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Интензитет на електрическото поле Интензитетът е основна векторна характеристика на електрическото поле; равна на отношението на силата, действаща на точков електрически заряд в дадена точка от полето, към големината на заряда. За да определим електрическото поле в определена точка на пространството, измерваме силата която това поле упражнява върху заряд , когато е поставен в тази точка. се нарича интензитет (напрегнатост) на електрическото поле. В учебниците по английски ще го срещнем само като електрическо поле Е. Да вземем заряд q поставен на разстояние r от тестов заряд . Съгласно закона на Кулон: Съгласно формула (1) Откъдето следва, че: - единичен вектор - определящ посоката на полето. Когато имаме електрическо поле, създадено от множество заряди, интензитета на полето е векторна сума от електрическите полета на всички заряди. -разстояние от заряда qi до точката в която измерваме електрическото поле. Единицата за интензитет е нютон на кулон.
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Electric Field Two field forces have been introduced into our discussions so far - the gravitational force and the electric force. As pointed out earlier, field forces can act through space, producing an effect even when no physical contact between the objects occurs. The gravitational field g at a point in space was defined earlier to be equal to the gravitational force Fg acting on a test particle of mass m divided by that mass: g = Fg/m. A similar approach to electric forces was developed by Michael Faraday and is of such practical value that we shall devote much attention to it in the next several chapters. In this approach, an electric field is said to exist in the region of space around a charged object. When another charged object enters this electric field, an electric force acts on it. As an example, consider Figure 23.10, which shows a small positive test charge q0 placed near a second object carrying a much greater positive charge Q. We define the strength (in other words, the magnitude) of the electric field at the location of the test charge to be the electric force per unit charge, or to be more specific the electric field E at a point in space is defined as the electric force Fe acting on a positive test charge q0 placed at that point divided by the magnitude of the test charge: Note that E is the field produced by some charge external to the test charge - it is not the field produced by the test charge itself. Also, note that the existence of an electric field is a property of its source. For example, every electron comes with its own electric field. The vector E has the SI units of newtons per coulomb (N/C), and, as Figure 23.10 shows, its direction is the direction of the force a positive test charge experiences when placed in the field. We say that an electric field exists at a point if a test charge at rest at that point experiences an electric force. Once the magnitude and direction of the electric field are known at some point, the electric force exerted on any charged particle placed at that point can be calculated from Equation 23.3. Furthermore, the electric field is said to exist at some point (even empty space) regardless of whether a test charge is located at that point. (This is analogous to the gravitational field set up by any object, which is said to exist at a given point regardless of whether some other object is present at that point to “feel” the field.) The electric field magnitudes for various field sources are given in Table 23.2. When using Equation 23.3, we must assume that the test charge q0 is small enough that it does not disturb the charge distribution responsible for the electric field. If a vanishingly small test charge q0 is placed near a uniformly charged metallic sphere, as shown in Figure 23.11a, the charge on the metallic sphere, which produces the electric field, remains uniformly distributed. If the test charge is great enough (qʹ0 >> q0) , as shown in Figure 23.11b, the charge on the metallic sphere is redistributed and the ratio of the force to the test charge is different: (Fʹe /qʹ0 ≠ Fe /q0) . That is, because of this redistribution of charge on the metallic sphere, the electric field it sets up is different from the field it sets up in the presence of the much smaller q0. To determine the direction of an electric field, consider a point charge q located a distance r from a test charge q0 located at a point P, as shown in Figure 23.12. According to Coulomb’s law, the force exerted by q on the test charge is Where r̂ is a unit vector directed from q toward q0. Because the electric field at P, the position of the test charge, is defined by we find that at P, the electric field created by q is If q is positive, as it is in Figure 23.12a, the electric field is directed radially outward from it. If q is negative, as it is in Figure 23.12b, the field is directed toward it. To calculate the electric field at a point P due to a group of point charges, we first calculate the electric field vectors at P individually using Equation 23.4 and then add them vectorially. In other words, at any point P, the total electric field due to a group of charges equals the vector sum of the electric fields of the individual charges. This superposition principle applied to fields follows directly from the superposition property of electric forces. Thus, the electric field of a group of charges can be expressed as where ri is the distance from the ith charge qi to the point P (the location of the test charge) and r̂i is a unit vector directed from qi toward P.