Electric field: the interaction between charges occurs through an electric field. As long as there is charge, there is an electric field around the charge, which is a form of matter. The basic characteristic of electric field is that it acts on static or moving charges, and the magnitude of its acting force is that the direction of positive charge is the same as that of field strength, and the direction of negative charge is opposite to that of field strength. Field strength is a physical quantity describing the characteristics of electric field, which is represented by symbol E. We use electric field lines to describe the distribution of electric field. There are two kinds of electric fields: one is electrostatic field and the other is induced electric field.

First, electrostatic field

Electrostatic field is an electric field excited by electrostatic charge. The electric field line of electrostatic field starts from positive charge and ends from negative charge, or from infinity to infinity, and its electric field force moves the charge to do work, which has the characteristics of being independent of the path. The potential difference is used to describe the electric field, or the equipotential surface is used to explain the distribution of the electric field visually.

Second, the induced electric field

The electric field excited by changing magnetic field is called induced electric field or eddy current electric field. The electric field line of the induced electric field is closed, with no starting point and no ending point. Closed electric field lines surround the changing magnetic field.

electric field intensity

A physical quantity describing the characteristics of an electric field at a certain point. The symbol is e, and e is a vector. The electric field strength, referred to as field strength, is the same as the stress direction of the measured charge. The definition of field strength is based on the characteristics of electric field acting on charge. It is suitable for electrostatic field excited by charge and eddy current field excited by changing magnetic field. The unit of field strength is Niu/ku or V/m. The two units have different names and the same size. The field strength is numerically equal to the electric field force of unit charge at this point, and the direction is the same as that of positive charge.

The characteristic of electric field is that it acts on electric charge. Electric field force, positive force in the same direction and negative force in the opposite direction. The electric field is a kind of energetic substance, and the energy of the electric field is great where the electric field is strong.

It is known that the strength of electric field can determine the force of electric field on charge, and the electrical breakdown of dielectric (insulator) is related to the field strength.

The electric field intensity of point charge is determined by point charge and has nothing to do with probe charge.

The formula of point charge field strength in vacuum is e = k * q/r 2.

Uniform electric field strength formula: E=U/d

Definition applicable to any electric field: E=F/q

Field strength of point charge in medium: kq/(ε * r 2)

electric field line

In order to describe the distribution of field strength vividly, some directional curves are drawn artificially in the electric field, and the tangent direction of a point on the curve indicates the direction of field strength at that point. The density of electric field lines is proportional to the field strength.

The electric field is a substance, and the electric field line is an auxiliary tool that we draw artificially, which is used to describe the electric field distribution easily and does not exist objectively.

In the space without charge, the electric field lines are disjoint and uninterrupted. The electric field lines of electrostatic field also have the following characteristics:

1, the electric field line is not closed, starting with positive charge and ending with negative charge;

2. The electric field line is perpendicular to the conductor surface;

3. The electric field line is perpendicular to the equipotential surface.

The electric field lines of the induced electric field have the following characteristics:

1, the electric field line is closed;

2. A closed electric field line surrounds the magnetic induction line.

Knowing the electric field line of an electric field can determine the direction and magnitude of the field strength, draw the equipotential surface and determine the potential level (the potential decreases along the electric field line).

It should be noted that the electric field line is not the trajectory of charge. According to the direction of electric field line, the direction of charge force and acceleration can be determined, but the direction of charge speed and trajectory cannot be determined. When the electric field line is a straight line, the speed of charge movement is parallel to the electric field line, and the trajectory of charge movement coincides with the electric field line.

Magnetic field: (simple definition: there is a magnetic field in the space where magnetic force can be generated. Magnetic field is a special substance. There is a magnetic field around the magnets, and the interaction between magnets is mediated by the magnetic field. )

A special substance existing in the space around current, moving charge, magnet or changing electric field. Because the magnetism of a magnet comes from current, which is the movement of charge. Simply put, it is to generate a magnetic field by moving the charge or changing the electric field.

The basic characteristic of magnetic field is that it can exert force on the moving charge, and the force or torque of magnetic field on current and magnet comes from this. Modern theory shows that magnetic force is the relativistic effect of electric field force.

Similar to the electric field, the magnetic field is a vector field distributed continuously in a certain spatial area. The basic physical quantity describing the magnetic field is the magnetic induction intensity vector b, which can also be expressed by the magnetic induction line graph. However, as a vector field, the nature of magnetic field is very different from that of electric field. The total magnetic field generated by moving charge or changing electric field, or the sum of the two, is a passive and rotating vector field, and the magnetic lines of force are a closed family of curves, which are uninterrupted and do not cross. In other words, there is no source of magnetic field lines, and there is no tail that converges magnetic field lines. The closure of the magnetic field lines shows that the loop integral along the magnetic field lines is not zero, that is, the magnetic field has a spin field instead of a potential field (conservative field), and there is no scalar function similar to the potential.

Magnetic induction intensity: the number of magnetic lines passing through unit area perpendicular to the direction of magnetic lines, also called magnetic line density, also called magnetic flux density, expressed by B, and the unit is Tesla T.

Magnetic flux: Magnetic flux is the total number of magnetic lines passing through a certain cross-sectional area, expressed by φ, and the unit is Wei (Bo) Wb. The expression of magnetic flux passing through the coil is: φ = b * s (where b is the magnetic induction intensity and s is the area of the coil. )

Magnetic field direction: specifies the direction of magnetic field force at a certain point in the magnetic field of the north pole of the small magnetic needle as the direction of electromagnetic field.

Magnetic induction line: draw some curves in the magnetic field so that the tangent direction of any point on the curve is the same as the magnetic field direction of that point. These curves are called magnetic field lines. The magnetic field lines are closed curves. The direction in which the north pole of the small magnetic needle points is defined as the direction of the magnetic field lines. The magnetic field lines around the magnet all come out from the N pole and enter the S pole. Inside the magnet, the magnetic field lines go from the S pole to the N pole.

Electromagnetic field is the medium of electromagnetic action and a unified whole. Electric field and magnetic field are closely related and interdependent. Changing electric field produces magnetic field, changing magnetic field produces electric field, and changing electromagnetic field propagates in space in the form of fluctuation. Electromagnetic wave propagation speed is limited, and energy and momentum are interchangeable. Interaction between electromagnetic waves and objects, mutual transformation between electromagnetic waves and particles, etc. It is proved that electromagnetic field is an objective substance, and its "special" is that it has no static mass.

Magnetic phenomenon is one of the earliest physical phenomena recognized by human beings, and compass is a great invention in ancient China. Magnetic fields exist widely in the earth, stars (such as the sun), galaxies (such as the Milky Way), planets, satellites and interstellar and intergalactic spaces. In order to understand and explain many physical phenomena and processes, we must consider the important factor of magnetic field. In modern science and technology and human life, magnetic fields can be seen everywhere. Generators, motors, transformers, telegrams, telephones, radios, accelerators, thermonuclear fusion devices and electromagnetic measuring instruments are all related to magnetic phenomena. Even in the human body, with the activities of life, some tissues and organs will produce weak magnetic fields. The magnetic level of the earth is opposite to the poles of geography.