JEE · Physics doubts
235 doubts found
What does “Forward & reverse bias” actually mean in Semiconductor Electronics (Diodes), and why does it matter for JEE?
forward bias (p to +, n to −) lowers the junction barrier and lets current flow; reverse bias widens the depletion region and blocks it.
I keep getting “Independent horizontal and vertical motion” questions wrong in Motion in a Plane. What's the trap?
using the resultant speed as the horizontal speed throughout the flight.
I keep getting “Thermal expansion” questions wrong in Thermal Properties of Matter. What's the trap?
treating a hole in a heated metal plate as shrinking.
I keep getting “Temperature and molecular kinetic energy” questions wrong in Kinetic Theory of Gases. What's the trap?
saying every molecule has the same speed at a given temperature.
I keep getting “Field-potential relationship” questions wrong in Electrostatic Potential & Capacitance. What's the trap?
assuming electric field points toward higher potential.
I keep getting “No isolated magnetic poles” questions wrong in Magnetism & Matter. What's the trap?
expecting a cut magnet to leave one north-only and one south-only piece.
I keep getting “Bohr's quantised orbits” questions wrong in Atoms. What's the trap?
imagining an electron radiating continuously while remaining in an allowed orbit.
I keep getting “Mass defect and binding energy” questions wrong in Nuclei. What's the trap?
subtracting masses in the wrong direction and obtaining negative binding energy.
I keep getting “Accuracy, precision and uncertainty” questions wrong in Experimental Skills. What's the trap?
calling tightly clustered but shifted readings accurate.
What does “Standing waves” actually mean in Waves & Sound, and why does it matter for JEE?
Two identical waves travelling oppositely form fixed nodes of zero displacement and antinodes of maximum displacement. A standing wave stores a spatial pattern and has no net energy transport along the medium.
What does “Biot-Savart law” actually mean in Moving Charges & Magnetism, and why does it matter for JEE?
Each current element contributes a magnetic field perpendicular to its direction and the line to the observation point. Integrating these vector contributions gives the field of the whole conductor.
What does “Young, bulk and shear moduli” actually mean in Mechanical Properties of Solids, and why does it matter for JEE?
Young's modulus resists length change, bulk modulus resists volume change, and shear modulus resists shape change. A larger modulus means less strain for the same corresponding stress.
What does “Speed and field ratio” actually mean in Electromagnetic Waves, and why does it matter for JEE?
In vacuum all electromagnetic frequencies travel at c. For a plane wave E_0/B_0 = c; frequency changes photon energy and wavelength, not vacuum speed.
I keep getting “Errors in measurement” questions wrong in Units, Dimensions & Measurement. What's the trap?
mixing up accuracy (closeness to the true value) with precision (repeatability).
I keep getting “Equations of motion” questions wrong in Kinematics. What's the trap?
using them when acceleration is changing.
I keep getting “Newton's third law” questions wrong in Laws of Motion. What's the trap?
thinking action and reaction cancel out — they can't, because they act on different objects.
I keep getting “Conservation of mechanical energy” questions wrong in Work, Energy & Power. What's the trap?
applying it when friction or other non-conservative forces are present.
I keep getting “Moment of inertia” questions wrong in Rotational Motion. What's the trap?
treating it as a fixed property regardless of the axis chosen.
I keep getting “Gravitational potential energy” questions wrong in Gravitation. What's the trap?
using mgh far from the surface, where it no longer holds.
I keep getting “Second law of thermodynamics” questions wrong in Thermodynamics & Kinetic Theory. What's the trap?
believing you can fully convert heat into work in a cycle.
I keep getting “Wave motion” questions wrong in Oscillations & Waves. What's the trap?
thinking the particles travel along with the wave.
I keep getting “Electric potential” questions wrong in Electrostatics. What's the trap?
thinking zero field means zero potential (or vice versa).
I keep getting “Series and parallel resistors” questions wrong in Current Electricity. What's the trap?
adding parallel resistances directly instead of their reciprocals.
I keep getting “Lenz's law” questions wrong in Magnetism & Electromagnetic Induction. What's the trap?
getting the direction of induced current backwards.