Have you noticed that when you take off a sweater or a T-shirt, sparks fly and crackles are heard? What about when you get out of the car and get electrocuted? This is static electricity or electrification of bodies. It results from the accumulation electric charges different signs on objects with their subsequent compensation. In this article, we will briefly consider this phenomenon, the causes of its occurrence, as well as methods of application both in everyday life and in industry.
Definition
Electrization is the process of separation of electric charges and their accumulation in certain places of objects and bodies. The phenomenon occurs as a result of friction, contact of bodies or as a result of electrostatic induction. In simple words when an object with an electric field is located nearby.
Recall: in physics, there are two types of charges - positive and negative, or protons and electrons. arises between them. Like charges attract and unlike charges repel.
The phenomenon is observed on power supplies and not only. Charges accumulate on dielectrics, everyone saw this in experiments illustrating the phenomenon with ebonite and glass rods, which were demonstrated at physics lessons at school.
Initially, all atoms, which make up everything that surrounds us, are electrically neutral. As a result of the phenomenon of electrification, positive or negative charges appear on the surface of objects. Recall a school experience: if you rub an ebonite stick with a woolen cloth, after the friction stops, the stick will remain charged. Then they say that the body is electrified.
Thus, during friction, electrons passed from one object to another. As a result, after the termination of friction, excess electrons remained "not on their" bodies and an excess charge was obtained, and it ceased to be neutral. As a result of friction of the stick on wool or fur, a negative charge formed on its surface.
Conditions for the occurrence of the phenomenon and methods of transferring charges
We have told how this phenomenon is explained in nature, and now let's look at how bodies can be electrified. We note right away that the fulfillment of all conditions is not necessary - electrification can occur for one reason or another, we will divide them into two main groups:
The first is mechanical interaction. During friction, the distance between objects is comparable to the distance between the molecules in it. Since the electrons in one of the bodies are weaker connected with the nucleus, they move “escape” to another body. Other methods of electrification are impact and contact.
The second group is electrification by influence, that is, the phenomenon is observed when external forces act on the body, including:
- Electric field. As a result of the action of the field on the conductor, charges appear on its surface, and the smaller the surface bending radius, the more charges will accumulate here. So there will be the most charges on the tip, we considered this issue in more detail in the article and here
- Exposure to light. Opened by Professor A.G. Stoletov in 1888, lies in the fact that when zinc, aluminum, cesium, sodium, lead, potassium and other metals are exposed to light, they lose electrons and become positively charged.
- Warmth. When the metal is heated, the electrons are given enough energy to leave the metal, as a result, it acquires a positive charge.
- Chemical reaction. In the presence of two electrodes from different metals, redox reactions occur, as a result, one of them becomes positively charged, and the second negatively. We considered this in more detail in an article about.
- Under pressure. In piezoelectrics (quartz, Rochelle salt, ammonium phosphate), under mechanical action (compression or tension), positive and negative charges are formed on the faces.
These are the main types of electrification.
What laws of physics are associated with electrification
The phenomenon of electrification is associated with such physical laws as:
- . Describes the force with which charges interact. Thus, it is possible to determine how strongly electrified bodies are attracted to each other.
- . It says that the algebraic sum of charges in a closed system is unchanged. This suggests that excess charges on electrified objects do not appear out of nowhere, but move from body to body.
We have already considered these laws, you can read more in the relevant articles to which we have referred.
Application in practice
The phenomenon of electrification has both positive and negative manifestations. Examples of positive application:
There are also a number of applications for cleaning, sorting, filtering, as well as in medicine to speed up treatment.
The negative impact of electrification can lead to fatal consequences:
- The occurrence of sparks when charged objects come into contact. Such cases include sparks in everyday life that slip when you take off your sweater, when you are shocked when you exit the car. For example, an airplane in flight is electrified and when a ladder is brought to it, sparks can slip through, and because of this, ignition is possible, so the charge is first removed from the aircraft. There are also known cases of ignition of oil tankers due to electrification.
- The phenomenon leads to the appearance of large electric charges, they can lead to failure of electronic components in equipment, both during the production of equipment and during operation or repair. This occurs as a result of discharging the tool onto the circuit board. Therefore, electronics repairmen must work with grounded electrical wristbands and grounded soldering irons and the like. In the modern element base there are a number of technical solutions to minimize the effect of electrification on their work. For example, installing Zener diodes in parallel with the GATE-SOURCE circuit of field-effect transistors.
Interesting! There is a known case when varnishing printed circuit boards after mounting electronic components, there was a large rejection, despite the fact that all products were tested before varnishing. The question arose: how to get rid of the problem of electrification? The problem was solved by grounding the spray gun.
REVIEW QUESTIONS FOR THE DISCIPLINE EXAM
_____________________PHYSICS_________________________
Electrification of tel. Methods of electrification of bodies. Coulomb's law. Dielectric permittivity of the medium.
To electrify the body is to charge.
Ways:
Friction (touch)-bodies are charged with the same name.
Influence - charge differently
Irradiation: ultraviolet, x-ray, etc.
The force of interaction of two point charges is directly proportional to the product of the magnitudes of these charges, inversely proportional to the square of the distance between them, depends on the medium, is directed along the straight line connecting these charges
ε=F_0/F_av
How many times the force of interaction of two point charges in a vacuum is greater than their interaction in a medium.
ε=ε_av/ε_0
Electric field as a special kind of matter. Graphic image electric field. Electric field strength. Homogeneous field.
An electric field is a special kind of matter through which static charges interact.
Properties:
Created by charge
Act on charge
charge related
Detect with a single positive test charge
It's limitless
Spreads in any environment
Represented by lines of force
E=F/q
The electric field strength at a given point is numerically equal to F acting on a single positive test charge placed at a given point of the electric field.
SI:
[E]=N/KL
A uniform electric field is a field in which the intensity is the same at every point.
The work of the electric field when moving the charge. Potential charge energy. Potential. Potential difference and voltage. Relationship between field strength and voltage.
φ=А_(1→∞)/q
The potential of the electric field at a point is numerically equal to A, which the electric field makes over a unit positive test charge when moving from one point to infinity.
φ=Е_р/q
SI:
[φ]=J/Cl=V
Voltage is the potential difference of two point charges of an electric field.
U=A_(1→2)/q
The potential of the electric field at a point is numerically equal to A, which the electric field makes over a unit positive test charge when moving from a given point to another.
A=E*q*l
A=U*q
U*q=E*q*l
U=E*l
Conductor in an electric field. equipotential surface. Dielectric in an electric field. Dielectric polarization. Electrostatic protection.
An electrified conductor has charges on the surface. An electrified conductor destroys E_ext (ϵ_(el.p) inside the conductor is equal to zero).
An equipotential surface is a surface of equal potential.
Dielectric polarization is the rotation of a dipole in an electric field.
Electrostatic protection - the placement of devices that are sensitive to an electric field inside a closed conductive sheath to shield from an external electric field.
The electrical capacity of the conductor. Capacitors. Types and connection of capacitors. The energy of the electric field of a charged capacitor.
The capacitance of a conductor is the ability of a conductor to accumulate charges on its surface.
С= q/φ
The electrical capacitance of the conductor is numerically equal to q, which must be placed on the conductor so that φ=1V.
In SI:
[C]=CL/V=F
Outside system units:
1pF=1*〖10〗^(-12)F
1nF=1*〖10〗^(-9)F
1uF=1*〖10〗^(-6)F
Capacitor - a system of two conductors separated by a dielectric
Types of capacitors:
Air
Paper
Electrolytic
Sluidny
Ceramic
They follow each other. Presence of nodal points.
W_el=(q*U)/2
W_el=(C*V^2)/2
Electric current and the condition of its existence. Strength and current density. Units of their measurement. The dependence of the current strength from an electronic point of view. Ohm's law for a circuit section.
Electric current-directed (ordered) movement of charged particles.
Existence conditions:
-the presence of free electric charges in the environment
- creation of an electric field in the environment.
The current strength is a value showing how much charge passed through the cross section of the conductor in 1 second.
I=q/t
Si: [I]=C/sec=A
Outside system units:
1uA=1*〖10〗^(-6)A
1mA=1*〖10〗^(-3) A
1kA=1*〖10〗^3 A
The current density shows the number of charges per unit area of the cross section of the conductor.
j=I/S
SI: [j]=A/m^2
Outside system units:
1A/〖mm〗^2 =1*〖10〗^(6 A/m^2)
1A/〖cm〗^2 =1*〖10〗^4 A/m^2
1A/〖dc〗^2 =1*〖10〗^2 A/m^2
Let's establish what the current strength in the conductor depends on from an electronic point of view
I=n_0*S*e*v
n_0-kind of conductor
S - thin or thick
e-type of conductor (TV, liquid, gas).
Ohm's law:
I=U/R
The current strength in a section of the circuit is directly proportional to the voltage at the ends of this section, inversely proportional to the resistance of this section of the circuit.
C:
[R]=V/A=Ohm
Outside system units:
1kΩ=1*〖10〗^3Ω
1mΩ=1*〖10〗^6Ω
Closed electrical circuit. External and internal sections of the chain. The electromotive force of a source of electrical energy. Ohm's law for a complete circuit with one E.D.S.
Closed circuit-consumer+source
The outer section of the circuit is a consumer of electricity
The internal section of the circuit is a source of electrical energy
ε=A_st/q
The EMF of the source is numerically equal to A, which is performed by external forces when moving a unit charge inside the source.
Ohm's law for a closed circuit
I=ε/(R+r)
The current strength in the entire circuit is directly proportional to the EMF of the source and inversely proportional to the sum of the external and internal sections of the circuit.
conductor resistance. The dependence of the resistance on the kind, size of the conductor and temperature. Superconductivity. Conductor resistivity and units of measure.
1/(n_0+e+u)=p-conductor resistivity
R=ρ*l/S
[p]=Ohm*m
Superconductivity is the phenomenon of a sharp drop in resistance to zero near absolute zero.
Serial and parallel connection of consumers and sources of electrical energy.
Consumer connection
Serial Parallel
I_total=I_1=I_2=I_3 I_total=I_1+I_2
U_total=U_1+U_2+U_3 U_total=U_1+U_2
R_total=R_1+R_2+R_3 1/R_total=1/R_1 +1/R_2
R_gen=(R_1*R_2)/(R_1+R_2)
Sign: one after the other Sign: nodal points
Linking sources
Serial Parallel
ε_b=ε_1+ε_2+ε_3=ε_1*nε_b=ε_1=ε_2=ε_3
r_b=r_1+r_2+r_3=r_1*n 1/r_b=1/r_1 +1/r_2 +1/r_3
I_b=(ε_1*n)/(R+r_1*n) I_b=ε_1/(R+r_1/m)
Work and power of electric current. Units of their measurement. Thermal action current. Joule-Lenz law. Short circuit.
A_(electric current)=U*I*t=P*t
A_ (electronic current) depends on the strength of the current, time and does not depend on what type of energy it turns into
Unit measurements:
[A]=V*A*sec=J=W*sec
Outside system units:
1Wh=3.6*〖10〗^3J
1 kWh=3.6*〖10〗^6J
1mWh=3.6*〖10〗^9J
Power is physical quantity, showing the unit of work done per unit of time.
P=U*I
SI:
[P]=W
Outside system units:
1kW=1*〖10〗^3W
1mW=1*〖10〗^6W
Joule Lenz's Law
Q=I^2*R*t
The amount of heat released in the conductors is directly proportional to the square of the current strength, resistance and the time it takes the current to pass through the conductor.
I_kz=ε/r
Thermionic emission. Exit work. Contact potential difference. Thermocouple and its application. thermoelectromotive force.
The phenomenon of the release of charge from the conductor under the action of high temperature is called emission.
A_out=e*∆φ
e=1.6*〖10〗^(-19)
Unit measurements: [A_out]=Cl*V=J
Off-system units: 1eV=e*1V=1.6*〖10〗^(-19)J
∆φ-contact potential difference arises:
With different work function
For different amounts of e
A thermocouple is a device consisting of two homogeneous metals, the ends of which are soldered.
Application:
1. Power source
2.Generator "Chamomile"
3. Thermometer
1.If t_a=t_0, then ∆φ_1=∆φ_2, I=0
2. t_a>t_b, then ∆φ_1>∆φ_2, I≠0
Thermo-EMF occurs in a thermocouple when one of the junctions is heated.
electrolytic dissociation. Electrolysis and its application. Faraday's laws. Application of electrolysis.
Electrolytic dissociation is a solution of salts, acids and alkalis.
Electrolysis is the process of separating a substance at the cathode during the passage of an electric current through an electrolyte.
Application:
To obtain refined metals
Electroplating is the coating of one metal with another
Galvanoplasty is the production of various impressions of bas-reliefs.
Faraday's laws:
m=k*I*t
The mass of the released substance on the cathode is directly proportional to the amount of electricity that has passed through the electrolyte per unit time.
M/N_A*q_1=k
k is the electrochemical equivalent.
Physical meaning:
k=m/q
The electrochemical equivalent is numerically equal to m of the substance, which was released on the cathode after passing q_ed ^ + through the electrolyte.
SI: [k]=Kg/Cl
k=1/F*x; k=e*N_A-Faraday number
k~x
The Faraday number shows the charge carried by a univalent ion contained in 1 mole of a substance.
F=9.7*〖10〗^4 Kg/mol
Electric current in gases at atmospheric pressure. Types of ranks. The concept of plasma. Electric current in rarefied gases. The concept of cathode rays. Electric current in vacuum. Two-, three-electrode lamp. Cathode-ray tube.
Gas at P_atm = dielectric
Discharge types:
Discharge types:
Non-self independent
Uch. 0.1; 1.2 account. 2.3
The presence of an ionizer (quiet) the presence of high U
Sound, light
Plasma is a substance in a state where it is generally electrically neutral, but contains equal numbers of free positive and negative charges.
It can be cold (up to 〖1000〗^° C-fire) and hot (over 1 〖million〗^° C-Sun)
Comparative characteristics of conductors, semiconductors and dielectrics. Intrinsic and impurity conductivity of semiconductors.
Electronic-hole transition. semiconductor diode. Direct and reverse inclusion of P - H - transition.
A magnetic field. Magnetic induction. Interaction of parallel currents. Magnetic permeability of the medium. Magnetic fields of direct and circular currents and a solenoid. Ampere power. Left hand rule.
magnetic flux. tension magnetic field. The action of a magnetic field on a moving charge. Lorentz force. The concept of PLASMA, prospects for its application.
Paramagnetic, diamagnetic, ferromagnetic substances. Curve of the initial magnetization of a ferromagnet. Curie point.
Electromagnetic induction. The law of electromagnetic induction. Flux linkage. Occurrence of emf with induction when a conductor moves in a magnetic field.
The direction of the induction current. Lenz's rule. Eddy currents, their use and measures to combat them.
Phenomena of self-induction. conductor inductance. Conditions on which the inductance of a conductor depends. Unit of measure for inductance.
Conditions for the occurrence of oscillations. Parameters of oscillatory motion. Own and forced vibrations. Harmonic oscillation, its equation and graph.
Propagation of vibrations in an elastic medium. Transverse and longitudinal waves. Wavelength. mechanical resonance.
The nature of the world. Wave and quantum theories of light. The speed of propagation of light in a vacuum, in various media. Determination of the speed of light by the Michelson method. Huygens principle.
OBJECTIVES TO REVIEW
§ 9 Nos. 14,18,20,21,24.
§10 Nos. 15,20,30,41,43,48.
§ 11 Nos. 8,24,27,35,38.
§ 12 Nos. 10,31,35,52,67,75,82,101,112,129,131,136.
§ 13 nos. 11,24,28,37,62,64.
§ 14 Nos. 13,15,17,31,41,42.
§ 17 Nos. 18,32,33,34.
As part of today's lesson, we will get acquainted with such a physical quantity as charge, see examples of the transfer of charges from one body to another, learn about the division of charges into two types and about the interaction of charged bodies.
Topic: Electromagnetic phenomena
Lesson: Electrification of bodies upon contact. Interaction of charged bodies. Two kinds of charges
This lesson is an introduction to the new section "Electromagnetic Phenomena", and in it we will discuss the basic concepts that are associated with it: charge, its types, electrification and the interaction of charged bodies.
The history of the concept of "electricity"
First of all, we should start with a discussion of such a thing as electricity. IN modern world we constantly encounter it at the household level and can no longer imagine our life without a computer, TV, refrigerator, electric lighting, etc. All these devices, as far as we know, work thanks to electric current and surround us everywhere. Even technologies not completely dependent on electricity initially, such as the operation of an internal combustion engine in a car, are slowly beginning to fade into history, and electric motors are actively taking their place. So where did the word "electric" come from?
The word "electric" comes from the Greek word "electron", which means "amber" (fossil resin, Fig. 1). Although it should, of course, be immediately stipulated that there is no direct connection between all electrical phenomena and amber, and we will understand a little later where such an association came from among ancient scientists.
The first observations of electrical phenomena date back to the 5th-6th centuries BC. e. It is believed that Thales of Miletus (the ancient Greek philosopher and mathematician from Miletus, Fig. 2) first observed the electrical interaction of bodies. He carried out the following experiment: he rubbed amber with fur, then brought it closer to small bodies (dust particles, shavings or feathers) and observed that these bodies began to be attracted to amber for no reason explainable at that time. Thales was not the only scientist who subsequently actively conducted electrical experiments with amber, which led to the emergence of the word "electron" and the concept of "electric".
Rice. 2. Thales of Miletus ()
We simulate similar experiments with the electrical interaction of bodies, for this we take finely chopped paper, a glass rod and a sheet of paper. If you rub a glass rod on a sheet of paper, and then bring it to finely cut pieces of paper, you will see the effect of attracting small pieces to the glass rod (Fig. 3).
An interesting fact is that for the first time such a process was fully explained only in the 16th century. Then it became known that there are two types of electricity, and they interact with each other. The concept of electrical interaction appeared in the middle of the 18th century and is associated with the name of the American scientist Benjamin Franklin (Fig. 4). It was he who first introduced the concept of electric charge.
Rice. 4. Benjamin Franklin ()
Definition.Electric charge- a physical quantity that characterizes the magnitude of the interaction of charged bodies.
What we had the opportunity to observe in the experiment with the attraction of pieces of paper to an electrified stick proves the presence of electrical interaction forces, and the magnitude of these forces is characterized by such a concept as charge. The fact that the forces of electrical interaction can be different is easily verified experimentally, for example, by rubbing the same stick with different intensities.
To carry out the next experiment, we need the same glass rod, a sheet of paper and a paper plume fixed on an iron rod (Fig. 5). If you rub the stick with a sheet of paper, and then touch it to the iron rod, then the phenomenon of repulsion of the strips of the sultan's paper from each other will be noticeable, and if you repeat rubbing and touching several times, you will see that the effect is enhanced. The observed phenomenon is called electrification.
Rice. 5. Paper sultan ()
Definition.Electrification- separation of electric charges as a result of close contact of two or more bodies.
Electrification can occur in several ways, the first two we considered today:
Electrification by friction;
Electrification by touch;
Electrization by guidance.
Consider electrification by guidance. To do this, take a ruler and put it on top of the iron rod on which the paper sultan is fixed, after that we touch the rod to remove the charge on it, and straighten the strips of the sultan. Then we electrify the glass rod by rubbing it against the paper and bring it to the ruler, the result will be that the ruler will begin to rotate on top of the iron rod. In this case, do not touch the ruler with a glass rod. This proves that there is electrification without direct contact between bodies - electrification by guidance.
The first studies of the values of electric charges date back to a later period in history than the discovery and attempts to describe the electrical interactions of bodies. At the end of the 18th century, scientists came to the conclusion that charge division leads to two fundamentally different results, and it was decided to conditionally divide charges into two types: positive and negative. In order to be able to distinguish between these two types of charges and determine which is positive and which is negative, we agreed to use two basic experiments: if you rub a glass rod on paper (silk), then a positive charge is formed on the rod; if you rub an ebonite stick against fur, then a negative charge is formed on the stick (Fig. 6).
Comment.Ebonite- rubber material with a high sulfur content.
Rice. 6. Electrization of sticks with two types of charges ()
In addition to the fact that the division of charges into two types was introduced, the rule of their interaction was noticed (Fig. 7):
Charges of the same name repel each other;
Opposite charges attract.
Rice. 7. Interaction of charges ()
Consider the following experiment for this interaction rule. We electrify the glass rod by friction (i.e., transfer a positive charge to it) and touch it to the rod on which the paper sultan is fixed, as a result we will see the effect that has already been discussed earlier - the strips of the sultan will begin to repel each other. Now we can explain why this phenomenon occurs - since the strips of the sultan are positively charged (of the same name), they begin to repel as far as possible and form a figure in the shape of a ball. In addition, for a more visual demonstration of the repulsion of like-charged bodies, you can bring a glass rod rubbed with paper to an electrified plume, and it will be clearly visible how the strips of paper will deviate from the rod.
At the same time, two phenomena - the attraction of oppositely charged bodies and the repulsion of similarly charged bodies - can be observed in the following experiment. For it, you need to take a glass rod, paper and a foil sleeve, fixed with a thread on a tripod. If you rub a stick with paper and bring it to an unloaded sleeve, then the sleeve will first be attracted to the stick, and after touching it will begin to repel. This is explained by the fact that at first the sleeve, until it has a charge, will be attracted to the stick, the stick will transfer part of its charge to it, and the similarly charged sleeve will repel from the stick.
Comment. However, the question remains why the initially uncharged cartridge case is attracted to the stick. It is difficult to explain this using the knowledge available to us at the current stage of studying school physics, however, let's try, looking ahead, to do this briefly. Since the sleeve is a conductor, then, once in an external electric field, the phenomenon of charge separation is observed in it. It manifests itself in the fact that free electrons in the material of the sleeve move to the side that is closest to the positively charged rod. As a result, the sleeve becomes divided into two conditional areas: one is negatively charged (where there is an excess of electrons), the other is positively charged (where there is a lack of electrons). Since the negative region of the sleeve is located closer to the positively charged rod than its positively charged part, the attraction between opposite charges will prevail and the sleeve will be attracted to the rod. After that, both bodies will acquire the same charge and repel.
This issue is considered in more detail in the 10th grade in the topic: "Conductors and dielectrics in an external electric field."
In the next lesson, the principle of operation of such a device as an electroscope will be considered.
Bibliography
- Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. Physics 8 / Ed. Orlova V.A., Roizena I.I. - M.: Mnemosyne.
- Peryshkin A. V. Physics 8. - M .: Bustard, 2010.
- Fadeeva A. A., Zasov A. V., Kiselev D. F. Physics 8. - M .: Education.
- Encyclopedia of Brockhaus F.A. and Efron I.A. ().
- youtube().
- youtube().
Homework
- Page 59: Questions #1-4. Peryshkin A. V. Physics 8. - M .: Bustard, 2010.
- The metal foil ball was positively charged. It was discharged, and the ball became neutral. Can we say that the ball's charge has disappeared?
- In production, to capture dust or reduce emissions, the air is cleaned using electrostatic precipitators. In these filters, air flows past oppositely charged metal rods. Why is dust attracted to these rods?
- Is there a way to charge at least part of a body positively or negatively without touching that body with another charged body? Justify the answer.
When bodies rub against each other, it is the electron shells of atoms that “rub” against each other, of which the bodies are composed. And since the electrons are weakly bound to the nuclei of atoms, the electrons can separate from "their" atoms and move to another body. As a result, an excess of electrons (negative charge) appears on it, and a lack of electrons (positive charge) appears on the first body.
So, electrification by friction is explained by the transition of part of the electrons from one body to another, as a result of which the bodies are charged differently. Therefore, bodies electrified by friction against each other always attract (see § 8-b). But, in addition to electrification by friction, there is electrification by induction (Latin "induction" - guidance). Let's experience it:
At the beginning of the experiment, there are two metal balls that touch each other (a). A charged glass rod is brought to one of them without touching it (b), after which the second ball is moved away (c). Now the wand can be removed - the balls will be oppositely charged (d).
Let us explain this experiment from the point of view of the electron-ion theory.
At first, the metal balls were not charged. This means that the electron gas was present in the balls in equal amounts (a). Since the rod is glass, we consider its charge to be positive (see § 8-b). It attracts negatively charged particles - electrons. As a result, the electron gas "flows" into the left side of the left ball, and an excess of negative charge is formed in this place (b).
All positive metal ions are firmly connected to each other (they are the metal), so they don’t “flow” anywhere. This means that in all other parts of the balls there is a lack of electrons, that is, a positive charge. And if at this moment, without removing the stick, move the balls apart (c) and only then remove it, the balls will remain oppositely charged (d).
So, electrification by induction is explained by the redistribution of electron gas between bodies (or parts of the body), as a result of which the bodies (or parts of the body) are charged differently. However, the question arises: can all bodies be electrified by induction? Experiments can be made and one can be convinced that plastic, wooden or rubber balls can be easily electrified by friction, but not by induction. Let's explain it.
The electrons in rubber, wood, and all plastics are not free, that is, they do not form an electron gas that can flow into other bodies. Therefore, to electrify bodies made of these substances, it is necessary to resort to their friction, which contributes to the separation of electrons from "their" atoms and the transition to another body.
So, according to the electrical properties, all substances can be divided into two groups. Dielectrics- substances that do not have free charged particles and therefore do not conduct a charge from one body to another. conductors – substances with free charged particles that can move, transferring charge to other parts of the body or to other bodies. This is illustrated in the drawing with electroscopes, plastic ruler and metal wire (see above).
Even in ancient times, it was known that if you rub amber on wool, it begins to attract light objects to itself. Later, the same property was discovered in other substances (glass, ebonite, etc.). This phenomenon is called electrification; bodies that are capable of attracting other objects to themselves after rubbing are electrified. The phenomenon of electrification was explained on the basis of the hypothesis of the existence of charges that an electrified body acquires.
3.1.2. Interaction of charges. Two types of electric charges
Simple experiments on the electrification of various bodies illustrate the following points.
1. There are two types of charges: positive (+) and negative (-). A positive charge arises when glass is rubbed against skin or silk, and a negative charge occurs when amber (or ebonite) is rubbed against wool.
2. Charges (or charged bodies) interact with each other. Like charges repel, and unlike charges attract.
