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fusion : A nuclear reaction in which two or more atomic nuclei join together, or “fuse”, to form a single heavier nucleus.

fundamental forces: Also called fundamental interactions.

freezing point : The temperature at which a substance changes state from liquid to solid.

lever : A type of machine consisting of a beam or rigid rod pivoted at a fixed hinge or fulcrum; one of six classical simple machines.

force (F) : Any interaction that, when unopposed, will change the motion of a physical body. A force has both magnitude and direction, making it a vector quantity. The SI unit used to measure force is the newton.

fission : Either a nuclear reaction or a radioactive decay process in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays) and releasing relatively large amounts of energy.

entropy : A quantity which describes the randomness of a substance or system.

fermion : A type of particle that behaves according to Fermi–Dirac statistics, obeys the Pauli exclusion principle, and possesses half-integer spin. Fermions include all quarks and leptons, as well as all composite particles made of an odd number of these (such as all baryons and many atoms and nuclei). Fermions constitute one of two main classes of particles, the other being bosons.

falling bodies : Objects that are moving towards a body with greater gravitational influence, such as a planet.

exothermic : An adjective used to refer to a process or reaction that releases energy from a system, usually in the form of heat but also in the form of light, electricity, or sound. Contrast endothermic.

escape velocity : The velocity at which the kinetic energy plus the gravitational potential energy of an object is zero. It is the speed needed to “escape” from a gravitational field without further propulsion.

endothermic : An adjective used to refer to a process or reaction in which a system absorbs energy from its surroundings, usually in the form of heat but also in the form of light, electricity, or sound. Contrast exothermic.

energy : The ability to do work.

electronics : A field that deals with electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes, and integrated circuits as well as associated passive interconnection technologies.

electronegativity : A chemical property that describes the tendency of an atom or a functional group to attract electrons (or electron density) towards itself.

electronvolt (eV) : A unit of energy equal to approximately 1.6×10⁻¹⁹ joule. By definition, it is the amount of energy gained by the charge of a single electron moved across an electric potential difference of one volt.

electron paramagnetic resonance:Also called electron spin resonance (ESR) and electron magnetic resonance (EMR).

A method for studying materials with unpaired electrons which makes use of the Zeeman effect. It shares some basic principles with nuclear magnetic resonance (NMR).

electron : A subatomic particle with a negative elementary electric charge.

electromotive force: Also abbreviated emf.

The electrical intensity or “pressure” developed by a source of electrical energy such as a battery or generator and measured in volts. Any device that converts other forms of energy into electrical energy provides electromotive force as its output.

electromagnetic radiation: Also abbreviated EM radiation or EMR.

A form of energy emitted and absorbed by charged particles, which exhibits wave-like behavior as it travels through space.

electromagnetic field: Also abbreviated EM field or EMF.

A physical field produced by moving electrically charged objects.

electromagnet : A type of magnet in which the magnetic field is produced by the flow of electric current.

electricity: The set of physical phenomena associated with the presence and flow of electric charges.

electrical resistance : The opposition to the passage of an electric current through an electrical element.

electrical network : An interconnection of electrical elements such as resistors, inductors, capacitors, voltage sources, current sources, and switches.

electrical insulator : Any material whose internal electric charges do not flow freely and which therefore does not conduct an electric current under the influence of an electric field.

electrical conductor : Any material which contains movable electric charges and therefore can conduct an electric current under the influence of an electric field.

electric power: The rate at which electric energy is transferred by an electric circuit.

electric field: The region of space surrounding electrically charged particles and time-varying magnetic fields. The electric field represents the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding.

electric current : A flow of electric charge through a conductive medium.

electric circuit : An electrical network consisting of a closed loop, giving a return path for the current.

electric charge : A physical property of matter that causes it to experience a force when near other electrically charged matter. There are two types of electric charge: positive and negative.

elasticity : The tendency of a material to return to its original shape after it is deformed.

dynamics : The branch of classical mechanics that studies forces and torques and their effects on motion, as opposed to kinematics, which studies motion without reference to these forces.

ductility : A solid material’s ability to deform under tensile stress; this is often characterized by the material’s ability to be stretched into a wire.

drag: Forces which act on a solid object in the direction of the relative fluid flow velocity. Unlike other resistive forces, such as dry friction, which is nearly independent of velocity, drag forces depend on velocity.

Doppler effect : The change in frequency of a wave (or other periodic event) for an observer moving relative to its source. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.

distance : A numerical description of how far apart objects are.

displacement : 1.  (fluid) Occurs when an object is immersed in a fluid, pushing it out of the way and taking its place. The volume of the immersed object will be exactly equal to the volume of the displaced fluid, so that the volume of the immersed object can be deduced if the volume of the displaced fluid is measured.2.  (vector) The shortest distance from the initial to the final position of a point. Thus, it is the length of an imaginary straight path, typically distinct from the path actually travelled by.

dielectric : An electrical insulator that can be polarized by an applied electric field. When a dielectric material is placed in an electric field, electric charges do not flow through the material as they would in a conductor but only shift slightly from their equilibrium positions, with positive charges displaced in the direction of the field’s flow and negative charges displaced in the opposite direction; this creates an internal electric field that reduces the larger field within the dielectric material.

derivative: For a mathematical function of a real variable, a measurement of the sensitivity to change of the function value (output) with respect to a change in its argument (input); e.g. the derivative of the position of a moving object with respect to time is the object’s velocity and measures how quickly the position of the object changes as time changes. Derivatives are a fundamental tool of calculus.

density: Also called mass density.

A physical property of a substance defined as its mass per unit volume.

deformation: 1.  (mechanics)2.  (engineering)

deflection : The degree to which a structural element is displaced under a load. It may refer to an angle or a distance.

DC motor : A mechanically commutated electric motor powered by direct current.

Damping ratio: Any influence upon or within an oscillatory system that has the effect of reducing, restricting, or preventing its oscillations. Damping is a result of processes that dissipate the energy stored in the oscillation.

cyclotron : A type of particle accelerator in which charged particles accelerate outwards from the center along a spiral path.

curvilinear motion: The motion of a moving particle or object that conforms to a known or fixed curve. Such motion is studied with two coordinate systems: planar motion and cylindrical motion.

cube of theoretical physics : See cGh physics.

critical mass : The smallest amount of fissile material needed for a sustained nuclear chain reaction.

crest : The point on a wave with the maximum value or upward displacement within a cycle.

coulomb (C) : The SI derived unit of electric charge, defined as the charge transported by a constant current of one ampere in one second.

convection : The transfer of heat by the actual transfer of matter.

condensed matter physics : A branch of physics that studies the physical properties of condensed phases of matter.

Compton scattering : A type of light–matter interaction in which a photon is scattered by a charged particle, usually an electron, which results in part of the energy of the photon being transferred to the recoiling electron; a resulting decrease in the energy of the photon is called the Compton effect. The opposite phenomenon occurs in inverse Compton scattering, when a charged particle transfers part of its energy to a photon.

cohesion : The tendency of similar particles or surfaces to cling to one another. Contrast adhesion.

classical mechanics: Also called Newtonian mechanics.

A sub-field of mechanics concerned with the set of physical laws describing the motion of bodies under the collective actions of a system of forces.

chemical physics : A branch of chemistry and physics that studies chemical processes from the point of view of physics by investigating physicochemical phenomena using techniques from atomic and molecular physics and condensed matter physics.

cGh physics : Any attempt in mainstream physics to unify existing theories of relativity, gravitation, and quantum mechanics, particularly by envisioning the three universal constants fundamental to each field – the speed of light ( c ), the gravitational constant ( G ), and the Planck constant ( h ) – as the edges of a three-dimensional cube, at each corner of which is positioned a major sub-field within theoretical physics according to which of the three constants are accounted for by that sub-field and which are ignored. One corner of this so-called “cube of theoretical physics”, where all three constants are accounted for simultaneously, has not yet been satisfactorily described: quantum gravity.

chain reaction : A sequence of reactions in which a reactive product or byproduct causes additional similar reactions to take place.

Centripetal force : A force which keeps a body moving with a uniform speed along a circular path and is directed along the radius towards the centre.

Centrifugal force: The apparent outward force that draws a rotating body away from the centre of rotation. It is caused by the inertia of the body as the body’s path is continually redirected.

Central-force problem: A classic problem in potential theory involving the determination of the motion of a particle in a single central potential field. The solutions to such problems are important in classical mechanics, since many naturally occurring forces, such as gravity and electromagnetism, are central forces.

Centigrade : See Celsius scale.

Center of mass: Within a given distribution of mass, the unique point in space at which the weighted relative position of the distributed mass sums to zero.

Center of gravity : The point in a body around which the resultant torque due to gravity forces vanish. Near the surface of the earth, where gravity acts downward as a parallel force field, the center of gravity and the center of mass are the same.

Celsius scale: Also centigrade scale.

A scale and unit of measurement of temperature.

Cation: A positively charged ion. Contrast anion.

Cathode : The electrode through which a conventional electric current flows out of a polarized electrical device; the direction of current flow is, by convention, opposite to the direction of electron flow, and so electrons flow into the cathode. In a galvanic cell, the cathode is the positive terminal or pole which accepts electrons flowing from the external part of an electrical circuit. However, in an electrolytic cell, the cathode is the wire or plate having excess negative charge, so named because positively charged cations tend to move towards it. Contrast anode.

Cartesian coordinate system : A coordinate system that specifies each point uniquely in a plane by a set of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in the same unit of length. Each reference line is called a coordinate axis or just axis (plural axes) of the system, and the point where they meet is called the origin, at ordered pair (0, 0). The coordinates can also be defined as the positions of the perpendicular projections of the point onto the two axes, expressed as signed distances from the origin.

Carnot cycle : A theoretical ideal thermodynamic cycle proposed by French physicist Nicolas Léonard Sadi Carnot in 1824 and expanded upon by others in the 1830s and 1840s. It provides an upper limit on the efficiency that any classical thermodynamic engine can achieve during the conversion of heat into work, or conversely, the efficiency of a refrigeration system in creating a temperature difference by the application of work to the system. It is not an actual thermodynamic cycle but is a theoretical construct.

Capacitor: An electrical circuit element consisting of two conductors separated by an insulator (also known as a dielectric).

Capacitance: The ratio of the change in the electric charge of a system to the corresponding change in its electric potential. There are two closely related notions of capacitance: self capacitance and mutual capacitance. Any object that can be electrically charged exhibits self capacitance. A material with a large self capacitance holds more electric charge at a given voltage than one with low capacitance. The notion of mutual capacitance is particularly important for understanding the operations of the capacitor, one of the three elementary linear electronic components (along with resistors and inductors).

Calculus : A branch of mathematics that studies change and has two major sub-fields: differential calculus (concerning rates of change and slopes of curves), and integral calculus (concerning accumulation of quantities and the areas under and between curves). These two branches are related to each other by the fundamental theorem of calculus.

Buoyancy : An upward force exerted by a fluid that opposes the weight of an immersed object.

Bulk modulus : A measure of a substance’s resistance to uniform compression defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. Its base unit is the pascal.

Brownian motion: Also called pedesis.

The presumably random movement of particles suspended in a fluid (liquid or gas) resulting from their bombardment by fast-moving atoms or molecules in the gas or liquid.

Brittleness: The tendency of a material to break without significant plastic deformation when subjected to stress. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound.