1,800+ MCAT terms, defined.

The photoelectric effect is the ejection of electrons from a metal surface by light. Electrons come out when the photon energy hf is at least the work function. Above that threshold, frequency sets the maximum kinetic energy and intensit...

Atmospheric pressure is the pressure exerted by the weight of the air above a point. At sea level it is 1 atm = 101.3 kPa = 760 mmHg (760 torr), and it decreases as altitude increases. The most testable point is that absolute pressure in...

Decay is the spontaneous change of an unstable nucleus into a more stable one by emitting alpha, beta or gamma radiation. Each nucleus decays at random, but a sample's undecayed amount falls exponentially, halving every half-life.

Coulomb's law gives the electrostatic force between two point charges: it is proportional to the product of the charges and inversely proportional to the square of their separation. Like charges repel and opposite charges attract. Doubli...

Buoyant force is the upward force a fluid exerts on an object in it. By Archimedes' principle, it equals the weight of the fluid displaced, so it depends on fluid density and displaced volume. For a given displaced volume it ignores the...

Magnetic describes fields and forces from moving charges (currents) and from the intrinsic magnetic moments in magnets. A magnetic field exerts a force on a moving charge perpendicular to both its velocity and the field, so it changes di...

The electromagnetic spectrum is the full range of electromagnetic waves, ordered by frequency and wavelength: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. All travel at c in vacuum, so higher frequency means shorter wa...

Conductors are materials, such as metals, whose charges move freely, so they carry current with low resistance. In electrostatic equilibrium, the electric field inside a conductor is zero, excess charge sits on its surface, and the whole...

Newton's first law (the law of inertia) says an object keeps its velocity, whether at rest or moving at constant speed in a straight line, unless a nonzero net external force acts on it. Most testable: constant velocity, including zero,...

The right hand rule gives the direction of the magnetic force on a moving positive charge (fingers along v, curl toward B, thumb along F) and of the field around a current-carrying wire. For a negative charge, reverse the force direction.

A neutron is an electrically neutral particle in the nucleus with a mass of about 1 u, slightly greater than a proton's. It sets the isotope without changing the element or charge. Testable point: N = A − Z, and β⁻ decay turns a neutron...

Equivalent resistance is the single resistance that could replace a network of resistors and draw the same current from the same source. Series resistances add; for parallel, reciprocals add, so adding a parallel branch lowers the total.

Faraday's law of induction says a changing magnetic flux through a loop of wire induces an emf, proportional to the rate of change of flux and to the number of turns. The key point is that the flux must change; a steady field gives no em...

Kirchhoff's rules are two conservation laws for analyzing circuits. The junction rule says current into a junction equals current out (charge is conserved). The loop rule says potential changes around any closed loop sum to zero (energy...

Amplitude is the maximum displacement of a wave or oscillating object from its equilibrium position. Energy and intensity scale with amplitude squared, so doubling the amplitude quadruples them, while frequency and wave speed do not change.

The index of refraction, n = c/v, compares the speed of light in vacuum to its speed in a medium. It is dimensionless, equals 1 for vacuum, and is greater than 1 for ordinary media. Light entering a higher-n medium slows and bends toward...

Conservation of energy says the total energy of an isolated system stays constant; energy changes form or moves between objects. Mechanical energy (KE + PE) is constant when only conservative forces do work; friction and other nonconserv...

The Venturi effect is the drop in a flowing fluid's pressure where a tube narrows and the fluid speeds up. By continuity the fluid moves faster in the narrow part, and by Bernoulli's principle its pressure is lower there (ideal fluid, ho...

The continuity equation states that for an incompressible fluid in steady flow, the volume flow rate is the same at every point along a pipe, so A₁v₁ = A₂v₂. The fluid speeds up where the cross-section narrows and slows where it widens.

A free body diagram is a sketch of one object, drawn as a point or box, showing only the forces acting on it as labeled arrows. Its most testable use is to sum the force components along each axis to get net force (ΣF = ma), which is zer...

Terminal velocity is the constant speed a falling object reaches in a fluid when the upward drag force (plus any buoyant force) equals its weight, so net force and acceleration are zero. Testable point: gravity still acts, but it is bala...

Bernoulli's equation is conservation of energy for an ideal fluid (incompressible, nonviscous, steady flow): P + ½ρv² + ρgh is constant along a streamline. At the same height, faster flow means lower pressure, as in a narrowed pipe.

A transverse wave is one in which the oscillation (of the medium, or of the fields in light) is perpendicular to the direction the wave travels. Light and waves on a string are transverse; sound in air is longitudinal. Transverse waves c...

Simple harmonic motion is oscillation about an equilibrium position where the restoring force is proportional to displacement and points back toward equilibrium (F = −kx). For an ideal spring, and a pendulum at small angles, the period d...

Gravity is the attractive force between objects with mass, proportional to the product of the masses and inversely proportional to the square of the distance between their centers. Near Earth's surface, freely falling objects share the s...

Conservation of momentum says the total momentum (p = mv) of a system stays constant when the net external force on it is zero. In collisions and explosions, total momentum before equals total momentum after, even when kinetic energy is...

Electromagnetic waves are transverse waves of oscillating electric and magnetic fields that need no medium and travel at c = 3.00 × 10⁸ m/s in vacuum. Every type has the same speed in vacuum, so higher frequency means shorter wavelength...

An insulator is a material with very few free charge carriers, so it has very high resistivity and passes almost no current. The testable point: it resists current, not the electric field, and a dielectric between capacitor plates is an...

The joule (J) is the SI unit of energy, work, and heat: 1 J = 1 N·m = 1 kg·m²/s². One joule is the work done when a 1 N force moves an object 1 m along the force. Expect to convert it to other units and to recognize it as the unit of ene...

A sound wave is a mechanical, longitudinal wave of alternating compressions and rarefactions that needs a medium. The source fixes its frequency, so at a boundary the frequency stays constant while speed and wavelength change.

A dielectric is an insulating material placed between capacitor plates. It polarizes and weakens the electric field inside, so capacitance rises by the dielectric constant κ (C = κC₀). The testable point is whether the battery stays conn...

The normal force is the contact force a surface exerts on an object, directed perpendicular to the surface and pushing the object away from it. Its size depends on the other forces and on any acceleration perpendicular to the surface. It...

The critical angle is the angle of incidence, in the medium with the higher index of refraction, at which the refracted ray travels along the boundary (refraction angle 90°). At larger incident angles, light undergoes total internal refl...

Laminar flow is smooth, orderly fluid flow in parallel layers that do not mix, favored by low speed, small radius and high viscosity. Poiseuille's law applies to it. Flow becomes turbulent when the Reynolds number is high, above roughly...

Turbulent flow is chaotic fluid motion with eddies and swirls, instead of smooth parallel layers. It becomes likely at a high Reynolds number (Re = ρvD/η, above roughly 3000 in a pipe) and wastes more energy, so laminar-flow laws such as...

Electric potential is the electric potential energy per unit charge at a point, V = U/q, measured in volts (J/C). For a point charge V = kQ/r, a scalar that adds algebraically. A positive charge released from rest moves toward lower pote...

A transverse wave is one in which the oscillation, of the medium's particles or of field vectors, is perpendicular to the direction the wave carries energy. Waves on a string and electromagnetic waves are transverse, while sound in air i...

The volt (V) is the SI unit of electric potential, potential difference and emf. One volt is one joule per coulomb, which is energy per unit charge. A charge q moving through a potential difference ΔV changes its potential energy by qΔV,...

Electric potential energy (U) is the energy a system of charges stores because of their positions. For two point charges, U = kq₁q₂/r, positive for like charges and negative for opposite charges, taking U = 0 at infinite separation. The...

The force of gravity is the attractive force between any two masses, given by Newton's law of universal gravitation. It is proportional to both masses and falls off as 1/r², so doubling the center-to-center distance cuts the force to one...

Mass defect is the amount by which a nucleus's measured mass is less than the combined mass of its separate protons and neutrons. The missing mass appears as the nuclear binding energy (E = Δmc²), and a larger defect per nucleon usually...

Final velocity (v) is an object's velocity at the end of the time interval being studied. In constant-acceleration kinematics, v = v₀ + at. It is a vector, so its sign gives direction, and at a projectile's peak the vertical component is...

Pitch is how high or low a sound seems to a listener, and it is set mainly by the wave's frequency: higher frequency means higher pitch. Amplitude changes loudness, not pitch, and a sound's frequency stays the same when it crosses betwee...

An antinode is a point on a standing wave where the oscillation amplitude is maximum, because the two opposite-moving waves are always in phase there. Adjacent antinodes are λ/2 apart, with a node midway between them.

Projectile motion is the curved, two-dimensional path of an object launched into the air and moving under gravity, with air resistance neglected. Horizontal and vertical motions are independent: horizontal velocity is constant, while ver...

A scalar is a quantity described by a magnitude and unit alone, with no direction. Distance, speed, mass, energy, work and temperature are examples. Testable point: scalars add as ordinary numbers, while vectors need direction, and a sca...

Elastic potential energy is energy stored in a deformed elastic object, such as a stretched or compressed spring. For an ideal spring U = ½kx², so it depends on the square of the displacement from equilibrium: doubling the stretch quadru...

Gauge pressure is pressure measured relative to the surrounding atmospheric pressure: Pgauge = Pabs − Patm. A gauge open to the air reads zero. The most testable point is that gas laws need absolute pressure, so add Patm to a gauge value...

Specific gravity is the ratio of a substance's density to the density of a reference substance, usually water (1000 kg/m³ at 4 °C). It has no units. The testable point: if it is below 1, an object floats in water; above 1, it sinks.

Potential difference (voltage) is the change in electric potential energy per unit charge between two points, measured in volts (J/C). Positive charges tend to move toward lower potential and negative charges toward higher, and ΔU = qΔV.