Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
A frequent error is adding potentials as vectors or dropping the sign of a negative source charge. In reality, Electric potential at a point is potential energy per unit test charge. Because it is a scalar, contributions from many source charges add algebraically with their signs.
Memory trick: fields need directions; potentials need signs.
A frequent error is reversing the magnetic moment direction because external field lines run north to south. In reality, A bar magnet behaves like a magnetic dipole. Its moment points from south to north inside the magnet and determines the torque m x B in an external field.
Memory trick: dipole moment points S to N through the magnet.
A frequent error is claiming Rutherford's model explains atomic stability or line spectra. In reality, Most alpha particles pass through foil because atoms are mostly empty space; rare large deflections show that positive charge and most mass occupy a tiny nucleus.
Memory trick: Rutherford found the nucleus; Bohr addressed stable energies.
A frequent error is thinking a nucleus with twice the nucleons has twice the radius. In reality, Nuclear radius scales as R = R_0 A^(1/3), so volume scales with nucleon number A. Because mass also scales with A, nuclear density is approximately constant across nuclei.
Memory trick: mass number grows with volume, so radius grows by the cube root.
A frequent error is adding a positive zero error instead of subtracting it from the observed reading. In reality, An instrument cannot honestly resolve changes smaller than its least count. A final reading combines the main-scale reading, coinciding-division contribution, and the zero-error correction with correct sign.
Memory trick: correct reading = observed reading - zero error.
A frequent error is saying frequency changes when a wave enters another medium. In reality, 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.
Memory trick: source keeps frequency; medium sets speed; wavelength adapts.
A frequent error is using qvB when velocity is parallel to the field. In reality, 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.
Memory trick: only the perpendicular velocity component bends.
A frequent error is expecting two ordinary bulbs to interfere. In reality, two sources are coherent if they keep a constant phase difference — a must for a steady interference pattern.
Memory trick: stable fringes need one source split in two (constant phase).
A frequent error is treating reactance as a fixed resistance. In reality, the opposition a capacitor (Xc = 1/ωC) or inductor (XL = ωL) offers to AC — frequency-dependent, unlike resistance.
Memory trick: reactance changes with frequency; resistance doesn't.
A frequent error is thinking a diode conducts equally both ways. In reality, forward bias (p to +, n to −) lowers the junction barrier and lets current flow; reverse bias widens the depletion region and blocks it.
Memory trick: a diode is a one-way valve for current — forward on, reverse off.
“Checking dimensions is the physicist's spell-check for equations.”
Force has dimensions [M L T^-2], from mass times acceleration.
[M L T^-2]
Memory trick: Force = mass × accel → M × (L/T²).
“It's the promise that steady acceleration adds speed at a fixed rate every second.”
For constant acceleration, final velocity v = u + at.
v = u + at
Memory trick: 'velocity = start + accel×time'.
“A rocket flies by throwing gas down; the gas pushes the rocket up.”
Every action has an equal and opposite reaction, acting on two different bodies.
Memory trick: Action & reaction: different bodies, never cancel.
“Brakes do negative work, draining a car's kinetic energy into heat until it stops.”
The net work done on a body equals its change in kinetic energy.
W = ΔKE
Memory trick: Net work = change in KE.
“A skater pulls their arms in, shrinks I, and spins faster — conserving L.”
With no external torque, angular momentum L = Iω is conserved.
L = Iω
Memory trick: Pull in → spin faster.
“Rockets must hit ~40,000 km/h to leave Earth for good.”
The minimum speed to break free of a planet's gravity; ≈ 11.2 km/s for Earth.
v = √(2GM/R)
Memory trick: Escape = √2 × orbital speed.
“A steel ship floats because its hull displaces enough water to match its weight.”
A submerged body feels an upward buoyant force equal to the weight of fluid it displaces.
Memory trick: Buoyancy = weight of fluid pushed aside.
Stress measures internal restoring force per area; strain measures fractional deformation. Stress has pressure units, while strain is dimensionless.
Memory trick: stress is cause per area; strain is fractional response.
“Tune a radio and you're picking a frequency; its wavelength adjusts to keep the speed of light.”
For any wave, speed = frequency × wavelength.
v = fλ
Memory trick: v = fλ.
“Coastal cities have mild weather because water heats and cools slowly.”
Heat needed to raise 1 kg of a substance by 1°C; water's is unusually high.
Q = mcΔT
Memory trick: High specific heat = slow to change temperature.
“Voltage is 'electric height' — charges roll from high to low potential.”
Work done per unit charge to bring it from infinity to a point; a scalar.
V = kQ/r
Memory trick: Potential is a scalar; just add with sign.
“Home wiring is parallel so one bulb failing doesn't kill the rest.”
In series the current is shared and resistances add; in parallel the voltage is shared and 1/R values add.
Memory trick: Parallel total < smallest branch.
“Drop a magnet down a copper pipe and it falls in slow motion, braked by induced currents.”
The induced current opposes the change that created it — conserving energy.
Memory trick: Nature resists the change.
“Optical fibres pipe internet light across oceans using total internal reflection.”
Beyond the critical angle, light in a denser medium reflects completely instead of refracting.
Memory trick: Dense→rare, past critical angle → all reflects.