1,800+ MCAT terms, defined.

Resistivity (ρ) is a material's intrinsic opposition to current flow, independent of the sample's size or shape. It sets a wire's resistance through R = ρL/A, so a longer or thinner wire of the same material has more resistance while ρ i...

Loudness is the perceived strength of a sound. It is set mainly by intensity (proportional to amplitude squared) and is measured on the logarithmic decibel scale. Each 10 dB increase means a tenfold increase in intensity. Pitch tracks fr...

Thermal energy is the energy of random particle motion in a substance, set by its temperature and by how much of it there is. Heat is thermal energy transferred because of a temperature difference. Testable point: temperature tracks aver...

Beta-negative (β⁻) decay is a radioactive process in which a neutron in the nucleus turns into a proton and emits an electron (the beta particle) and an antineutrino. The mass number A stays the same and the atomic number Z rises by 1, s...

Beta-positive (β+) decay is a nuclear decay in which a proton becomes a neutron, and the nucleus emits a positron and a neutrino. The atomic number Z falls by 1 and the mass number A stays the same. It typically occurs in proton-rich nuc...

The dielectric constant κ is a dimensionless number for an insulator that tells how much it multiplies a capacitor's capacitance (C = κC₀) by weakening the field inside. Testable point: with the battery connected, V stays fixed and Q ris...

The ampere (A) is the SI unit of electric current, the rate of charge flow: 1 A = 1 coulomb per second. The most testable point is that current in amperes follows I = V/R, so it depends on the voltage and the resistance in the circuit.

Electrostatic describes electric charges at rest and the forces, fields and potentials they produce. The most testable point is Coulomb's law: the force between two point charges falls off as 1/r², repelling for like charges and attracti...

A frequency shift is the change between the frequency a source emits and the frequency a detector measures, caused by their relative motion (the Doppler effect). Approach raises the observed frequency and separation lowers it, while the...

Electric charge is the property of matter that produces electric forces. It is positive or negative, measured in coulombs, quantized in units of e = 1.60 × 10⁻¹⁹ C, and conserved. Most testable: like charges repel, opposite charges attra...

Electric field lines are a picture of an electric field: the tangent to a line shows the force direction on a positive test charge, and closer lines mean a stronger field. They leave positive charges and enter negative ones, and where th...

A charge's velocity is the vector v (speed and direction) of a moving charge. It matters because a magnetic field exerts a force F = qvB sinθ only on a moving charge, perpendicular to v, so it changes the charge's direction but not its s...

An equipotential is a line, surface, or region where every point has the same electric potential, so the field does no work on a charge moved along it. Field lines are perpendicular to equipotentials and point toward lower potential.

A harmonic is a standing-wave frequency that is a whole-number multiple of the fundamental (lowest) frequency, fn = nf₁. Strings fixed at both ends and pipes open at both ends allow every integer n; a pipe closed at one end allows odd n...

A voltage drop is the decrease in electric potential across a circuit element, such as a resistor, as current passes through it. It follows V = IR. In series the drops typically add up to the source emf, while parallel branches share the...

The spring constant k measures a spring's stiffness: the restoring force it exerts per meter of stretch or compression, in N/m. In Hooke's law, F = −kx, so a larger k means a greater force for the same displacement.

Fission is the splitting of a heavy nucleus, such as uranium-235, into smaller nuclei, usually after it absorbs a neutron, releasing energy and more neutrons. The energy comes from a small mass loss (E = Δmc²), because the products have...

Internal resistance (r) is the resistance inside a real battery that makes its terminal voltage lower than its emf when it delivers current. Terminal voltage is V = ε − Ir, so it falls as current rises and equals the emf when no current...

This is the bulk modulus (B), the pressure change needed per fractional change in volume. A large B means the medium is hard to compress. The testable point is that sound speed is v = √(B/ρ), so stiffer media such as solids and liquids u...

When one object exerts a force on a second, the second exerts a force of equal magnitude and opposite direction on the first. The two forces act on different objects, so they do not cancel when you find the net force on either one.

Perceived loudness is how strong a sound seems to a listener. It increases with sound intensity (which scales with amplitude squared), but on a logarithmic scale, so it is tracked by the decibel level β = 10 log(I/I₀). Every 10 dB is a t...

Slit width (a) is the size of the opening a wave passes through in single-slit diffraction. The narrower the slit, the more the wave spreads, so the central bright band gets wider. Dark fringes obey a sin θ = mλ.

A conservative force does work that depends on the start and end points, not the path, so the net work over any closed loop is zero. Gravity, spring forces and electrostatic forces are examples. Friction and air resistance are not, so th...

Damping is the loss of mechanical energy from an oscillating system, usually as thermal energy through friction or drag, so the amplitude decays over time. Most testable: with light damping, amplitude and energy fall but the frequency ch...

Diamagnetic materials have no permanent atomic magnetic moments, usually because their electrons are all paired, and they are weakly repelled by an external magnetic field. The field induces a small opposing moment. Unpaired electrons po...

A virtual image is a point from which light rays only appear to diverge; no light actually passes through it, so it cannot be projected on a screen. Testable point: a virtual image has a negative image distance (i < 0) in the standard si...

Light refraction is the bending of a light ray as it crosses a boundary between two media, caused by a change in the light's speed. Entering a medium with a higher index of refraction, the ray bends toward the normal; entering a lower on...

Convection is heat transfer by the bulk motion of a fluid (liquid or gas): warmer fluid, usually less dense, rises while cooler, denser fluid sinks, forming a current. It needs a fluid that can flow, so it cannot occur through a solid or...

A dielectric is an insulating material placed between capacitor plates. Its molecules polarize and reduce the electric field inside, so capacitance is multiplied by the dielectric constant κ (κ ≥ 1). Whether Q or V changes depends on whe...

Inertia is an object's tendency to resist changes in its velocity, and mass is its measure. By Newton's first law, an object at rest stays at rest and a moving one keeps constant velocity unless a net external force acts. For the same ne...

Newton's third law says that if object A exerts a force on object B, then B exerts a force on A that is equal in magnitude and opposite in direction. The two forces act on different objects, so they do not cancel when you find the net fo...

An ohmmeter is a meter that measures the resistance of a component, in ohms. It uses its own internal battery to drive a current through the component, so the component must be isolated from the circuit's power source and any parallel br...

A pressure difference (ΔP) is the pressure at one point minus the pressure at another. It is what drives fluid flow, from higher toward lower pressure. For viscous flow in a tube, flow rate is proportional to ΔP and to r⁴, so small radiu...

Image distance (i) is the distance from a mirror or lens to its image, measured along the principal axis. Its sign is the testable point: positive i means a real image, negative i means a virtual image, found from 1/f = 1/o + 1/i.

The normal line is an imaginary line perpendicular (90°) to a surface at the point where a light ray hits it. All angles of incidence, reflection and refraction are measured from it, not from the surface itself.

Visible light is the band of electromagnetic waves the human eye detects, roughly 400 nm (violet) to 700 nm (red). Violet has the shortest wavelength and highest frequency and energy; red has the longest wavelength and lowest. All colors...

A diopter (D) is the unit of optical power of a lens or curved mirror, equal to 1/f with f in meters (1 D = 1 m⁻¹). Shorter focal length means greater power; converging lenses are positive, diverging negative. Powers of thin lenses in co...

Maximum kinetic energy is the largest kinetic energy of electrons ejected from a metal in the photoelectric effect. It equals the photon energy minus the work function, Kmax = hf − φ. It rises with light frequency, not intensity.

Critical speed is the fluid speed above which smooth laminar flow in a tube becomes turbulent. It rises with viscosity and falls with fluid density and tube diameter. Turbulence is likely when the actual speed exceeds it.

Sound intensity is the power a sound wave delivers per unit area perpendicular to its travel, in W/m². The most testable point: for a point source radiating equally in all directions it falls as 1/r², and it is proportional to amplitude...

Archimedes' principle says the upward buoyant force on an object in a fluid equals the weight of the fluid it displaces. Most testable point: the force depends on the fluid's density and the volume displaced, not on the object's own mass.

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, so doubling the distance cuts the force to one quarter....

Electric current is the rate at which charge flows past a point, measured in amperes (coulombs per second). By convention it points the way positive charge would move, from high to low potential, which is opposite to electron flow in met...

Kirchhoff's junction rule says the total current flowing into a circuit junction equals the total current flowing out, which is conservation of charge. On the MCAT, use it to find a missing branch current: parallel branch currents add to...

Newton's second law says the net force on an object equals its mass times its acceleration (Fnet = ma). Acceleration points along the net force and is smaller for larger mass. Testable point: zero net force means zero acceleration (const...

Snell's law relates the angles of a light ray on the two sides of a boundary between transparent media: n₁ sin θ₁ = n₂ sin θ₂, with angles measured from the normal. Light entering a higher-index medium bends toward the normal; entering a...

Binding energy is the energy needed to split a nucleus into separate protons and neutrons, equal to the mass defect times c². Higher binding energy per nucleon means a more stable nucleus. The curve peaks near iron-56, so fusion of light...

Equivalent capacitance is the capacitance of a single capacitor that could replace a network and store the same charge at the same voltage. Capacitors in parallel add directly, while in series the reciprocals add, so the series value is...

Vascular resistance is the opposition to blood flow in vessels, caused by viscous friction, and ΔP = QR. For idealized laminar flow, R = 8ηL/(πr⁴), so resistance varies as 1/r⁴: halving a vessel's radius raises its resistance 16-fold.

The electrostatic force is the force between two electrically charged objects, given by Coulomb's law. Like charges repel and opposite charges attract, and the magnitude falls off as 1/r², so doubling the distance cuts the force to one-q...