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Physics Theory : Static Electricity

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Static electricity is produced when a suitable combination of substances (e.g., glass rod and silk; ebonite rod and cat's skin; sealing wax and wool) are rubbed together and get electrified due to friction.


Atom consists of three sub-atomic particles, namely protons, neutrons and electrons.
Electrons have a negative charge, protons are positively-charged and neutrons have no charge (neutral).
The protons and neutrons together form the atom's central core which is known as the nucleus.
These protons and neutrons are held together by strong attractive forces known as nuclear forces.
The electrons away from the nucleus experience hardly any attractive force.
These electrons present in the outermost orbit, revolving around the nucleus, are held by weak forces and are known as free electrons.
On the other hand, the electrons close to the nucleus are held strongly by the electric pull of the protons and are called bound electrons.
The normal atom of an element is electrically neutral because the number of protons (having positive charge) equal the number of electrons (having negative charge). Hence the charges get canceled and the net charge is zero. Also, the neutrons (having no charge) do not affect the charge on an atom.


Ion is an atom (or group of atoms) carrying a positive or negative charge due to loss or gain of electrons.
Positively-charged ions are called cations.
Negatively-charged ions are called anions.


Law of Conservation of Electric Charges states that the sum of the total electrons in any system is a constant quantity.
However, when a suitable combination of bodies are rubbed together, the electrons from one body may be transferred to the other body.


Negative electrification is due to the excess of electrons (compared to the normal number of electrons in a neutral atom).
For example, on rubbing a glass rod with silk, the electrons from the glass rod get transferred to the silk. The silk now has an excess of electrons and so is negatively-charged. On the other hand, the glass rod is deficient in electrons and hence is positively-charged.
In the above example, the silk undergoes negative electrification.


Positive electrification is due to the deficiency of electrons (compared to the normal number of electrons in a neutral atom).
For example, on rubbing an ebonite rod with fur or cat's skin, the electrons from the cat's skin get transferred to the ebonite rod. The cat's skin now has a deficiency of electrons and so is positively-charged. On the other hand, the ebonite rod has an excess of electrons and hence is negatively-charged.
In the above example, the cat's skin undergoes positive electrification.
Note that it is the electrons and not the protons that get transferred during positive (and negative) electrification.


Gold Leaf Electroscope is the device that can be used to find out whether a given body has an electric charge or not, as well as whether a given body is positively-charged or negatively-charged.


When two charged bodies are brought close to each other, attraction or repulsion does take place (depending on the charges of both the bodies) even if these bodies are placed in vacuum.
Like charges or similar charges always repel each other.
Unlike charges or dissimilar charges always attract each other.


Tendency of electrons to start drifting from one end of a substance (conductor) to the other end depends on the difference in the electrical potential (voltage) of those ends.


Conductor is a substance having a large number of free electrons.
Insulator is a substance that has only few free electrons, such that these free electrons do not easily drift from one end of the substance to the other end when connected to a source of electricity.


Lightning (a dazzling bluish-white light that is produced in the clouds) is followed by a loud noise called thunder.
When water vapor (present in the atmosphere) condenses, clouds are formed.
These clouds contain huge amounts of static electric charges (which may be either positive or negative).
Lightning is caused when oppositely-charged clouds come close to each other.
A lightning conductor provides a path by which all the electric discharge from the clouds flows safely into the earth without damaging the building.


Coulomb is the SI unit of electric charge.
The charge on an electron is 1.602 × 10-19 Coulombs.


Farads and microfarads are units of capacitance (electrical capacity).
1 microfarad = 10-6 farads.
A capacitor is represented by the symbol: ____| |____


Physics Quiz on Static Electricity.
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