JEE · Physics doubts
235 doubts found
A ball is projected at an angle. How should I find its velocity at a later time without mixing up speed and direction?
Think of the launch velocity as two teammates.
Why does a metal chair feel colder than a wooden chair when both have been in the same room?
They begin at nearly the same room temperature.
Hydrogen and oxygen are at the same temperature. Which molecules move faster, and does that mean they have more kinetic energy?
At the same absolute temperature both gases have the same average translational kinetic energy per molecule, 3k_BT/2.
A dielectric slab is inserted into a charged capacitor. How do I know whether stored energy rises or falls?
Do not start from a memorised result; identify the connection.
Why is soft iron used in a transformer core but steel is used for a permanent magnet?
A transformer reverses magnetisation every AC cycle, so its core should magnetise and demagnetise easily with little energy loss.
How can I quickly decide the longest-wavelength line in a hydrogen spectral series?
Within one named series, the final level n_f is fixed.
A sample has passed through three half-lives. Why is the amount left one-eighth, not zero?
A half-life removes the same fraction, not the same amount.
My screw gauge shows a positive reading when fully closed. What correction should I apply?
That nonzero closed reading is a positive zero error.
How do I decide whether a pipe question uses all harmonics or only odd harmonics?
Look at the boundary conditions, not the drawing style.
A charged particle enters a uniform magnetic field at an angle. How do I find its path?
Split velocity into v_parallel along B and v_perp across B.
Two wires of the same material carry the same load. How can I tell which stretches more?
Start from Delta L = FL/(AY).
Why can radio waves and X-rays travel at the same speed but behave so differently?
In vacuum both are electromagnetic waves, so Maxwell's equations give the same speed c.
What does “Dimensional analysis” actually mean in Units, Dimensions & Measurement, and why does it matter for JEE?
expressing every quantity in powers of mass (M), length (L) and time (T) so you can check or derive relations.
What does “Instantaneous velocity” actually mean in Kinematics, and why does it matter for JEE?
the velocity at a single instant — the slope of the position–time graph at that point.
What does “Inertia” actually mean in Laws of Motion, and why does it matter for JEE?
the tendency of a body to resist any change in its state of rest or uniform motion (Newton's first law).
What does “Work” actually mean in Work, Energy & Power, and why does it matter for JEE?
energy transferred by a force: W = F·d·cosθ, where θ is the angle between force and displacement.
What does “Centre of mass” actually mean in Rotational Motion, and why does it matter for JEE?
the single point where the whole mass of a system can be treated as concentrated.
What does “Universal law of gravitation” actually mean in Gravitation, and why does it matter for JEE?
every two masses attract with F = G·m₁m₂/r² along the line joining them.
What does “First law of thermodynamics” actually mean in Thermodynamics & Kinetic Theory, and why does it matter for JEE?
energy conservation for gases: ΔU = Q − W, where W is work done BY the gas.
What does “Simple harmonic motion” actually mean in Oscillations & Waves, and why does it matter for JEE?
oscillation where the restoring force is proportional to and opposite the displacement, F = −kx.
What does “Coulomb's law” actually mean in Electrostatics, and why does it matter for JEE?
the force between two point charges, F = kq₁q₂/r², along the line joining them.
What does “Ohm's law” actually mean in Current Electricity, and why does it matter for JEE?
for many conductors V = IR at constant temperature.
What does “Magnetic force on a moving charge” actually mean in Magnetism & Electromagnetic Induction, and why does it matter for JEE?
F = qvB·sinθ, always perpendicular to the velocity, so it changes direction but not speed.
What does “Photoelectric effect” actually mean in Modern Physics, and why does it matter for JEE?
light ejects electrons from a metal only if its frequency exceeds a threshold, regardless of intensity.