Electrostatic Charging by Friction
Experiments:
Rubbing a cloth and rod makes them electrically charged. This can be demonstrated by holding both the items near the hair – they attract the hair. Static electricity is generated by rubbing. Also, a charged object may attract an uncharged object. By hanging the rod in a cradle and bringing the cloth near to it makes the rod move towards the cloth
Rubbing a second rod and bringing this near the hanging in the cradle will produce a repulsive action by both the rods.
Conclusion:
like charges repel;
Unlike charges attract.
It is the force of friction which causes charging. When the two items are rubbed, friction transfers tiny particles called electrons from one material to the other.
Measurement of electric charge
Electric charge is measured in coulombs C, which is the SI unit for electric charge.
Electric charge (C) = current (amps) × time (sec)
Therefore, one coulomb is the amount of electric charge passing through a section in a circuit in when a current of one ampere flows for one second. The charge of one electron or proton is 1.6×10-19; and 6.25×1018 electrons are required to make up one coulomb. Coulombs are large quantities and therefore mill coulombs (1 mC = 10-3 C) and micro coulombs (1µC = 10-6C) are commonly used.
Electric Field
The force between electric charges act over a distance without the charges being in contact. This force also exists across a vacuum. An electric field exists in a region of space where a small positive charge experiences an electric force. The direction of a field is described as the direction of the force on a small positive charge.
Lines of force:
Lines of force are used to show the direction of an electric field. The lines of force are directed outwards for a positive charge and inwards for a negative charge. The strength of the force is shown by the distance between the lines, with closer lines indicating a stronger electric field. An electric field between two parallel oppositely charged plates is uniform at the central region, with field lines starting from positive charges.
Induction
Induction is the process of charging a conductor without any contact with the charging body.
a) To charge two conductors with equal and opposite charges i. The two conductors (metallic spheres) on insulator stands are brought into contact with one another. ii. A negatively charged rod is brought into contact with sphere A, which causes the electrons from sphere A to be repelled to the furthest side of sphere B. Now, sphere A alone will have excess positive charge (electron loss) and sphere B will have excess negative charge (electron gain). iii. With the negative rod yet in place, the two spheres A and B are placed a distance apart. iv. The sphere A will now have induced positive charges while B will have induced negative charges, and the charge on the rod will remain the same. b) To charge a single conductor by induction i. Bring a charged rod (e.g. positively charged) to the area of the conductor. Held by an insulating stand. ii. The free electrons in the conductors will be drawn towards the end of the conductor nearer to the positively charged rod, whilst the other end will be positively charged. However, the conductor remains neutral. iii. Keeping the rod in place, the conductor to be charged is earthed, which can be done by touching the conductor. This path will allow the electrons to flow to the conductor to neutralise the excess positive charge on the other end of the conductor. The conductor will carry an excess negative charge. iv. When the rod is removed the electrons will redistribute on the surface of the conductor to attain electrostatic equilibrium.
Insulators and Conductors
Insulators are materials which do not carry electrons , thus not able to conduct electricity. Such materials inc. glass, silk, ebonite, hard rubber and fur. These materials can be electrified by rubbing. The electrons are transferred from one material to another and remain on the surface. They do not move within the material and are called electric insulators.
Electric conductors do permit electrons to flow through them. They are not easily charged by rubbing unless first insulated. Electric conductors inc. metals, electrolytes and ionised gases which contain flowing positive and negative ions.
All electrons of electrical insulators are bound to their nuclei and the displacement of electrons does not cause electrons to flow i.e. the charge is confined to the area where it was produced or placed (e.g. rubbing). The valence electrons in in electrical conductors are delocalised. If these materials gain electrons, the electrons can move about them. A loss of electrons by conduction causes a redistribution of those left behind.
Neutralising charged insulators and conductors
Discharging is the neutralisation of charged bodies.
Charged insulators: to discharge a charged rod (glass) it can be heated. The heat causes the surrounding air of the rod to be ionised into positive and negative ions. These negative ions will neutralise the excess positive charges on the rod. Charged insulators also discharge slowly when left in moist conditions. Water vapour in the air causes the excess charges on the insulator to leak away slowly.
Charged conductors: a charged conductor e.g. a metal sphere with excess electrons, the electrons can be removed by earthing it. To earth a charged conductor is to provide a path for the electrons to flow away or towards the charged conductor and cause it to become electrically neutral. For a positively charged sphere, earthing causes electrons from the earth to flow towards the sphere and neutralise it. The human body can also act as a conducting path for electrons.