Electric Field

Electric field is defined as the electric force per unit charge. The direction of the field is taken to be the direction of the force it would exert on a positive test charge. The electric field is radially outward from a positive charge and radially in toward a negative point charge.

Electric Field of Point Charge

The electric field of a point charge can be obtained from Coulomb's law: 点电荷的电场可通过库仑定律获得： The electric field is radially outward from the point charge in all directions. The circles represent spherical equipotential surfaces. 电场从点电荷向各个方向径向向外延伸。这些圆圈表示的是球形等势面。

The electric field from any number of point charges can be obtained from a vector sum of the individual fields. A positive number is taken to be an outward field; the field of a negative charge is toward it.

This electric field expression can also be obtained by applying Gauss' law.

Electric and Magnetic Constants

In the equations describing electric and magnetic fields and their propagation, three constants are normally used. One is the speed of light c, and the other two are the electric permittivity of free space ε₀ and the magnetic permeability of free space, μ₀. The magnetic permeability of free space is taken to have the exact value

See also relative permeability 参见相对磁导率

With the magnetic permeability established, the electric permittivity takes the value given by the relationship

This gives a value of free space permittivity

These expressions contain the units F for Farad, the unit of capacitance, and C for Coulomb, the unit of electric charge.

In the presence of polarizable or magnetic media, the effective constants will have different values. In the case of a polarizable medium, called a dielectric, the comparison is stated as a relative permittivity or a dielectric constant. In the case of magnetic media, the relative permeability may be stated.

Physical Connections to Electric Permittivity and Magnetic Permeability

Expressions for the electric and magnetic fields in free space contain the electric permittivity ε₀ and magnetic permeability μ₀ of free space. As indicated in the section on electric and magnetic constants, these two quantities are not independent but are related to "c", the speed of light and other electromagnetic waves.

The electric permittivity is connected to the energy stored in an electric field. It is involved in the expression for capacitance because it affects the amount of charge which must be placed on a capacitor to achieve a certain net electric field. In the presence of a polarizable medium, it takes more charge to achieve a given net electric field and the effect of the medium is often stated in terms of a relative permittivity. 电容率与电场中储存的能量有关。它参与电容的表达式，因为它影响了必须加在电容器上的电荷量，以产生一定的净电场。在存在极化介质的情况下，要产生给定的净电场需要更多的电荷，而介质的影响通常用相对电容率来表述。

The magnetic permeability is connected to the energy stored in a magnetic field. It is involved in the expression for inductance because in the presence of a magnetizable medium, a larger amount of energy will be stored in the magnetic field for a given current through the coil. The effect of the medium is often stated in terms of a relative permeability. 磁导率与磁场中储存的能量有关。它参与电感的表达式，因为在存在磁性介质时，对于给定的线圈电流，磁场中储存的能量会更大。介质的影响通常用相对磁导率来表述。