Which one among the following correctly defines a unit magnetic pole in SI units ? It is the pole which when placed in air at a distance of—

1. Basics of Magnetism and Magnetic Poles (basic)

Magnetism is one of the most fascinating forces in nature because it acts over a distance without any physical contact. At the heart of this phenomenon are magnetic poles—specific regions in a magnet where the magnetic strength is most concentrated. Every magnet, regardless of its shape, possesses two poles: a North Pole and a South Pole Science, Class VIII, Electricity: Magnetic and Heating Effects, p.51. A fundamental rule of magnetics is that like poles repel each other (North repels North), while unlike poles attract each other Science, Class VIII, Exploring Forces, p.69.

To quantify the "strength" of a magnet, physicists developed the concept of a unit magnetic pole. This is a theoretical standard used to define pole strength based on the force of repulsion. In the modern SI system (International System of Units), a unit pole is defined as one that, when placed in a vacuum (or air) at a distance of 1 metre from an identical pole, repels it with a force of 1 Newton. This is a significant jump from the older CGS system, as shown in the comparison below:

System	Distance	Force required for "Unit Pole"
CGS (Centimetre-Gram-Second)	1 Centimetre	1 Dyne
SI/MKS (Metre-Kilogram-Second)	1 Metre	1 Newton

Interestingly, these poles are not limited to solid metal bars. We see the same behavior in electromagnets and solenoids (coils of copper wire). When electricity flows through a solenoid, one end behaves as a North pole and the other as a South pole, creating a magnetic field identical to that of a bar magnet Science, Class X, Magnetic Effects of Electric Current, p.201. You can always identify a pole using a simple magnetic compass: because opposite poles attract, the North pole of the compass needle will be drawn toward the South pole of your magnet Science, Class X, Magnetic Effects of Electric Current, p.196.

Sources: Science, Class VIII (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.51; Science, Class VIII (Revised ed 2025), Exploring Forces, p.69; Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.196, 201

2. Magnetic Field and Field Lines (basic)

To understand magnetism, we must first visualize the invisible. A magnetic field is the region surrounding a magnet where its influence—a force—can be detected by another magnet or a magnetic material. While we cannot see the field itself, we represent it using magnetic field lines. These are imaginary paths that follow a set of strict rules: they emerge from the North pole and enter the South pole outside the magnet, but inside the magnet, they travel from South to North. This means magnetic field lines are continuous closed curves Science, Class X, p.197.

The visual density of these lines tells us about the field's intensity. We say the field is stronger where the lines are crowded together, and weaker where they are spread out. This is why the magnetic force is most potent near the poles Science, Class X, p.206. A fundamental rule to remember is that no two field lines ever cross each other. If they did, a compass needle placed at the intersection would have to point in two different directions simultaneously, which is physically impossible Science, Class X, p.197.

In the world of physics, we often need a standard of measurement. In the SI system (International System of Units), we define a unit magnetic pole based on force. A unit magnetic pole is one which, when placed in a vacuum (or air) at a distance of 1 metre from an identical pole, repels it with a force of 1 Newton. This helps scientists quantify the strength of magnetic interactions beyond just visual maps.

Feature	Field Line Property
Direction (Outside)	North Pole to South Pole
Direction (Inside)	South Pole to North Pole
Intersection	Never (would imply two directions at one point)
Closeness	Proportional to magnetic field strength

Sources: Science, class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.197; Science, class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.206

3. Systems of Units: CGS vs SI (MKS) (intermediate)

In the study of physics, consistency is everything. To measure the world, we use two primary systems: the CGS system (Centimetre, Gram, Second) and the SI system (International System, often referred to as the MKS system—Metre, Kilogram, Second). While CGS was traditionally used in many laboratory calculations, the SI system is the global standard for modern science and engineering. For example, in the SI system, the standard unit of force is the Newton (N) Science, Class VIII, Exploring Forces, p.65, and the standard unit of length is the metre (m) Science, Class VII, Measurement of Time and Motion, p.113.

When we talk about magnetism, we often define a "unit magnetic pole" to establish a benchmark for magnetic strength. This definition is based on Coulomb's Law for Magnetism, which describes the force between two poles. Because force and distance are measured differently in CGS and SI, the definition of a unit pole changes depending on which system you are using. Just as density can be expressed in g/cm³ or kg/m³ depending on the chosen units for mass and volume Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.141, magnetic units must scale accordingly.

Feature	CGS System	SI (MKS) System
Unit of Force	Dyne	Newton (N)
Unit of Distance	Centimetre (cm)	Metre (m)
Unit Magnetic Pole Definition	Repels an identical pole with 1 dyne force at 1 cm distance.	Repels an identical pole with 1 Newton force at 1 metre distance.

In the SI/Rationalized MKS system, we define a unit magnetic pole as that pole which, when placed in a vacuum (or air) at a distance of 1 metre from an equal and similar pole, repels it with a force of 1 Newton. This transition ensures that our magnetic calculations align perfectly with other SI units, such as those for pressure (Pascal or N/m²) Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.82, allowing for a unified approach to physical laws.

Sources: Science, Class VIII, Exploring Forces, p.65; Science-Class VII, Measurement of Time and Motion, p.113; Science, Class VIII, The Amazing World of Solutes, Solvents, and Solutions, p.141; Science, Class VIII, Pressure, Winds, Storms, and Cyclones, p.82

4. Electromagnetism and Current Effects (intermediate)

In the early 19th century, electricity and magnetism were treated as separate provinces of physics. This changed in 1820 when Hans Christian Oersted noticed a compass needle deflect near a current-carrying wire. This accidental discovery proved that moving charges (current) generate a magnetic field, effectively birthing the field of Electromagnetism Science, Class X, Magnetic Effects of Electric Current, p.195. To visualize this field, we use the Right-Hand Thumb Rule: if you point your right thumb in the direction of the current, your fingers wrap around the wire in the direction of the magnetic field lines Science, Class X, Magnetic Effects of Electric Current, p.200.

To move from qualitative observation to quantitative science, we must define the strength of a magnetic source. This is done through the concept of a Unit Magnetic Pole. While magnetic poles always exist in pairs (North and South), we use a theoretical "isolated" pole to define units of force. Historically, the CGS system defined a unit pole based on centimeters and dynes. However, in the modern SI system (or the rationalized MKS system), we scale these units to the standard meter and Newton to maintain consistency across all scientific measurements.

System of Units	Distance	Repulsive Force	Medium
CGS System	1 centimeter	1 dyne	Vacuum/Air
SI (Rationalized MKS)	1 meter	1 Newton	Vacuum/Air

This definition allows scientists to calculate the exact magnetic field strength produced by different configurations, such as a circular loop. In such a loop, the magnetic field lines form concentric circles around the wire; as you move toward the center of the loop, these circles appear as straight lines because the magnetic effects from all sides of the loop reinforce each other Science, Class X, Magnetic Effects of Electric Current, p.200.

Sources: Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.195; Science, Class VIII, NCERT (Revised ed 2025), Electricity: Magnetic and Heating Effects, p.48; Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.200

5. Earth's Magnetism and Elements (intermediate)

To understand Earth's magnetism, we must first look at the planet as a massive, albeit slightly imperfect, bar magnet. This geomagnetic field originates from the Earth's interior and extends far into space, creating a protective bubble known as the magnetosphere that shields us from solar winds Physical Geography by PMF IAS, Earths Magnetic Field, p.65. While we often imagine a magnet perfectly aligned with the Earth's North and South poles, the reality is more nuanced. The magnetic axis is currently tilted at approximately 11° relative to the Earth's rotational (geographic) axis Physical Geography by PMF IAS, Earths Magnetic Field, p.72. Because of this tilt, your compass does not point to the 'True North' (the North Pole), but rather toward the Magnetic North.

To precisely describe the Earth's magnetic field at any specific location, scientists use three specific parameters known as the Elements of Earth's Magnetism:

• Magnetic Declination: This is the horizontal angle between True North (geographic) and Magnetic North. For navigators, knowing this angle is critical because it tells them how much to correct their compass reading to stay on a true course Physical Geography by PMF IAS, Earths Magnetic Field, p.76.
• Magnetic Inclination (or Dip): This is the vertical angle that a magnetic needle makes with the horizontal plane. If you are at the magnetic equator, the needle stays perfectly horizontal (0° dip). As you move toward the magnetic poles, the needle tilts further downward until it stands perfectly vertical (90° dip) Physical Geography by PMF IAS, Earths Magnetic Field, p.77.
• Horizontal Component (Bₕ): This is the intensity of the Earth's magnetic field acting in the horizontal direction. It is the force that actually drives the compass needle to align with the magnetic meridian.

Finally, it is important to understand how we quantify magnetic strength. In the SI system (Standard International), a unit magnetic pole is defined as a pole which, when placed in a vacuum at a distance of 1 metre from an identical pole, repels it with a force of exactly 1 Newton. This provides the mathematical foundation for measuring the magnetic intensities we observe across the globe.

Feature	Magnetic Equator	Magnetic Poles
Magnetic Dip (Inclination)	0° (Horizontal)	90° (Vertical)
Magnetic Field Lines	Parallel to the surface	Perpendicular to the surface

Sources: Physical Geography by PMF IAS, Earths Magnetic Field, p.65; Physical Geography by PMF IAS, Earths Magnetic Field, p.72; Physical Geography by PMF IAS, Earths Magnetic Field, p.76; Physical Geography by PMF IAS, Earths Magnetic Field, p.77

6. Coulomb’s Law of Magnetism (exam-level)

In our journey through magnetism, we have seen that magnets exert forces without any physical contact—a phenomenon you may have observed by trying to push two repelling ring magnets together Science, Class VIII, Exploring Forces, p.69. To measure this interaction precisely, we use Coulomb’s Law of Magnetism. This law provides the mathematical backbone for understanding how "pole strength" dictates the intensity of magnetic attraction or repulsion.

Coulomb’s Law states that the force (F) between two magnetic poles is directly proportional to the product of their pole strengths (m₁ and m₂) and inversely proportional to the square of the distance (r) between them. This can be written as:

F = k · (m₁ · m₂ / r²)

While modern physics teaches us that magnetic fields are actually produced by moving charges—as seen in the uniform field inside a solenoid Science, Class X, Magnetic Effects of Electric Current, p.201—the concept of a "pole" remains a vital tool for calculation. The definition of a unit magnetic pole depends entirely on the system of measurement used:

Feature	CGS System	SI (Rationalized MKS) System
Unit of Distance	1 centimetre (cm)	1 metre (m)
Unit of Force	1 dyne	1 Newton (N)
Definition	Repels an identical pole 1 cm away with 1 dyne of force.	Repels an identical pole 1 m away with 1 Newton of force.

By moving from the CGS to the SI system, we align magnetic measurements with the standard international units of force and length, making it easier to integrate magnetism with other branches of physics and engineering.

Sources: Science, Class VIII, Exploring Forces, p.69; Science, Class X, Magnetic Effects of Electric Current, p.201

7. Defining the Unit Magnetic Pole (exam-level)

To understand magnetism quantitatively, we must first define a standard measure for the 'strength' of a magnetic pole. Just as we use a 'unit charge' in electrostatics, we use the Unit Magnetic Pole as a fundamental building block in magnetostatics. While a magnet always exists as a dipole—meaning a pair of equal and opposite poles as seen in Earth's own magnetic field Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.74—defining a single unit pole allows us to calculate the specific forces these poles exert on one another. Historically, scientists used the CGS (centimetre-gram-second) system, where a unit pole was defined by its ability to repel an identical pole with a force of 1 dyne at a distance of 1 centimetre. However, for modern applications and competitive exams, we focus on the SI (International System) or rationalized MKS system. In this framework, the definitions are scaled up to match standard units of force and length. A unit magnetic pole is defined as that pole which, when placed in a vacuum or air at a distance of 1 metre from an equal and similar pole, experiences a repulsive force of 1 Newton.

System of Units	Force Requirement	Distance Requirement
CGS System	1 dyne	1 centimetre
SI / MKS System	1 Newton	1 metre

Understanding this unit is crucial because it bridges the gap between theoretical physics and observable phenomena, such as how a compass needle interacts with the Earth's magnetic north—the point where field lines are directed vertically downwards into the Earth Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.71. By defining the strength of these poles precisely, we can map the complex distribution of magnetic fields across the globe.

Sources: Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.74; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.71

8. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamentals of Coulomb’s Law of Magnetostatics, this question serves as a perfect application of how scientific definitions are standardized. You learned that the force between two magnetic poles depends on their pole strengths and the square of the distance separating them. To define a unit magnetic pole, physicists simply set all variables in the force equation to unity (one) within a specific system of measurement. While the physical concept of magnetic repulsion remains the same, the specific values for distance and force change depending on whether you are using the SI or the CGS system.

To arrive at the correct answer, you must apply the SI standards you have studied: the unit of distance is the metre and the unit of force is the newton. Therefore, a unit pole is logically defined as one that, when placed 1 metre away from an identical pole in a vacuum or air, results in a repulsion of exactly 1 newton. This logical 1-1-1 relationship is why Option (B) is the correct choice. When you see "unit" definitions in UPSC, always look for the standard base units of the system specified in the question stem.

UPSC often includes "distractor" options to test your precision under pressure. Option (C) represents the CGS system definition (1 cm and 1 dyne), which is the classic trap for students who memorize the definition but overlook the unit system requested. Option (A) uses imperial units (feet and pounds) which are irrelevant for modern scientific definitions, and Option (D) incorrectly introduces newton/m², which is a unit of pressure rather than force. Recognizing these dimensional inconsistencies allows you to eliminate wrong answers with confidence. ScienceDirect: Magnetic Pole