Glossary

Absolute zero is the thermodynamic limit corresponding to the lowest possible temperature. Absolute zero is defined to be 0 K.

Absorption of light occurs when light waves completely transfer their energy to a medium as they attempt to pass through it. The light is therefore blocked from transmitting through the medium.

An absorption spectrum shows the wavelengths of light that are absorbed by a material. It appears as a continuous spectrum with dark lines caused by light passing through a substance.

Acceleration quantifies the rate at which an object changes its velocity. (symbol: a, unit: m/s^2)

An adiabatic process is a specific type of thermodynamic process in which the heat added to or removed from a system is zero.

An alpha particle is a helium nucleus (consisting of two protons and two neutrons).

Alternating current (AC) is a property of a voltage or current source in which the value of the voltage or current changes periodically with time.

Amorphous solids have atomic or molecular arrangements that are spatially disordered, or that otherwise lack a regular periodic arrangement.

Wave amplitude describes the maximum displacement from either side of the equilibrium position.

Apparent weight is equal to the value of the support force acting on a person. This causes our perception of having weight.

Archimedes’ principle states that the buoyant force that acts on a substance is equal to the weight of the fluid that is displaced when that substance is placed into a fluid.

Atomic mass number is equal to the number of nucleons (protons and neutrons) in an atom. (symbol: A)

The atomic number uniquely defines an element and is equal to the number of protons in atoms of that element. (symbol: Z)

Ballistics is the study and analysis of how projectiles travel.

A beat is an interference phenomenon resulting from the time interference of two sound waves at very similar frequencies. It is perceived as a periodic change in amplitude of the composite wave.

Bernoulli’s principle relates pressure to the speed and elevation of a fluid. If the elevation is constant, then as the speed of a fluid increases, the pressure decreases and vice versa.

Binding energy describes the energy required to dissociate a nucleus into its constituent nucleons.

Birefringence is a property of some materials that causes the index of refraction to be different based on the polarization of light.

Boyle’s law describes the relationship between pressure and volume of a gas when the temperature and number of molecules in a closed system is held constant. It states that the pressure times the volume is constant.

When unpolarized light reflects off of a surface at a particular angle, called Brewster's angle, the reflected light is polarized 100% in a direction parallel to the reflecting surface.

A capacitor is a circuit element that stores energy in the form of an electric field. It is commonly used in circuits to smooth out changes in voltage to create a DC signal.

Carnot efficiency describes the maximum efficiency that can result from any heat engine, even if it is perfectly ideal in every other way: no friction or other losses. (symbol: e_carnot, unit: none)

The center of mass is the point in an object (or system of objects) around which it rotates freely. It is considered to be the average position of all the mass in a system.

Centripetal force is a force, pointing toward the center of the rotational motion of an object, that causes an object to move in a circular path. (symbol: F_c, unit: N)

A chain reaction is a self-sustaining fission process whereby one fission reaction provokes one or more subsequent chain reactions.

Electric charge is a fundamental property of matter that causes particles that carry a charge to experience a force when in the presence of an electric field.

A circuit is an interconnection of elements such as batteries, light bulbs, resistors, capacitors, and diodes.

Classical physics describes the natural phenomena that can be explained by Newton’s laws of motion, Maxwell’s equations, and the laws of thermodynamics.

Coherent light means that all of the waves have the same frequency and phase.

Color addition refers to the ability to use light to create secondary colors from primary colors (red, green, and blue).

Color subtraction is a color model based on light reflection and absorption. Colors that are not reflected back into our eyes are subtracted out and modify the color that we perceive.

Compression is the force that exists when something is pushed or squeezed together.

Condensation is the phase change from a gas to a liquid.

Conduction is the motion of electrical charge within a material or between different materials.

Conduction is heat transfer that occurs due to collisions among atoms, electrons, or molecules.

Convection is heat transfer that comes about due to the motion of molecules themselves.

Critical mass describes the quantity of a fissionable material above which energy is released so quickly that an explosion can result.

A crystalline solid has constituent atoms or molecules that are arranged in an orderly, repeating manner.

Current describes the flow of electric charge through a circuit. (symbol: I, unit: A)

Density describes the compactness of matter in a substance, defined as the mass per unit volume. (symbol: D, unit: kg/m^3)

Deposition is the phase change process when a gas turns directly into a solid without passing through the liquid form.

Diffraction is a bending of a wave as it travels around a corner or through a narrow opening.

A diode is a circuit element that acts like a one-way valve in that it only allows current to flow in one direction but not the other.

Direct current (DC) is a property of a voltage or current source in which the value of the voltage or current remains constant over time.

Displacement describes the straight-line distance from one point to another. Displacement is a vector quantity. (symbols: Δd [for general displacement], Δx [for horizontal displacement], Δy [for vertical displacement]. unit: m).

The Doppler effect is the perceived change in frequency of a sound wave as heard by an observer when there is motion between the observer and the source.

Dynamic equilibrium is a condition of mechanical equilibrium where either the velocity or rotational velocity of an object is constant and not equal to zero.

Efficiency quantifies how well work input is converted to work output. (symbol: e, unit: none)

An elastic collision occurs when there is neither lasting deformation of objects in the collision, nor generation of heat during the collision. Both momentum and kinetic energy are conserved in elastic collisions.

Elasticity describes how much an object deforms when experiencing a stress or force.

Electric potential describes the amount of work it takes to move a charge through an electric field divided by the amount of charge. (symbol: EP, unit: V)

Electromagnetic radiation is heat that is transferred from one place to another through light waves.

The electromagnetic spectrum describes the entire range of electromagnetic waves. Each subset of waves can be defined in different categories based on its frequency or wavelength.

An electromagnetic wave is a transverse wave that contains electric and magnetic fields oscillating at right angles to each other.

Electromagnetism describes the interactions that occur between charged objects.

An electron is a fundamental building block of matter that has a negative charge and is found surrounding the nucleus of an atom.

Electron deexcitation is the process by which an electron loses energy and jumps down one or more energy levels. In the process, the electron either transfers energy to another particle, emits a photon, or generates heat.

Electron excitation is the process by which an electron gains energy and moves up one or more energy levels.

An element is a substance that consists of atoms whose nuclei have the same atomic number. An element cannot be broken down further by chemical means.

The collection of frequencies of light emitted by a substance via the electron deexcitation process is known as its emission spectrum. It appears as bright lines on a dark background.

Energy is defined as the capability of an object (or collection of objects) to do useful work. (symbol: E, unit: J)

Energy levels describe the quantity of energy that each electron orbiting an atom can have. Each atom is uniquely defined by its discrete energy level values. The lowest energy level is known as the ground state.

Entropy is a thermodynamic property that describes the amount of disorder in a system. In particular, it is related to the amount of energy that becomes unavailable to do work.

A fissile nucleus is an isotope that can undergo fission after being bombarded with a slow neutron (known as a “thermal” neutron).

Fission is the splitting of heavy atomic nuclei into lighter pieces.

A fissionable nucleus describes an unstable nucleus that will undergo fission after being bombarded with a suitable neutron.

Fluorescence is a process of light emission where an object absorbs light at a high frequency, causing electron excitation, and during the deexcitation process emits light at a lower frequency.

A force is a push or a pull that causes an object to change its motion. More fundamentally, force is an interaction between two objects. (symbol: F, unit: N)

A forced vibration occurs when one object causes another object to vibrate at a certain frequency.

Fourier analysis decomposes a wave into individual frequency components.

Free fall is defined as a situation where gravity is the only force acting on an object. Forces such as friction and air drag are ignored and assumed to be equal to zero. The acceleration of an object in free fall is defined by the acceleration due to gravity (g).

Freezing is the phase change from a liquid to a solid.

Frequency describes how many oscillations occur per second in a wave. (symbol: f, unit: Hz)

Fusion is the combining of light atomic nuclei into a single heavier nucleus.

A galvanometer is a mechanical instrument capable of measuring electrical current by detecting a needle proportional to the amount of current flowing through the circuit.

Gas is a state of matter where molecules are very free to move about and generally do not interact with each other except during collisions. This means that the shape and volume of a gas is free to change. Gas is one of the four most common phases of matter.

A generator converts mechanical work to electrical energy.

Gravity is the attractive force experienced by objects of mass. It is one of the four fundamental forces.

Grounding is a special type of conduction where charges redistribute themselves to an area that can accept lots of charges without causing a large buildup of electrons or positive ions.

Half-life is a description of the amount of time it takes, on average, for half of a particular unstable isotope to decay.

Heat is energy that is transferred from one object to another in response to a difference in temperature. (symbol: Q, unit: cal)

Huygens’ principle states that every point on a wave can be treated as a source of a spherical wave that itself propagates through time and space. Each of these individual spherical waves interact with each other through interference, with the sum representing the total wave.

A hypothesis is a proposed explanation of how the universe works that is based on observations and can be tested experimentally.

Impulse is the measure of the change in momentum of an object or of a system of objects. (symbol: Δp, unit: kgm/s or Ns)

Incandescence is a form of light emission and heat transfer process by which an object emits electromagnetic radiation with a peak frequency that corresponds to the temperature of that object.

Electromagnetic induction is the process by which changing magnetic fields produce electric fields, and vice versa.

Induction is a redistribution of electric charge due to the presence of a nearby electric field.

In an inelastic collision, there is either lasting deformation of the objects in the collision, or heat/sound generated during the collision (or both). In an inelastic collision, momentum is conserved, but kinetic energy is not (as it is converted to other forms of energy during the collision).

Inertia is a property of matter whereby objects resist changes in their motion. Inertia is related to the mass of an object.

Infrasound is the set of sound waves that have frequencies below 20 Hz. They are not audible to humans.

Intensity describes the amount of energy per unit of area that reaches a certain location every second (unit: W/m^2)

Interference is the phenomenon that occurs when two waves overlap. The total amount of wave energy present at a point is equal to the sum of each individual wave's energy at that point.

An ion is a charged atom; there is an unequal number of protons and electrons in the atom.

Ionization is the process by which a gas is converted into a plasma.

Ionizing radiation refers to electromagnetic waves (UV, x-rays, and gamma rays) that contain sufficient energy to ionize atoms. These waves can cause damage to human DNA and other tissue.

Elements that have the same number of protons but different numbers of neutrons are known as isotopes.

Kinetic energy is the energy that an object has due to its motion. (symbol: KE, unit: J)

Kinetic friction is the force that occurs between surfaces as they slide past each other. The force acts parallel to the contact surfaces.

Latent heat of fusion describes the amount of heat required to convert one gram of a substance from a solid form to a liquid form. Alternatively, it defines the amount of heat that must be removed from one gram of a substance to convert it from a liquid to a solid.

Latent heat of vaporization describes the amount of heat required to convert one gram of a substance from a liquid form to a gaseous form. Alternatively, it defines the amount of heat that must be removed from one gram of a substance to convert it from a gas to a liquid.

A scientific law is a statement that describes relationships among the quantities that we observe or measure.

The law of reflection states that the angle of reflection of a wave is equal to the angle of incidence of that wave.

A liquid is a state of matter in which the constituent molecules will change their shape or arrangement but cannot be easily compressed to change their volume. Liquid is one of the four most common phases of matter.

In a longitudinal wave, the oscillations of the wave travel in the same direction as the wave itself.

Loudness has to do with the subjective quality of the amplitude of a sound wave.

A magnetic domain is an area inside of a material where the magnetic effect of each individual atom couples to those of neighboring atoms, causing the internal magnets to be aligned.

Magnetism is a phenomenon that occurs when moving electric charges generate attractive and repulsive forces.

Mass is a property of physical objects that relates to resistance to changes in motion: inertia. (symbol: m, unit: kg)

Mechanical advantage is a force multiplication that can be provided by a simple machine. (symbol: MA, unit: none)

Mechanical energy is the sum of kinetic energy and potential energy of a system. When ignoring dissipative forces such as air drag and friction, mechanical energy will be conserved.

Mechanical equilibrium is a condition where the net force and net torque acting on an object are equal to zero. Two cases of mechanical equilibrium are static and dynamic. An object in mechanical equilibrium has a constant velocity (constant speed in one direction).

Melting is the phase change that occurs when a substance converts from a solid to a liquid.

A mixture is a substance where the properties of that substance (such as density, melting point, and specific heat capacity) vary based on the concentration of each component in the mixture.

Momentum is a vector property that quantifies the motion of an object. It is sometimes called "inertia in motion" as it is the product of mass and velocity. (symbol: p, unit: kgm/s or Ns)

A motor converts electrical energy to mechanical work.

The natural frequency of an object refers to the frequency that the object will tend to produce.

The net force is the vector sum of all forces acting on an object.

A neutron is a subatomic particle that has no charge and resides in the nucleus of an atom.

Newton’s first law is sometimes called the law of inertia. Newton’s first law states that “an object in motion tends to stay in motion, and an object at rest tends to stay at rest, unless acted upon by an outside force.”

Newton's second law states that the acceleration of an object is proportional to the net force acting on the object, and inversely proportional to the mass of the object. In equation form, Newton's second law can be stated as F_net = ma.

Newton's third law states that for every action, there is an equal and opposite reaction. All forces exist in action-reaction pairs that are equal in magnitude and opposite in direction.

A nuclide is a nucleus of a particular isotope.

The observer effect describes the process that occurs when measuring a quantum process. When making an observation, the process is influenced in some way, and therefore affects the outcome.

Ohm's law describes the relationship between voltage, resistance, and current in an electric circuit. Ohm's law states that V = IR.

An object is opaque if it does not allow any light to transmit through it.

An optical aberration occurs when light (incident on a mirror or traveling through a lens) is not focused at a single point. Instead, light rays are spread out around the focal point.

The orbit of an electron describes the properties of the electron wave as it exists around the nucleus of an atom.

The orbit of a satellite describes its path around a planet or star.

A parallel circuit is one where circuit elements are all contained in their own path in the circuit.

Path describes the total distance that an object travels as it moves from one point to another, measured along its trajectory. It is a scalar quantity. (symbols: d, x [for horizontal path], y [for vertical path], unit: m)

In a perfectly inelastic collision, the maximum amount of kinetic energy is lost in a collision. In a perfectly inelastic collision between two objects, the two objects will travel together after the collision.

The period of a wave describes how long it takes for one complete oscillation to occur. (symbol: T, unit: s)

Phosphorescence is a light emission phenomenon similar to fluorescence. However, the deexcitation process takes place on a timescale that extends after the excitation source is removed.

The photoelectric effect is the emission of electrons when light of a particular frequency (especially UV light) strikes a metal target.

Physics is a branch of science that focuses on the fundamentals of the workings of our universe.

Plasma is ionized gas. Plasma is the most common phase of matter in the universe.

Polarization is the property of transverse waves which allows them to have their electric field components oscillating in a single direction.

Potential energy is energy that exists based on the arrangement or configuration of a substance.

Power quantifies how quickly work is done. (symbol: P, unit: W)

Pressure defines the amount of force applied over an area of a substance. (symbol: P, unit: Pa)

The principle of flotation states that any object that floats displaces a weight of fluid equal to its own weight.

The principle of least time describes how light travels. Light does not necessarily take the shortest distance between two points. Instead, it will travel whatever path takes the least amount of time.

A proton is a subatomic particle that has a positive charge and resides in the nucleus of an atom.

Pseudoscience is the misuse of scientific phrases and terminology, usually to mislead others into purchasing a product that sounds “high tech.”

In a pure substance, the properties of a substance (density, melting point, specific heat capacity, etc.) are inherent to the material.

Quantum physics describes the physical laws used to describe the properties of matter and energy that occur on a very small scale.

A quark is a fundamental building block of matter, described by the standard model of physics. It is the building block of protons and neutrons.

Radioactivity is the process by which an unstable nucleus releases energy and emits a particle.

Recombination is the process by which a plasma changes phase to a gas.

Reflection occurs when a wave bounces off of a surface and subsequently changes its direction.

Refraction occurs when a wave changes speed (and therefore bends) at an interface between two different media.

Resistance is the property of a material that impedes the flow of electrons throughout that material. (symbol: R, unit: Ω)

Resonance occurs when an object is forced to vibrate at its natural frequency, which could lead to vibrations of increasing amplitude.

Rotational inertia is a measure of how much an object resists changes to its rotational speed.

Rotational speed quantifies how quickly (or slowly) something rotates about an axis. (symbol: ω, unit: RPM, degrees/s, rad/s, etc.)

A scalar quantity is a variable that can be conveyed with a numerical quantity indicating magnitude or strength.

Scattering is a process where the direction that a wave travels in is changed without affecting the wavelength.

Science is the systematic pursuit of knowledge that comes about through asking questions, making careful observations, and carrying out experiments about how the universe works.

The scientific method is the process by which scientists come to answer questions about the universe.

A series circuit is one where all of the circuit elements are contained in a single path.

A short circuit is a circuit that contains a very low-resistance path along which electrons will travel. It can result in excess current, which may cause the circuit to overheat.

A simple machine is used to change either the direction or magnitude (or both) of an applied force. There are six types of simple machines: levers, wheels and axles, pulleys, inclined planes, wedges, and screws.

The solar constant is the rate at which we obtain energy from the sun. The solar constant is approximately 1,400 W/m^2.

A solid is a substance where the molecules or atoms are very tightly bound together. This gives a solid a very rigid volume and shape. Solid is one of the four most common phases of matter.

A sound wave is a longitudinal wave that is caused by vibrations in air pressure between two points.

Specific heat capacity defines how difficult it is to change the temperature of a substance. It describes the amount of heat required to change the temperature of a certain mass of a substance by a certain temperature. (symbol: c, unit: cal/(g°C))

Speed is the scalar quantity that describes the rate at which an object changes its position. Speed is equal to position divided by time. (symbols: s, |v|, unit: m/s)

The spring constant defines the stiffness of an elastic object such as a spring. The spring constant is equal to the force required to deflect a spring a certain distance. (symbol: k, unit: N/m)

The standard atomic weight is the weighted average of the mass of naturally occurring isotopes of the element.

A standing wave is a wave that oscillates in time but that does not move in space. Standing waves occur due to interference between two waves that have equal amplitude and frequency that travel in opposite directions.

Static equilibrium is a special case of mechanical equilibrium where both the velocity and rotational speed of an object are constant and equal to zero.

Static friction is the force that must be overcome in order to get an object at rest to start moving.

Sublimation is the process by which a substance goes directly from a solid to a gas without going through the liquid phase.

A system is any collection of objects that we define. The concept of a system is used to determine what is internal or external when discussing concepts such as momentum conservation.

Tangential velocity quantifies the linear distance that something travels as it rotates per unit of time. The direction of the tangential velocity is tangent to the motion of the rotation. (symbol: v_T, unit: m/s)

Technology is the outcome of using scientific principles to solve problems. This can be a product, innovation, or other object whose creation is based on science such as physics, chemistry, or biology.

Temperature defines the average kinetic energy of an object. It quantifies the “hotness” or “coldness” of something. (symbol: T, unit: °C or K)

Tension is the force that exists when something is pulled or stretched apart.

A theory is an explanation of how the universe works that is supported by multiple, repeated experiments.

Thermodynamics is the study of heat flow and its relationship to temperature, work, energy, and entropy.

The timbre of a sound wave describes the qualities of that wave that make it distinct from other sound waves that have the same frequency and amplitude.

Torque is a twist or a turn that causes an object to change its rotational motion. Torque creates rotational acceleration and causes the rotation of an object to speed up, slow down, or change direction. (symbol: τ, unit: Nm)

Total internal reflection describes the phenomenon that occurs when light travels from an object with a high index of refraction to an object with lower index of refraction at an angle greater than the so-called critical angle. Light that is incident at angles greater than the critical angle will not refract at all, but will instead completely reflect off of the interface.

A transformer is an electronic device that is capable of transferring energy from one circuit (known as a primary circuit) to another (known as a secondary circuit) with no physical connection between the two circuits. It can increase the voltage, decrease the voltage, or keep the voltage constant between primary and secondary devices.

An object is translucent if it allows light to pass through, but scatters that light in the process.

Transmission occurs when light waves pass through an object.

The term transmutation refers to the change of one nuclide into a completely different nuclide.

An object is transparent if it allows visible light to pass through without getting scattered or absorbed.

In a transverse wave, the vibrations of the wave travel perpendicular to the direction of motion of the wave.

Ultrasound is the set of sound waves that have frequencies above 20,000 Hz. They are not audible to humans.

The uncertainty principle theorizes the extent to how well we can know information about related properties.

Vaporization is the phase change that occurs when a substance converts from a liquid to a gas.

A vector quantity is a variable that must be conveyed with both a numerical quantity (indicating magnitude or strength) and a direction.

Velocity is the vector quantity that describes the rate at which an object changes its position. Velocity is equal to displacement divided by time. (symbol: v, unit: m/s)

The venturi effect states that as the area of fluid flow decreases, the speed of fluid flow must increase.

A vibration is a periodic back-and-forth motion that remains fixed in one location.

Voltage is a difference in electric potential between two points in a circuit. It can be considered as a "push" that causes current to flow. (symbol: V, unit: V)

A wave is a periodic oscillation that transfers energy from one place to another.

The wavelength of a wave describes the shortest distance between two identical repeating points on a wave. (symbol: λ, unit: m)

The weight of an object is proportional to its mass as well as the force of gravity acting on that object, related by Newton's second law. More accurately, weight arises due to a supporting force acting on the object.

Work is energy that is transferred to an object by exerting a force on that object over a distance. (symbol: W, unit: J)

The work-energy theorem states that work is equal to the change in kinetic energy of an object, W = ΔKE.