RRB JE Science Questions with Detailed Solutions

The correct answer is (A) 2.2 kWh

Explanation:
• The formula to calculate electrical energy consumed is: Energy (E) = Power (P) × Time (t).
• First, calculate Power. The formula for electric power is P = Voltage (V) × Current (I).
• Given: V = 220 Volts, I = 2 Amperes. So, P = 220 × 2 = 440 Watts.
• Next, convert power into Kilowatts (kW) since the answer requires kWh. 440 Watts = 0.44 kW.
• Finally, multiply by time. Time (t) = 5 hours. E = 0.44 kW × 5 h = 2.2 kWh.

Information Booster:
• Commercial Unit: The Kilowatt-hour (kWh) is the commercial unit of electrical energy. On electricity bills, "1 Unit" is exactly equal to 1 kWh.
• Joule Conversion: 1 kWh represents a massive amount of energy when converted to standard SI units: 1 kWh = 3.6 × 10^6 Joules (or 3,600,000 Joules).
• Power Rating: A device rated at 1000W (1 kW) operating continuously for 1 hour consumes 1 kWh of energy.

Additional Knowledge:
• Miscalculations often occur by failing to convert watts to kilowatts. Multiplying 440 Watts by 5 hours gives 2200 Watt-hours (Wh). Shifting the decimal three places for the "kilo" prefix gives the correct 2.2 kWh.

The correct answer is (D) It is equal to the sum of the currents through each resistor

Explanation:
• In a parallel circuit, the electrical current has multiple paths to flow through.
• According to Kirchhoff's Current Law, the total current entering a junction must equal the total current leaving it. Therefore, the total current (I) supplied by the voltage source equals the sum of the individual currents flowing through each parallel branch (I = I1 + I2 + I3).
• The potential difference (voltage), however, remains constant across all resistors in a parallel configuration.

Information Booster:
• Equivalent Resistance: The reciprocal of the equivalent resistance of a parallel combination is equal to the sum of the reciprocals of the individual resistances (1/Req = 1/R1 + 1/R2 + 1/R3). The equivalent resistance is always less than the smallest individual resistance in the network.
• Series Circuit Contrast: In a series circuit, the current remains constant throughout the circuit, while the voltage is divided among the resistors.
• Household Wiring: Domestic electrical appliances are connected in parallel so that each appliance receives the same voltage and can operate independently.

Additional Knowledge:
• Option A (Average of currents): Incorrect, as currents add up, they do not average out.
• Option B (Same as current through each): This describes a series circuit, not a parallel one.
• Option C (Equal to current of lowest resistance): The branch with the lowest resistance will carry the *highest* individual current, but it does not represent the *total* current of the circuit.

The correct answer is (C) The pendulum exhibits uniform motion

Explanation:
• Uniform motion occurs when an object travels in a straight line with a constant speed, meaning its velocity (and direction) never changes.
• A swinging pendulum constantly changes both its speed and its direction. It stops momentarily at its extreme positions (speed = zero) and moves fastest as it passes through the mean (central) position.
• Because the velocity is constantly changing, a pendulum absolutely cannot be described as exhibiting uniform motion.

Information Booster:
• Periodic Motion: A pendulum's motion is periodic because it repeats its path in equal intervals of time (its time period).
• Oscillatory Motion: It is a specific type of periodic motion involving a to-and-fro movement about a fixed mean position.
• Simple Harmonic Motion (SHM): For small angles of swing, a pendulum executes SHM, where the restoring force is directly proportional to its displacement from the mean position.

Additional Knowledge:
• Accelerated motion (Option A): Since the velocity is constantly changing, the pendulum is continuously accelerating (specifically, angular acceleration driven by gravity). Thus, this statement is true.
• Periodic motion (Option B): True, it swings back and forth in a set time interval.
• Non-uniform motion (Option D): True, because its speed fluctuates between zero at the ends and maximum at the center.

The correct answer is (A) 2 m/s²

Explanation:
• Acceleration is defined as the rate of change of velocity with respect to time.
• The mathematical formula for uniform acceleration is: a = (v - u) / t
• Where:
- Initial velocity (u) = 6 m/s
- Final velocity (v) = 12 m/s
- Time taken (t) = 3 seconds
• Plugging in the values: a = (12 - 6) / 3
• a = 6 / 3 = 2 m/s².

Information Booster:
• SI Unit: The standard international unit for acceleration is meters per second squared (m/s²).
• Vector Quantity: Acceleration is a vector quantity, meaning it has both magnitude and direction. Since the cyclist is speeding up in a straight line, the acceleration is positive and acts in the direction of motion.
• Deceleration: If the cyclist were braking and slowing down from 12 m/s to 6 m/s, the acceleration would be -2 m/s², commonly called deceleration or retardation.

Additional Knowledge:
• Mathematical Trap (Option C): 6 m/s is the total change in velocity (v - u), but failing to divide by time (3s) results in this incorrect answer.
• Mathematical Trap (Option D): 0.5 is obtained by inverting the formula to time / change in velocity (3 / 6), which violates the physical definition of acceleration.

The correct answer is (D) Because the switch completes the circuit, allowing current to flow

Explanation:
• An electric circuit must be completely closed (unbroken) for electrical current to flow from the power source through the load (the bulb) and back to the source.
• A switch acts as a mechanical bridge in the circuit. When turned 'on', the bridge closes, allowing electrons to move continuously.
• This flow of electrons (current) passes through the bulb's filament, generating heat and light due to resistance.

Information Booster:
• Open vs. Closed Circuit: An 'open circuit' means there is a break in the path (switch is off), halting current. A 'closed circuit' has an uninterrupted path.
• Electrical Resistance: The filament in the bulb provides high resistance, causing electrical energy to be converted into light and heat energy.

Additional Knowledge:
• Option A & C: While a functional bulb and a charged battery are prerequisites, they cannot cause the bulb to glow unless the electrical pathway is complete.
• Option B: Being in series is a wiring configuration, but without a closed switch, no current will flow through the series circuit.

The correct answer is (D) A constant inward (centripetal) force is needed on the riders to keep them in uniform circular motion

Explanation:
• According to Newton's First Law of Motion, an object will move in a straight line unless acted upon by an external force.
• To make an object (or a rider) move in a circular path, its direction must be continuously changed. This requires a continuous force directed toward the center of the circle.
• This center-seeking force is called Centripetal Force. In an amusement park ride, the seats, harnesses, or friction must supply this inward force to prevent the riders from flying off in a straight tangential line.

Information Booster:
• Centripetal Acceleration: The formula for centripetal force is F = mv²/r (where m is mass, v is velocity, and r is the radius). Because velocity is a vector (having magnitude and direction), uniform circular motion is constantly accelerated motion, even if speed is constant.
• Centrifugal 'Force': Riders feel like they are being pushed "outward." This is not a real force, but rather the inertia of their bodies trying to travel in a straight line while the seat pulls them in.

Additional Knowledge:
• Outward force (Option C): Engineering designs must provide inward (centripetal) force, not outward force.
• Gravity alone (Option B): Gravity acts downwards. It cannot provide the horizontal centripetal force needed for flat circular motion.

The correct answer is (D) Negative

Explanation:
• According to the New Cartesian Sign Convention used in optics, the Pole (P) of the mirror is taken as the origin (0,0) of the coordinate system.
• The principal axis is taken as the X-axis.
• Any distances measured upward and perpendicular to the principal axis (along the +Y axis) are considered positive (typically the height of the object).
• Any distances measured downward and perpendicular to the principal axis (along the -Y axis) are considered negative. Therefore, a point located below the axis has a negative sign.

Information Booster:
• Real vs. Virtual Images: Because real images in spherical mirrors are always inverted (formed below the principal axis), their image height (h') is always negative. Virtual images are erect (above the axis), so their height is positive.
• Distance direction: Distances measured in the direction of incident light are positive; those measured against the incident light (to the left of the mirror) are negative.

Additional Knowledge:
• Depends on mirror type (Option C): The sign convention is a universal mathematical coordinate system applied uniformly to both concave and convex mirrors, as well as lenses. It does not change based on mirror type.

The correct answer is (A) Controls the size of the pupil

Explanation:
• The iris is the prominent, pigmented (colored) muscular ring located just behind the cornea of the eye.
• Situated in the very center of the iris is an adjustable opening called the pupil.
• The primary anatomical function of the iris is to involuntarily expand and contract its radial and circular muscles, thereby dynamically adjusting the diameter of the pupil to regulate the exact amount of light permitted to enter the deeper chambers of the eye.

Information Booster:
• In bright, harsh light, the iris contracts, shrinking the pupil to protect the sensitive retina from damage. In dim light, it dilates (widens) the pupil to maximize light capture and improve vision.
• The specific concentration and distribution of the pigment melanin within the iris dictate a person's unique eye color.

Additional Knowledge:
• Option B (Refracts light): The cornea and the crystalline lens are responsible for bending (refracting) incoming light.
• Option C (Transmits signals): The optic nerve acts as the biological cable that transmits visual electrical signals from the retina to the brain's occipital lobe.
• Option D (Forms the image): The lens dynamically focuses light to project a sharp, inverted image onto the retina.

The correct answer is (B) Gas particles move randomly and strike the walls

Explanation:
• According to the Kinetic Molecular Theory of Gases, gas molecules are in a state of continuous, random, and high-speed motion in all possible directions.
• As these particles zip around, they frequently collide with each other and, crucially, with the interior walls of their container.
• Every time a gas particle impacts the wall, it exerts a tiny force. The sum total of all these billions of microscopic collisions over the surface area of the container generates the macroscopic, measurable phenomenon we call gas pressure (Pressure = Force/Area).

Information Booster:
• Elastic Collisions: The collisions made by gas particles are perfectly elastic. This means there is no net loss of kinetic energy during the impact; they bounce off with the same energy they struck with.
• Effect of Temperature: Heating a gas increases the kinetic energy of its particles. They move faster, strike the walls harder and more frequently, resulting in an increase in pressure (Gay-Lussac's Law).
• Effect of Volume: Shrinking the container forces the same number of particles into a smaller space, increasing the frequency of wall collisions and driving up pressure (Boyle's Law).

Additional Knowledge:
• Attract the walls (Option A): Intermolecular forces between ideal gas molecules and the wall are considered negligible. Pressure is due to kinetic impact, not static attraction.
• Fixed inside container (Option C): Only molecules in a solid state stay in relatively fixed positions.
• Low kinetic energy (Option D): Gases possess the *highest* kinetic energy compared to liquids and solids.

The correct answer is (C) Water

Explanation:
• The Law of Constant Proportions (or Definite Proportions), proposed by Joseph Proust, states that in a pure chemical compound, elements are always present in definite proportions by mass.
• The chemical formula for water is H2O. It contains 2 atoms of Hydrogen and 1 atom of Oxygen.
• Mass calculation: The atomic mass of Hydrogen (H) is roughly 1 u, so 2 atoms = 2 u. The atomic mass of Oxygen (O) is 16 u.
• The ratio by mass is 2:16, which mathematically simplifies to exactly 1:8. This means every 9 grams of pure water strictly contains 1 gram of hydrogen and 8 grams of oxygen.

Information Booster:
• Hydrogen Peroxide (Option D): The formula is H2O2. Mass of hydrogen = 2u, mass of oxygen = 32u. The mass ratio here is 2:32, which simplifies to 1:16.
• Ammonia (Option B): Formula is NH3. It contains Nitrogen (14u) and Hydrogen (3u). Its fixed mass ratio is 14:3.
• Methane (Option A): Formula is CH4. It contains Carbon (12u) and Hydrogen (4u). Its fixed mass ratio is 12:4, simplifying to 3:1.

Additional Knowledge:
• John Dalton's Theory: The law of constant proportions provided vital experimental backing for Dalton's Atomic Theory, specifically the postulate that "relative number and kinds of atoms are constant in a given compound."

The correct answer is (C) Surface area is increased

Explanation:
• Drying clothes is a real-world example of the physical process of evaporation.
• Evaporation is strictly a surface phenomenon; particles at the surface of a liquid absorb enough kinetic energy to break free from attractive forces and convert into a gas.
• When wet clothes are spread out flat, the exposed surface area of the water trapped in the fabric is drastically increased. A larger surface area allows more water molecules to escape into the air simultaneously, thus accelerating the drying rate.

Information Booster:
• Factors Affecting Evaporation: Evaporation rate depends on four major factors: surface area (increases rate), temperature (increases rate), wind speed (increases rate), and humidity (decreases rate).
• Cooling Effect: Evaporation inherently causes cooling. As high-energy water molecules escape into the vapor phase, the average kinetic energy (temperature) of the remaining liquid drops.
• Sublimation: A related concept where a solid turns directly into a gas without becoming a liquid (e.g., dry ice or naphthalene balls disappearing over time).

Additional Knowledge:
• Sunlight (Option B): While sunlight provides heat which aids evaporation, spreading clothes out accelerates drying even in a dark room or at night solely because of the increased surface area.
• Water boils off (Option D): Boiling is a bulk phenomenon that happens at a specific temperature (100°C for water). Clothes dry due to evaporation, which happens at any temperature below the boiling point.

The correct answer is (D) Liquids have fixed volume but no fixed shape

Explanation:
• The defining macroscopic characteristic of the liquid state is its fluidity and adaptability.
• The intermolecular forces in a liquid are strong enough to hold the particles close together, granting the liquid a definite, fixed volume. However, these forces are not strong enough to lock the particles into a rigid crystal lattice like in a solid.
• Because the particles can slide past one another freely, a liquid lacks a fixed shape; it flows and automatically assumes the exact shape of the container it is poured into.

Information Booster:
• Solids: Possess strong intermolecular forces locking particles in place, granting them both a rigid, fixed shape and a fixed volume.
• Gases: Possess negligible intermolecular forces, granting them neither a fixed shape nor a fixed volume (they expand to fill their container completely).
• Intermolecular Space: The space between particles in a liquid is slightly greater than in solids, allowing for movement, but vastly smaller than in gases.

Additional Knowledge:
• Liquids cannot diffuse (Option A): Incorrect. Liquids can easily diffuse into other liquids (like ink mixing into water), though slower than gases.
• Completely incompressible (Option B): While liquids are highly incompressible compared to gases, they are not "completely" incompressible. Under extreme pressure, they compress slightly.
• Fixed shape, no fixed volume (Option C): This is a physical impossibility. Nothing holds a shape while simultaneously changing its mass/volume capacity.

The correct answer is (B) Magnesium loses electrons to form Mg3+ ions.

Explanation:
• Magnesium (Mg) is an alkaline earth metal located in Group 2 of the periodic table, possessing an atomic number of 12. Its electronic configuration is 2, 8, 2.
• To achieve a stable noble gas configuration (like Neon), an atom of magnesium loses its 2 valence electrons, forming a dipositive magnesium ion (Mg2+).
• Magnesium does not possess the ionization energy or necessary valence shell structure to lose three electrons and form an Mg3+ ion. Thus, this statement is chemically impossible and represents the correct answer for the 'NOT correct' option.

Information Booster:
• Ionic Bonding: The chemical reaction (2Mg + O2 → 2MgO) represents the formation of an ionic compound. The electrostatic attraction between oppositely charged Mg2+ and O2- ions forms the strong ionic bond.
• Oxygen's Role: Oxygen (atomic number 8, configuration 2, 6) requires 2 electrons to complete its octet. It gains the exact 2 electrons lost by a magnesium atom, forming an oxide ion (O2-).
• Redox Reaction: This is a classic redox reaction where Magnesium is oxidized (loses electrons) and Oxygen is reduced (gains electrons).

Additional Knowledge:
• Options A, C, and D: All these statements are perfectly accurate scientific descriptions of the formation of Magnesium Oxide, making them the "correct" statements.

The correct answer is (B) Nitrogen gas inhibits oxidation by replacing oxygen.

Explanation:
• Foods that contain fats and oils, like potato chips, undergo chemical oxidation when exposed to the oxygen present in standard atmospheric air.
• This oxidation process is called rancidity, which ruins the taste, changes the color, and causes a foul smell.
• To prevent this, manufacturers flush the bags with nitrogen gas. Nitrogen is an inert (unreactive) gas at normal temperatures. By displacing the oxygen, it entirely starves the chemical oxidation process, preserving the food's freshness and shelf life.

Information Booster:
• Cushioning Effect: Besides preventing rancidity, filling the bag with nitrogen gas acts as a physical "pillow," protecting fragile chips from being crushed during shipping and handling.
• Rancidity Prevention: Other methods to prevent rancidity include using airtight containers, refrigeration, keeping food away from direct light, and adding chemical antioxidants (like BHA and BHT).
• Nitrogen properties: Nitrogen makes up about 78% of the Earth's atmosphere. It requires extreme conditions (like high heat or pressure) to react, due to its very strong triple covalent bond (N≡N).

Additional Knowledge:
• Makes chips crispier/Enhances taste (Options A & D): Nitrogen is odorless, colorless, and tasteless. It does not chemically interact with the food to alter flavor or texture; it merely preserves the original state.
• Accelerates oxidation (Option C): This is the exact opposite of what nitrogen does. Oxygen accelerates oxidation.

The correct answer is (D) Chlorine

Explanation:
• According to Graham's Law of Effusion and Diffusion, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass (Rate ∝ 1/√M).
• This means heavier molecules travel and diffuse slower than lighter ones under the same conditions of temperature and pressure.
• Calculating the molar masses: Hydrogen (H2) = 2 g/mol, Ammonia (NH3) = 17 g/mol, Oxygen (O2) = 32 g/mol, and Chlorine (Cl2) = 71 g/mol.
• Since Chlorine has the highest molar mass by a significant margin, its molecules are the heaviest and will diffuse the slowest.

Information Booster:
• Hydrogen's Speed: Because hydrogen is the lightest of all gases, it diffuses the fastest among the given options.
• Kinetic Energy Constant: At a given temperature, all gas molecules possess the same average kinetic energy (KE = 1/2 mv²). For KE to remain constant, a higher mass (m) demands a lower velocity (v).
• Practical Application: The difference in diffusion rates is utilized in uranium enrichment, separating the lighter U-235 isotope from the heavier U-238.

Additional Knowledge:
• If asked to arrange these gases in increasing order of diffusion rate (slowest to fastest), the sequence would be: Chlorine < Oxygen < Ammonia < Hydrogen.