JEE doubts
700 doubts found
When I half-cover a tap or hose with my thumb, why does the water suddenly spray out much faster?
This is the equation of continuity, A1v1 = A2v2.
Why does a straw in a glass of water look bent or broken at the surface?
Light from the underwater part of the straw bends as it leaves the water (denser) into air (rarer), refracting away from the normal.
Why does a soap bubble or a film of oil on water show swirling rainbow colours?
Light reflects off both the top and bottom surfaces of the thin film.
Why is electricity sent across the country at very high voltage instead of the 230 V we use at home?
Power lost in the wires is I²R.
How does a diode turn AC into DC (rectification)?
A diode conducts only when forward-biased, so it passes current in just one direction.
For a swimmer crossing a flowing river, when do I aim upstream and when do I aim straight across?
First decide the target.
How do I solve a calorimetry question that includes ice and warm water?
Work in stages and ask what phases can coexist.
Why is gamma usually larger for a monatomic gas than for a diatomic gas?
Adding heat to a monatomic gas mainly feeds three translational modes, so f = 3 and gamma = 5/3.
Why is the equivalent capacitance in series smaller than the smallest individual capacitor?
In series, every capacitor carries the same charge Q, but the total voltage is the sum of each Q/C_i.
A compass is carried from the magnetic equator toward a magnetic pole. What changes?
At the magnetic equator, dip is approximately zero and Earth's field is nearly horizontal.
Why is hydrogen's ground-state energy negative, and how much energy ionises it?
We define a stationary electron and proton infinitely far apart as zero energy.
How do I decide whether a nuclear reaction releases energy using atomic masses?
Calculate Q = (total initial mass - total final mass)c^2.
How should I use a graph to find an experimental constant reliably?
First rearrange the physical law into y = mx + c and decide what each axis represents.
What does “Wave speed, frequency and wavelength” actually mean in Waves & Sound, and why does it matter for JEE?
A travelling wave carries energy while the medium's particles oscillate around equilibrium. Its speed is v = f lambda; the source fixes frequency, while the medium fixes wave speed, so wavelength adjusts when the medium changes.
What does “Magnetic force on a moving charge” actually mean in Moving Charges & Magnetism, and why does it matter for JEE?
A charge moving in magnetic field feels q(v x B). The force is perpendicular to both velocity and field, so a magnetic field changes direction of motion but does no work and cannot change speed by itself.
What does “Stress and strain” actually mean in Mechanical Properties of Solids, and why does it matter for JEE?
Stress measures internal restoring force per area; strain measures fractional deformation. Stress has pressure units, while strain is dimensionless.
What does “Changing fields sustain an electromagnetic wave” actually mean in Electromagnetic Waves, and why does it matter for JEE?
A changing electric field produces magnetic effects and a changing magnetic field produces an electric field. This coupled disturbance can travel through vacuum without charges continuously carrying it forward.
I keep getting “Dimensional analysis” questions wrong in Units, Dimensions & Measurement. What's the trap?
thinking a dimensionally-correct equation must be numerically correct — dimensionless constants (like 2 or π) are invisible to it.
I keep getting “Instantaneous velocity” questions wrong in Kinematics. What's the trap?
confusing average velocity (total displacement/total time) with instantaneous velocity.
I keep getting “Inertia” questions wrong in Laws of Motion. What's the trap?
thinking a moving body needs a continuous force to keep moving — it doesn't, unless friction acts.
I keep getting “Work” questions wrong in Work, Energy & Power. What's the trap?
forgetting the cosθ — a force perpendicular to the motion does zero work.
I keep getting “Centre of mass” questions wrong in Rotational Motion. What's the trap?
assuming it always lies inside the body (for a ring it's at the empty centre).
I keep getting “Universal law of gravitation” questions wrong in Gravitation. What's the trap?
forgetting the force depends on 1/r² — doubling the distance quarters the force.
I keep getting “First law of thermodynamics” questions wrong in Thermodynamics & Kinetic Theory. What's the trap?
getting the sign of W wrong (work done by vs on the gas).