A number of simple experiments demonstrate the existence of electric forces and charges. You can easily electrify your body by vigorously rubbing your shoes on a wool rug. Evidence of the electric charge on your body can be detected by lightly touching (and startling) a friend. Your friend might experience a small spark due to electromagnetism.
Fig: Electric Charge
Electromagnetism is a science of the combination of electrical and magnetic phenomena.
Electromagnetism can be divided into 2 parts:
(i) Electrostatics: It deals with the study of charges at rest.
(ii) Electrodynamics: It deals with the study of charges in motion
In this chapter, we will be dealing with charges at rest i.e. electrostatics. Electric and magnetic phenomena are generally bracketed together, since both derive from charged particles. James Clarke Maxwell in late 19th century unified the laws of Electricity and Magnetism.
STRUCTURE OF ATOM
An atom consists of two parts:
(ii) Extra nuclear part
Nucleus consists of neutrons and protons and the extra nuclear part has electrons revolving around the nucleus.
Fig: Structure of atom
In a neutral atom, Number of electrons = Number of protons
Charge of a material body or particle is the property (acquired or natural) due to which it produces and experiences electrical and magnetic effects. Some of the naturally charged particles are electrons, proton, α-particle etc.
Types of Charges:
Fundamentally, the electron is a negatively charged particle, a proton is a positively charged particle while a neutron is a neutral particle with no electric charge.
(i) Positive charge: It is a deficiency of electrons compared to protons. (number of protons > number of electrons)
(ii) Negative charge: It is the excess of electrons compared to protons. (number of protons < number of electrons)
(Note: that the quantity of negative charge on an electron = quantity of positive charge on a proton.)
Units of Charge
A charge is a derived physical quantity. The charge is measured in coulomb in S.I. unit. In practice we use,
Charge of a single electron = -1.602 × 10-19 C
Charge of a single proton = + 1.602 × 10-19 C
Charge of a single neutron = 0 C
PROPERTIES OF CHARGE
(i) Electric Charge comes in two varieties, which are called “plus” and “minus”. Like charges repel each other and unlike charges attract each other.
Fig: Forces between charges
(ii) Electric Charge is a scalar quantity: It follows scalar laws of operations i.e. it adds algebraically and represents excess of electrons in a negatively charged atom or a deficiency of electrons in a positively charged atom.
(iii) A charge is transferable: Electric charge can be transferred from one body to another but there is a restriction to the charge transfer. Only electrons are transferred from one body to another because protons are tightly bound to the nucleus of every atom. Hence, the body which loses electrons in the transfer becomes positively charged and the body which receives electrons becomes negatively charged.
Thus, A neutral body has number of electrons = number of protons
A positively charged body has number of electrons < number of protons
A negatively charged body has number of electrons > number of protons.
(iv) Charge is conserved: In an isolated system, the total charge (sum of positive and negative) remains constant whatever charge transfer takes place in the system internally. It is called the principle of charge conservation.
Fig: Conservation of charge
(v) Charge is quantized: Charge on any body always exists in integral multiples of a fundamental unit of electric charge. This unit is equal to the magnitude of charge on one electron (1e = 1.6 × 10-19 C). So charge on any body Q = ± ne, where n is an integer and e is the charge on a single electron. This was proved by the Millikan's oil drop experiment.
Recently, the existence of particles of charge +(2/3) e and -(1/3) e has been postulated. These particles are called quarks but still this is not considered as the quantum of charge because these are unstable (They have a very short span of life.)
(vi) Charge is always associated with mass: Yes! electrons, protons and neutrons also have masses.
Their value is determined, experimentally, to be following:
Mass of an electron = 9.109 × 10-31 Kg = 5.49 × 10-4 amu
Mass of a proton = 1.6726 × 10-27 Kg = 1.007 amu
Mass of a neutron = 1.6749 × 10-27 Kg = 1.008 amu
It is recommended to remember these values in Kg (SI units). Also, please note that mass of neutron is slightly greater than mass of proton.
This also shows that mass of a negatively charged body is greater than mass of a positively charged identical body as it would have excess number of electrons than the positively charged bodies.
(vii) Charge is relativistically invariant: This means that charge is independent of frame of reference i.e., charge on a body does not change whatever be its speed. This property is worth mentioning as in contrast to charge, the mass of a body depends on its speed and increases with increase in speed. You will be exposed to this property later when you will learn The Special Theory of Relativity.
(viii) A charge at rest produces only electric field around itself: A charge at rest creates a region of influence called Electric Field around itself in space.
Fig: Electric field lines
While a charge having uniform motion (constant velocity) produces electric as well as magnetic field around itself. Accelerated charges produce a special combination of electric and magnetic fields called electromagnetic waves. We will study about electric field in detail in the coming section.
CONCEPT BASED QUESTIONS
Q.1. Why have we defined only two types of charges? Why not three or more?
Ans. Only two kinds of electric charges exist because any unknown charge that is found experimentally to be attracted to a positive charge is also repelled by a negative charge. No one has ever observed a charged object that is repelled by both a positive and a negative charge.
Q.2. What happens
(a) When two like charges are brought together?
(b) When two unlike charges are brought together?
(a) When two like charges are brought together they repel each other with an electrostatic force.
(b) When two unlike charges are brought together they attract each other with an electrostatic force.
Q.3. What does neutral in electric charge mean?
Ans. Neutral does not refer to any third type of charge. It is the absence of any excess or deficiency of electrons in a body i.e. number of electrons = number of protons.
Q.4. Does the mass of the body get affected while charging?
Ans. Yes, the mass of the body gets affected because electrons have a definite mass so the mass of the body slightly increases when it gains electrons while the mass decreases when it loses electrons.
Q.5. Two identical metallic spheres of exactly equal masses are taken. One is given positive charge q coulombs and other an equal negative charge. Are their masses after charging equal?
Ans. No. A body is positively charged due to the deficit of electrons while the negative charge is due to a surplus of electrons. Hence, the mass of the negatively charged sphere will be slightly more than that of the positively charged sphere.
Q.6. During a nuclear reaction, what happens to electric charge?
Ans. In the event of a nuclear reaction, the electric charge is conserved considering an isolated system. This is true for any nuclear or chemical reaction. In a nuclear reaction the parent nuclei undergoes a transformation into daughter nuclei but the total algebraic charge remains constant.
Q.7. Explain the statement: ‘For a body, an electric charge is quantized’.
Ans. Considering a particular body, ‘electric charge is quantized’ refers to the number of electrons which can be transferred from that body to another. It should be noted that charges don’t get transported in fractions. Therefore, the overall charge on a body is simply an integral multiple of charge on an electron.