1,800+ MCAT terms, defined.

Resistance (R) is a measure of how strongly a conductor or circuit element opposes the flow of charge, in ohms (Ω). For an ohmic element V = IR, and R = ρL/A, so resistance rises with length and falls with cross-sectional area. Parallel...

Frequency is the number of complete cycles of a wave or oscillation per second, measured in hertz, and it is the inverse of the period. The most testable point: the source sets it, so frequency stays the same when a wave enters a new med...

A magnetic field B is the vector field produced by moving charges (currents) and by magnets, measured in tesla. It exerts a force on a moving charge perpendicular to both v and B, so it changes the particle's direction but does no work a...

Work is energy transferred to or from an object by a force acting through a displacement. Only the force component along the displacement counts, so W = Fd cosθ. Work is positive when the force has a component along the motion and negati...

Electric current is the rate at which charge flows past a point in a circuit, measured in amperes (1 A = 1 C/s). By convention it points the way positive charge would move, from high to low potential, even though electrons usually carry...

An electric field is the force per unit positive test charge that a source charge or charges would exert at a point in space. It is a vector pointing away from positive source charges and toward negative ones. Field strength from a point...

Power is the rate at which work is done or energy is transferred, measured in watts (1 W = 1 J/s). The most testable point is that power depends on time as well as work: the same work done faster takes more power.

Intensity is the power a wave delivers per unit area perpendicular to its travel, in W/m². It is proportional to amplitude squared, and for a point source it falls as 1/r², so doubling the distance cuts intensity to one quarter.

Potential energy is energy stored in a system by the position or arrangement of its parts, such as height in a gravitational field or a stretched spring. Only changes in U matter, and when only conservative forces act, a loss of potentia...

Sound is a longitudinal mechanical wave: a pressure disturbance of compressions and rarefactions that travels through a medium and cannot travel through a vacuum. Its speed is set by the medium (generally faster in solids than liquids th...

Displacement is a vector giving the straight-line change in position from start to finish, with both magnitude and direction. It depends only on the endpoints, not the path, so it can be zero on a round trip even though the distance trav...

Velocity is the rate of change of displacement with time. It is a vector with magnitude and direction, unlike speed, which is a scalar. The most testable point is that an object moving in a circle at constant speed still has changing vel...

The Doppler effect is the change in observed wave frequency when a source and an observer move relative to each other. The observed frequency is higher when they approach and lower when they separate, while the wave speed in the medium s...

A force is a push or pull, a vector measured in newtons, that can change an object's velocity. The net force (vector sum of all forces on the object) sets acceleration through Fnet = ma. Zero net force means constant velocity, which may...

A lens is a transparent curved object that refracts light to form an image. In air, a convex lens converges rays (f > 0) and a concave lens diverges them (f < 0). Most testable: use 1/o + 1/i = 1/f, where the sign of i tells real from vi...

Kinetic energy is the energy an object has because of its motion, K = ½mv². It depends on the square of speed, so doubling speed quadruples kinetic energy. The work-energy theorem says net work on an object equals its change in kinetic e...

Voltage is the electric potential difference between two points: the change in electric potential energy per unit charge, measured in volts (1 V = 1 J/C). A positive charge tends to move toward lower potential, and across a resistor the...

Acceleration is the rate at which velocity changes with time, a vector measured in m/s². It is nonzero whenever speed or direction changes, so an object moving in a circle at constant speed is still accelerating.

A wave is a disturbance that carries energy from place to place without net transport of the medium itself. Mechanical waves need a medium; electromagnetic waves do not. Most testable: v = fλ, and at a boundary the frequency stays fixed...

Refraction is the change in direction of a wave when it crosses a boundary into a medium where its speed differs. Light entering a medium with a higher index of refraction slows and bends toward the normal. Frequency stays the same; spee...

Capacitance is a capacitor's ability to store charge per unit potential difference, C = Q/V, measured in farads. For a parallel-plate capacitor, C = κε₀A/d, so it rises with plate area and dielectric constant and falls with plate separat...

Density is mass per unit volume of a substance, ρ = m/V, in kg/m³. The most testable point is that an object's average density compared with the fluid's density usually decides whether it floats or sinks.

Electric charge is a property of matter that produces and responds to electric force. It is positive or negative, measured in coulombs, with like charges repelling and opposite charges attracting. Net charge is conserved and comes in who...

A proton is a positively charged particle in the nucleus, with charge +1.60 × 10⁻¹⁹ C and mass about 1.67 × 10⁻²⁷ kg. Its count is the atomic number Z. Most testable: it has the same charge size as an electron but about 1836 times the ma...

Torque is the rotational analog of force: a measure of how effectively a force twists an object about a pivot. Its magnitude is τ = rF sinθ, so it is largest when the force is perpendicular to the lever arm and zero when the force acts a...

Friction is a contact force parallel to the surfaces that resists their relative sliding (kinetic) or the tendency to slide (static). Static friction adjusts up to a maximum of μsN, while kinetic friction is μkN, and μk is usually smalle...

A standing wave is the pattern made when two waves of the same frequency travel in opposite directions, usually a wave and its reflection. It has fixed nodes and antinodes and ideally carries no net energy along the medium. Only waveleng...

Heat is energy transferred between systems because of a temperature difference, flowing spontaneously from hotter to colder. It is a transfer, not something an object contains. Adding heat usually raises temperature (Q = mcΔT), but not d...

Light is electromagnetic radiation: a transverse wave of oscillating electric and magnetic fields that travels at c = 3.00 × 10⁸ m/s in vacuum. The most testable point: entering a new medium, its frequency stays the same while speed and...

Pascal's principle says a pressure change applied to an enclosed, incompressible fluid at rest is transmitted undiminished to every point in the fluid. In a hydraulic lift, F₁/A₁ = F₂/A₂, so a small input force can balance a large load,...

Surface tension is the force per unit length (or energy per unit area) that makes a liquid surface act like a stretched sheet, because cohesive forces pull surface molecules inward. It generally falls with higher temperature and with sur...

A vector is a quantity with both magnitude and direction, such as displacement, velocity, acceleration, force and momentum. Vectors add by components or head-to-tail, so direction changes the result; magnitudes alone do not simply add.

Ohm's law states that the potential difference across a conductor equals the current through it times its resistance, V = IR. It holds for ohmic materials, where R stays constant, but not for every device. The MCAT tests how current chan...

Viscosity (η) is a fluid's internal resistance to flow, caused by friction between layers moving at different speeds; its SI unit is Pa·s. The most testable point: in Poiseuille's law, flow rate is inversely proportional to η and proport...

The electric dipole moment p measures a pair of equal and opposite charges separated by a distance: p = qd, in C·m, pointing from the negative to the positive charge in the physics convention. In a uniform electric field the dipole feels...

Total internal reflection is the complete reflection of light back into its original medium when it meets a boundary with a lower-index medium at an angle of incidence greater than the critical angle. It needs n₁ > n₂ and θ > θc, and the...

Hooke's law says an ideal spring exerts a restoring force proportional to its displacement from equilibrium and opposite in direction: F = −kx. It holds within the elastic limit; doubling the stretch doubles the force but quadruples the...

A vacuum is a region with no matter, ideally at zero absolute pressure. Electromagnetic waves, including light at c = 3.00 × 10⁸ m/s, travel through it, but sound and other mechanical waves cannot, and with no air resistance, objects fal...

Perpendicular means at a 90° angle to a line, surface, or vector. The most testable consequence is that a force perpendicular to the displacement does no work (cos 90° = 0), so it can change direction without changing speed.

Poiseuille's law gives the volume flow rate of a viscous fluid in steady laminar flow through a long cylindrical tube: Q = πr⁴ΔP / (8ηL). Most testable point: flow depends on the fourth power of the radius, so a small change in radius ch...

Centripetal force is the net force directed toward the center of a circular path that keeps an object in circular motion. It is not a new kind of force but the net of real forces (tension, gravity, friction, normal force), and it is alwa...

A node is a point in a standing wave where the medium has zero displacement at all times, because the two oppositely traveling waves always interfere destructively there. Adjacent nodes are λ/2 apart; fixed string ends are nodes, but ope...

Archimedes' principle says the buoyant force on an object that is fully or partly submerged in a fluid equals the weight of the fluid it displaces. Most testable: the force depends on the fluid's density and the submerged volume, not on...

An image is the point where light rays from an object meet (real) or appear to come from (virtual) after reflection or refraction. Real images form where rays actually converge and can be projected on a screen; virtual images cannot. A n...

A resistor is a circuit element that opposes current, producing a potential drop across it and dissipating electrical energy as heat. The most testable point is V = IR combined with the series and parallel rules: adding a resistor in ser...

Focal length is the distance from a mirror or thin lens to its focal point, where rays parallel to the axis converge or appear to diverge from. It is positive for converging elements and negative for diverging ones. Key relation: 1/f = 1...

Diffraction is the spreading of a wave as it passes through an opening or around an edge. It is most noticeable when the opening is comparable to or smaller than the wavelength, so longer wavelengths spread more.

Half-life is the time for half of the nuclei in a radioactive sample to decay. It is constant for a given isotope and does not depend on sample size, so after n half-lives the fraction remaining is (1/2)ⁿ.

Hydrostatic pressure is the pressure in a fluid at rest caused by the weight of the fluid above a point. It increases linearly with depth, P = P₀ + ρgh, and depends on depth, density and g, not on container shape or width.

Snell's law relates the angles of a light ray to the refractive indices of two media at a boundary: n₁ sin θ₁ = n₂ sin θ₂. Angles are measured from the normal. Light typically bends toward the normal when it enters a medium with higher n...