Doubtify – JEE Ki Taiyaari, Simple Bhasha Mein

  ⚙️ de-Broglie Wavelength vs Time – Charged Sheet Problem | Doubtify JEE 📌 Question: An electron is released from rest near an infinite non-conducting sheet of uniform negative charge density (−σ) . The rate of change of de-Broglie wavelength associated with the electron varies inversely as the nᵗʰ power of time . 👉 Find the numerical value of n . 📸 Question Image:

  ⚛️ Bohr’s Atomic Model in 59 Seconds | Doubtify JEE Shorts 📌 Question: What explains the fixed energy levels of electrons in an atom and why do atoms emit light? Bohr’s Atomic Model gives us the first quantum explanation of atomic structure! 📸 Image: 📕 Concept & Formula: Stationary Orbits: Electrons revolve around the nucleus in fixed circular paths called orbits. No radiation is emitted in these orbits. Quantized Angular Momentum: m v r = n h 2 π mvr = \frac{nh}{2\pi} Where: m = mass , v = velocity , r = radius , n = orbit number (1, 2, 3…) , h = Planck's constant Energy Transitions: When an electron jumps between orbits: Δ E = E 2 − E 1 = h ν \Delta E = E_2 - E_1 = h\nu Energy is absorbed or emitted in the form of light/spectral lines Energy of Orbit: E n ∝ − 1 n 2 E_n \propto -\frac{1}{n^2} → Lower orbit → More negative energy → More stability 🔢 Example: An electron jumps from n = 3 to n = 2 , and emits energy. This corre...

  🔬 Bulk Modulus Concept – Find Initial Volume | Doubtify JEE 📌 Question: A sample of a liquid is kept at 1 atm . It is compressed to 5 atm , which leads to a change of volume of 0.8 cm³ . If the bulk modulus of the liquid is 2 GPa , the initial volume of the liquid was ______ litre . (Take 1 atm = 10⁵ Pa) 🖼️ Question Image:

  ⚙️ A Thin Rotating Disk – Torque & Diameter Problem | Doubtify JEE 📌 Question: A thin solid disk of mass 1 kg is rotating along its diameter axis at the speed of 1800 rpm . An external torque of 25π Nm is applied for 40 seconds , increasing the speed to 2100 rpm . Find the diameter of the disk. 📸 Question Image:

🚆 Train Crossing Concept in 59 Seconds | Doubtify JEE Shorts 📌 Question: Ever wondered how long one train takes to pass another or a platform? The Train Crossing Concept helps solve such questions in seconds using speed-distance fundamentals.

  🧲 Magnetic Field at Centre of Square Loop with Unequal Resistance | JEE Concept 📌 Question: Figure shows a current-carrying square loop ABCD of edge length ‘a’ lying in a plane. If the resistance of the path ABC is r and the path ADC is 2r, find the magnitude of the resultant magnetic field at the centre of the square loop. 🖼️ Question Image: 💡 Concept Used: Unequal resistance in different paths means current divides unequally in branches. Use current division rule and Biot-Savart Law or symmetry to find net magnetic field at center. Magnetic field due to a straight segment at center of square is: B = μ 0 I 4 π R ⋅ 2 sin ⁡ ( θ / 2 ) or use full loop formula if current is symmetrical. 🧠 Approach: Let total current be I. Since resistances are in parallel: Current in ABC: I 1 = 2 3 I I_1 = \frac{2}{3}I I 1 ​ = 3 2 ​ I Current in ADC: I 2 = 1 3 I I_2 = \frac{1}{3}I I 2 ​ = 3 1 ​ I Both paths form half-loops and contribute to B at center ....

  📌 Newton’s Law of Cooling – Explained for JEE | Formula + Graph + Example 🖼️ Image:

⚡ Gauss’s Law – Flux Through a Cube | JEE Physics | Doubtify 📌 Question: An infinitely long wire has uniform linear charge density λ = 2 nC/m. The net flux through a Gaussian cube of side length √3 cm, if the wire passes through any two corners of the cube, that are maximally displaced from each other, would be x Nm²C⁻¹, where x is: 🖼️ Question Image:

  🧪 Young’s Modulus from Wire Extension | JEE Physics | Doubtify 📌 Question: A 3 m long wire of radius 3 mm shows an extension of 0.1 mm when loaded vertically by a mass of 50 kg in an experiment to determine Young's modulus. The value of Young’s modulus of the wire as per this experiment is P × 10¹¹ N/m² , where the value of P is: (Take g = 3π m/s²) 🖼️ Question Image:

  🌊 Superposition of Two Harmonic Waves – Find Resultant Amplitude & Phase | JEE Physics | Doubtify 📌 Question: The amplitude and phase of a wave that is formed by the superposition of two harmonic travelling waves, y₁(x, t) = 4sin(kx − ωt) and y₂(x, t) = 2sin(kx − ωt + 2π/3) are: 🖼️ Question Image:

  🌈 YDSE with Red and Green Filters – Interference of Light | JEE Physics | Doubtify 📌 Question: In a Young's double slit experiment (YDSE) , the source is white light . One of the slits is covered by a red filter and the other by a green filter . What will be observed? Options: There shall be an interference pattern for red distinct from that for green. There shall be an interference pattern, where each fringe’s pattern center is green and outer edges is red. There shall be no interference fringes . There shall be alternate interference fringes of red and green. 🖼️ Question Image:

  🔥 Adiabatic Compression of Monatomic Gas | Thermodynamics | JEE Physics | Doubtify 📌 Question: A monatomic gas having γ = 5 3 \gamma = \frac{5}{3} ​ is stored in a thermally insulated container , and the gas is suddenly compressed to 1 8 th \frac{1}{8}^\text{th} of its initial volume. What is the ratio of final pressure to initial pressure ? 🖼️ Question Image:

  🔍 Focal Length of a Lens with Liquid Combination – Optics | JEE Physics | Doubtify JEE 📌 Question: A concave-convex lens has a refractive index of 1.5 , and the radii of curvature of its surfaces are 30 cm and 20 cm , respectively. The concave surface is upwards and is filled with a liquid of refractive index 1.3 . What will be the focal length of the liquid-glass combination ? 🖼️ Question Image:

  ⚡ Surface Charge Distribution on Irregular Conductor – Electrostatics | JEE Physics | Doubtify JEE 📌 Question: Electric charge is transferred to an irregular metallic disk as shown in the figure. If the surface charge densities at four points are σ₁, σ₂, σ₃, and σ₄ , then which of the following options is correct ?

  ⚡ Surface Charge Density After Contact – Electrostatics | JEE Physics | Doubtify JEE 📌 Question: Two metal spheres of radius R and 3R have the same surface charge density σ . They are brought in contact and then separated . After separation, let the surface charge densities be σ₁ (for smaller sphere) and σ₂ (for larger sphere). What is the ratio σ₁ / σ₂ ? 🖼️ Question Image:

  🏐 Momentum vs Height Graph – Vertical Motion | JEE Physics | Doubtify JEE 📌 Question: A ball is thrown vertically upwards (take +z-axis as upwards) from the ground. 👉 Which of the following momentum-height (p–h) graphs correctly represents the motion? This is a concept-based question that checks your understanding of vector momentum and energy conservation . 🖼️ Question Image:

  🚀 Uniform Acceleration Motion Graphs | Kinematics | Doubtify JEE 📌 Question: A particle starts from origin O from rest and moves with a uniform acceleration along the positive x-axis . 👉 Identify all the graphs that correctly represent the motion qualitatively: a = acceleration v = velocity x = displacement t = time This is a conceptual question testing your understanding of motion graphs! 🖼️ Question Image:

  🧭 Relative Motion of Ships – Minimum Distance Concept | JEE Physics | Doubtify JEE 📌 Question: Ship A is sailing towards north-east with velocity v = 30 î + 50 ĵ km/h , where î points east and ĵ points north . Ship B is at a distance of 80 km east and 150 km north of Ship A and is sailing west at 10 km/h . 🔍 Find the time at which Ship A is at minimum distance from Ship B. 🖼️ Question Image:

  🚆 Train Crossing Time Ratio – Relative Motion | Speed, Distance & Time | JEE Physics | Doubtify JEE 💡 Question: A passenger train of length 60 m travels at a speed of 80 km/hr . Another freight train of length 120 m travels at a speed of 30 km/hr . Find the ratio of times taken by the passenger train to completely cross the freight train when: (i) They are moving in the same direction , and (ii) They are moving in opposite directions . 🖼️ Question Image:

  ⚖️ Tension in Strings in Orbiting Satellite – Electrostatics in Space | JEE Physics | Doubtify JEE 💡 Question: Two identical small bodies each of mass = m and charge = q are suspended from two strings of length = l from a fixed point. The system is taken inside an orbiting artificial satellite . 👉 Question: What is the tension in the strings inside the satellite? 🖼️ Question Image:

  🚢 Minimum Distance Between Two Moving Ships – Relative Motion | JEE Physics | Doubtify JEE 💡 Question: Ship A is sailing towards the north-east with velocity v = 30 î + 50 ĵ km/h , where î points east and ĵ points north . Ship B is located at a distance of 80 km east and 150 km north of ship A and is sailing towards the west at 10 km/h . Question: After how much time will Ship A be at a minimum distance from Ship B ? 🖼️ Question Image:

  🔢 Matrices & Determinants | JEE Maths | Doubtify JEE 📌 Question: The number of all 3 × 3 matrices A , with entries from the set {-1, 0, 1} , such that the sum of the diagonal elements of AAᵀ is 3 , is: 🖼️ Question Image:

🌀 Centroid of a Circular Plate with a Hole | System of Particles | JEE Physics | Doubtify JEE ❓ Question: A circular hole of radius (a/2) is cut out of a circular disc of radius ‘a’ , as shown in the figure. What will be the position of the centroid of the remaining circular portion with respect to point ‘O’ ? 🖼️ Question Image: 🧠 Solution Image: ✍️ Detailed Explanation: To solve this problem, we apply the concept of mass moment (or area moment) . When a portion is cut out from a symmetric object, we use the principle of superposition — treat the hole as a negative mass or negative area. Let’s break it down: 🧮 Step 1: Assume mass per unit area = σ Area of full disc = πa² Area of hole = π(a/2)² = (πa²)/4 Remaining area = πa² – (πa²)/4 = (3πa²)/4 🧮 Step 2: Find centroid of each part Full disc’s centroid = At origin (O) Hole’s centroid is at a distance of a/2 from O along the radius. 🧮 Step 3: Apply formula for center of mass (COM): Let x be the position of the c...

  📈 Velocity-Time Graph Problem | Motion in One Dimension | Physics | Doubtify JEE 💡 Question: The velocity (v) and time (t) graph of a body in a straight-line motion is shown in the figure. Point S is at 4.333 sec. The total distance covered by the body in 6 seconds is? 🖼️ Question Image: 🧠 Solution Image:

  🔥 Balance Redox Reaction Using Oxidation Number Method | JEE Chemistry | Doubtify ⚛️ Question : Balance the following equation by the oxidation number method: K₂Cr₂O₇ + HCl → KCl + CrCl₃ + H₂O + Cl₂

  📈 Acceleration as a Function of Distance – Motion in One Dimension | JEE Physics | Doubtify JEE 🧮 Question: The distance x covered by a particle in one-dimensional motion varies with time t as: x² = at² + 2bt + c . If the acceleration of the particle depends on x as x⁻ⁿ , where n is an integer, what is the value of n ? 🖼️ Question Image: ✅ Detailed Concept & Solution: This is a JEE-level conceptual problem involving the relation between distance, velocity, and acceleration — with a twist of functional dependence. We are given: x² = at² + 2bt + c — (1) To find acceleration and its dependency on x , let’s differentiate both sides of equation (1): Differentiate w.r.t. time t : 2x(dx/dt) = 2at + 2b ⇒ v = dx/dt = (at + b)/x — (2) Now differentiate velocity v w.r.t time to find acceleration: a = dv/dt = d/dt[(at + b)/x] Use quotient rule: a = [a·x - (at + b)(dx/dt)] / x² Plug dx/dt = (at + b)/x from (2): a = [a·x - (at + b)·((at + b)/x)] / x² ⇒ ...

  🧲 The Sum of Two Forces P and Q is R | Angle Between 2P and Q | JEE Physics Doubt Solved

  🚴 Velocity-Displacement Graph of a Bicycle | Find the Acceleration-Displacement Graph | Motion in 1D – Doubtify JEE 📌 Question : The velocity-displacement graph describing the motion of a bicycle is shown in the figure. What will the corresponding acceleration-displacement graph look like? 🎯 Concept Tested: This question tests your graphical understanding of motion — a frequently asked and often misunderstood area in JEE Mains and Advanced . It focuses on converting one type of graph (v-s) into another (a-s) using calculus concepts. 🔍 How to Approach: If you're given a velocity-displacement (v-s) graph , and you need to find the acceleration-displacement (a-s) graph , you must apply: a = v ⋅ d v d s a = v \cdot \frac{dv}{ds} This is derived from the chain rule: a = d v d t = d v d s ⋅ d s d t = v ⋅ d v d s a = \frac{dv}{dt} = \frac{dv}{ds} \cdot \frac{ds}{dt} = v \cdot \frac{dv}{ds} Depending on the slope of the v-s graph: If velocity increases with displaceme...

  ⚗️ Balance the Equation: Mg + HNO₃ ⟶ Mg(NO₃)₂ + N₂O + H₂O ❓ Question: Balance the following chemical equation: Mg + HNO₃ ⟶ Mg(NO₃)₂ + N₂O + H₂O This question appears simple at first glance, but it actually requires deep understanding of redox reactions and balancing complex equations . 🎯 Key Concepts Tested: Oxidation and reduction processes Balancing redox equations using ion-electron method or oxidation number method Conservation of mass and conservation of charge Identifying oxidizing and reducing agents Understanding product formation in acidic medium (presence of HNO₃) 🧠 Step-by-Step Solution Strategy: Step 1: Identify oxidation states Let’s determine oxidation states of all elements in reactants and products: Mg (0) → Mg²⁺ in Mg(NO₃)₂ → Oxidation (Mg is losing electrons) N in HNO₃: +5 N in N₂O: +1 → Reduction (N is gaining electrons) Step 2: Write unbalanced half-reactions Oxidation (Mg): Mg → Mg²⁺ + 2e⁻ Reduction (N): 2...

🔥 Combustion Analysis – Percentage of Oxygen in an Organic Compound | JEE Chemistry | Doubtify JEE

  ⏱️ Acceleration-Time Graph – Find Time When Initial Velocity is Regained | JEE Physics | Doubtify JEE ❓ Question: The acceleration-time graph of a particle is as shown. At what time does the particle acquire its initial velocity again?

🔄 Circular Motion | Physics | Doubtify JEE

🧪 Question: The ratio of mass percent of C and H of an organic compound (CxHyOz) is 6:1. If one molecule of the above compound contains half as much oxygen as required to burn one molecule of compound CxHy completely to CO₂ and H₂O, then what is the empirical formula of the compound? 📌 Why This Question is Important: This question combines two highly testable concepts in JEE Chemistry – combustion analysis and empirical formula derivation . You’re not only working with mass percent ratio , but also applying oxygen balancing logic in a combustion reaction. Such questions are frequently seen in organic chemistry chapters , particularly under Basic Principles of Organic Chemistry and Stoichiometry . Mastering this helps you crack more complex analysis-based questions in JEE Mains and even Advanced . 💡 Concept Used: To find the empirical formula , we first convert the mass percent ratio of C and H into moles , assuming 100 g of substance. Then we apply a stoichiometric appr...