DC current can be controlled by which one of the following components?

1. Fundamentals of Electric Current: AC vs. DC (basic)

At its heart, electricity is the flow of charge, specifically electrons, through a conductor. This flow is driven by a potential difference created by a power source, such as a battery or a generator Science, class X, Electricity, p.188. However, not all electricity flows in the same way. We categorize it into two main types based on the direction of flow: Direct Current (DC) and Alternating Current (AC).

Direct Current (DC) flows steadily in only one direction. Think of it like water flowing through a garden hose from the tap to the nozzle. Common sources of DC include electric cells and batteries, where chemical reactions create a constant push for electrons Science-Class VII, Electricity: Circuits and their Components, p.36. DC is essential for portable electronics like mobile phones, torches, and laptops Science-Class VII, Electricity: Circuits and their Components, p.40. To control the amount of current in a DC circuit, we primarily use resistance; by increasing resistance (using components like rheostats), we can decrease the current flow Science, class X, Electricity, p.181, 192.

Alternating Current (AC), on the other hand, changes its direction periodically. In the electrical grid that powers our homes, the current reverses direction many times every second. In India, the standard household supply is 220 V with a frequency of 50 Hz, meaning the current changes direction 100 times per second Science, class X, Magnetic Effects of Electric Current, p.206. AC is preferred for large-scale power distribution because it can be easily stepped up or down in voltage for long-distance transmission.

Sources: Science, class X (NCERT 2025 ed.), Electricity, p.188; Science-Class VII . NCERT(Revised ed 2025), Electricity: Circuits and their Components, p.36; Science-Class VII . NCERT(Revised ed 2025), Electricity: Circuits and their Components, p.40; Science, class X (NCERT 2025 ed.), Electricity, p.181, 192; Science, class X (NCERT 2025 ed.), Magnetic Effects of Electric Current, p.206

2. Ohm’s Law and the Concept of Resistance (basic)

To understand electricity, we must first understand how we control it. Think of electricity like water flowing through a pipe. For water to flow, you need pressure; in electricity, this "pressure" is the Potential Difference (V). The flow itself is the Current (I). Ohm’s Law, the most fundamental principle in circuit theory, states that the current flowing through a conductor is directly proportional to the potential difference applied across its ends, provided its temperature remains constant. This is expressed by the famous formula: V = IR Science, Class X (NCERT 2025 ed.), Chapter 11, p. 192.

The constant R in this equation is Resistance. It is the inherent property of a material to oppose the flow of electric charges. While every conductor has some resistance, we use specific components called resistors to deliberately regulate current. In Direct Current (DC) circuits, resistance is the primary tool for control. By increasing the resistance, we decrease the current for a fixed voltage, and vice versa. This is why devices like rheostats (variable resistors) are used in laboratories and machinery to change the current flow without changing the power source Science, Class X (NCERT 2025 ed.), Chapter 11, p. 176.

The resistance of a specific conductor isn't random; it depends on three physical factors: its length (longer wires have more resistance), its area of cross-section (thicker wires have less resistance), and the nature of the material itself (quantified as resistivity, ρ). As we move into more complex electronics, we combine these resistors in different ways to achieve desired results Science, Class X (NCERT 2025 ed.), Chapter 11, p. 181:

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.192; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.176; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.181

3. Heating and Chemical Effects of Electric Current (basic)

When an electric current flows through a conductor, it isn't a frictionless journey. As electrons move, they constantly collide with the atoms of the conductor. These collisions transfer kinetic energy to the atoms, which manifests as an increase in temperature. This phenomenon is known as the Heating Effect of Electric Current. While this heat is often an "inevitable consequence" that leads to energy loss in transmission wires, we have ingeniously harnessed it for everyday utility Science, Class X, Chapter 11, p.190.

The mathematical backbone of this effect is Joule’s Law of Heating. It states that the heat (H) produced in a resistor is directly proportional to three factors: the square of the current (I²), the resistance (R) of the conductor, and the time (t) for which the current flows. Formally, this is expressed as H = I²Rt. This law implies that even a small increase in current leads to a significantly larger increase in heat production Science, Class X, Chapter 11, p.189.

In practical applications, we use specific materials called heating elements (like Nichrome) which have high resistance and high melting points. These elements become red-hot without melting, as seen in room heaters, electric irons, and toasters Science, Class VIII, Chapter 4, p.53. Beyond heating, electricity can also trigger Chemical Effects. When current passes through a conducting liquid (an electrolyte), it causes chemical reactions that may result in the deposition of metal on electrodes (electroplating), the evolution of gas bubbles, or changes in the color of the solution.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.189-190; Science, Class VIII (NCERT Revised ed 2025), Chapter 4: Electricity: Magnetic and Heating Effects, p.53-54

4. Electromagnetic Induction and Inductance (intermediate)

In our previous steps, we explored how an electric current creates a magnetic field—a discovery that transformed our understanding of physics. As noted in Science, Class X, Magnetic Effects of Electric Current, p.195, if electricity can produce magnetism, it stands to reason that the reverse should also be possible. This "reverse" effect is what we call Electromagnetic Induction (EMI). It is the process by which a changing magnetic field within a conductor induces an electric current. It is important to remember that a static magnet near a wire does nothing; it is the relative motion or the change in the magnetic environment that "pushes" the electrons into motion.

To determine the direction of this induced current, we use Fleming’s Right-Hand Rule. While the Left-Hand Rule helps us understand motors (force on a current-carrying wire), the Right-Hand Rule is for generators. If you stretch your thumb, forefinger, and middle finger of your right hand mutually perpendicular to each other, the thumb points toward the motion of the conductor and the forefinger toward the magnetic field; your middle finger will then point in the direction of the induced current Science, Class X, Magnetic Effects of Electric Current, p.206. This principle is the backbone of the 220V, 50 Hz AC power supply that reaches our homes Science, Class X, Magnetic Effects of Electric Current, p.206.

Inductance is a related concept that describes a conductor's "laziness" or resistance to changes in current. Think of it as electrical inertia. When the current through a coil changes, the magnetic field around it also changes, which—by electromagnetic induction—creates a back-voltage (EMF) that opposes the very change that created it. There are two types:

• Self-Inductance: When a single coil opposes changes in its own current.
• Mutual Inductance: When a change in current in one coil induces a voltage in a neighboring coil. This is the fundamental principle behind transformers, which allow us to step up or step down voltages for efficient power transmission.

Sources: Science, Class X, Magnetic Effects of Electric Current, p.195; Science, Class X, Magnetic Effects of Electric Current, p.206

5. Electrostatics and Capacitance (intermediate)

Electrostatics begins with the understanding of electric potential (V). When we move a charge (Q) between two points with a potential difference, work (W) must be done, expressed as W = VQ Science, Class X (NCERT 2025 ed.), Chapter 11, p.173. This work represents energy stored in the system. While Current is the flow of charge, Capacitance is the ability of a component to store that charge and energy within an electric field.

A capacitor typically consists of two conducting surfaces separated by an insulating material (dielectric). The measure of its storage capacity is Capacitance (C), defined by the ratio of the charge stored (Q) to the potential difference (V) applied across it: C = Q / V. In practical terms, capacitors are essential in consumer electronics like televisions and tube lights Understanding Economic Development, Class X (NCERT Revised ed 2025), GLOBALISATION AND THE INDIAN ECONOMY, p.67, where they release energy quickly or filter electronic noise.

It is crucial to understand how capacitors behave in Direct Current (DC) circuits compared to resistors. While a resistor actively regulates or limits current according to Ohm's Law (V = IR) Science, Class X (NCERT 2025 ed.), Chapter 11, p.181, a capacitor's opposition to current is time-dependent. In a steady-state DC circuit, once a capacitor is fully charged to the supply voltage, it acts as an open circuit, meaning it blocks any further flow of current. Therefore, for the active control and regulation of DC, we rely on resistance rather than capacitance.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.173; Understanding Economic Development, Class X (NCERT Revised ed 2025), GLOBALISATION AND THE INDIAN ECONOMY, p.67; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.181; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.188

6. Reactance and Impedance in AC Circuits (exam-level)

7. Component Behavior in Steady-State DC (exam-level)

In a steady-state Direct Current (DC) circuit, the behavior of electrical components is dictated by how they respond to a constant, unchanging flow of charge. While Direct Current is defined as the unidirectional flow of electric charge Science, Class VII, Electricity: Circuits and their Components, p.36, "steady-state" refers to the condition where the circuit has settled after being switched on, and voltages and currents are no longer changing over time.

The primary tool for controlling this flow is resistance. According to Ohm’s Law (V = IR), the current (I) is inversely proportional to the resistance (R). To actively regulate or "throttle" the current in a DC circuit, we use components like rheostats or variable resistors Science, Class X, Electricity, p.181. By increasing the resistance, we decrease the rate of flow of electric charges Science, Class X, Electricity, p.171. However, when we introduce capacitors and inductors into DC circuits, they exhibit unique "extreme" behaviors once steady-state is reached.

It is crucial to distinguish this from Alternating Current (AC) systems. In AC, we speak of reactance and impedance because the current is constantly changing. In steady-state DC, reactance effectively disappears: the capacitor's reactance becomes infinite (blocking current), and the inductor's reactance becomes zero (allowing current freely) Science, Class X, Electricity, p.192. Therefore, for the active regulation of a functioning DC circuit, resistance remains the fundamental control mechanism.

Sources: Science, Class VII, Electricity: Circuits and their Components, p.36; Science, Class X, Electricity, p.171; Science, Class X, Electricity, p.181; Science, Class X, Electricity, p.192

8. Solving the Original PYQ (exam-level)

You have just mastered the building blocks of Ohm’s Law and circuit behavior; this question is the perfect test of how those concepts integrate. To control a current means to have the ability to increase or decrease its flow predictably. In a Direct Current (DC) environment, the voltage remains constant, so the governing principle is $I = V/R$. As taught in Science, class X (NCERT 2025 ed.), the only way to manipulate the current ($I$) without changing the source is to adjust the Resistance (B). This is why devices like rheostats or variable resistors are the standard tools for active current regulation in DC electronics.

It is crucial to understand why the other options serve as classic UPSC conceptual traps. While Impedance sounds like a more comprehensive answer, it is a vector sum that includes Reactance—a property that only opposes current in Alternating Current (AC) systems where the frequency is non-zero. In a steady-state DC circuit (where frequency is zero), an Inductor acts as a short circuit (offering no control), and a Capacitor acts as an open circuit (blocking current entirely once charged). Because these components do not provide a stable, intermediate opposition to DC, they cannot be used for active control, leaving Resistance as the only functional choice.