Electric Charges: Definition, Formula, Properties, Unit & More

Table of Contents
1. What is an Electric Charge?
2. Historical note
3. Types of Electric Charges
4. How to Measure an Electric Charge?
5. Common multiples and other units
6. Is Electric Charge a Vector Quantity?
7. Properties of Electric Charge
8. Simple Activities and Demonstrations to Understand Charge
9. Static Electricity
10. Conductors and Insulators
11. Concept-Based Questions
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All of us have the experience of seeing a spark or hearing a crackle when we take off our synthetic clothes or sweaters, particularly in dry weather. Have you ever tried to find an explanation for this phenomenon? It can be attributed to electric charges.

In this document we examine electric charges and related ideas: their definition, measurement, units, types, basic properties, and simple applications and demonstrations suitable for school and competitive-exam preparation.

What is an Electric Charge?

Electric charge is a fundamental property of matter that determines how it interacts with other charged bodies and with electric and magnetic fields. This property is carried by subatomic particles: protons carry a positive charge, electrons carry a negative charge, and neutrons are electrically neutral. The interactions of these charges explain phenomena such as static electricity, attraction and repulsion between bodies, and the origin of electric fields.

Historical note

The word "electricity" is derived from the Greek word elektron, meaning amber. Ancient observations showed that rubbing amber produces static charges that attract light objects - the first recorded instances of electrostatic effects.

Types of Electric Charges

• Positive (+) charge: A body is said to be positively charged when it has a deficiency of electrons relative to protons.
• Negative (-) charge: A body is negatively charged when it has an excess of electrons relative to protons.

When the number of positive and negative charges in a body is equal, the net charge is zero and the body is neutral.

How to Measure an Electric Charge?

The SI unit of electric charge is the coulomb (C).

By definition, one coulomb is the quantity of charge that passes through a conductor when a steady current of one ampere flows for one second.

In this relation, \(Q\) is the electric charge (in coulombs), \(I\) is the electric current (in amperes), and \(t\) is the time (in seconds) during which the current flows.

Common multiples and other units

• millicoulomb (mC) = \(10^{-3}\) C
• microcoulomb (µC) = \(10^{-6}\) C
• nanocoulomb (nC) = \(10^{-9}\) C
• C.G.S. unit of charge = electrostatic unit (esu)
• 1 coulomb = \(3 \times 10^{9}\) esu (approx.)
• Dimensional formula of charge = [M⁰ L⁰ T¹ A¹]

Fundamental values:

• Charge of a single electron = \(-1.602 \times 10^{-19}\) C
• Charge of a single proton = \(+1.602 \times 10^{-19}\) C
• Charge of a neutron = 0 C

Is Electric Charge a Vector Quantity?

No. Electric charge is a scalar quantity. It has magnitude and sign (positive or negative) but no direction in space. Charges add algebraically (taking sign into account) rather than following vector addition laws.

Example demonstrating algebraic addition of charges:

This shows we simply add signed numerical values. No spatial direction is involved, which confirms that charge is scalar in nature.

Properties of Electric Charge

1. Like charges repel and unlike charges attract. This empirical law is the basis for electrostatic interactions observed between charged bodies.

2. Charge is a scalar quantity. Charges add algebraically and the sign (positive or negative) indicates whether electrons are deficient or in excess.
3. Charge is transferable. Electric charge can be transferred from one body to another. In common contact or rubbing processes it is the electrons that move because protons are tightly bound within atomic nuclei. A body that loses electrons becomes positively charged; a body that gains electrons becomes negatively charged.

• A neutral body: number of electrons = number of protons
• Positively charged body: number of electrons < number of protons
• Negatively charged body: number of electrons > number of protons

5. Charge is conserved. In an isolated system, the algebraic sum of all charges remains constant. Internal transfers do not change the total charge of the system. Systems reach electrostatic equilibrium when charges redistribute until there is no net motion of charge without external influence.
6. Charge is quantized. The charge on any object is always an integral multiple of the elementary charge \(e\). That is,
   \[ Q = \pm n e \]
   where \(n\) is an integer and \(e = 1.602 \times 10^{-19}\ \mathrm{C}\). This quantisation was demonstrated experimentally by Millikan's oil-drop experiment.
7. Elementary substructure and quarks. Fundamental particles called quarks are known (from particle physics) to carry fractional charges such as \(+\tfrac{2}{3}e\) and \(-\tfrac{1}{3}e\). These quarks, however, are confined within composite particles (hadrons) and do not appear as free charges; therefore the macroscopic quantum of charge remains the electron charge \(e\).
8. Charge is associated with mass. Charged particles also have mass. Experimentally measured masses are:
   Mass of electron = \(9.109 \times 10^{-31}\ \mathrm{kg}\) = \(5.49 \times 10^{-4}\ \mathrm{u}\)
   Mass of proton = \(1.6726 \times 10^{-27}\ \mathrm{kg}\) ≈ \(1.007\ \mathrm{u}\)
   Mass of neutron = \(1.6749 \times 10^{-27}\ \mathrm{kg}\) ≈ \(1.008\ \mathrm{u}\)
   Note: It is convenient to remember these values in SI units (kg).
9. Charge is relativistically invariant. The electric charge of a particle is independent of the observer's inertial frame; unlike relativistic mass (or energy), charge does not change with speed.
10. Charges produce fields. A charge at rest produces only an electric field around it. A charge moving with constant velocity produces both electric and magnetic fields; accelerated charges produce changing electric and magnetic fields that propagate as electromagnetic waves.

Electric field lines

Simple Activities and Demonstrations to Understand Charge

• Balloon attraction: Inflate a balloon, rub it against your hair or a woollen sweater, and hold it near small bits of paper or Styrofoam. The balloon will attract these objects due to static charge.

Balloon Attraction

• Salt and pepper attraction: Sprinkle salt and pepper on a table. Rub a plastic comb through your hair and hold it near the mixture. The comb attracts the lighter particles demonstrating electrostatic forces between charged objects.

Salt and pepper attraction

• Lemon battery: Remove the pulp from a lemon, insert a copper strip into one side and a zinc strip into the other, and connect a small LED across the strips. The chemical reaction establishes a potential difference and allows current to light the LED, demonstrating electricity generation.

• Comb and small pieces of paper: Rub a plastic comb on a woollen cloth or fur and bring it near small paper pieces. The paper pieces are attracted and stick to the comb, showing static electricity in action.

Charged Comb attracting small pieces of paper

Static Electricity

Static electricity refers to an imbalance of electric charges on the surface or in the body of a material. Typical causes include contact, separation, and friction between different materials.

• In static electricity, electrons (not protons) are transferred between objects; electrons are comparatively loosely bound in many materials and can move from one body to another when rubbed or contacted.
• When two materials are rubbed, one may lose electrons and become positively charged while the other gains electrons and becomes negatively charged.
• All the usual rules hold: like charges repel and unlike charges attract.

Conductors and Insulators

Materials are classified by how easily they allow electric charge to move through them.

Conductors

Conductors are materials that allow free movement of electric charge (electrons) within them. Metals are typical conductors because they have free electrons that can move under the influence of an electric field. Conductors also usually conduct heat well.

Examples: Most metals (copper, aluminium, iron), carbon in the form of graphite, human body, earth.

Applications of Conductors

• Electrical wiring and power transmission use conductors such as copper and aluminium.
• Metallic components (heating elements, engine parts, radiators) conduct heat efficiently and are used where rapid heat transfer is required.
• Thermometers with mercury use the conducting property of mercury for temperature measurement.
• Cookware and iron soleplates use conductive metals to absorb and transfer heat.

Insulators

Insulators

Insulators are materials that resist the flow of electric charge; they have no (or very few) free charge carriers. Insulators are used to prevent unwanted flow of charge and to protect against electric shocks.

Examples: Glass, rubber, plastic, mica, wood, and dry air.

Applications of Insulators

An insulator is used to protect wire opening

• Electrical insulation of wires and cables in circuits using plastic or rubber coatings.
• High-voltage insulators on power lines prevent current from leaking to supporting structures.
• Thermal insulators (polystyrene, glass wool) reduce heat flow and are used in bottles, building insulation, and cookware handles.
• Sound-insulating materials are used in auditoria and recording studios to absorb noise.

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 experimentally observed charge that is attracted to a positive charge is repelled by a negative charge and vice versa. No charged object has been observed that is repelled by both a positive and a negative charge; therefore two types (positive and negative) suffice to describe observed behaviour.

Q.2. What happens

(a) When two like charges are brought together?

(b) When two, unlike charges, are brought together?

Ans. (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 means the absence of net 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. Because electrons have definite mass, when a body gains electrons its mass increases slightly; when it loses electrons its mass decreases slightly.

Q.5. Two identical metallic spheres of exactly equal masses are taken. One is given a positive charge q coulombs and the other an equal negative charge. Are their masses after charging equal?

Ans. No. The negatively charged sphere has gained electrons and therefore its mass is slightly more than that of the positively charged sphere which has lost electrons.

Q.6. During a nuclear reaction, what happens to electric charge?

Ans. In an isolated system, electric charge is conserved in nuclear reactions as in chemical reactions: the algebraic sum of charges before the reaction equals the algebraic sum after the reaction.

Q.7. Explain the statement: 'For a body, an electric charge is quantized.'

Ans. Quantisation of charge means that the net charge on a body changes only by integer multiples of the elementary charge \(e\). Charges are transferred in whole numbers of electrons; fractional parts of the electron charge are not observed as free charge carriers in ordinary matter. Therefore the charge on any macroscopic object is \(Q = \pm n e\) where \(n\) is an integer.