, it has been customary to use lines of force.
Electric field lines are lines indicating the direction of the force acting in this field on a positively charged particle placed in it. The lines of force of the field created by a positively charged body are shown in Figure 16, a. Figure 16, b shows the lines of force of the field created by a negatively charged body.
A similar picture can be observed using a simple device called electric sultan. Having informed him of the charge, we will see how all his paper strips will disperse in different directions and will be located along the lines of force of the electric field (Fig. 17).
When a charged particle enters an electric field, its speed in this field can either increase or decrease. If the particle charge q>0, then when moving along the lines of force, it will accelerate, and when moving in the opposite direction, it will slow down. If the particle charge q<0, то все будет наоборот ее скорость будет уменьшаться при движении в направлении силовых линий и увеличиваться при движении в противоположном направлении.
It's interesting to know
From today's topic about the electric field, we learned that it exists in the space that is around the electric charge.
Let's see how, with the help of lines of force having a direction, this electric field can be depicted using graphs:
You will probably be interested to know that electric fields of various strengths function in our atmosphere. If we consider the electric field from the point of view of the universe, then usually the Earth has a negative charge, but the bottom of the clouds is positive. And such charged particles as ions are contained in the air and its content varies depending on various factors. These factors depend both on the time of year and on weather conditions and the frequency of the atmosphere.
And since the atmosphere is permeated with these particles, which, being in continuous motion and which are characterized by changes either to positive or negative ions, tend to affect the well-being and health of a person. And the most interesting thing is that a large predominance of positive ions in the atmosphere can cause discomfort in our body.
The biological effect of the electromagnetic field
And now let's talk about the biological effect of EMF on human health and its impact on living organisms. It turns out that living organisms that are in the zone of influence of the electromagnetic field are subject to strong factors of its influence.
A long stay in the field of the electromagnetic field negatively affects the health and well-being of a person. So, for example, in a person with allergic diseases, such exposure to EMF can cause an epileptic attack. And if a person stays in an electromagnetic field for a longer time, diseases can develop not only of the cardiovascular and nervous systems, but also cause oncological diseases.
Scientists have proven that where there is a strong action of the electric field, behavioral changes can also be observed in insects. This negative impact can manifest itself in the form of aggression, anxiety and decreased performance.
Under such influence, abnormal development can also be observed among plants. Under the influence of an electromagnetic field in plants, the size, shape and number of petals can change.
Interesting Facts Related to Electricity
Discoveries in the field of electricity are one of the most important achievements of man, because modern life without this discovery is now even hard to imagine.
Do you know that in some parts of Africa and South America there are villages where there is still no electricity. And do you know how people get out of this situation? It turns out that they light up their homes with the help of insects such as fireflies. They fill glass jars with these insects and get light with the help of fireflies.
Do you know about the ability of bees to accumulate a positive charge of electricity during the flight? But flowers have a negative electric charge and due to this, their pollen itself is attracted to the body of the bee. But the most interesting thing is that the field of such a contact between a bee and a flower changes the electric field of the plant and, as it were, gives a signal to other bee individuals about the absence of pollen on this plant.
But in the world of fish, the most famous electric hunters are stingrays. To neutralize its prey, the stingray paralyzes it with electrical discharges.
Did you know that electric eels have the strongest electrical discharge. These freshwater fish have a discharge voltage of which it can reach 800 V.
Homework
1. What is an electric field?
2. What is the difference between a field and a substance?
3. List the main properties of the electric field.
4. What do the electric field lines indicate?
5. How is the acceleration of a charged particle moving in an electric field?
6. In what case does the electric field increase the speed of the particle and in what case does it decrease it?
7. Why are neutral pieces of paper attracted to an electrified body?
8. Explain why, after the electric sultan is charged, his paper strips diverge in different directions.
Experimental task.
Electrify the comb on the hair, then touch it with a small piece of cotton wool (fluff). What will happen to the cotton? Shake the fluff from the comb and, when it is in the air, make it soar at the same height, substituting an electrified comb from below at some distance. Why does the fluff stop falling? What will keep her in the air?
S.V. Gromov, I.A. Motherland, Physics Grade 9
An electric field arises around a charge or a charged body in space. In this field, any charge is affected by the electrostatic Coulomb force. A field is a form of matter that transmits force interactions between macroscopic bodies or particles that make up a substance. In an electrostatic field, the force interaction of charged bodies takes place. An electrostatic field - a stationary electric field, is a special case of an electric field created by stationary charges.
The electric field is characterized at each point in space by two characteristics: force - the vector of electric intensity and energy - potential, which is a scalar quantity. The strength of a given point of the electric field is a vector physical quantity, numerically equal and coinciding in direction with the force acting from the field on a unit positive charge placed at the considered point of the field:
The force line of the electric field is the line, the tangents to which at each point determine the directions of the intensity vectors of the corresponding points of the electric field. The number 0 of lines of force passing through a unit area normal to these lines is numerically equal to the magnitude of the electric field strength vector at the center of this area. The electrostatic field strength lines start at a positive charge and go to infinity for the field created by this charge. For a field created by a negative charge, lines of force come from infinity to the charge.
The potential of the electrostatic field at a given point is a scalar value numerically equal to the potential energy of a single positive charge placed at a given point of the field:
The work that is done by the forces of the electrostatic field when moving a point electric charge is equal to the product of this charge and the potential difference between the start and end points of the path:
where and are the potentials of the initial and final points of the field when the charge moves.
The intensity is related to the potential of the electrostatic field by the relation:
The potential gradient indicates the direction of the most rapid change in potential when moving in a direction perpendicular to the surface of equal potential.
The field strength is numerically equal to the change in potential per unit length , counted in the direction perpendicular to the surface of equal potential, and directed in the direction of its decrease (minus sign):
The locus of electric field points whose potentials are the same is called an equipotential surface or a surface of equal potential. The intensity vector of each point of the electric field is normal to the equipotential surface drawn through this point. On fig. 1 graphically shows the electric field formed by a positive point charge and a negatively charged plane R.
Solid lines are equipotential surfaces with potentials , , etc., dotted lines are field lines of force, their direction is shown by an arrow.
What allows us to assert that there is an electric field around a charged body?
- The presence of electromagnetic stress and vortex fields.
- the action of an electric field on a charge.
simple experience:
1. You take a wooden stick and tie a sleeve made of a shiny chocolate wrapper to it with a silk thread.
2. rub the handle on hair or wool
3. bring the handle to the sleeve - the sleeve will deviate
this allows us to assert that around a charged body (in this case, a pen, there is an electric field))) - someone help me to solve the problem
http://answer.mail.ru/question/94520561 - it's in the textbook)
- Link (electrono.ru Electric field strength, electric. .)
- In the space around an electrically charged body there is an electric field, which is one of the types of matter. The electric field has a store of electrical energy, which manifests itself in the form of electrical forces acting on charged bodies in the field.
The electric field is conventionally depicted in the form of electric lines of force, which show the direction of action of the electric forces created by the electric field.
Electric lines of force diverge in different directions from positively charged bodies and converge at bodies with a negative charge. The field created by two flat oppositely charged parallel plates is called uniform.
An electric field can be made visible by placing gypsum particles suspended in liquid oil in it: they rotate along the field, located along its lines of force. A homogeneous field is an electric field in which the intensity is the same in magnitude and direction at all points in space.Wikipedia: To quantify the electric field, a force characteristic is introduced - the electric field strength - a vector physical quantity equal to the ratio of the force with which the field acts on a positive test charge placed at a given point in space to the magnitude of this charge. The direction of the tension vector coincides at each point in space with the direction of the force acting on the positive test charge.
Approximately uniform is the field between two oppositely charged flat metal plates. In a uniform electric field, the lines of tension are parallel to each other. - Charge yourself and pour out the fluff on yourself from the pillow. Everything will be very clear.
- If you bring to the first electrically charged object another, also el. charged object, you can see their interaction, which proves the existence of an electric field.
- Allows you to read the laws of physics
- An electric field is a special form of matter that exists around bodies or particles that have an electric charge, as well as in a free form in electromagnetic waves. The electric field is directly invisible, but can be observed by its action and with the help of instruments. The main action of the electric field is the acceleration of bodies or particles that have an electric charge.
The electric field can be considered as a mathematical model that describes the value of the electric field strength at a given point in space. Douglas Giancoli wrote: “It should be emphasized that the field is not a kind of matter; more correctly, this is an extremely useful concept ... The question of the "reality" and existence of an electric field is actually a philosophical, rather even a metaphysical question. In physics, the concept of the field has proved extremely useful - it is one of the greatest achievements of the human mind.
The electric field is one of the components of a single electromagnetic field and a manifestation of electromagnetic interaction.
Physical properties of the electric field
At present, science has not yet reached an understanding of the physical essence of such fields as electric, magnetic and gravitational, as well as their interaction with each other. So far, the results of their mechanical action on charged bodies have only been described, and there is also a theory of an electromagnetic wave, described by Maxwell's Equations.Field effect - The field effect lies in the fact that when an electric field acts on the surface of an electrically conductive medium in its surface layer, the concentration of free charge carriers changes. This effect underlies the operation of field-effect transistors.
The main action of the electric field is the force effect on stationary (relative to the observer) electrically charged bodies or particles. If a charged body is fixed in space, then it does not accelerate under the action of a force. A magnetic field (the second component of the Lorentz force) also exerts a force on moving charges.
Observation of the electric field in everyday life
In order to create an electric field, it is necessary to create an electric charge. Rub some kind of dielectric on wool or something similar, such as a plastic pen on your own hair. A charge will be created on the handle, and an electric field around it. A charged pen will attract small scraps of paper to itself. If you rub an object of greater width, for example, a rubber band, on wool, then in the dark you can see small sparks resulting from electrical discharges.An electric field often occurs near the television screen when the TV set is turned on or off. This field can be felt by its action on the hairs on the arms or face.
As you know, a characteristic feature of conductors is that they always contain a large number of mobile charge carriers, i.e., free electrons or ions.
Inside the conductor, these charge carriers, generally speaking, move randomly. However, if there is an electric field in the conductor, then their ordered movement in the direction of the action of electric forces is superimposed on the chaotic movement of carriers. This directed movement of mobile charge carriers in a conductor under the influence of a field always occurs in such a way that the field inside the conductor is weakened. Since the number of mobile charge carriers in the conductor is large, the metal contains the order of free electrons), their movement under the action of the field occurs until the field inside the conductor disappears completely. Let's find out in more detail how this happens.
Let a metal conductor, consisting of two parts tightly pressed to each other, be placed in an external electric field E (Fig. 15.13). The free electrons in this conductor are affected by field forces directed to the left, i.e., opposite to the field strength vector. (Explain why.) As a result of the displacement of electrons by these forces, an excess of positive charges appears on the right end of the conductor, and an excess of electrons on the left end. Therefore, an internal field arises between the ends of the conductor (field of displaced charges), which in Fig. 15.13 is shown with dotted lines. Inside
conductor, this field is directed towards the outer one, and each free electron remaining inside the conductor acts with a force directed to the right.
At first, the force is greater than the force and their resultant is directed to the left. Therefore, the electrons inside the conductor continue to shift to the left, and the internal field gradually increases. When enough free electrons accumulate at the left end of the conductor (they still make up an insignificant fraction of their total number), the force becomes equal to the force and their resultant will be equal to zero. After that, the free electrons remaining inside the conductor will move only randomly. This means that the field strength inside the conductor is zero, i.e., that the field inside the conductor has disappeared.
So, when a conductor enters an electric field, it is electrified so that a positive charge arises at one of its ends, and a negative charge of the same magnitude at the other. Such electrification is called electrostatic induction or electrification by influence. Note that in this case only the conductor's own charges are redistributed. Therefore, if such a conductor is removed from the field, its positive and negative charges will again be evenly distributed over the entire volume of the conductor and all its parts will become electrically neutral.
It is easy to verify that at the opposite ends of a conductor electrified by influence, there are indeed equal amounts of charges of the opposite sign. We divide this conductor into two parts (Fig. 15.13) and then remove them from the field. By connecting each of the parts of the conductor to a separate electroscope, we will make sure that they are charged. (Think about how you can show that these charges are of opposite signs.) If you put both parts back together so that they form one conductor, you will find that the charges are neutralized. This means that before the connection, the charges on both parts of the conductor were the same in magnitude and opposite in sign.
The time during which the conductor is electrified by influence is so short that the balance of charges on the conductor occurs almost instantly. In this case, the tension, and hence the potential difference inside the conductor, everywhere becomes equal to zero. Then for any two points inside the conductor, the relation
Therefore, when the charges on the conductor are in equilibrium, the potential of all its points is the same. This also applies to a conductor electrified by contact with a charged body. Take a conducting ball and place a charge at point M on its surface (Fig. 15.14). Then a field appears in the conductor for a short time, and at the point M - an excess of charge. Under the influence of the forces of this field
the charge is evenly distributed over the entire surface of the sphere, which leads to the disappearance of the field inside the conductor.
So, no matter how the conductor is electrified, when the charges are in equilibrium, there is no field inside the conductor, and the potential of all points of the conductor is the same (both inside and on the surface of the conductor). At the same time, the field outside the electrified conductor, of course, exists, and its lines of tension are normal (perpendicular) to the surface of the conductor. This can be seen from the following discussion. If the line of tension were somewhere inclined to the surface of the conductor (Fig. 15.15), then the force acting on the charge at this point on the surface could be decomposed into components. Then, under the action of a force directed along the surface, the charges would move along the surface of the conductor, which, if there should be no equilibrium of charges. Therefore, when the charges on the conductor are in equilibrium, its surface is an equipotential surface.
If there is no field inside a charged conductor, then the volume density of charges in it (the amount of electricity per unit volume) must be zero everywhere.
Indeed, if there was a charge in any small volume of the conductor, then an electric field would exist around this volume.
In field theory, it is proved that at equilibrium, the entire excess charge of an electrified conductor is located on its surface. This means that the entire interior of this conductor can be removed and nothing will change in the arrangement of charges on its surface. For example, if you equally electrify two solitary metal balls of equal size, one of which is solid and the other is hollow, then the fields around the balls will be the same. M. Faraday first proved this experimentally.
So, if a hollow conductor is placed in an electric field or electrified by contact with a charged body, then
when the charges are in equilibrium, the field inside the cavity will not exist. This is the basis of electrostatic protection. If a device is placed in a metal case, then external electric fields will not penetrate inside the case, i.e., the operation and readings of such a device will not depend on the presence and change of external electric fields.
Let us now find out how the charges are located on the outer surface of the conductor. Take a metal mesh on two insulating handles, to which paper sheets are glued (Fig. 15.16). If you charge the grid and then stretch it (Fig. 15.16, a), then the leaves on both sides of the grid will disperse. If the mesh is bent into a ring, then only the leaves on the outer side of the mesh deviate (Fig. 15.16, b). By giving the grid a different bend, one can make sure that the charges are located only on the convex side of the surface, and in those places where the surface is more curved (smaller radius of curvature), more charges accumulate.
So, the charge is distributed evenly only over the surface of a spherical conductor. With an arbitrary shape of the conductor, the surface charge density and, hence, the field strength near the surface of the conductor is greater where the curvature of the surface is greater. The charge density is especially high on the protrusions and on the edges of the conductor (Fig. 15.17). This can be verified by touching the various points of the electrified conductor with the probe, and then the electroscope. An electrified conductor, having points or provided with a point, quickly loses its charge. Therefore, the conductor, on which the charge must be stored for a long time, should not have points.
(Think about why the rod of an electroscope ends in a ball.)
