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Main concepts. Electric field is usually described by two basic quantities – vector of electric intensity and electric potential
Electric field is usually described by two basic quantities – vector of electric intensity and electric potential
where Intensity and potential of electric fieldof the point charge q at a distance of r of the charge
where e0 = 8, 85 × 10–12 F / m – permittivity of vacuum; e – relative permittivity of medium (for air e=1). For the uniform field the dependence between the intensity and potential of the electric field
where r 1, 2 – the distance between equipotential lines with the potentials j1 and j2, E – the electric intensity in the middle of the equipotential lines with the potentials j1 and j2. The modulus of electric field intensity: 1) uniformly charged sphere with a radius of R at a distance of r from the centre of sphere а) E = 0, for r < R; б) where Q= s × S – a charge of sphere, s – surface charge density of sphere (charge of 1 m 2 of a surface), S= 4p R2 – surface area of sphere. 2) endless uniformly charged cylinder with a radius of R at a distance of r from the axis of the cylinder. а) E = 0, for r < R; б) where Q= t × L – a charge of cylinder, t – linear charge density (charge of 1 m of a length), L – length of cylinder. 3) endless uniformly charged plate
s – surface charge density of plate. The superposition principle: the electric field created by one charge is independent from positions of other charges. Then the resultant vector is a vector sum of vectors of electric field individual charges:
therefore: a) the direction of the resultant vector is defined by parallelogram rule of the vectors’ additions (see example 1); b) the magnitude of the resultant vector is defined by cosine theorem:
where a – angle between vectors.
The electrostatic force acting on a test charge q F=q × E RES.
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