Electrostatics

Electrostatics is the branch of physics that deals with the forces exerted by a static (i.e. unchanging) electric field upon charged objects.

Overview

In electrostatics conditions of charge need not be 'static' and unchanging. Instead 'static' implies that the dynamic portion is being ignored, and we analyze frozen snapshots of the situation. In electrostatics we study e-fields, voltage, and charge but ignore any currents and magnetism which may also be present. Because of its relationship and interaction with magnetism, the two fields are often combined as electromagnetism.

Coulomb's law

The fundamental equation of electrostatics is Coulomb's law, which describes the force between two point charges:

$F={\frac {\left|Q_{1}Q_{2}\right|}{4\pi \epsilon _{0}r^{2}}}$

Gauss's law

Gauss' law states that "the total electric flux through a closed surface is proportional to the total electric charge enclosed within the surface." The constant of proportionality is the permittivity of free space.

Mathematically, Gauss's law takes the form of an integral equation:

$\oint _{S}\varepsilon _{o}\mathbf {E} \cdot d\mathbf {A} =\int _{V}\rho \cdot dV$

Alternatively, in differential form, the equation becomes

$\nabla \cdot \varepsilon _{o}\mathbf {E} =\rho$

Electrostatic potential

The electrostatic potential (also known as the voltage) is another common and significant topic in electrostatics. The electrostatic potential Φ is related to the electric field E by the gradient operator:

$\mathbf {E} =-\nabla \Phi$

This equation, combined with the differential form of Guass's law (above), provides a relationship between the potential and the charge density ρ:

${\nabla }^{2}\Phi =-{\rho \over \epsilon _{0}}$

This last relationship is a form of Poisson's equation.

Static Charge generation

Charge separation by contact

The presence of surface charge imbalance means that the objects will exhibit attractive or repulsive forces. This surface charge imbalance, which leads to static electricity, can be generated by touching two differing surfaces together and then separating them due to the phenomena of contact electrification and the triboelectric effect. Rubbing two non-conductive objects generates a great amount of static electricity. This is not just the result of friction; two non-conductive surfaces can become charged by just being placed one on top of the other. Since most surfaces have a rough texture, it takes longer to achieve charging through contact than through rubbing. Rubbing objects together increases amount of adhesive contact between the two surfaces. Usually insulators, e.g., substances that do not conduct electricity, are good at both generating, and holding, a surface charge. Some examples of these substances are rubber, plastic, glass, and pith. Conductive objects only rarely generate charge imbalance except, for example, when a metal surface is impacted by solid or liquid nonconductors. The charge that is transferred during contact electrification is stored on the surface of each object. Static electric generators, devices which produce very high voltage at very low current (such as the Van de Graaf generator or Wimshurst machine) and used for classroom physics demonstrations, rely on this effect. Note that the presence of electric current does not detract from the electrostatic forces nor from the sparking, from the corona discharge, or other phenomena. Both phenomena can exist simultaneously in the same system.

Triboelectric series

Main article: triboelectric effect

Charge neutralisation

Natural electrostatic phenomena are most familiar as an occasional annoyance in seasons of low humidity, but can be destructive and harmful in some situations (e.g. electronics manufacturing.) When working in direct contact with integrated circuit electronics (especially delicate MOSFETs), or in the presence of flammable gas, care must be taken to avoid accumulating and discharging a static charge.

'Static' electricity

Static can be a serious nuisance in the processing of analog recording media, because it can attract dust to sensitive materials. In the case of photography, dust accumulating on lenses and photographic plates degrades the resulting picture. Dust also permanently damages vinyl records because it can be embedded into the grooves as the stylus passes over. In both cases, several approaches exist to combat such dust deposition. Some brushes, particularly those with carbon fiber bristles, are advertised as possessing anti-static properties. Also available are handheld static guns which shoot streams of ions to discharge static on records and lenses. Static electricity is a class of phenomena involving objects with a net charge; typically referring to charged objects with voltages of sufficient magnitude to produce visible attraction, repulsion, and sparks.

Natural electrostatic phenomena are most familiar as an occasional annoyance in seasons of low humidity, but can be destructive and harmful in some situations (e.g. electronics manufacturing.) When working in direct contact with integrated circuit electronics (especially delicate MOSFETs), or in the presence of flammable gas, care must be taken to avoid accumulating and discharging a static charge.

Note that the presence of electric current does not detract from the electrostatic forces nor from the sparking, from the corona discharge, or other phenomena. Both phenomena can exist simultaneously in the same system.

See also

• Electromagnetism
• Electrostatic force
• Ionic bond

Electronics

• Electroscope
• Wimshurst machine
• Van de Graaff generator
• First photoconductive copy
• List of electronics topics

Natural

• Triboelectric effect
• Bioelectricity
• Pith
• Cumulonimbus clouds (and Relative humidity)
• Lightning
• Chemical Solutions
• Examples of electrical phenomena

Historical

• Luigi Galvani
• Pseudodoxia Epidemica

Other

• Static

External links

• RMCybernetics - High Voltage Physics Homemade projects & experiments.
• Humans and sparks, causes and cures
• "Man's static jacket sparks alert" at BBC News, 16 September 2005

References

• . ISBN 0-471-80457-6. {{cite book}}: Missing or empty |title= (help); Unknown parameter |Author= ignored (|author= suggested) (help); Unknown parameter |Publisher= ignored (|publisher= suggested) (help); Unknown parameter |Title= ignored (|title= suggested) (help); Unknown parameter |Year= ignored (|year= suggested) (help)
• . ISBN 0-13-805326-X. {{cite book}}: Missing or empty |title= (help); Unknown parameter |Author= ignored (|author= suggested) (help); Unknown parameter |Publisher= ignored (|publisher= suggested) (help); Unknown parameter |Title= ignored (|title= suggested) (help); Unknown parameter |Year= ignored (|year= suggested) (help)