An electric bulb is connected to 220 V generator. The current drawn is 600 mA. What is the power of the bulb?

1. Electric Current and the Flow of Charges (basic)

To understand electricity, we must first look at the tiny particles that power our world: electrons. In a metal wire, like copper, an electric current is essentially a collective stream of electrons moving through the conductor Science, Class X (NCERT 2025 ed.), Electricity, p.192. However, these electrons don't just wander aimlessly to create a current; they need a "push." Think of a horizontal pipe filled with water—the water won't flow unless there is a pressure difference between the two ends. Similarly, in a circuit, electrons only move when there is a difference in electric pressure, which we call Potential Difference, typically provided by a cell or a battery Science, Class X (NCERT 2025 ed.), Electricity, p.173.

One of the most important concepts to master for your conceptual foundation is the direction of flow. Historically, scientists discovered electricity before they knew about electrons. They assumed current was the flow of positive charges. Today, we know it is actually negative electrons that move, but we still keep the old "conventional" direction for consistency. This leads to a unique rule in physics:

Entity	Direction of Flow
Electrons (Negative Charges)	From Negative terminal to Positive terminal
Electric Current (Conventional)	From Positive terminal to Negative terminal

As these electrons move, they aren't entirely free. They must weave through the atoms of the conductor, which act like obstacles that "retard" their motion. This property is known as resistance Science, Class X (NCERT 2025 ed.), Electricity, p.177. The rate at which these charges flow is measured in Amperes (A), the SI unit of electric current Science, Class X (NCERT 2025 ed.), Electricity, p.192.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.171; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.173; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.177; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.192

2. Potential Difference and Electromotive Force (basic)

To understand why electricity flows through a wire, imagine water flowing through a pipe. Water only moves if there is a pressure difference between two ends. In electricity, this "pressure" is known as Electric Potential. When we talk about the work required to move a unit charge from one point to another in a circuit, we are defining the Potential Difference (V). Scientifically, if W is the work done to move a charge Q, then V = W/Q. The SI unit for this is the Volt (V), named after Alessandro Volta. One volt is defined as the potential difference between two points when 1 Joule of work is done to move a charge of 1 Coulomb Science, Class X (NCERT 2025 ed.), Chapter 11, p.173.

While "Potential Difference" is a general term for any two points in a circuit, Electromotive Force (EMF) specifically refers to the maximum potential difference a source (like a battery or generator) can provide when no current is flowing through it. Think of EMF as the "intrinsic capacity" of the battery and Potential Difference as the "actual pressure" measured across a component like a bulb or resistor. In a functioning circuit, the potential difference across a device is what drives the current through it, and according to Ohm's Law, this is directly proportional to the current flow, provided temperature remains constant Science, Class X (NCERT 2025 ed.), Chapter 11, p.192.

Feature	Electromotive Force (EMF)	Potential Difference (PD)
Definition	Work done by the source to move a unit charge around the whole circuit.	Work done to move a unit charge between two specific points.
Circuit Condition	Measured when the circuit is "open" (no current flowing).	Measured when the circuit is "closed" (current is flowing).
Nature	It is the cause of the current.	It is the effect of the current flowing through a resistance.

In practical terms, the potential difference is the energy converted from electrical form to other forms (like light in a bulb or heat in a heater) per unit charge. For instance, if a heater is connected to a 60V source, it indicates the amount of electrical energy available to be converted into heat Science, Class X (NCERT 2025 ed.), Chapter 11, p.180.

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.173; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.180; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.192

3. Ohm’s Law and Electrical Resistance (intermediate)

Imagine electricity as water flowing through a pipe. The "pressure" pushing the water is the Potential Difference (Voltage, V), and the flow of water itself is the Current (I). Ohm’s Law establishes the fundamental relationship between these two: it states that the current flowing through a conductor is directly proportional to the potential difference applied across its ends, provided its temperature remains constant. Mathematically, this is expressed as V = IR, where R is the constant of proportionality known as Resistance Science, Class X, Electricity, p.192.

Resistance is the internal "friction" a conductor offers to the flow of electrons. Its SI unit is the Ohm (Ω). By definition, if a potential difference of 1 Volt across a conductor produces a current of 1 Ampere, the resistance of that conductor is 1 Ω Science, Class X, Electricity, p.176. This law is crucial because it allows us to predict how much current will flow through a component like a bulb or a heater when connected to a specific power source.

The resistance of a conductor isn't just a fixed number; it depends on four specific physical factors:

• Length (l): Resistance is directly proportional to length. A longer wire offers more obstacles to electrons.
• Area of Cross-section (A): Resistance is inversely proportional to the area. A thicker wire is like a wider highway, allowing current to flow more easily.
• Nature of Material: This is represented by Resistivity (ρ), a property unique to each material.
• Temperature: Generally, for pure metals, resistance increases as temperature rises.

Combining these, we get the formula R = ρ(l/A) Science, Class X, Electricity, p.178.

Factor	Relationship with Resistance (R)	Effect on Current (I)
Length (l) ↑	Increases	Decreases
Thickness (A) ↑	Decreases	Increases

Sources: Science, Class X (NCERT 2025 ed.), Electricity, p.192; Science, Class X (NCERT 2025 ed.), Electricity, p.176; Science, Class X (NCERT 2025 ed.), Electricity, p.178

4. Heating Effect of Current and Joule's Law (intermediate)

When an electric current flows through a conductor, it isn't a frictionless journey. Electrons constantly collide with the atoms of the material, and during these collisions, a portion of their kinetic energy is transferred to the atoms. This increased atomic vibration manifests as thermal energy, causing the conductor to warm up. This phenomenon is known as the Heating Effect of Electric Current. In many devices, like computers or power lines, this is an undesirable loss of energy; however, we purposefully harness it in appliances like electric irons, toasters, and water heaters Science, Class VIII, Electricity: Magnetic and Heating Effects, p.53.

To quantify this heat, we look to Joule's Law of Heating. It states that the heat (H) produced in a resistor is mathematically expressed as H = I²Rt. This law reveals three critical relationships: heat is directly proportional to the square of the current (I²), directly proportional to the resistance (R) of the conductor, and directly proportional to the time (t) for which the current flows Science, Class X, Electricity, p.189. This explains why doubling the current doesn't just double the heat—it actually quadruples it!

Closely linked to heating is Electric Power (P), which is the rate at which electrical energy is consumed or dissipated. It is defined as the product of potential difference (V) and current (I), or P = VI. By substituting Ohm's Law (V = IR), we can also express power as P = I²R or P = V²/R. The standard unit of power is the Watt (W), representing one joule of energy consumed per second Science, Class X, Electricity, p.192. For example, in an incandescent bulb, the filament's high resistance causes it to become so hot that it glows and emits light, converting a significant portion of electrical power into heat Science, Class X, Electricity, p.190.

Sources: Science, Class VIII, Electricity: Magnetic and Heating Effects, p.53; Science, Class X, Electricity, p.189; Science, Class X, Electricity, p.190; Science, Class X, Electricity, p.192

5. Domestic Circuits and Electrical Safety (exam-level)

In a domestic electrical circuit, the way we connect our appliances is a matter of both functional logic and life-saving safety. Unlike the simple circuits you might build in a lab, a home requires a parallel arrangement. The primary reason is independence: in a parallel circuit, the potential difference (voltage) remains constant across every appliance (usually 220 V in India), ensuring each device operates at its rated power. If we used a series connection, turning off one light would break the entire circuit, plunging the whole house into darkness Science, Class X (NCERT 2025 ed.), Chapter 11, p.187. Furthermore, different appliances like a toaster and a bulb require vastly different current values; a parallel circuit allows the current to divide according to each gadget's needs Science, Class X (NCERT 2025 ed.), Chapter 11, p.188.

To protect these circuits from damage, two critical safety mechanisms are employed: Fusing and Earthing. An electric fuse is a safety device placed in series with the live wire. It consists of a wire with a low melting point; if the current exceeds a safe limit (due to overloading or a short circuit), the wire melts and breaks the circuit instantly Science, Class X (NCERT 2025 ed.), Chapter 11, p.190. On the other hand, Earth wires are connected to the metallic bodies of appliances. This ensures that if there is any insulation failure and the live wire touches the metal casing, the current flows safely into the earth rather than through the user, preventing a fatal electric shock Science, Class X (NCERT 2025 ed.), Chapter 12, p.205.

Feature	Series Connection	Parallel Connection (Domestic)
Current	Same through all components	Divides based on appliance resistance
Voltage	Divided across components	Same (220 V) for all appliances
Failure Impact	One fail = All fail	One fail = Others continue working

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.187, 188, 190; Science, Class X (NCERT 2025 ed.), Chapter 12: Magnetic Effects of Electric Current, p.205, 206

6. Commercial Units of Energy and Power (intermediate)

When we discuss electricity in a professional or domestic context, it is crucial to distinguish between Power and Energy. Power is the rate at which electrical work is done, while Energy is the total amount of work performed over a period of time. In the SI system, the unit of power is the Watt (W). Formally, one watt of power is consumed when 1 Ampere (A) of current flows at a potential difference of 1 Volt (V) Science, Class X (NCERT 2025 ed.), Electricity, p.192. However, because a Watt is a very small unit, we often use Kilowatts (kW), where 1 kW = 1000 W, for practical engineering applications.

For billing and commercial purposes, measuring energy in Joules (Watt-seconds) becomes unwieldy because the numbers would be astronomically high. Instead, we use the Kilowatt-hour (kWh), which is commonly referred to as a 'unit' on your electricity bill. One kilowatt-hour represents the energy consumed when an appliance with a power rating of 1000 Watts is used for exactly one hour Science, Class X (NCERT 2025 ed.), Electricity, p.191. To understand the scale of this, we can convert it back to the SI unit of energy, the Joule:

• 1 kWh = 1 kW × 1 hour
• 1 kWh = 1000 W × 3600 seconds
• 1 kWh = 3.6 × 10⁶ Joules (J)

Interestingly, the consumption of electricity is not just a technical metric but a significant socio-economic indicator. In India, the per capita consumption of electricity is approximately 350 kWh. While this has grown significantly from an installed capacity of just 2.3 thousand MW in 1950-51 to over 264 thousand MW in recent years, it remains well below the global average of 1000 kWh and the USA's high consumption of 7000 kWh Geography of India, Majid Husain, Energy Resources, p.17. This disparity highlights the ongoing journey toward energy security and industrial development in the country.

Sources: Science, Class X (NCERT 2025 ed.), Electricity, p.191-192; Geography of India, Majid Husain, Energy Resources, p.17

7. Unit Conversions and Scientific Prefixes (basic)

In scientific calculations, especially within the realm of electricity and magnetism, we often encounter values that are either extremely large or incredibly small. To make these numbers manageable, we use the Metric System of Prefixes. These prefixes act as shorthand for powers of ten. For instance, instead of saying 0.001 Amperes, we say 1 milliampere (mA). Understanding these is vital because standard physics formulas, such as the power formula (P = VI) or Ohm’s Law (V = IR), require the input values to be in their base SI units (like Amperes, Volts, and Ohms) to yield a correct result in Watts or Joules.

The most common prefixes you will encounter in basic electrical circuits are milli- and micro-. According to Science, Class X (NCERT 2025 ed.), Chapter 11, p.172, a milliampere is one-thousandth of an Ampere (1 mA = 10⁻³ A), while a microampere is one-millionth of an Ampere (1 µA = 10⁻⁶ A). To convert from a prefixed unit to a base unit, you simply multiply by the corresponding power of ten (or divide if moving from a smaller unit to a larger one). For example, if a problem mentions a current of 2.5 mA Science, Class X (NCERT 2025 ed.), Chapter 11, p.193, you must convert it to 0.0025 A before proceeding with any calculations involving resistance or power.

Here is a quick reference table for the prefixes most relevant to competitive exams:

Prefix	Symbol	Value (Multiplier)	Example
Mega	M	10⁶ (1,000,000)	1 MW = 1,000,000 Watts
Kilo	k	10³ (1,000)	1 kV = 1,000 Volts
Milli	m	10⁻³ (0.001)	1 mA = 0.001 Amperes
Micro	µ	10⁻⁶ (0.000001)	1 µA = 0.000001 Amperes

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.172; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.193

8. Calculating Electrical Power (P = VI) (intermediate)

In our study of electricity, Electric Power represents the rate at which electrical energy is consumed or dissipated in a circuit Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.191. Think of it as the "speed" at which a device does its work. While we measure total energy in Joules, power tells us how many Joules are being used every single second. This concept is vital for understanding why different appliances have different "ratings" and how our electrical infrastructure is designed to handle various loads.

The fundamental relationship for calculating power is given by the formula P = VI, where P is Power, V is the potential difference (Voltage), and I is the current. This equation shows us that the power of a device depends on both the electrical "pressure" pushing the charges (Voltage) and the actual flow rate of those charges (Current). If you increase either the voltage or the current, the total power consumed by the device increases proportionally.

The SI unit of electric power is the Watt (W). We define one watt as the power consumed by a device that carries 1 Ampere (A) of current when operated at a potential difference of 1 Volt (V) Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.192. When performing calculations, it is critical to ensure your units are consistent. For example, if a current is given in milliamperes (mA), you must convert it to Amperes (by dividing by 1,000) before applying the formula. Because the Watt is a relatively small unit for industrial use, we often use the Kilowatt (kW), which is equivalent to 1,000 Watts Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.191.

Variable	Description	Standard Unit
P	Power (Rate of energy use)	Watt (W)
V	Potential Difference (Voltage)	Volt (V)
I	Current (Flow of charge)	Ampere (A)

Sources: Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.191; Science, Class X (NCERT 2025 ed.), Chapter 11: Electricity, p.192

9. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental building blocks of electricity, this question serves as the perfect synthesis of those concepts. In your recent lessons, you learned that Electrical Power (P) is the rate at which electrical energy is consumed, defined mathematically as the product of potential difference (V) and current (I). This specific problem tests your ability to not only recall the formula P = VI but also your precision in handling SI units, a critical skill for the UPSC Prelims where examiners often hide traps in unit conversions. Science, class X (NCERT 2025 ed.) > Chapter 11: Electricity

To arrive at the correct answer, think like a physicist: first, identify your known variables and ensure they are in standard form. You have a voltage of 220 V and a current of 600 mA. Before applying the power formula, you must convert the current into Amperes (A). Since 1000 mA equals 1 A, 600 mA becomes 0.6 A. By multiplying 220 V by 0.6 A, you find the power is exactly 132 W. This confirms that (A) 132 W is the correct answer, aligning with the definition that one watt is the power consumed by a device when 1 A of current flows through it at a potential difference of 1 V. Science, class X (NCERT 2025 ed.) > Chapter 11: Electricity > What you have learnt

UPSC often designs distractors based on common calculation slips. Option (C) 1320 W and Option (D) 13200 W are classic traps for students who forget the unit conversion and simply multiply 220 by 6 or 600. Option (B) 13.2 W is a decimal error designed to catch those who might misplace the decimal point during the division by 1000. Avoiding these traps requires you to be disciplined with your units—always convert to SI units (Amperes, Volts, Watts) before you begin your final calculation.