﻿ Electrostatics: Coulomb's law

# What does Coulomb's law say?

Electrostatics is called the doctrine about electrified bodies and forces of the vzaimoyodeystviye between them provided that bodies and charges remain motionless from each other. The set of the electric forces created by motionless zayoryada is called electrostatic field.

Force of interaction between two motionless point charges.

Electrostatic field can exist only in dielectrics as creation of the field in the conductor will cause movement of charges (electric current), i.e. will break the electrostatic state.

Coulomb's law: two charges which are in the homogeneous environment and remote from each other at some distance work at each other with the force, proportsioyonalny to the work of these charges and inversely proportional to the square the rasstoyayoniya between them:

F = K(Q1 x Q2) : R2

where F — force of interaction (N),

To = 1: 4? є (here є = є0 є ' — dielectric permittivity of the environment, õ/m),

Q1 and Q2 — charges (C),

R - distance between charges (m),

Interaction of like charges is expressed in their mutual pushing away with F force, and heteronymic — in the attraction with the same force. At the charge equal to 1 C and acting on the charge equal to it which is at distance of 1 m interaction with the force of 1 N takes place. The charge in 1 C ра­вен to the charge 6.28 x 1018 elekyotron. Charge size in 1 C is the electricity unit of quantity in MKSA force system.

Field gradient characterizes the elekyotrichesky field, under kotoyory some area of space where forces created by motionless charges work is understood. Electric field can exist only in dielectrics as priyovodit sozdayony fields in the conductor to movement of zaryayod, i.e. causes elektriyochesky current. In number the veliyochina of field gradient is defined by the relation of force operating in the danyony point of the field on the charge to charge size:

E = (F: Q) = Q: 4? єR2 (in oil),

where E — strength по­ля (In oil),

Q - the charge creating the field (C)

R — distance (m),

Breakdown strength of insulating materials.

The extreme value of field gradient at which there comes the dielectric breakdown is called breakdown strength.

Breakdown strength usually is expressed in kilovolts on millimeter (kV/mm) and is very important characteristic of insulating materials. Znayocheniya of breakdown strength for the number of the materials applied in electrical equipment is brought in fig. 1. The condenser represents the system consisting of the metal carrying-out plates (electrodes) and the isolating material (dielectric) between them.

The condenser is the device for electricity accumulation. If to connect the condenser to the source of the direct current, then it will be loaded and charges, equal and opposite on the sign, will focus on its electrodes. After the condenser otklyuyocheniye from the current source it keeps the stock of electrical energy which can be received from the condenser as from the current source.

The capacitor capacitance is defined by the size measured by the charge relation on one of its electrodes to tension between electrodes:

WITH = Q: U,

where With - the capacitor capacitance (ф or to/in),

Q - the charge on one of electrodes (C),

U — tension between electrodes (V),

The capacitor capacitance depends on three factors: the areas of electrodes, the rasstoyayoniya between electrodes and properties of the dielectric which is between them. Let's note that at the capacitor capacitance 1 ф the charge in 1 C creates the napryayozheniye of 1 Century on its electrodes.

Stored energy of electric field of the condenser is defined by the following ratio:

W = (Q2: 2C) = (CU2: 2)

where W — field energy of the condenser (Vt*sek).

Depending on the form of electrodes, distinguish condensers flat, tsilindriyochesky and spherical; on dielectric material — mica, oil, paper, electrolytic, air, etc. On constructive performance condensers can be the fixed and variable capacitor.

Capacity of the parallel-plate capacitor is determined by the following formula:

C = (є xS): d

where With - the capacitance (cm),

є - the dielectric constant of the insulator,

S — the surface of each electrode (cm2);

d — distance between electrodes, i.e. thickness of the insulator (cm).