The polarity of an unmarked horse shoe magnet can be determined by using

1. Fundamentals of Magnets and Magnetic Poles (basic)

Welcome to your first step in mastering Electricity and Magnetism. To understand complex systems like motors or MRI machines, we must start with the humble magnet. At its core, a magnet is any material that produces a magnetic field, attracting materials like iron, nickel, and cobalt Science, Class VIII, Electricity: Magnetic and Heating Effects, p.47. Every magnet, regardless of its shape—be it a bar, a horse-shoe, or a ring—possesses two distinct regions of maximum magnetic strength called poles: the North (N) pole and the South (S) pole.

The most critical principle to internalize is the Fundamental Law of Magnetism: Like poles repel each other, while unlike poles attract each other. This means if you bring two North poles together, they will push apart, but a North pole and a South pole will pull together. This interaction is a non-contact force, meaning magnets can exert a push or pull on each other even through a distance or a physical barrier like a wooden stick Science, Class VIII, Exploring Forces, p.69.

How do we identify these poles if they aren't labeled? We use a magnetic compass. A compass needle is actually a tiny, pivoted magnet. By default, its North pole points toward the Earth's geographic North. When you bring a compass near an unmarked magnet, the South pole of the needle will be drawn toward the North pole of the magnet Science, Class X, Magnetic Effects of Electric Current, p.196. This property is also why the Earth itself is considered a giant magnet, though there is a catch: the Earth’s North geomagnetic pole actually represents the South pole of its internal magnetic field Physical Geography by PMF IAS, Earths Magnetic Field, p.73.

Interaction	Result	Example
Like Poles	Repulsion	North-North or South-South
Unlike Poles	Attraction	North-South

Sources: Science, Class VIII (NCERT 2025 ed.), Electricity: Magnetic and Heating Effects, p.47, 51; Science, Class VIII (NCERT 2025 ed.), Exploring Forces, p.69; Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.196; Physical Geography by PMF IAS, Earths Magnetic Field (Geomagnetic Field), p.73

2. Characteristics of Magnetic Field Lines (basic)

To understand magnetism, we visualize the invisible force using magnetic field lines. Think of these lines as a map that tells us two things: the direction of the force and how strong it is at any given point. A fundamental characteristic is that these lines are continuous closed curves. By convention, they emerge from the North pole and enter the South pole outside the magnet; however, inside the magnet, the field travels from the South pole back to the North pole to complete the loop Science, Class X (NCERT 2025 ed.), Chapter 12, p.197.

The density or closeness of these lines is a direct indicator of the magnetic field's strength. Where the lines are crowded together—usually near the poles—the magnetic force is at its most powerful. As the lines spread out, the field grows weaker. In specific configurations, such as inside a current-carrying solenoid, the field lines are parallel and straight, which tells us the magnetic field is uniform (constant) at all points in that region Science, Class X (NCERT 2025 ed.), Chapter 12, p.201.

Perhaps the most critical rule for any UPSC aspirant to remember is that magnetic field lines never intersect. If two lines were to cross, it would imply that a compass needle placed at the point of intersection would point in two different directions at once. Since a magnetic field can only have one resultant direction at any single point, such an intersection is physically impossible Science, Class X (NCERT 2025 ed.), Chapter 12, p.197.

Feature	Outside the Magnet	Inside the Magnet
Direction	North Pole → South Pole	South Pole → North Pole
Shape	Curved arcs	Parallel straight lines (approx.)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 12: Magnetic Effects of Electric Current, p.197; Science, Class X (NCERT 2025 ed.), Chapter 12: Magnetic Effects of Electric Current, p.201; Science, Class X (NCERT 2025 ed.), Chapter 12: Magnetic Effects of Electric Current, p.206

3. Earth's Magnetism and the Magnetic Compass (intermediate)

Have you ever wondered why a simple needle, when allowed to pivot freely, always points toward the horizon in a specific direction? This happens because our Earth behaves like a giant bar magnet. This magnetic field isn't just a curiosity for hikers; it is a fundamental shield that protects our atmosphere from high-energy cosmic rays and solar winds Science, Class VIII, Electricity: Magnetic and Heating Effects, p.51. The origin of this field lies deep within the Earth's core. It is believed that the movement of molten iron in the outer core creates electric currents, which in turn generate the geomagnetic field through what is known as the dynamo effect Science, Class VIII, Our Home: Earth, a Unique Life Sustaining Planet, p.217.

To understand the magnetic compass, we must distinguish between the Geographic Poles (defined by Earth's rotation) and the Magnetic Poles. The axis of Earth’s magnetic field is currently tilted at an angle of about 11 degrees relative to the rotational axis Physical Geography by PMF IAS, Earths Magnetic Field, p.72. This means that if you follow a compass strictly "North," you won't end up exactly at the North Pole (True North), but rather at the Magnetic North Pole.

Feature	Geographic North Pole	North Magnetic Pole
Definition	The fixed point where Earth's axis of rotation meets the surface.	The point where Earth's magnetic field lines are vertical.
Stability	Fixed (True North).	Wanders over time due to core fluid changes.
Magnetic Nature	Non-magnetic concept.	Physically acts as the South Pole of Earth's internal magnet.

The logic of a magnetic compass relies on the fundamental law: Like poles repel, and unlike poles attract. A compass needle is itself a tiny magnet with its own North and South poles. The end of the needle we call the "North Pole" is actually a North-seeking pole. Because it is attracted to the North Magnetic Pole of the Earth, it reveals a fascinating scientific paradox: the Earth's North Magnetic Pole is physically the South Pole of the Earth's internal magnetic dipole Physical Geography by PMF IAS, Earths Magnetic Field, p.75. Without this invisible field, not only would navigation be impossible for humans and migratory animals, but our planet would also be stripped of its atmosphere by the constant bombardment of solar particles.

Sources: Science, Class VIII, Electricity: Magnetic and Heating Effects, p.51; Science, Class VIII, Our Home: Earth, a Unique Life Sustaining Planet, p.217; Physical Geography by PMF IAS, Earths Magnetic Field, p.72; Physical Geography by PMF IAS, Earths Magnetic Field, p.75

4. The Magnetic Effect of Electric Current (Electromagnetism) (intermediate)

For centuries, electricity and magnetism were studied as two distinct branches of science. This changed in 1820 when Hans Christian Oersted noticed a curious phenomenon: a compass needle, which is essentially a small magnet, deflected when placed near a wire carrying an electric current Science, Class X, Chapter 12, p.195. This accidental discovery proved that moving electric charges (current) generate a magnetic field in the space around them. This unity of forces is the foundation of modern technology, from the motors in your fans to the MRI machines in hospitals Science, Class VIII, Chapter 4, p.48.

To visualize the shape of this invisible field around a straight wire, we use the Right-Hand Thumb Rule. Imagine gripping the wire with your right hand: if your thumb points in the direction of the current, your fingers curl in the direction of the magnetic field lines Science, Class X, Chapter 12, p.200. Because the magnetic field is a vector quantity (having both magnitude and direction), reversing the current direction will immediately flip the direction of the magnetic field, causing a compass needle to deflect in the opposite direction.

Identifying the unknown poles of a magnet (like a horseshoe magnet) relies on the fundamental law of magnetism: Like poles repel and unlike poles attract. A magnetic compass is the most reliable tool for this because its needle has a clearly defined North and South pole. When brought near an unmarked magnet, the needle's North pole will be drawn toward the magnet's South pole, and vice versa. Note that tools like electroscopes or charged glass rods are useless here, as they interact with static electric charges, not magnetic fields.

When we place a current-carrying wire inside an external magnetic field, the two fields interact, exerting a mechanical force on the wire. This is the principle behind electric motors. To predict the direction of this force, we use Fleming’s Left-Hand Rule: stretch your thumb, forefinger, and middle finger perpendicularly. If the forefinger points to the magnetic Field and the middle finger to the Current, the thumb points to the Motion or Force Science, Class X, Chapter 12, p.203.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 12: Magnetic Effects of Electric Current, p.195, 200, 203; Science, Class VIII (NCERT 2025 ed.), Chapter 4: Electricity: Magnetic and Heating Effects, p.48

5. Electrostatics vs. Magnetism: Distinguishing Tools (intermediate)

In our journey through physics, it is vital to distinguish between electrostatic forces (stationary charges) and magnetic forces (poles). While both can exert attractive or repulsive forces, they require specific tools for identification. For instance, an electroscope is a precision device used solely to determine whether an object carries an electric charge Science, Class VIII, Exploring Forces, p. 79. It cannot, however, tell you if a metal bar is a magnet or simply a piece of charged plastic. Conversely, to identify the North and South poles of a magnet, we must use a tool that interacts with magnetic fields.

The most reliable tool for identifying magnetic polarity is the magnetic compass. The needle of a compass is actually a tiny bar magnet with a pre-defined North and South pole Science, Class X, Magnetic Effects of Electric Current, p. 196. By applying the fundamental Law of Magnetism—which states that like poles repel and unlike poles attract—we can easily map an unmarked magnet. If the North-seeking end of the compass needle is repelled by one end of a horseshoe magnet, that end is definitively the North pole.

Feature	Electrostatic Tools (e.g., Electroscope)	Magnetic Tools (e.g., Compass)
Primary Target	Stationary Electric Charges (+ or -)	Magnetic Poles (North or South)
Mechanism	Repulsion of charged foils/leaves	Alignment with external magnetic fields
Key Interaction	Like charges repel; Unlike charges attract	Like poles repel; Unlike poles attract

It is a common misconception that a charged object (like a glass rod rubbed with silk) can identify magnetic poles. While moving charges do create magnetic effects Science, Class VIII, Electricity: Magnetic and Heating Effects, p. 51, a stationary charge will not show the specific polar attraction or repulsion needed to label a magnet's ends. To resolve an "unknown," you must always use a "known" reference, such as a marked compass needle.

Sources: Science, Class VIII, Exploring Forces, p.79; Science, Class X, Magnetic Effects of Electric Current, p.196; Science, Class VIII, Electricity: Magnetic and Heating Effects, p.51

6. Testing Magnetism: The Sure Test of Polarity (exam-level)

When we encounter an unmarked magnet—whether it is a bar magnet or a horseshoe magnet—we cannot simply look at it to determine which end is North or South. To solve this, we rely on the Fundamental Law of Magnetism: Like poles repel each other, while unlike poles attract each other Science, Class VIII, Exploring Forces, p.69. While attraction is a common observation, it can be misleading because a magnet will attract any magnetic material (like iron) regardless of its own polarity. Therefore, to truly identify poles, we need a reference magnet with known polarity.

The most practical tool for this is a magnetic compass. A compass needle is actually a very small, pivoted magnet. By convention, the end that points toward the Earth's geographic north is its North pole, and the opposite end is its South pole Science, Class X, Magnetic Effects of Electric Current, p.206. When you bring this compass near one end of an unmarked magnet, you observe the interaction: if the North pole of the compass needle is attracted to the magnet's end, that end must be a South pole. Conversely, if the North pole of the needle is repelled (pushed away), that end is a North pole Science, Class X, Magnetic Effects of Electric Current, p.196.

In the laboratory or field testing, we often say that repulsion is the sure test for magnetism. Why? Because a piece of ordinary iron will be attracted to both the North and South poles of a magnet. However, only another magnet can experience repulsion. Therefore, if you see two objects pushing each other away, you can be absolutely certain that both are magnets and that their facing poles are identical. Tools like charged glass rods or electroscopes are ineffective here, as they deal with static electricity (electric charges) rather than magnetic fields.

Sources: Science, Class VIII (NCERT 2025 ed.), Exploring Forces, p.69; Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.196; Science, Class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.206

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental Law of Magnetism—the principle that like poles repel and unlike poles attract—this question tests your ability to apply that knowledge in a practical scenario. You've learned that every magnet has a North and South pole, and to identify these in an unmarked object, you require a reference magnet with known polarity. As noted in Science, Class X (NCERT 2025 ed.), a magnetic compass is essentially a small, pivoted magnet where the North-seeking end is already established, making it the perfect diagnostic tool for this purpose.

To arrive at the correct answer (B), think like a scientist: if you bring the North pole of the compass needle near one end of the horse shoe magnet and it is attracted, that end must be the South pole. Conversely, if the needle's North pole is repelled, you are looking at the magnet's North pole. This systematic application of magnetic interaction, described in Science, Class VIII (NCERT 2025 ed.), allows for definitive identification.

UPSC often uses distractors from related but distinct fields to test your conceptual clarity. Options (A) and (C), a charged glass rod and an electroscope, are tools used in electrostatics to detect electric charges, not magnetic poles; they would not provide a consistent directional response to a magnetic field. Finally, another unmarked bar magnet (Option D) is a logical trap—while it would react to the horse shoe magnet, you would be unable to label the poles because neither object has a known reference point. Always look for the known constant in the experiment!