JEE · Physics doubts
235 doubts found
I keep getting “Bohr model” questions wrong in Modern Physics. What's the trap?
using a positive energy for a bound electron — bound states are negative.
Why does water climb up a thin glass tube on its own, while mercury actually goes down?
It's capillarity, set by whether the liquid wets the glass.
Why can a magnifying glass make text look bigger, and where must I hold it?
A magnifying glass is a convex lens.
Why can't the headlights of two different cars ever produce an interference pattern?
Interference needs coherent sources — a constant phase relationship.
To dim a light on AC, why is a choke (coil) better than a resistor?
A resistor would limit the current but waste the extra energy as heat.
I keep getting “Forward & reverse bias” questions wrong in Semiconductor Electronics (Diodes). What's the trap?
thinking a diode conducts equally both ways.
What does “Projectile trajectory” actually mean in Motion in a Plane, and why does it matter for JEE?
Eliminating time from x = u cos(theta)t and y = u sin(theta)t - gt^2/2 gives a quadratic y(x), so the ideal projectile path is a parabola.
What does “Calorimetry and thermal equilibrium” actually mean in Thermal Properties of Matter, and why does it matter for JEE?
In an insulated mixture, energy lost by hotter bodies equals energy gained by colder bodies until they reach a common temperature. Use signed heat changes or a clear lost = gained equation, never both conventions at once.
What does “Root-mean-square speed” actually mean in Kinetic Theory of Gases, and why does it matter for JEE?
The rms speed is the square root of the mean of v^2, not the ordinary mean speed. It links the molecular picture to measurable pressure and temperature.
What does “Equipotential surfaces” actually mean in Electrostatic Potential & Capacitance, and why does it matter for JEE?
Moving a charge along an equipotential surface requires no work by or against the electrostatic field. Electric-field lines cross equipotentials at right angles.
What does “Earth as a magnetic field” actually mean in Magnetism & Matter, and why does it matter for JEE?
Locally, Earth's field is described by declination, dip (inclination), and horizontal component. Dip is measured from the horizontal plane, not from the vertical.
What does “Hydrogen energy levels” actually mean in Atoms, and why does it matter for JEE?
Hydrogen bound-state energies are negative because zero is assigned to a free electron at infinity. Larger n means less negative energy and weaker binding.
What does “Binding energy per nucleon” actually mean in Nuclei, and why does it matter for JEE?
Binding energy per nucleon measures average tightness of binding. Its curve rises toward iron/nickel and then slowly falls, explaining why both fusion of light nuclei and fission of heavy nuclei can release energy.
What does “Graph slope and intercept” actually mean in Experimental Skills, and why does it matter for JEE?
A graph can transform a law into y = mx + c. The slope must be calculated with two well-separated points on the best-fit line, not necessarily two raw data points, and it carries units.
I keep getting “Standing waves” questions wrong in Waves & Sound. What's the trap?
placing a node and adjacent antinode half a wavelength apart.
I keep getting “Biot-Savart law” questions wrong in Moving Charges & Magnetism. What's the trap?
treating magnetic-field contributions as scalars without direction.
I keep getting “Young, bulk and shear moduli” questions wrong in Mechanical Properties of Solids. What's the trap?
saying a larger modulus means the material deforms more easily.
I keep getting “Speed and field ratio” questions wrong in Electromagnetic Waves. What's the trap?
claiming gamma rays travel faster than radio waves in vacuum.
What does “Least count” actually mean in Units, Dimensions & Measurement, and why does it matter for JEE?
the smallest value an instrument can reliably read — e.g. 0.01 mm for a screw gauge.
What does “Projectile motion” actually mean in Kinematics, and why does it matter for JEE?
motion under gravity where horizontal and vertical motions are independent — horizontal a=0, vertical a=−g.
What does “Friction” actually mean in Laws of Motion, and why does it matter for JEE?
static friction adjusts itself up to a maximum μₛN to prevent motion; kinetic friction μₖN opposes sliding.
What does “Power” actually mean in Work, Energy & Power, and why does it matter for JEE?
the rate of doing work, P = W/t = F·v.
What does “Angular momentum” actually mean in Rotational Motion, and why does it matter for JEE?
L = Iω, conserved when the net external torque is zero.
What does “Escape velocity” actually mean in Gravitation, and why does it matter for JEE?
the minimum speed to leave a planet's gravity, v = √(2GM/R) ≈ 11.2 km/s for Earth.