70. Basic properties of the electric field. Field strength

Properties of the field.

Our understanding of what is an electric field is the result of an experimental research of its properties. There are many of these properties, and one would think not all of them are known now.

The main property of an electric field is the ability to act on electric charges with some force. In action on the charge establish the presence of the field, its distribution in space, study all its characteristics. The electrostatic field is created only by electric charges. The electrostatic field is the electric field of stationary charges. In electrostatics we deal only with an electrostatic field - a special case of an electric field. Further, speaking about the properties of the field, we will not call it electrostatic if this property is equally inherent in both static and variable fields.

Electric field strength.

The quantitative electric field is characterized by a value called the field strength. Unlike the field itself, the field strength can be determined accurately and concisely.

If you place small charged bodies at the same point in the field and measure the forces acting on them, you will find that the force acting on charge \(q\) from the field is proportional to the value of the charge. The force-to-charge ratio is therefore independent from the value of the charge and can be considered as a field characteristic. It is called electric field strength. Like force, electric field strength is a vector. We will denote it by \(\overrightarrow{E}\).
By definition

\( \overrightarrow{E} ~= ~\frac{\overrightarrow{F}}{q} \) (8-9)

From here, the force acting on the charge \(q\) equals

\( \overrightarrow{F} ~= ~{q}\overrightarrow{E} \)

According to \((8-9)\) the field strength is the value measured by the ratio of the force acting from the field to some test charge.
It is not difficult to find the field strength of a point charge \(q\). Let the charge \(q_0\) be placed at a distance \(R\) from the charge \(q\). Force will act on it in an environment with dielectric permittivity \(\varepsilon\)

\( F ~= ~\frac{1}{4 \pi \,\varepsilon{_0} \,\varepsilon} \,\frac{q \,q_0}{R^2}\)

The field strength at a distance \(R\) according to the definition \((8-9)\) in SI units is equal to

\( \overrightarrow{E} ~= ~\frac{\overrightarrow{F}}{q_0} ~= ~\frac{q}{4 \pi \,\varepsilon{_0} \,\varepsilon \,R^2} \) (8-10)

The principle of superposition.

The electric field has another very important property. If at a given point of space different charged particles create fields \(\overrightarrow{E}_1, \,\overrightarrow{E}_2, \,\overrightarrow{E}_3\), etc., then the full field strength at this point will be, as experience shows, equal to the geometric sum of the fields of individual particles.

\( \overrightarrow{E} ~= ~\overrightarrow{E}_1 \,+ \,\overrightarrow{E}_2 \,+ \,\overrightarrow{E}_3 \,+ \,...\) (8-11)

Equality \((8-11)\) means that when the fields are overlapping, they do not affect each other. This is the meaning of the field superposition principle (superposition means overlapping). Thanks to the superposition principle, to find the field of a charged body system at any point, it is sufficient to know the expression \((8-10)\) that defines the field created by a single point charged body.