The current (I)-voltage (V) plot of a certain electronic device is given above. The device is

1. Electric Current and Potential Difference (basic)

To understand electricity, we must first distinguish between the 'flow' and the 'push.' Electric current (I) is defined as the rate of flow of electric charges through a cross-section of a conductor Science, Class X (NCERT 2025 ed.), Chapter 11, p.171. In metallic wires, these charges are electrons. By convention, we say current flows from the positive terminal to the negative terminal, which is exactly opposite to the direction of the actual electron flow Science, Class X (NCERT 2025 ed.), Chapter 11, p.192. The SI unit for current is the Ampere (A).

While current is the flow, Potential Difference (V) is the electrical 'pressure' that causes that flow. It represents the work done to move a unit charge from one point to another. We use devices like cells or batteries to create this difference; chemical reactions inside the cell generate a potential difference across its terminals even when no current is being drawn Science, Class VIII (NCERT 2025 ed.), Chapter 11, p.58. It is measured in Volts (V). Without this 'push,' electrons would move randomly, and there would be no net current.

The relationship between these two is the foundation of circuit theory. For most metallic conductors, the current is directly proportional to the potential difference applied across its ends, provided physical conditions like temperature remain the same. This is known as Ohm’s Law. If you were to plot a graph of Voltage (V) on one axis and Current (I) on the other, you would get a straight line passing through the origin. The slope of this line represents the Resistance (R), which is the property of a conductor to oppose the flow of charges Science, Class X (NCERT 2025 ed.), Chapter 11, p.192.

Feature	Electric Current (I)	Potential Difference (V)
Core Concept	The actual movement of charges.	The cause/pressure behind the movement.
SI Unit	Ampere (A)	Volt (V)
Measuring Device	Ammeter (connected in series)	Voltmeter (connected in parallel)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.171, 192; Science, Class VIII (NCERT 2025 ed.), Chapter 11: Electricity: Magnetic and Heating Effects, p.58

2. Ohm’s Law and Electrical Resistance (basic)

To understand how electricity behaves in a circuit, we look at the relationship between 'pressure' (Voltage) and 'flow' (Current). In 1827, Georg Simon Ohm discovered that for metallic conductors, this relationship is beautifully simple and linear. This is known as Ohm’s Law: the potential difference (V) across the ends of a metallic wire is directly proportional to the current (I) flowing through it, provided its temperature remains constant Science, Class X (NCERT 2025 ed.), Chapter 11, p. 176.

When we express this proportionality as an equation, we introduce a constant called Resistance (R). The formula becomes:
V = IR

Resistance is the property of a conductor to resist the flow of charges through it. While voltage 'pushes' the electrons, resistance 'opposes' them, similar to how friction opposes motion on a floor. The SI unit of resistance is the ohm, represented by the Greek letter Ω. If 1 Volt of potential difference leads to 1 Ampere of current, the resistance of that conductor is exactly 1 Ω Science, Class X (NCERT 2025 ed.), Chapter 11, p. 176.

Visually, if you plot a graph with Voltage on one axis and Current on the other for an 'ohmic' conductor, you will see a straight line passing through the origin. This indicates a constant ratio (V/I). However, it is important to note that not all devices follow this law. For example, semiconductors like diodes exhibit non-linear behavior where the current does not rise proportionally with voltage. Identifying a linear I-V plot is the primary way to confirm a device obeys Ohm's Law Science, Class X (NCERT 2025 ed.), Chapter 11, p. 192.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.176; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.192

3. Factors Affecting Resistance (intermediate)

Welcome back! Now that we understand Ohm’s law, let’s dive into what actually determines how much a conductor resists the flow of current. Imagine electrons trying to move through a wire like people trying to walk through a hallway. The difficulty they face—the electrical resistance (R)—isn't random; it depends on four specific physical factors of the conductor.

According to precise observations, the resistance of a uniform metallic conductor is directly proportional to its length (l) and inversely proportional to its area of cross-section (A) Science, Class X (NCERT 2025 ed.), Chapter 11, p. 178. Think of it this way: a longer wire offers more obstacles (atoms) for the electrons to bump into, increasing resistance. Conversely, a thicker wire (larger cross-section) provides more "lanes" for electrons to travel through, thereby decreasing resistance. These relationships are captured in the master formula:

Here, ρ (rho) is the constant of proportionality called electrical resistivity. Unlike resistance, which changes with the shape of the object, resistivity is an intrinsic property of the material itself Science, Class X (NCERT 2025 ed.), Chapter 11, p. 178. For example, silver and copper have very low resistivity (10⁻⁸ Ω m), making them excellent conductors, while insulators like glass have incredibly high resistivity (10¹² to 10¹⁷ Ω m) Science, Class X (NCERT 2025 ed.), Chapter 11, p. 179.

Factor	Relationship with Resistance (R)	Practical Logic
Length (l)	Directly Proportional (R ∝ l)	Longer path = more collisions.
Area (A)	Inversely Proportional (R ∝ 1/A)	Wider path = easier flow.
Material (ρ)	Depends on nature of substance	Atomic structure dictates electron mobility.
Temperature	Directly Proportional (for metals)	Heat causes atoms to vibrate more, blocking electrons.

It is also important to note that alloys (like Nichrome) typically have higher resistivity than their constituent pure metals. They also do not oxidize or "burn" easily at high temperatures, which is why they are the preferred choice for heating elements in electric irons or toasters Science, Class X (NCERT 2025 ed.), Chapter 11, p. 179.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.178; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.179

4. Superconductors and Insulators (intermediate)

To master the flow of electricity, we must understand that materials don't just 'allow' or 'block' current—they exist on a spectrum defined by how they manage resistance. Resistance is the inherent property of a material to resist the flow of charges through it (Science, Class X, Chapter 11, p.176). While Ohm's Law (V = IR) provides a perfect linear relationship for standard metallic conductors, Insulators and Superconductors represent the two extreme bookends of this physical property.

Insulators are materials with extremely high electrical resistivity (Science, Class X, Chapter 11, p.178). In these substances, like rubber, glass, or dry wood, electrons are tightly bound to their atoms and cannot move freely. Even if you apply a significant potential difference (voltage), the resulting current is negligible. If you were to look at an I-V graph for an insulator, the line would barely lift off the horizontal axis, reflecting that it takes massive 'push' to get even a tiny 'flow'.

On the opposite end of the spectrum are Superconductors. These are materials that, when cooled below a specific 'critical temperature,' lose all electrical resistance (R = 0). This is a quantum mechanical phenomenon, not just 'very low' resistance, but absolute zero resistance. Because R is zero, Ohm's Law (V = IR) tells us that the potential difference (V) across a superconductor is also zero, even while a steady current is flowing. This allows for incredible applications, such as Maglev trains and MRI machines, where electricity can circulate indefinitely without losing energy as heat.

Material Type	Resistance Level	I-V Characteristic	Example
Insulator	Extremely High	Negligible current regardless of voltage.	Rubber, Ebonite
Conductor	Low / Constant	Linear (Straight line through origin).	Copper, Aluminum
Superconductor	Zero (R = 0)	Zero voltage for a range of current flow.	Mercury (below 4.2 K)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.176; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.178

5. Introduction to Semiconductors (intermediate)

In our journey through electricity, we have seen how metals allow current to flow easily (conductors) and how materials like rubber block it (insulators). However, there is a fascinating middle ground: Semiconductors. These materials, such as Silicon (Si) and Germanium (Ge), are the building blocks of modern electronics, from your smartphone to solar panels.

The defining characteristic of a semiconductor lies in its atomic structure. Elements like Silicon have a valency of four, meaning they have four electrons in their outermost shell Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.62. Unlike metals, where electrons are "free" to roam, the electrons in a semiconductor are held in covalent bonds. At absolute zero temperature, a semiconductor acts like a perfect insulator. However, as temperature increases or as specific impurities are added (a process called doping), these bonds break, allowing electrons to move and conduct electricity.

A critical way to identify a semiconductor is by looking at its I-V Characteristic Plot (the graph of Current vs. Voltage). While a standard metallic conductor obeys Ohm’s Law—showing a straight, linear line passing through the origin—a semiconductor device like a diode is non-linear Science, Class X (NCERT 2025 ed.), Electricity, p.176. It often requires a specific "threshold voltage" to begin conducting, and the current does not rise in direct proportion to the voltage.

Feature	Conductor (e.g., Copper)	Semiconductor (e.g., Silicon)
Ohm's Law	Obeys (Linear I-V plot)	Does not obey (Non-linear I-V plot)
Valence Electrons	Usually 1, 2, or 3	Typically 4
Temperature Effect	Resistance increases with heat	Resistance decreases with heat

Beyond physics, semiconductors have a significant environmental footprint. The manufacturing process involves industrial gases like Perfluorocarbons (PFCs) and Sulfur hexafluoride (SF₆). These are potent greenhouse gases with very high Global Warming Potentials (GWPs) and long atmospheric lifetimes Environment, Shankar IAS Academy (ed 10th), Climate Change, p.257. Understanding semiconductors is therefore not just about circuits, but also about the global challenge of sustainable industrial growth.

Sources: Science, Class X (NCERT 2025 ed.), Carbon and its Compounds, p.62; Science, Class X (NCERT 2025 ed.), Electricity, p.176; Environment, Shankar IAS Academy (ed 10th), Climate Change, p.257

6. Ohmic vs. Non-Ohmic Devices (exam-level)

In the study of electricity, we categorize materials based on how they respond to electrical pressure (voltage). At the heart of this distinction is Ohm’s law, which states that the potential difference (V) across the ends of a metallic conductor is directly proportional to the current (I) flowing through it, provided its temperature remains constant (Science, Class X, Chapter 11, p.176). Devices that strictly adhere to this linear proportionality are known as Ohmic devices. For these materials, the ratio of V to I is a constant value called Resistance (R). If you were to plot their behavior on a graph, you would see a perfectly straight line passing through the origin.

In contrast, Non-Ohmic devices do not show a constant resistance. In these components, the current does not rise proportionally with the voltage. Instead, the relationship is non-linear, meaning their I-V characteristic is a curve. A common example found in modern torches is the Light Emitting Diode (LED) (Science, Class VII, Chapter 3, p.27). These devices might offer high resistance at low voltages but suddenly allow current to flow freely once a specific threshold is reached. While metallic conductors like nichrome or copper are generally Ohmic, complex electronic components like diodes, transistors, and even vacuum tubes are non-Ohmic.

To help you distinguish between the two for your exam, consider this comparison of their fundamental properties:

Feature	Ohmic Devices	Non-Ohmic Devices
V-I Relationship	Linear (V ∝ I)	Non-linear
Resistance (R)	Constant (at fixed temperature)	Variable (Dynamic)
Graph Shape	Straight line through the origin	Curves or non-proportional slopes
Examples	Copper wire, Nichrome wire, Silver	LEDs, Solar cells, Transistors

Sources: Science, class X (NCERT 2025 ed.), Chapter 11: Electricity, p.176; Science-Class VII, NCERT (Revised ed 2025), Chapter 3: Electricity: Circuits and their Components, p.27

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental relationship between potential difference and current, this question brings those building blocks together through the lens of graphical analysis. The core concept here is direct proportionality. In your studies, you learned that for certain materials, the current flowing through them increases linearly with the voltage applied. This specific behavior is the hallmark of Ohm's Law, and being able to identify it on a plot is a crucial skill for the UPSC Civil Services Examination.

To arrive at the correct answer, look closely at the shape of the graph. Because the plot is a straight line passing through the origin, it indicates that the ratio of voltage to current (V/I) is constant. As described in Science, class X (NCERT 2025 ed.) > Chapter 11: Electricity, this constant ratio represents electrical resistance (R). If the resistance remains unchanged regardless of the voltage applied, the device is functioning as an Ohmic conductor. Therefore, the device is (B) a conductor which obeys Ohm's law.

Understanding why the other options are incorrect is vital for avoiding UPSC traps. A semiconductor would exhibit a non-linear curve because its resistance changes with voltage (non-Ohmic behavior). A superconductor would show current flowing with zero potential difference, while an insulator would show a flat line along the voltage axis with effectively zero current. Always remember: a straight line through the origin is the unique visual signature of a device following Ohm's Law.