1,800+ MCAT terms, defined.

Electric power is the rate at which a circuit element transfers or converts electrical energy, measured in watts (J/s). The key relation is P = IV = I²R = V²/R; which form to use depends on whether current or voltage is the same across t...

Fluorescence spectroscopy excites a molecule with light and measures the light it re-emits. The emitted photon typically has a longer wavelength and lower energy than the absorbed one, because some energy is lost as heat first. This diff...

Image formation is how a mirror or lens redirects light from an object so the rays either meet (real image) or only appear to diverge from a point (virtual image). Most testable: 1/f = 1/o + 1/i gives image distance, and the sign of i te...

Ionisation energy is the minimum energy needed to remove an electron from an atom or ion, leaving it at rest far away. In the Bohr model of hydrogen it is 13.6 eV from the ground state (n = 1) and smaller from higher levels, because it e...

Mechanical efficiency is the fraction of the work or energy put into a machine that comes out as useful work, e = Wout/Win. Real machines lose some input as thermal energy through friction, so efficiency is below 100% and cannot exceed it.

Nuclear binding energy is the energy needed to separate a nucleus into its protons and neutrons, and equally the energy released when they combine. It corresponds to the mass defect by E = Δm c². Greater binding energy per nucleon means...

A particle is an idealized object treated as a single point with mass (and sometimes charge) but no size or rotation. Most testable: a charged particle moving perpendicular to a magnetic field follows a circle, r = mv/(qB), at constant s...

The Reynolds number is a dimensionless ratio of inertial to viscous forces in a flowing fluid, Re = ρvD/η. Low values mean smooth laminar flow and high values mean turbulent flow. Higher speed, density or diameter, or lower viscosity, ra...

A thin lens is an idealized lens whose thickness is negligible, so light bends at a single plane. Image position follows 1/f = 1/o + 1/i, with f positive for converging and negative for diverging lenses. A converging lens gives a real or...

Ultrasound imaging builds a picture from echoes of high-frequency sound pulses (above 20,000 Hz) reflected at boundaries between tissues of different acoustic impedance. Echo delay gives depth, d = vt/2; higher frequency gives finer reso...

An isobaric process is a change that occurs at constant pressure. For a gas, volume and temperature change together, and the work done by the gas is W = PΔV, the area under a horizontal line on a PV diagram.

Radioactive decay is the spontaneous change of an unstable nucleus into a more stable one, emitting alpha, beta or gamma radiation. Most testable: it is first-order, so each half-life halves the number of undecayed nuclei, whatever the s...

The thin lens equation, 1/f = 1/o + 1/i, links a lens's focal length to the object distance and image distance. The testable point is the sign of i: positive means a real image on the opposite side of the lens from the object, negative m...

Cosine (cos) is the trigonometric ratio adjacent/hypotenuse. In physics it gives a vector's component along the axis the angle is measured from, such as Fx = F cos θ. Most testable: cos is 1 at 0° and 0 at 90°, so that component shrinks...

Refracted light is light that bends as it crosses a boundary into a different medium, because its speed changes. Entering a medium with a larger index of refraction, it bends toward the normal. Light hitting along the normal is not bent.

The SI unit of electric charge is the coulomb (C), the charge delivered by 1 ampere in 1 second. The smallest free charge is the elementary charge, e = 1.60 × 10⁻¹⁹ C, so the net charge on an ordinary object is a whole-number multiple of e.

The coulomb (C) is the SI unit of electric charge, equal to one ampere-second (1 C = 1 A·s). The elementary charge is e = 1.60 × 10⁻¹⁹ C. The most testable uses are Q = IΔt and the charge unit in Coulomb's law.

Radiation is heat transfer by electromagnetic waves, which needs no medium and so works across a vacuum. The power an object radiates scales with T⁴ (kelvin), so doubling absolute temperature raises it sixteenfold. In nuclear physics, ra...

An isolated system exchanges neither matter nor energy (heat or work) with its surroundings, so its total energy stays constant. A closed system differs: it can exchange energy but not matter. In mechanics it also means no net external f...

A closed system can exchange energy (heat and work) with its surroundings but not matter. Its internal energy changes according to ΔU = Q − W. Closed is not the same as isolated, which exchanges neither matter nor energy.

Electric potential (V) is the electric potential energy per unit charge at a point, measured in volts (J/C). Positive charges tend to move toward lower potential and negative charges toward higher. Potential is a scalar, so contributions...

The junction rule (Kirchhoff's first rule) says the total current flowing into a junction equals the total current flowing out. It expresses conservation of charge. Most testable point: currents in parallel branches split but always add...

The loop rule (Kirchhoff's second rule) says the potential changes around any closed loop in a circuit sum to zero, which expresses conservation of energy. On the MCAT, you write one such equation per loop, with signs set by the directio...

An open system can exchange both energy (heat and work) and matter with its surroundings. The MCAT tests this by contrasting it with closed systems (energy only) and isolated systems (neither).

Nuclear fusion is the joining of two light nuclei into a heavier nucleus, releasing energy because the product has a higher binding energy per nucleon. It powers stars and needs extreme temperature to overcome the repulsion between posit...

A traveling wave is a disturbance that carries energy through a medium (or, for light, through space) while the medium's particles only oscillate about fixed positions. Its speed follows v = fλ and is set by the medium; the source sets t...

Centripetal acceleration is the acceleration of an object moving on a circular path. It points toward the center and changes the direction of the velocity, not the speed. Its magnitude is v²/r, so doubling the speed at a fixed radius qua...

Kinematics describes motion using displacement, velocity, acceleration and time, without considering the forces that cause it. Most testable: the standard equations, such as v = v₀ + at, apply when acceleration is constant, as in free fa...

When object A exerts a force on object B, B exerts a force on A that is equal in magnitude and opposite in direction. The two forces act on different objects, so they never cancel each other. This is the key point the MCAT tests.

Absolute pressure is the total pressure at a point, measured from a perfect vacuum as zero. It equals gauge pressure plus atmospheric pressure, and at depth h in a static liquid open to the air, P = P₀ + ρgh. Gas laws need absolute press...

Buoyancy is the net upward force a fluid exerts on an object placed in it, equal to the weight of the fluid the object displaces (Archimedes' principle). Most testable: it depends on the fluid's density and the submerged volume, not on t...

Charged particles (electrons, protons, ions) carry net charge q, in multiples of e = 1.60 × 10⁻¹⁹ C, and feel forces in electric and magnetic fields. Most testable: the electric force qE can do work, while the magnetic force qvB sinθ is...

Electric potential difference (voltage), ΔV, is the change in electric potential energy per unit charge between two points, measured in volts (J/C). Released in an electric field, a positive charge tends to move toward lower potential an...

Ultrasound is sound with frequency above about 20,000 Hz, beyond human hearing. In imaging, pulses reflect off tissue boundaries and the echo time gives depth (d = vt/2). Higher frequency gives finer resolution but less penetration.

A voltmeter measures the potential difference between two points in a circuit and is connected in parallel with the element being measured. An ideal voltmeter has infinite resistance so it draws no current; real ones have very large resi...

Cohesion is the attraction between molecules of the same substance, such as hydrogen bonding among water molecules. It produces surface tension. The testable point is cohesion versus adhesion: when adhesion to the container wall exceeds...

A dielectric is an insulator placed between capacitor plates; it polarizes, weakens the field, and raises capacitance by the factor κ (C = κC₀). With the battery connected, V stays fixed and Q rises; if the battery is disconnected, Q sta...

Electrostatics is the study of electric charges at rest and the forces, fields, and potential energies they produce. The most testable point: like charges repel, opposite charges attract, and the force between point charges falls off as...

A membrane potential is the voltage across a cell membrane, set by uneven ion distribution and selective permeability. An ion's electrochemical gradient combines its concentration gradient and the electrical gradient, so its net flow dep...

Polarization means restricting a transverse wave's oscillation, for light the electric field, to one plane. A polarizer cuts unpolarized light to half its intensity. Polarized light meeting a second polarizer at angle θ follows I = I₀cos...

The refractive index n = c/v compares the speed of light in a vacuum with its speed in a medium, so n is unitless and at least 1. A higher n means slower light. A ray entering a higher-n medium bends toward the normal.

An ammeter is a meter that measures the current through a circuit element, and it is connected in series with that element. An ideal ammeter has zero resistance, so it does not change the current it measures.

Kinetic friction is the force along the contact surface that opposes the relative sliding of two surfaces. Its magnitude is fk = μk N, which depends on the normal force but not on speed or contact area, and μk is usually smaller than μs.

The ohm (Ω) is the SI unit of electrical resistance, equal to one volt per ampere. In an ohmic conductor, a larger resistance in ohms means less current for the same potential difference, and resistances in series add.

An attractive force pulls two objects toward each other along the line joining them. Gravity between masses and the Coulomb force between opposite charges are attractive. The most testable point is that these forces weaken as 1/r², so do...

A closed pipe is closed at one end and open at the other, so it has a displacement node at the closed end and an antinode at the open end. Its resonances are only odd harmonics, f = nv/4L with n = 1, 3, 5, …, so its fundamental is half t...

Electric field lines are drawn curves whose tangent at any point shows the direction of the force on a positive test charge. They leave positive charges and end on negative charges, and closer spacing means a stronger field. Lines do not...

A paramagnetic material is weakly attracted to a magnetic field because its atoms have unpaired electrons whose magnetic moments partly align with the field. The alignment fades when the field is removed. Diamagnetic materials are weakly...

Bulk modulus (B) measures how strongly a material resists uniform compression: the pressure change divided by the fractional volume change. A larger B means the material is harder to compress. Liquids and solids generally have much large...

Gravitational force is the attractive force between any two objects that have mass, directed along the line joining their centers. Its magnitude is proportional to the product of the masses and inversely proportional to the square of the...