JEE · Physics doubts
235 doubts found
What does “Newton's second law” actually mean in Laws of Motion, and why does it matter for JEE?
the net force equals mass times acceleration, F=ma, in the direction of the net force.
What does “Work–energy theorem” actually mean in Work, Energy & Power, and why does it matter for JEE?
the net work done on a body equals its change in kinetic energy.
What does “Torque” actually mean in Rotational Motion, and why does it matter for JEE?
the turning effect of a force, τ = r×F = rF·sinθ — the rotational analogue of force.
What does “Acceleration due to gravity g” actually mean in Gravitation, and why does it matter for JEE?
g = GM/R² at a planet's surface; it decreases with both altitude and depth.
What does “Internal energy” actually mean in Thermodynamics & Kinetic Theory, and why does it matter for JEE?
the total microscopic energy of a system; for an ideal gas it depends only on temperature.
What does “Time period of a pendulum” actually mean in Oscillations & Waves, and why does it matter for JEE?
T = 2π√(L/g) — it depends on length and gravity, not on mass or (small) amplitude.
What does “Electric field” actually mean in Electrostatics, and why does it matter for JEE?
the force per unit positive charge, E = F/q; field lines point away from positive charges.
What does “Resistance and resistivity” actually mean in Current Electricity, and why does it matter for JEE?
R = ρL/A: resistance grows with length and shrinks with cross-section; resistivity ρ is the material's own property.
What does “Faraday's law of induction” actually mean in Magnetism & Electromagnetic Induction, and why does it matter for JEE?
a changing magnetic flux induces an EMF, EMF = −dΦ/dt.
What does “Photon energy” actually mean in Modern Physics, and why does it matter for JEE?
light comes in packets of energy E = hf = hc/λ.
A steel ship floats but a steel needle sinks — both are steel, so how?
Floating depends on average density, not the material alone.
How does an optical fibre carry light around bends without leaking it out?
The light hits the fibre's inner wall at an angle greater than the critical angle, so it undergoes total internal reflection instead of escaping.
If I do the double-slit experiment underwater, do the fringes change?
Yes.
How does tuning a radio pick out just one station from all the signals in the air?
The radio's LCR circuit is set to resonance by adjusting C (or L).
Why does an LED light up but only when connected the right way round?
An LED is a p–n junction that emits light when forward-biased: electrons and holes recombine across the junction and release the energy as photons, with the colour set by the semiconductor's band gap.
I keep getting “Resolving a vector into components” questions wrong in Motion in a Plane. What's the trap?
writing both components positive even when the arrow points left or downward.
I keep getting “Heat versus temperature” questions wrong in Thermal Properties of Matter. What's the trap?
using heat and temperature as if they were the same physical quantity.
I keep getting “Microscopic origin of gas pressure” questions wrong in Kinetic Theory of Gases. What's the trap?
imagining pressure as a stored substance inside the gas.
I keep getting “Potential as energy per charge” questions wrong in Electrostatic Potential & Capacitance. What's the trap?
adding potentials as vectors or dropping the sign of a negative source charge.
I keep getting “Magnetic dipole moment” questions wrong in Magnetism & Matter. What's the trap?
reversing the magnetic moment direction because external field lines run north to south.
I keep getting “Rutherford scattering and nuclear atom” questions wrong in Atoms. What's the trap?
claiming Rutherford's model explains atomic stability or line spectra.
I keep getting “Nuclear size and density” questions wrong in Nuclei. What's the trap?
thinking a nucleus with twice the nucleons has twice the radius.
I keep getting “Least count and instrument reading” questions wrong in Experimental Skills. What's the trap?
adding a positive zero error instead of subtracting it from the observed reading.
What does “Superposition” actually mean in Waves & Sound, and why does it matter for JEE?
When waves overlap, their displacements add algebraically at each point. Constructive and destructive interference are consequences of this linear addition, not permanent destruction of energy.