1,800+ MCAT terms, defined.

A state function is a property set by the system's current state, not by the path taken to reach it. P, V, T, internal energy U, enthalpy H, entropy S and Gibbs free energy G are state functions; work and heat are path-dependent, so they...

The work function (φ) is the minimum energy needed to free an electron from a metal's surface. In the photoelectric effect, electrons are ejected only if photon energy hf is at least φ, so the threshold frequency is f₀ = φ/h.

Magnetic force is the force a magnetic field exerts on a moving charge or a current-carrying wire. It points perpendicular to both the velocity and the field, so it does no work and changes a charge's direction but not its speed.

Intensity of sound is the power a sound wave delivers per unit area, in W/m². It grows with the square of the wave's amplitude, and for a point source it falls as 1/r². The most testable point: a 10 dB rise in sound level means a tenfold...

Dispersion is the spreading of white light into its colors because a medium's index of refraction depends on wavelength. In most transparent materials, such as glass and water, shorter wavelengths have a larger n, so violet bends more th...

The Lorentz force is the total electromagnetic force on a charge q moving at velocity v through electric field E and magnetic field B. On the MCAT it usually means the magnetic part, F = qvB sinθ, which is perpendicular to v, so it bends...

Cross-sectional area is the area of the face exposed when a pipe, vessel, or wire is cut perpendicular to the flow or current. For a steady incompressible fluid, Av is constant, so a smaller area means faster flow. In a wire of fixed len...

Laminar flow is smooth fluid motion in parallel layers (streamlines) that slide past one another without mixing. It is favored at low Reynolds number (roughly below 2000 in a pipe), and Poiseuille's law describes it. Higher Re leads to t...

Magnification (m) is the ratio of image height to object height, m = hi/ho = −i/o. Its sign gives orientation (negative = inverted, positive = upright) and its magnitude gives size (|m| > 1 enlarged, |m| < 1 reduced).

Adhesion is the attraction between molecules of different substances, such as water and glass. Cohesion is the attraction between like molecules. When adhesion exceeds cohesion, a liquid wets the wall, forms a concave meniscus and rises...

Efficiency is useful output divided by total input (work or energy), e = Wout/Win, often given as a percentage. Real machines fall below 100% because some input is dissipated as heat; a heat engine's maximum is the Carnot value, 1 − TC/T...

Threshold frequency (f₀) is the minimum frequency of light that can eject electrons from a particular metal in the photoelectric effect. Below f₀ no electrons are emitted, however intense the light. Above f₀, the extra photon energy beco...

Circular motion is motion along a circular path. Even at constant speed the velocity direction keeps changing, so there is a centripetal acceleration a = v²/r toward the center. Most testable: centripetal force is not a new force. It is...

Angular frequency (ω) is the rate at which the phase of an oscillation or rotation advances, in radians per second. One cycle is 2π radians, so ω = 2πf = 2π/T. The testable point is converting between ω (rad/s), f (Hz) and T (s).

Elastic force is the restoring force a stretched or compressed elastic object, such as a spring, exerts to return toward its equilibrium length. Within the elastic limit it is proportional to the displacement and opposite in direction (F...

Nuclear fission is the splitting of a heavy nucleus, such as uranium-235, into two smaller nuclei, usually after it absorbs a neutron. It releases neutrons and a large amount of energy because the products are more tightly bound per nucl...

Unpaired electrons are electrons alone in an orbital, so their spin magnetic moments are not canceled by a partner. Atoms with them are attracted to magnetic fields (paramagnetic, or ferromagnetic in domain-forming metals like iron); ful...

Faraday's law says a changing magnetic flux through a loop of wire induces an emf equal to the number of turns times the rate of change of flux. The most testable point is that only a change in flux induces an emf. If the flux stays cons...

The farad (F) is the SI unit of capacitance: a capacitor has a capacitance of 1 F if it stores 1 coulomb of charge per volt across it (1 F = 1 C/V). Capacitance is set by geometry and the dielectric, not by the charge or voltage applied.

The watt (W) is the SI unit of power, the rate at which energy is transferred or work is done. One watt is one joule per second. The testable point is that a watt measures a rate, not an amount of energy.

The newton (N) is the SI unit of force: the net force that gives a 1 kg mass an acceleration of 1 m/s², so 1 N = 1 kg·m/s². The most testable point is that newtons measure force, including weight, not mass.

A quantity is exponential when its variable sits in the exponent, so its rate of change is proportional to its current value. Equal time steps multiply it by the same factor, as in radioactive decay and half-life. A straight line on a se...

Newton's laws are three rules of classical mechanics: velocity stays constant unless a net force acts (first), Fnet = ma (second), and forces come in equal, opposite pairs on different objects (third). Most testable: acceleration depends...

A longitudinal wave is one in which the medium's particles oscillate parallel to the direction the wave travels, forming compressions and rarefactions. Sound in fluids is the classic example. Light and string waves are transverse, and on...

A current-carrying wire is a stream of moving charges that produces a circular magnetic field around it, with direction from the right-hand rule. Its strength is B = μ0I/(2πr), so it falls off as 1/r. Parallel wires with currents in the...

Destructive interference is the superposition of overlapping waves so their displacements partly or fully cancel, giving a smaller net amplitude. For two in-phase sources, it occurs where the path difference is a half-integer multiple of...

Cohesive forces are attractions between molecules of the same substance, such as hydrogen bonds between water molecules. They produce surface tension. Testable point: if adhesion to the wall exceeds cohesion the meniscus is concave; if c...

Gravitational potential energy is energy stored by a mass because of its position in a gravitational field; near Earth's surface U = mgh. It rises as the object moves higher, and only the change in U matters because the zero level is cho...

A longitudinal wave is one in which particles of the medium oscillate parallel to the direction the wave travels, forming compressions and rarefactions. Sound is the key example, and it needs a medium, so it cannot travel through a vacuum.

Conduction is heat transfer through a material, or between materials in contact, as colliding particles pass kinetic energy from hotter to cooler regions with no bulk motion. Testable point: the rate rises with area and temperature diffe...

A test charge is a small, imaginary positive point charge used to probe an electric field without disturbing the source charges. The field at a point is the force per unit charge on it, E = F/q₀, so it points the way a positive test char...

Atomic mass is the mass of one atom of a nuclide, measured in atomic mass units (u), where 1 u is 1/12 the mass of a carbon-12 atom (about 1.66 × 10⁻²⁷ kg). It is close to the mass number but slightly less than the sum of its parts, and...

Volumetric flow rate (Q) is the volume of fluid passing a cross-section per unit time, Q = Av, in m³/s. For an incompressible fluid with no leaks, Q is the same at every point along the pipe, so the fluid speeds up where the area narrows.

An absorption spectrum shows which wavelengths a substance absorbs. Because energy levels are quantized, an atom absorbs photons whose energy matches the gap between two levels, leaving dark lines. Those lines fall at the same wavelength...

A lever is a rigid bar that pivots about a fixed point (the fulcrum), so a small effort force can balance or move a larger load. At rotational equilibrium the torques balance: Feffort × deffort = Fload × dload. A longer effort arm lowers...

Turbulent flow is irregular, swirling fluid motion with eddies, in contrast to smooth laminar layers. It becomes likely at a high Reynolds number (fast, wide, dense, or low-viscosity fluid), and Poiseuille's law no longer applies.

A non-conservative force is one whose work depends on the path taken, so the work over a closed loop is not zero and no potential energy can be defined for it. Friction and air resistance are the standard examples, and they usually drain...

Photon energy is the energy carried by one photon, a single quantum of electromagnetic radiation. It is E = hf = hc/λ, so it rises with frequency and falls with wavelength. Brightness changes the number of photons, not the energy of each...

Wave speed is how fast a wave pattern, such as a crest, advances through a medium: v = fλ. At MCAT level the medium sets it, not the source. When a wave enters a new medium, frequency stays constant while speed and wavelength change in p...

Harmonics are the standing-wave frequencies a string or air column can sustain: whole-number multiples of the lowest one, the fundamental (first harmonic). Strings and open pipes support every integer multiple, but a pipe closed at one e...

The Rydberg formula gives the wavelength of the photon emitted or absorbed when a hydrogen electron changes between levels n₁ and n₂. Most testable point: a bigger energy gap means a shorter wavelength, so transitions to n = 1 (Lyman) ar...

Energy dissipation is the conversion of organized energy, such as mechanical or electrical energy, into thermal energy that spreads out and is hard to recover. Total energy is still conserved, but the mechanical energy of the system typi...

Magnetism is the interaction produced by moving electric charges, described by magnetic fields (B, in tesla). A field exerts force F = qvB sinθ on a moving charge, perpendicular to its velocity, so it changes the charge's direction but d...

Parallel resistors connect across the same two nodes, so each branch has the same voltage and the current splits among them. Adding a resistor in parallel lowers the equivalent resistance, which is smaller than the smallest single resistor.

Resistance to flow is how strongly a tube opposes the movement of a viscous fluid, so a pressure drop is needed to keep the fluid moving. For steady laminar flow it scales as 1/r⁴, so halving a tube's radius raises its resistance 16-fold.

Positive charge is the kind of electric charge a proton carries; an object is positively charged when it has a net deficit of electrons. Positive charges repel each other and attract negative charges, and electric field lines point away...

Thermal equilibrium is the state in which objects in thermal contact have reached the same temperature, so there is no net heat flow between them. The most testable point: for an isolated pair, the final temperature comes from setting he...

Corrective lenses are lenses placed in front of the eye so the image lands on the retina. Myopia (image in front of the retina) is corrected with a diverging lens of negative power; hyperopia (image behind it) with a converging lens of p...

DC circuits are closed loops in which a constant emf source, such as a battery, drives current in one direction through resistors and other elements. The most testable point: series resistors share the same current, while parallel resist...

The decibel scale is a base-10 logarithmic scale for sound intensity level, β = 10 log(I/I₀), where I₀ = 10⁻¹² W/m² is the threshold of hearing (0 dB). Most testable: every 10 dB increase multiplies intensity by 10, and 3 dB roughly doub...