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Coulomb's law
 Electrostatic Force
 Electric Field
 Source and Reference

Coulomb's law

Coulomb's law or Coulomb's inverse-square law is a low of physics used to quantify the amount of force between two stationary elctrically charged particles.

Electrostatic Force


Electrostatic force or Coulomb force is defined as the electric force between two charged bodies at rest. The quantity of electrostatic force between stationary charges is always described by Coulomb's law.


The direction of force is attractive if charges have opposite sign as above and is repulsive if charges have same sign as below. The electrostatic force always acts along a line connecting tht charges


The magnitude of electrostatic force is proportional to the magnitude of each charge and inversely proportional to the distance squared


The unit of electric charge, 𝑄 in SI unit is Coulomb, 𝐢. The unit of electrostatic force, 𝐹 is 𝑁. The unit of distance, π‘Ÿ is π‘š. Therefore the unit of permittivity, πœ€ is 𝐢2/π‘π‘š2.

The vacuum permittivity, πœ€0 is equal to 8.85Γ—10-12𝐢2/π‘π‘š2=8.85Γ—10-12πΉπ‘š-1. The Coulomb's constant, π‘˜π‘’ for vacuum, 1/4πœ‹πœ€0 is 9Γ—109π‘π‘š2/𝐢2

The electric charge of an electron, 𝑒 is 1.602Γ—10-19𝐢. The electric charge quantity of an electron, 𝑄 is βˆ’π‘’, while the electric charge quantity of a proton, 𝑄 is +𝑒.

Electric Field

From Coulomb's law, a particle with electric charge, 𝑄0 at position, π‘₯0 exerts a force on a particle with electric charge, 𝑄1 at position, π‘₯1 can be expressed as 𝐹=14πœ‹πœ€0𝑄0𝑄1(π‘₯1βˆ’π‘₯0)2π‘Ÿ The effect of the source charged particles 𝑄0 at position π‘₯0 on charged particles, 𝑄1 at position π‘₯1can be expressed as 𝐹0,1𝑄1=14πœ‹πœ€0𝑄0(π‘₯1βˆ’π‘₯0)2π‘Ÿ0,1 Since charged particles exerts electrostatic forces in all direction at all times, whether or not there is another charged particles around, the effect of the source charged particles at position, π‘₯ on charged particles, 𝑄 throughout space at all time can be expressed as 𝐸π‘₯=𝐹π‘₯𝑄=14πœ‹πœ€0𝑄0(π‘₯βˆ’π‘₯0)2π‘Ÿπ‘₯ This is the electric field which is equal to Electrostatic force per unit charge that a point charge π‘ž of the same sign would exprerience at position π‘₯ due to the charged particles, 𝑄0 of positive charge π‘ž0 at position π‘₯0. The unit of electric field is Newton/Coulomb. image In other words, the electrostatic force that acts on any charge can be expressed as 𝐹=π‘žπΈ=14πœ‹πœ€0π‘ž0π‘Ÿ2π‘Ÿ The direction of electric field due to a charged particle is only affected by the sign of charges. A positive charged particle generates a pointing outward electric field, while a negative charged particle generates a pointing inward electric field. image The direction of electrostatic force acting on an charaged observed particle due to the electic field generated by a charged source particle depends on the signes of the two charged particles. However, a positively charged observed particle alway experences an electrostatic force which is along with the same direction of electric field, while a negatively charged observed particle particle alway experences an electrostatic force which is against the direction of electric field, i.e. in the opposite direction of electric field. image

Source and Reference


ID: 191003002 Last Updated: 10/30/2019 Revision: 0

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