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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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Motion of Charged Particles in a Uniform Electric Field When a particle of charge q and mass m is placed in an electric field E, the electric force exerted on the charge is qE. If this is the only force exerted on the particle, it must be the net force and so must cause the particle to accelerate. In this case, Newton’s second law applied to the particle gives The acceleration of the particle is therefore If E is uniform (that is, constant in magnitude and direction), then the acceleration is constant. If the particle has a positive charge, then its acceleration is in the direction of the electric field. If the particle has a negative charge, then its acceleration is in the direction opposite the electric field. The electric field in the region between two oppositely charged flat metallic plates is approximately uniform (Fig. 23.25). Suppose an electron of charge - e is projected horizontally into this field with an initial velocity vii. Because the electric field E in Figure 23.25 is in the positive y direction, the acceleration of the electron is in the negative y direction. That is, Because the acceleration is constant, we can apply the equations of kinematics in two dimensions with vxi = vi and vyi = 0. After the electron has been in the electric field for a time t, the components of its velocity are Its coordinates after a time t in the field are Substituting the value t = x/vi from Equation 23.11 into Equation 23.12, we see that y is proportional to x2. Hence, the trajectory is a parabola. After the electron leaves the field, it continues to move in a straight line in the direction of v in Figure 23.25, obeying Newton’s first law, with a speed v = vi . Note that we have neglected the gravitational force acting on the electron. This is a good approximation when we are dealing with atomic particles. For an electric field of 104 N/C, the ratio of the magnitude of the electric force eE to the magnitude of the gravitational force mg is of the order of 1014 for an electron and of the order of 1011 for a proton. The Cathode Ray Tube The example we just worked describes a portion of a cathode ray tube (CRT). This tube, illustrated in Figure 23.26, is commonly used to obtain a visual display of electronic information in oscilloscopes, radar systems, television receivers, and computer monitors. The CRT is a vacuum tube in which a beam of electrons is accelerated and deflected under the influence of electric or magnetic fields. The electron beam is produced by an assembly called an electron gun located in the neck of the tube. These electrons, if left undisturbed, travel in a straight-line path until they strike the front of the CRT, the “screen,” which is coated with a material that emits visible light when bombarded with electrons. In an oscilloscope, the electrons are deflected in various directions by two sets of plates placed at right angles to each other in the neck of the tube. (A television CRT steers the beam with a magnetic field) An external electric circuit is used to control the amount of charge present on the plates. The placing of positive charge on one horizontal plate and negative charge on the other creates an electric field between the plates and allows the beam to be steered from side to side. The vertical deflection plates act in the same way, except that changing the charge on them deflects the beam vertically.