Electrostatics
(A) Electric charge
Electrostatics refers to the study of interaction between static electric charges.
(1) Types of charges and electric forces
There are two types of electric charges: positive (+) and negative (-).
There are electric forces between charges: like charges repel, unlike charges attract.
(2) Quantity of Charge
The unit of charge is coulomb (C). One coulomb of charge is equal to the charge of 6 1018 electrons. In fact, it is defined as the amount of charge that passes through a cross-section of an electrical wire in a circuit with a current of 1 ampere in 1 second.
(3) Atomic Model
Matter is made up of tiny particles called atoms. In the centre of each atom, there is a nucleus, which is made up of protons (positively charged) and neutrons (electrically neutral). Tinier particles called electrons (negatively charged) are orbiting round the nucleus.
Electron / negatively charged (-1.6 x 10-19 C)Proton / positively charged (+1.6 x 10-19 C)
Neutron / neutral
An object is neutral, positively charged or negatively charged according to the difference in the number of protons and the number of electrons as shown in the following table:
Neutral objects / no. of protons = no. ofelectronsNegatively-charged objects / electrons are transferredo them from other objects
no. of electrons > no. of protons
Positively-charged objects / electrons are removedfrom them
no. of electrons < no.of protons
(4) Conductors and Insulators
Materials which allow electrons to flow through them easily are called conductors; those that do not allow electrons to flow through them easily are called insulators.
(5) Earthing
The Earth is like an extremely huge conductor, which is ready to supply and take away extra electrons from other conductors. In other words, it can share charge with other conductors. It remains practically at all times.
Like charges repel each other so that they repel to move apart as far as possible. Therefore most of the charge on a small conductor will be distributed to a huge conductor when they are connected.
An object is earthed when it is connected to the earth through a conducting wire or other conductors. In the absence of other charged objects, conductors become neutral when earthed.
(5) Induced Charges
A neutral object can be attracted by a charged object because of the presence of induced charges.
(i) Electrostatic induction in a conductor:
Normally free electrons in an aluminium foil are uniformly distributed. / Free electrons are pulled towards the positively-charged rod. / Induced charges result in a net upward force on the aluminium foil.(ii) Electrostatic induction in an insulator
Normally electrons within each atom are uniformly distributed. / Electrons within each atom are pulled towards the positively-charged rod. / Induced charges result in a net upward force on the paper.(B) Electric field
A charged object sets up an electric field in the space around it and it can exert an electric force on other charges or objects even if they are not in contact. An electric field set up by a charged object or charged objects is represented by a series of lines called field lines.
Electric field lines of a negative charge / Electric field lines of a positive charge / Electric field lines between a negative charge and a positive chargeThe direction of field lines shows the direction of the electric force experienced by a positive point charge placed in the field. The density of the field lines shows the strength of the field.
(C) Problems and Applications of Electrostatics
(1) Problems
In paper and weaving factories, sheets of charged paper stick together and charged cotton and artificial fibres attract dust.
Large amount of charges accumulated on the tyres and the body of a petrol tanker due to friction will cause sparks. This is very dangerous if the sparks ignite petrol vapour. Aircrafts also have the same problem.
Sparks produced by the movement of blankets and apparatus in an operation theatre may ignite the flammable gas used to anaesthetize the patient.
(2) Applications
(a) Electrostatic precipitation
(b) Electrostatic paint spraying
(c) Photocopying
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Electrostatics