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Physics Definitions G481 Mechanics Scalar quantity Vector quantity Displacement Instantaneous speed Average speed Velocity Acceleration The Newton Terminal Velocity Centre of gravity Torque of a couple Couple Moment of a force Principle of moments Conditions necessary for an object to be in equilibrium Thinking Distance Braking distance Stopping distance Work done The Joule principle of conservation of energy Power The Watt Stress Strain Young’s modulus Ultimate tensile strength Hooke’s law Elastic deformation Plastic deformation Elastic Limit A quantity that has magnitude / size only A quantity that has (both) magnitude / size and direction Distance moved in a certain direction Speed at a particular time. This may be distance ÷ time or the gradient of a tangent on a speed – time graph. Rate of change of distance OR distance travelled ÷ time taken Rate of change of displacement OR change in displacement ÷ time Rate of change of velocity OR change in velocity ÷ time (Force is 1 N ) when a 1 kg mass has an acceleration of 1 ms-2 This occurs when the weight of a body = the drag /air resistance forces on the body. Point on a body where the entire weight appears to act one of the forces × perpendicular distance (between forces) A pair of equal and opposite forces (with their lines of action separated by a distance) moment = force x perpendicular distance from pivot / axis / point For equilibrium of an object the sum of clockwise moments about a point = sum of anticlockwise moments about the same point Net / total / resultant force = 0 Net / total torque / moment = 0 The distance travelled (by the car) from when the driver sees a problem and the brakes are applied. The distance travelled (by the car) whilst the brakes are applied and the car stops. thinking distance + braking distance force x distance moved in the direction of the force The energy needed to move an object one metre with a force of 1N (1Nm). Energy cannot be created or destroyed; it can only be transferred/transformed into other forms or The (total) energy of a system remains constant or (total) initial energy = (total) final energy Work (done)/time or energy/time or rate of work done One Joule per second. Force per unit area Extension per unit length Stress / strain Maximum stress material can withstand (before fracture) extension (or compression) α force as long as elastic limit is not exceeded Material returns to its original length / shape/ size when the force / stress is removed Material does not return to its original length / shape/ size when the force / stress is removed Point beyond which an object will not return to its original length/shape/size when the load is removed. G482 Electrons Photons and Waves Conventional current Current moves from + to – (of battery in circuit) Electron flow in a circuit Electrons move from – to + (terminals of a battery in a circuit) The Coulomb The ampere second (As) Mean drift velocity The average displacement/distance traveled of the electrons along the wire per second. Kirchoff’s 1st law (sum of/total) current into a junction equals the (sum of/total) current out. This is an example of conservation of charge. Potential difference The electrical energy converted to other forms per coulomb of charge flowing between two points in a circuit. Electromotive force The electrical energy given to each coulomb of charge in the battery /power supply. Resistance p.d./current The Ohm The volt per ampere (VA-1) Resistivity ρ = RA/l with terms defined Kilowatt hour (a unit of) energy equal to 3.6 MJ or 1 kW used for 1 h Kirchoff’s 2nd Law (sum of) e.m.f.s = sum /total of p.d.s/sum of voltages (in a loop). This is an example of conservation of energy. Terminal p.d. Potential difference between the terminals of a power supply Amplitude Maximum displacement from the rest position. Wavelength Distance between (neighbouring) identical points/points with same phase (on the wave) i.e. crest – crest. Period The time taken for one complete oscillation. Frequency number of oscillations (at a point) per unit time/second. Intensity rate of transfer of energy per unit area at right angles to the wave velocity i.e. power ÷ cross-sectional area Phase difference The amount by which one wave leads or lags behind another wave. Speed of a wave distance traveled by the wave (energy) per unit time/second Reflection Change in direction of a wave when it meets an impenetrable barrier (bounces off) Refraction The sudden change in direction of a wave as it crosses the boundary from one material to another. It is caused by a sudden change in speed. NB wavelength changes but frequency does not. Diffraction The spreading out of a wave after passing through a gap or around an edge Interference when (two) waves meet/combine/interact/superpose, etc. (at a point) there is a change in overall intensity/displacement Plane polarised waves (transverse) waves with oscillations in one plane only. Malus law Intensity of polarized light transmitted I = I0 cos2 θ Principle of superposition When two waves pass through the same space at the same time the of waves resultant displacement is the algebraic sum of the displacements produced by each wave. Coherence Path difference Constructive interference Destructive interference Progressive wave Stationary wave constant phase difference/relationship (between the waves) The extra distance travelled by one wave compared to the other. Two waves arrive at a point in phase so their displacements add up. Two waves arrive at a point out of phase so their displacements cancel. Waves which always travel away from their source. A wave produced from the interference of a progressive wave and its reflection. A wave with zero speed of travel, because it is fixed between two points. Node node occurs where the amplitude/displacement is (always) zero Antinode antinode occurs where the amplitude (of the standing wave) takes the maximum (possible) value Fundamental mode of The lowest frequency standing wave that can be produced (on a string vibration or in a pipe). Harmonics Odd or even integer multiples of the fundamental frequency that produce a stationary wave. Photon a quantum/packet/particle of energy of electromagnetic radiation. The electron volt (eV) an eV is the energy acquired by an electron accelerated/moves through a p.d. of 1 V. 1 eV = 1.6 x 10-19 J Work function The minimum energy (of a photon) that will produce a photoelectron. Threshold frequency The minimum frequency (of light) needed to produce photoelectrons. Intensity intensity is the (incident) energy per unit area per second or power per unit area or power per m2 Emission line spectrum The light emitted from (excited isolated) atoms producing a series of (sharp/bright/coloured) lines against a dark background absorption spectrum a series of dark lines (appears against a bright background /within a continuous spectrum). Continuous spectrum A spectrum where all wavelengths/frequencies/colours are present (in the radiation). The de Broglie Electrons are observed to behave as waves/show wavelike properties wavelength of an electron after passing through a gap. The electron wavelength depends on its speed/momentum. G484 Newtonian World Newton’s 1st Law Newton’s 2nd Law Newton’s 3rd Law Impulse S.H.M. Conservation of linear momentum Inelastic collision Conditions necessary for circular motion In SHM what is the difference between displacement and amplitude. In SHM what is the difference between frequency and angular frequency. Boyle’s law Assumptions of the kinetic model of ideal gases How a gas exerts a pressure Conclusions about air molecule motion from Brownian Motion observations of smoke particles. Thermal equilibrium An object will remain at rest or travel at constant velocity unless acted on by a (an external unbalanced) force. Force is proportional to the rate of change of momentum When one body exerts a force on another the other body exerts an equal (in magnitude) and opposite (in direction) force on the first body. Change in momentum or product of force and time acceleration is (directly) proportional to displacement and is directed in the opposite direction to the displacement / towards the equilibrium position. Total momentum is constant or total momentum before = total momentum after a collision. One in which there is some loss of kinetic energy or KE before is not equal to the KE after the collision. The resultant or net or overall force acts (on object) perpendicular to the velocity or towards the centre of the circle Displacement is the distance (of the body) from an equilibrium position. Amplitude is the maximum displacement Frequency is the number of oscillations/cycles per unit time/second Angular frequency is product of 2π x frequency or 2πperiod. Pressure is inversely proportional to volume for a fixed mass of gas at constant temperature 1. particles move with rapid, random motion (WTTE) 2. elastic collisions. 3. negligible (or zero) volume of atoms (compared with volume of container). 4. no intermolecular forces (except during collisions)/all internal energy is KE. 5. collision time negligible (compared to time between collision). 1. molecules make collisions with walls/surface 2. (hence) exerts a force on the wall (or each collision has a change of momentum) 3. Pressure = force/area 1. air molecules are moving in different directions/randomly with different speeds. 2. mass/size of air molecules is smaller than smoke particles No net heat flow between objects or objects are at the same temperature. G485 Fields, Particles and Frontiers of Physics Capacitance charge per (unit) potential difference allow charge / potential difference, charge/pd, charge/voltage but not charge / volt, coulomb /pd (a mixture of quantities and units). Assumptions of Olbers Paradox Universe is; static / homogeneous Universe is: infinite / infinite number of stars magnetic flux magnetic flux density x area (perpendicular to field direction) Allow equation (Φ = BA) with the symbols identified correctly Faraday’s law Induced e.m.f is proportional to the rate of change of (magnetic) flux The Farad coulomb per volt Allow: 1 F = 1 CV-1 Hubble’s Law the process of nuclear fusion in the core of the Sun The speed of recession of a galaxy is proportional to its distance (from Earth / observer) 1.Protons / hydrogen nuclei to produce He nuclei (positrons and neutrinos) 2. There is electrostatic repulsion (between the protons) / The protons repel (each other because of their positive charge) 7 the formation of a star such as our Sun evolution of a star that is much more massive than our Sun Observations supporting the idea of the big bang 3. High temperatures / 10 K needed (for fusion) 4. (At high temperatures some of the fast moving) protons come close enough to each other for the strong (nuclear) force (to overcome the electrostatic repulsion) 5. High density / pressure (in the core of the Sun) 6. There is a decrease in mass, hence energy is released / products have greater binding energy 1. Gas / dust (cloud) drawn together by gravitational forces 2. Loss in (gravitational) PE / KE increases / PE changes KE / temperature increase 3. Fusion of protons / hydrogen nuclei (produces helium nuclei and energy) 4. A stable star is formed when radiation pressure is equal to gravitational pressure 5. When hydrogen runs out the outer layers of the star expands / core shrinks 6. Red giant formed / eventually (the core becomes) a white dwarf As above plus Supernova followed by Neutron star or Black hole 1. Spectra from galaxies show shift to longer wavelengths (suggests galaxies are moving away from the Earth) 2. The more distant galaxies are moving faster (than the ones The Parsec closer to our galaxy). 3. Existence of microwave background radiation (which is the same in all directions) / The temperature of universe is 3K (after cooling due to expansion) / gamma (radiation) became microwaves (as the universe expanded). 4. Existence of primordial helium (produced in the early stages of the universe). 5. Temperature fluctuations (predicted and observed) Parsec is a distance that gives a (stellar) parallax of 1 second (of o Piezoelectric effect Acoustic impedance matching Spontaneous and random nature of radioactive decay of unstable nuclei The decay constant The technique of radioactive carbon-dating The use of image intensifiers and contrast media when X-rays are used to produce images of internal body structures Difference between a CAT scan and an X-ray How ultrasound scanning is used to obtain diagnostic information about internal structures of a body. arc) / 1/3600 The application of a p.d. across a material / crystal causes an expansion / contraction / vibration (Acoustic) impedances of media are similar / identical Spontaneous: the decay cannot be induced / occurs without external influence. Random: cannot predict when / which (nucleus) will decay next The probability of decay of a nucleus per unit time Allow = A/N with symbols defined. 1. Living plants / animals absorb carbon(-14) 2. Once dead, the plant does not take in any more carbon(-14) 3. The fraction of C-14 to C-12 (nuclei) or number of C-14 (nuclei) or activity of C-14 (nuclei) is measured in dead and living samples 4. x =xo e-t used to estimate the age 1. Intensifier used as X-ray would pass through film 2. Intensifier converts X-ray photon to many visible (light) photons (which are absorbed by film) 3. Lower exposure / fewer X-rays needed 4. Iodine / barium (used as contrast material) 5. High Z number / large attenuation coefficient / large absorption coefficient (used to improve image contrast) 6. Contrast media are ingested / injected into the body 7. Scan shows outline / shape of soft tissue Simple X-ray is one directional / produces single image CT image(s) taken at different angles / X-ray tube is rotated Computer processes data / image constructed from many slices 1. Pulses of ultrasound (sent into the body) 2.Wave / ultrasound / pulse / signal is reflected (at boundary of tissue) 3. Time of delay used to determine depth / thickness 4. The fraction of reflected signal is used to identify the tissue