Physics Glossary — Key Terms and Definitions

Acceleration: The rate of change of velocity with respect to time, measured in m/s². Defined as a = Δv/Δt. See also Newton’s Second Law.

Angular Momentum: The rotational equivalent of linear momentum, calculated as L = r × p = Iω. Conserved in isolated systems with no external torque.

Centripetal Force: The net force directed toward the center of a circular path, given by F = mv²/r. This force keeps objects in uniform circular motion.

Conservation of Energy: The principle that total energy in an isolated system remains constant. Energy can transform between forms but cannot be created or destroyed. Related: First Law of Thermodynamics.

Displacement: A vector quantity representing the change in position of an object, measured from initial to final position. Unlike distance, displacement has both magnitude and direction.

Elastic Collision: A collision in which both momentum and kinetic energy are conserved. Objects bounce apart without permanent deformation.

Equilibrium: A state where the net force and net torque on an object are zero. Objects in equilibrium have zero acceleration.

Free Fall: Motion under the sole influence of gravity. Near Earth’s surface, all objects accelerate at approximately 9.81 m/s² regardless of mass.

Friction: A force that opposes relative motion between surfaces. The frictional force is f = μN, where μ is the coefficient of friction and N is the normal force.

Impulse: The change in momentum of an object, equal to the force applied multiplied by the time interval: J = FΔt = Δp. Related: Momentum.

Inertia: The tendency of an object to resist changes in its state of motion. Mass is a quantitative measure of inertia.

Kinetic Energy: The energy of motion, given by KE = ½mv². Depends on both mass and velocity squared.

Momentum: The product of mass and velocity, p = mv. A conserved quantity in isolated systems. Related: Impulse.

Newton’s First Law: An object at rest stays at rest, and an object in motion stays in uniform motion, unless acted upon by a net external force. Also called the law of inertia.

Newton’s Second Law: The net force on an object equals its mass times its acceleration: F = ma. This is the fundamental equation of classical mechanics.

Newton’s Third Law: For every action, there is an equal and opposite reaction. Forces always occur in pairs.

Potential Energy: Stored energy due to position or configuration. Gravitational PE = mgh; elastic PE = ½kx². Related: Kinetic Energy.

Power: The rate at which work is done or energy is transferred: P = W/t = F·v. Measured in watts (W).

Simple Harmonic Motion: Periodic motion where the restoring force is proportional to displacement: F = -kx. Examples include springs and pendulums.

Torque: The rotational equivalent of force, calculated as τ = r × F = rF sin θ. Causes angular acceleration.

Velocity: The rate of change of displacement with respect to time. A vector quantity with both magnitude and direction.

Work: Energy transferred by a force acting over a distance: W = F·d = Fd cos θ. Measured in joules (J).

Absolute Zero: The lowest possible temperature (0 K or -273.15°C), at which particles have minimum thermal motion. Cannot be reached by any physical process.

Adiabatic Process: A thermodynamic process with no heat transfer between the system and surroundings (Q = 0). Related: Isothermal Process.

Carnot Cycle: The most efficient theoretical heat engine cycle, consisting of two isothermal and two adiabatic processes. Maximum efficiency = 1 - T_cold/T_hot.

Entropy: A measure of disorder or randomness in a system. The second law of thermodynamics states that total entropy of an isolated system always increases.

First Law of Thermodynamics: Energy cannot be created or destroyed; the change in internal energy equals heat added minus work done: ΔU = Q - W. This is conservation of energy applied to thermal systems.

Heat: Energy transferred between objects due to a temperature difference. Flows from higher to lower temperature.

Ideal Gas Law: The equation of state for an ideal gas: PV = nRT, where P is pressure, V is volume, n is moles, R is the gas constant, and T is temperature.

Isochoric Process: A thermodynamic process at constant volume (ΔV = 0). No work is done; all heat goes to changing internal energy.

Isothermal Process: A thermodynamic process at constant temperature (ΔT = 0). For an ideal gas, PV = constant (Boyle’s Law).

Second Law of Thermodynamics: Heat cannot spontaneously flow from a colder to a hotter body. The total entropy of an isolated system never decreases.

Third Law of Thermodynamics: As temperature approaches absolute zero, the entropy of a perfect crystal approaches zero.

Capacitance: The ability of a system to store electric charge per unit voltage: C = Q/V. Measured in farads (F). Related: Capacitor.

Capacitor: A device that stores electrical energy in an electric field between two conductors. Energy stored: U = ½CV².

Coulomb’s Law: The force between two point charges: F = k|q₁q₂|/r², where k is Coulomb’s constant (8.99 × 10⁹ N⋅m²/C²).

Electric Field: A region around a charged object where other charges experience a force. E = F/q = kQ/r².

Electric Potential Energy: The energy a charged particle has due to its position in an electric field: U = kq₁q₂/r.

Electromagnetic Wave: A wave consisting of oscillating electric and magnetic fields. Light, radio waves, and X-rays are all electromagnetic waves.

Gauss’s Law: The net electric flux through a closed surface equals the enclosed charge divided by ε₀: ∮E·dA = Q_enc/ε₀.

Inductance: The property of a circuit that opposes changes in current, measured in henries (H). Related: Inductor.

Inductor: A component that stores energy in a magnetic field when current flows through it. Induced EMF: ε = -L(dI/dt).

Kirchhoff’s Current Law: The total current entering a junction equals the total current leaving: ΣI_in = ΣI_out.

Kirchhoff’s Voltage Law: The sum of all voltages around any closed loop in a circuit is zero: ΣV = 0.

Magnetic Flux: The measure of magnetic field lines through a surface: Φ_B = B·A = BA cos θ. Measured in webers (Wb).

Ohm’s Law: The current through a conductor is proportional to the voltage across it: V = IR, where R is resistance.

Resistor: A component that opposes the flow of electric current, converting electrical energy to heat. Resistance: R = ρL/A.

Bohr Model: An atomic model where electrons orbit the nucleus in quantized energy levels. Explains hydrogen spectrum but fails for multi-electron atoms.

de Broglie Wavelength: Every particle has wave-like properties with wavelength λ = h/p, where h is Planck’s constant and p is momentum.

Heisenberg Uncertainty Principle: It is impossible to simultaneously know both the exact position and exact momentum of a particle: ΔxΔp ≥ ℏ/2.

Photon: A quantum of electromagnetic radiation, carrying energy E = hf = hc/λ, where f is frequency and λ is wavelength.

Planck’s Constant: The fundamental constant h = 6.626 × 10⁻³⁴ J⋅s that relates photon energy to frequency.

Quantum Entanglement: A phenomenon where particles become correlated so that measuring one instantly determines the state of the other, regardless of distance.

Quantum Tunneling: The probability of a particle passing through a potential energy barrier that it classically could not overcome.

Wave Function: A mathematical function ψ(x,t) that describes the quantum state of a particle. The square of its magnitude gives probability density.

General Relativity: Einstein’s theory describing gravity as the curvature of spacetime caused by mass and energy. Field equations: G_μν + Λg_μν = (8πG/c⁴)T_μν.

Length Contraction: Objects moving at relativistic speeds appear shorter in the direction of motion: L = L₀/γ, where γ = 1/√(1-v²/c²).

Mass-Energy Equivalence: The famous equation E = mc² showing that mass and energy are interchangeable. A small amount of mass contains enormous energy.

Reference Frame: A coordinate system used to measure positions and events. Observers in different frames may measure different values for the same event.

Special Relativity: Einstein’s theory stating that the laws of physics are the same in all inertial frames and the speed of light is constant. Leads to time dilation and length contraction.

Time Dilation: Time passes more slowly for a moving observer relative to a stationary one: Δt = γΔt₀. Significant at speeds approaching c.

• Mechanics Problems
• Thermodynamics Examples
• Electromagnetism Guide
• Quantum Physics Overview
• Relativity Introduction

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