List of Required Definitionshkamichaelhemsley.weebly.com/uploads/8/4/4/9/8449053… · Web viewPhysics and Physical Measurement. Fundamental Units – seven basic units of the SI

Required Vocabulary – IB 12 Standard LevelPhysics and Physical Measurement

1. Fundamental Units – seven basic units of the SI measurement system: kilogram, second, mole, meter, ampere, Kelvin, candela.

2. Derived Units – units that are combinations of fundamental units. These combinations may or may not have a separate name. (eg. 1 kg m/s2 = 1 N)

3. Accuracy - An indication of how close a measurement is to the accepted value (a measure of correctness).

4. Precision - An indication of the agreement among a number of measurements made in the same way (a measure of exactness).

5. Random Uncertainty - An uncertainty produced by unknown and unpredictable variations in the experimental situation, such as temperature fluctuations and estimations when reading instruments. (Affects the precision of results - Can be reduced by taking repeated trials but not eliminated – shows up as error bars on a graph)

6. Systematic Error - An error associated with a particular instrument or experimental technique that causes the measured value to be off by the same amount each time. (Affects the accuracy of results - Can be eliminated by fixing source of error – shows up as non-zero y-intercept on a graph)

7. Vector – a quantity with both a magnitude and a direction

8. Scalar – a quantity with magnitude only

Mechanics

9. *Displacement (s) - distance traveled in a particular direction (change in position)

10. *Velocity (u,v) - rate of change of displacement

11. *Speed (u,v) - rate of change of distance

12. *Acceleration (a) - rate of change of velocity

13. *Newton’s First Law of Motion – An object at rest remains at rest and an object in motion remains in motion at a constant speed in a straight line unless acted on by an unbalanced force.

14. *Newton’s Second Law of Motion – An unbalanced force will cause an object to accelerate in the direction of the net force. The acceleration of the object is proportional to the net force and inversely proportional to its mass. (Fnet = ma or Fnet = Δ p/Δ t)

-----------------------------------------------------------------------------------------------------A ‘*’ indicates a required definition – that means, know this “word for word.”

Terms in parentheses do not need to be memorized OR indicates an alternate definition

15. *Newton’s Third Law of Motion - When two bodies A and B interact (push or pull), the force that A exerts on B is equal and opposite to the force that B exerts on A.

16. Translational Equilibrium - net force acting on a body is zero

17. *Linear Momentum (p) - product of mass and velocity

18. *Impulse (J) - change in momentum

19. *Law of Conservation of Linear Momentum - The total momentum of an isolated system (no external forces) remains constant.

20. *Work (W) - The product of a force on an object and the displacement of the object in the direction of the force.

21. Kinetic Energy (EK) – product of ½ times the mass of an object times the square of an object’s speed

22. Change in Gravitational Potential Energy – product of an object’s mass times the gravitational field strength times the change in height

23. *Principle of Conservation of Energy – The total energy of an isolated system (no external forces) remains constant. (OR – Energy can be neither created nor destroyed but only transformed from one form to another or transferred from one object to another.)

24. *Elastic Collision – a collision in which kinetic energy is conserved

25. Inelastic Collision – a collision in which kinetic energy is not conserved

26. *Power (P) - The rate at which work is done or the rate at which energy is transferred.

27. *Efficiency (eff) - The ratio of the useful energy (or power or work) output to the total energy (or power or work) input.

Gravitation

28. *Newton’s Universal Law of Gravitation – The force of gravity between two objects is directly proportional to the product of the two masses and inversely proportional to the square of the distance between them and acts along a line joining their centers. (NOTE: The objects are point masses. If they are not point masses but are very far apart, that is, the distance between them is very much greater than their radii, they can be treated like point masses.)

29. *Gravitational Field Strength (g) – gravitational force per unit mass on a point mass (g = Fg

/ m)

30. *Gravitational Potential Energy (EP) - the work done in moving a mass from infinity to a point in space (NOTE: the work done is path independent)



Thermal Physics

31. *Temperature (T) – a. The property that determines the direction of thermal energy transfer between two

objects.b. A measure of the average random kinetic energy of the particles of a substance.

32. Thermal Equilibrium - two objects are in thermal equilibrium when they are at the same temperature so that there is no transfer of thermal energy between them

33. *Internal Energy of a substance (U) - The total potential energy and random kinetic energy of the molecules of the substance.

34. *Thermal Energy (Heat) (Q) - Energy transferred between two substances in thermal contact due a temperature difference.

35. *Mole - An amount of a substance that contains the same number of atoms as 0.012 kg of 12C.

36. *Molar Mass - The mass of one mole of a substance.

37. *Avogadro constant (NA) - The number of atoms in 0.012 kg of 12C ( = 6.02 x 1023).

38. *Thermal Capacity (C) - energy required to raise the temperature of a substance by 1K

39. *Specific Heat Capacity (c) - energy required per unit mass to raise the temperature of a substance by 1K

40. Boiling – a phase change of a liquid into a gas that occurs at a fixed temperature

41. Evaporation – when faster moving molecules have enough energy to escape from the surface of a liquid that is at a temperature less than its boiling point, leaving slower moving molecules behind which results in a cooling of the liquid

42. *Specific Latent Heat (L) - energy per unit mass absorbed or released during a phase change

43. *Pressure (P) – force per unit area acting on a surface

44. *Ideal Gas - a gas that follows the ideal gas equation of state (PV = nRT) for all values of P, V, and T (an ideal gas cannot be liquefied)

45. Real Gas – a gas that does not follow the ideal gas equation of state for all values of P, V, and T (a real gas can approximate an ideal gas in some circumstances)

46. Absolute Zero of Temperature – temperature at which a gas would exert no pressure

47. Kelvin scale of Temperature – an absolute scale of temperature in which 0 K is the absolute zero of temperature



Oscillations and Waves

48. *Displacement (for waves) – distance a particle moves in a particular direction from its mean (equilibrium) position

49. *Amplitude – maximum displacement from the mean position

50. *Frequency (f) – number of oscillations per unit time

51. *Period (T) – time taken for one complete oscillation (cycle) (OR: time taken for one cycle to pass a given point)

52. *Phase Difference – difference in phase between two points

53. *Simple Harmonic Motion – motion that takes place when the acceleration of an object is proportional to its displacement from its equilibrium position and is always directed toward its equilibrium position (NOTE: this motion is defined by the equation a = -ω2x)

54. Damping – involves a force that is always in the opposite direction to the direction of motion of the oscillating particle (NOTE: this force is a dissipative force)

55. Critical Damping – when a resistive force is applied to an oscillating system that causes the particle to return to zero displacement in a minimum amount of time

56. Natural Frequency of Vibration – when a system is displaced from equilibrium and allowed to oscillate freely, it will do so at its natural frequency of vibration

57. Forced Oscillations – a system may be forced to oscillate at any given frequency by an outside driving force that is applied to it

58. *Resonance – a transfer of energy in which a system is subject to an oscillating force that matches the natural frequency of the system resulting in a large amplitude of vibration

59. *Wave Pulse - single oscillation or disturbance in a medium

60. *Continuous Progressive (Traveling) Wave – series of periodic pulses (NOTE: involves a transfer of energy) (NOTE: each point on the wave has the same amplitude)

61. *Transverse Wave – wave in which the direction of motion of the energy transfer (the wave) is perpendicular to the direction of motion of the particles of the medium (NOTE: light waves are transverse) (NOTE: transverse waves cannot be propagated in gases)

62. *Longitudinal Wave – wave in which the direction of motion of the energy transfer (the wave) is parallel to the direction of motion of the particles of the medium (NOTE: sound waves are longitudinal)

63. Wavefront - collection of neighboring points on a wave that are in phase



64. Ray - line drawn perpendicular to a wavefront indicating the direction of motion of the energy transfer

65. Crest - top of a transverse wave

66. Trough - bottom of a transverse wave

67. Compression - area of high pressure in a longitudinal wave

68. Rarefaction - area of low pressure (expansion) in a longitudinal wave

69. *Wavelength (λ) - shortest distance along the wave between two points in phase with one another (OR: distance traveled by the wave in one period)

70. *Wave Speed (v) - speed of transfer of the energy of the wave

71. *Intensity (I) – power received per unit area (NOTE: for a wave, its intensity is proportional to the square of its amplitude)

72. Law of Reflection - The angle of incidence is equal to the angle of reflection when both angles are measured with respect to the normal line

73. *Snell’s Law - The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for a given frequency.

74. *Refractive Index (Index of Refraction) (n) – a. the ratio of the speed of the wave in the refracted medium to the speed of the wave in the

incident medium

b. the ratio of the sine of the angle of incidence to the sine of the angle of refraction

75. Diffraction – the bending of a wave around an obstacle or the spreading of a wave through an opening (NOTE: diffraction is only noticeable when the size of the opening is smaller than or on the same order of the size of the wavelength)

76. *Principle of Superposition – When two waves meet, the resultant displacement is the vector sum of the displacements of the component waves.

77. Constructive Interference – superposition of two waves which are in phase with each other

78. Destructive Interference - superposition of two waves which are out of phase with each other

79. Path Difference – difference in the distances two waves must travel from their sources to a given point

80. *Doppler Effect - The change of frequency of a wave due to the movement of the source or the observer relative to the medium of wave transmission.



Electrostatics

81. *Law of Conservation of Charge – The total electric charge of an isolated system remains constant.

82. Conductor – material through with electric charge flows freely

83. Insulator – material through which electric charge does not flow freely

84. *Coulomb’s Law – The electric force between two point charges is directly proportional to the product of the two charges and inversely proportional to square of the distance between them, and directed along the line joining the two charges. (F = k q1 q2 / r2)

85. Insulator – material through which electric charge does not flow freely

86. *Electric Field Strength (E) - Electric force per positive unit test charge (E = F/q)

87. Radial Field – field that extends radially (like the electric field around a point charge or the gravitational field around a planet)

88. *Electric Potential (V) - work done per unit charge moving a small positive test charge in from infinity to a point in an electric field. (V = W/q) (V = kq/r) (NOTE: the work done is path independent)

89. *Electric Potential Energy (Ee) - energy that a charge has due to its position in an electric field

Electric Currents

90. *Electric Potential Difference (ΔV) – electric potential energy difference per unit charge between two points in an electric field (ΔV = ΔEe / q OR ΔV = W / q)

91. *Electronvolt (eV) – energy gained by an electron moving through an electric potential difference of one volt. (OR: Work done moving an electron through an electric potential difference of one volt.) (1 eV = 1.60 x 10-19 J)

92. *Electric Current (I) – current is defined in terms of the force per unit length between parallel current-carrying conductors (NOTE: one ampere of current is the amount of current in each of two infinitely long straight wires one meter apart experiencing a magnetic force per unit length of 2 x 10-7 newtons)

93. *Resistance (R) - ratio of potential difference applied to a device to the current through the device (R = V/I)

94. Resistor - device with a constant resistance (Ohmic device) over a wide range of potential differences



95. *Ohm’s Law – The current flowing through a device is proportional to the potential difference applied across it providing the temperature is constant. (NOTE: R = V/I is not a statement of Ohm’s Law)

96. Ohmic Device – one whose resistance remains constant over a wide range of potential differences (eg – resistor)

97. Non-Ohmic Device – one whose resistance does not remain constant over a wide range of potential differences (eg – filament lamp)

98. *Electromotive Force (emf) (ε) - Total energy difference per unit charge around a circuit (total energy per unit charge made available by the chemical reaction in the battery)

99. Internal Resistance (r) – resistance inside a battery that causes the battery’s terminal potential difference to be less than its emf

100. Ideal Ammeter – one with zero internal resistance – must be placed in series

101. Ideal Voltmeter – one with infinite internal resistance – must be placed in parallel

102. Potential Divider – two resistors placed in series that divide up the battery’s potential difference (R1 / R2 = V1 / V2)

103. Light-Dependent Resistor (LDR) – sensor whose resistance depends on amount of light shining on its surface – increase in light causes a decrease in resistance

104. Negative Temperature Coefficient (NTC) Thermistor – sensor whose resistance depends on its temperature – increase in temperature causes decrease in resistance

105. Strain Gauge – sensor whose output voltage depends on any small extension or compression that occurs which results in a change of length

Electromagnetism

106. *Magnitude of a Magnetic Field (magnetic field strength, magnetic field intensity, magnetic flux density) (B) – ratio of magnetic force on a current carrying conductor to the product of the current and length of wire and sine of the angle between the current and the magnetic field (B = FB / Ilsinθ) (OR: ratio of magnetic force on a charged particle to the product of the charge and its velocity and the sine of the angle between the velocity and the magnetic field) (B = FB / qvsinθ)

107. *Direction of a Magnetic Field – the direction that the North pole of a small test compass would point if placed in the field (N to S)



Atomic and Nuclear Physics

108. Geiger-Marsden experiment – also known as the Rutherford Alpha Particle Scattering or Gold Foil Experimet

109. Photon – a discrete unit or package of light energy

110. *Nuclide – a particular type of nucleus with a certain number of protons and neutrons

111. *Isotope - nuclei with the same number of protons (Z) but different number of neutrons (N)

112. *Nucleon – a proton or neutron (NOTE: Do not say “a particle in the nucleus” since that would include quarks as well.)

113. *Nucleon Number (Mass Number) (A) - number of nucleons (protons + neutrons) in nucleus

114. *Proton Number (Atomic Number) (Z) - number of protons in nucleus

115. *Neutron Number (N) - number of neutrons in nucleus (N = A – Z)

116. Coulomb interaction (Coulomb force, electrostatic force) – electrostatic force of repulsion between the protons in the nucleus

117. Radioactive Decay – when an unstable nucleus emits a particle (alpha, beta, gamma) (NOTE: Radioactive decay is both a random and a spontaneous process.) (NOTE: The rate of radioactive decay decreases exponentially with time.)

118. Alpha Particle (α)– helium nucleus (2 protons + 2 neutrons)

119. Beta Positive Particle (β+) – electron

120. Beta Negative Particle (β-) – positron (antielectron)

121. Gamma Radiation (γ) – high energy (high frequency) electromagnetic radiation

122. *Radioactive Half-life (T1/2) – a. the time taken for ½ the number of radioactive nuclei in sample to decay

b. the time taken for the activity of a sample to decrease to ½ its initial value

123. Artificial (Induced) Transmutation – when a nucleus is bombarded with a nucleon, an alpha particle or another small nucleus, resulting in a nuclide with a different proton number (a different element).

124. *Unified Atomic Mass Unit – 1/12th the mass of a carbon-12 nucleus



125. *Mass Defect – difference between the mass of the nucleus and the sum of the masses of its individual nucleons

126. *Binding Energy – energy released when a nuclide is assembled from its individual components (OR: energy required when nucleus is separated into its individual components)

127. *Binding Energy per Nucleon - energy released per nucleon when a nuclide is assembled from its individual components (OR: energy required per nucleon when nucleus is separated into its individual components)

128. *Nuclear Fission - a heavy nucleus splits into two smaller nuclei of roughly equal mass

129. *Nuclear Fusion - two light nuclei join to form a heavier nuclei (NOTE: This is the main source of the Sun’s energy.)

130. *Photoelectric Effect - the emission of electrons from a metal when electromagnetic radiation of high enough frequency falls on the surface

131. Activity (A) - number of radioactive disintegrations (decays) per unit time

Energy, Power and Climate Change

132. Second Law of Thermodynamics – Thermal energy may be completely converted to work in a single process, but that continuous conversion of this energy into work requires a cyclical process and the transfer of some energy from the system.

133. Degraded Energy – In any process that involves energy transformations, the energy that is transferred to the surroundings (thermal energy) is no longer available to perform useful work.

134. Sankey Diagram – energy flow diagram

135. Renewable Energy Source – source of energy that cannot be used up (eg. –hydroelectric, photovoltaic cells, active solar heaters, wind, biofuels) (NOTE: In most instances, the Sun is the primary energy source for world energy.)

136. Non-renewable Energy Source – source of energy that can be used up (eg. – coal, oil, natural gas, nuclear)

137. *Energy Density (of a fuel) – the ratio of the energy released from the fuel to the mass of the fuel consumed

138. Fossil Fuels – coal, oil, and natural gas (NOTE: Industrialization led to a high rate of energy usage leading to industry being developed near large deposits of fossil fuels.)

139. Chain Reaction – neutrons released from one fission reaction go on to initiate further reactions (NOTE: Only low-energy neutrons (≈ 1 eV) favor nuclear fission.)



140. Critical Mass – minimum mass of radioactive fuel block needed for a chain reaction to occur

141. Controlled Nuclear Fission – used for power production

142. Uncontrolled Nuclear Fission – used for nuclear weapons

143. Fuel Enrichment – process by which the percentage composition of a desirable radioactive nuclide (eg. – uranium-235) is increased in order to make nuclear fission more likely

144. Moderator – Most neutrons released in fission are fast neutrons, so a moderator is used to reduce their energy down to thermal levels to ensure that the fission is self-sustaining. (eg. – may be made of solid graphite or steam)

145. Control Rods – are used to remove any excess neutrons to ensure the fission reaction continues safely (eg. – may be made of cadmium or boron steel)

146. Heat Exchanger – This allows the nuclear reactions to occur in a place that is sealed off from the rest of the environment. Reactions increase temperature in the core and this thermal energy is transferred to water and the steam that is produced turns the turbines.

147. Photovoltaic Cell (solar cell, photocell) – converts a portion of the solar radiation directly into a potential difference (voltage) using a semiconductor (NOTE: A typical photovoltaic cell produces a very small voltage and is not able to provide much current so is usually used to run electrical devices that do not require a great deal of energy.)

148. Active Solar Heater (solar panel) – designed to capture as much thermal energy as possible by allowing solar radiation to heat water running through a pipe in the panel.

149. Oscillating Water Column (OWC) Ocean-Wave Energy Converter – device built on land that uses the kinetic energy of waves to force air in and out of a turbine which generates electrical energy

150. *Albedo (α)– fraction of the total incoming solar radiation received by a planet that is reflected back out into space (OR: ration of total solar radiation power scattered by a planet to total solar radiation received by a planet) (NOTE: global annual mean albedo is 0.3 for Earth)

151. Greenhouse Effect –Short wavelength radiation received from the Sun causes the Earth’s surface to warm up. Earth will then emit longer wavelength radiation (infra-red) which is absorbed by some gases (eg. - methane, water vapor, carbon dioxide, nitrous oxide) in the atmosphere and re-radiated in all directions. This extra warming of the Earth’s atmosphere is known as the Greenhouse Effect.

152. Global Warming – increase in mean temperature of the Earth in recent years

153. Enhanced (Anthropogenic) Greenhouse Effect – Human activities, mainly related to the burning of fossil fuels, have released extra carbon dioxide into the atmosphere, thereby enhancing or amplifying the greenhouse effect (a possible cause of global warming).



154. Black-Body Radiation – radiation emitted by a “perfect” emitter of radiation

155. Stefan-Boltzmann Law – The total power radiated by a black-body per unit area is proportional to the fourth power of the temperature of the body. (power = σAT4)

156. Emissivity (ε) – ratio of power emitted by an object to the power emitted by a black-body at the same temperature.

157. *Surface Heat Capacity (CS) – energy required to raise the temperature of a unit area of a planet’s surface by 1 K. (CS = Q / (A ΔT))

158. *Coefficient of Volume Expansion (γ) – fractional change in volume per degree change in temperature (γ = ΔV / (V0 ΔT))

159. Intergovernmental Panel on Climate Change (IPCC) – panel established by the World Meteorological Organization and the United Nations Environment Programme in 1988 in which hundreds of governmental scientific representatives from more than one hundred countries regularly assess the up-to-date evidence from international research into global warming and human induced climate change.

160. Kyoto Protocol – an amendment to the United Nations Framework Convention on Climate Change in which signatory countries agree to work towards achieving a stipulate reduction in greenhouse gas emissions (NOTE: Notable non-signers are the United States and Australia.)

161. Asia-Pacific Partnership on Clean Development and Climate (APPCDC) – agreement between six countries (representing approximately 50% of the worlds energy use – Australia, China, India, Japan, Republic of Korea, and USA) to “work together and with private sector partners to meet goals for energy security, national air pollution reduction, and climate change in ways that promote sustainable economic growth and poverty reduction.”

Digital Technology

162. Decimal Number – number written using base-10

163. Binary Number – number written using base-2

164. Least-Significant Bit (LSB) – right hand digit representing the smallest power

165. Most-Significant Bit (MSB) – left hand digit representing the largest power

166. Bumps and Pits – high and low areas of a CD used to encode data (NOTE: Destructive interference occurs when light is reflected from the edge of a pit.)

167. Analog – technique involving codes or signals that can take on a large number of different values between given limits – analog signals vary continuously with time



168. Digital – technique involving codes or signals made up of a large number of binary digits (bits) that can each take only one of two possible values

169. Bit – binary digit that can only take one of two possible values (1 or 0; ON or OFF; High or Low; True or False)

170. Byte – eight separate bits of information

171. *Capacitance (C) – ratio of charge stored in a device to the potential difference across the device (C = q / V)

172. Charge-Coupled Device (CCD) – silicon ship divided into small areas called pixels (NOTE: CCDs are used for image capturing in a large range of the electromagnetic spectrum. They are used in digital cameras, video cameras, medical X-ray imaging, and telescopes, such as the Hubble Telescope.)

173. Pixel – small area of a CCD that acts as a capacitor

174. *Quantum Efficiency (of a pixel) – ratio of the number of photoelectrons emitted to the number of photons incident on the pixel

175. *Magnification – ratio of the length of the image on the CCD to the length of the object

analogue vs digital An analogue signal encodes the information using a range of possible values. A digital signal encodes the information using only two possible values (1 or 0). The digital transmission of information as compared to the analogue transmission of information is not as susceptible to the effects of noise, thus increasing the quality of broadcast information.

analogue-to-digital converter (ADC) An analogue-to-digital converter (ADC) takes a single inputted analogue signal and converts it into a parallel digital signal outputted on several digital output lines.

bit-rate A greater number of bits used for each sample allows for a greater quality of transmitted signal. Thus an increase in bit-rate (the speed at which bits need to be transmitted) will improve the quality of the reproduction of a transmitted signal.

cellular exchange The cellular exchange is connected to all of the base stations in a given area. It selects and monitors the base stations and arranges the allocations of frequency channels to individual base stations and mobile phones.

comparator The output of a comparator circuit depends on how the input to the circuit compares with a fixed known value.

digital-to-analogue converter (DAC) A digital-to-analogue converter (DAC) takes a parallel digital signal inputted on several digital lines and converts it into a single outputted analogue signal.



gain The gain of an amplifier is defined by: G = VOUT / VIN G is the gain (and has no units: it is a ratio), VOUT is the output voltage in V, VIN is the input voltage in V.

gain of a non-inverting amplifier The gain of non-inverting amplifier is given by: G = 1 + RF / R G is the gain (and has no units: it is a ratio), RF is the feedback resistor in Ω, R is the input resistor (the resistor between non-inverting input and 0 V) in Ω.

gain of an inverting amplifier The gain of an inverting amplifier is given by: G = - RF / RG is the gain (and has no units: it is a ratio), RF is the feedback resistor in Ω, R is the input resistor in Ω.

ideal operational amplifier (op-amp) An ideal operational amplifier (op-amp) has infinite gain and draws zero current on its inputs.

inverting amplifier circuit using an op-amp An inverting amplifier circuit has the non-inverting input of the op-amp connected to 0 V. There is an input resistor between the inverting input of the op-amp and the input signal, and a feedback resistor connected between the output and the inverting input of the op-amp.

LSB and MSB The least-significant bit (LSB) is the bit representing 20 and is the furthest on the right when a binary number is written down. The most-significant bit (MSB) is the bit representing the highest power of 2 and is the furthest on the left when a binary number is written down.

mobile phone cells In the mobile phone system, any area is divided into a number of cells (each with its own base station) to which is allocated a range of frequencies for communications.

noise Noise, in any signal, is the term used to describe unwanted additions that degrade the quality of the signal. Dispersions and scatterings that take place within an optical fibre will mean that the output signal is not identical to the input signal, but also includes some noise.

non-inverting amplifier circuit using an op-amp An inverting amplifier circuit has the inverting input of the op-amp connected to 0 V via an input resistor. The input signal is connected directly to the non-inverting input of the op-amp and a feedback resistor connected between the output and the inverting input of the op-amp.

optical fibre transmission, role of amplifiers in Amplifiers are used to increase signal strength and thus correct for attenuation. An amplifier will also increase any noise that has been added to the electrical circuit



optical fibre transmission, role of reshapers in A reshaper can reduce the effects of noise on a digital signal, but returns the signal to a series of 1s and 0s with sharp transitions between the allowed levels.

output circuit using light-emitting diodes (LEDs) Light emitting diodes (LEDs) emit light when current flows through them. If a reverse p.d. is applied across the LED, no current flows. Most LEDs require a protective resistor in series to limit the forward current.

public switched telephone network (PSTN) The public switched telephone network (PSTN) is the arrangement of phones that are physically connected with wires (landline). Different users are connected to one another by the exchange switching connections.

sample-and-hold The analogue output of a sample-and-hold system follows the analogue input. On receiving a control signal, the output stops varying and retains the same fixed value. The input signal is said to have been sampled.

satellite communication Satellite communication is relaying information from source to receiver via an artificial satellite in orbit around the Earth. Communications are in the high-frequency radio part of the EM spectrum.

Schmitt trigger The output of a Schmitt trigger has two possible values. The voltage on the input, which triggers the change between these two values, has a different value for a rising signal when compared with a falling signal. It can be used to reshape digital pulses that have been subjected to noise or dispersion.

up-link frequency Different frequencies are used for sending information to and from a satellite. The up-link frequency is the frequency used for sending information to a satellite.



Astrophysics Option

absolute magnitude The absolute magnitude of a star is the apparent magnitude it would have if it were observed from a distance of 10 pc.

apparent brightness The apparent brightness of a star is the power per unit area received by an observer on the Earth in W m–2. The link between brightness and luminosity is: b = L / 4πd2

b is the apparent brightness in W m-2 L is the luminosity in W d is the distance between the star and Earth in m

apparent magnitude scale The brightness of stars is compared using the (apparent) magnitude scale. This is a logarithmic scale with a magnitude 1 (the brightest stars in the night sky) star being 100 times brighter than a magnitude 6 (just visible to the naked eye) star. One ‘step’ on the magnitude scale is equivalent to a brightness ratio of 2.51 between stars.

apparent vs absolute magnitude There is a relationship linking apparent and absolute magnitude.

asteroid belt Lies between Mars and Jupiter and contains many small rocky objects orbiting the sun.

Big BangThe Big Bang is the creation of the Universe – mass, space, and time. Approximately 15 billion years ago, all of the observable matter of the universe was crushed together at a very high density and temperature. Since then, the universe has been expanding, and thus the temperature and the density have both been decreasing. [Option E]

binary starsOften what appears to be a single star is, in fact, a binary star – two stars in orbit around their common centre of mass.

black holes Black holes are regions of space-time with extreme curvatures due to the presence of a mass. A large-mass star ends its red giant phase in a supernova. The remnant in the centre could be a black hole if its mass is large enough.

blueshift Light from distant stars and galaxies that are coming towards the Earth will show a Doppler shift towards the shorter wavelengths. This is known as blueshift.

Cepheids Cepheid variable stars are a particular type of unstable star that have an observed regular variation in brightness, as a result of a periodic expansion and contraction in their outer layers. There is a mathematical link between the period of brightness variation and their average luminosity: the greater the period of the variation, the greater the luminosity. This relationship allows astronomers to calculate the distance to some galaxies that contain Cepheid variable stars.

Chandrasekhar limit The Chandrasekhar limit is the maximum size for a white dwarf star to be able to exist (approximately 1.4 solar masses). Below this mass, electron degeneracy pressure



allows the white dwarf to be stable. Above this mass, further gravitational collapse must take place.

closed universe A closed universe is one whose expansion is brought to a stop as a result of the force of gravity. The universe will then collapse back on itself. This would happen if the density of the universe were high.

comets Comets are mixtures of rock and ice in very elliptical orbits around the Sun.

constellation A constellation is a group of stars that, to an observer on the Earth, are in close angular proximity to one another for a region of the sky with a specific pattern of stars.

cosmic microwave background radiation Cosmic microwave background radiation is the microwave radiation that is coming towards us from all directions in space and provides evidence for the existence of the Big Bang. The spectrum of this radiation is in agreement with black-body radiation for an object at a temperature of 2.7 K. Radiation released soon after the Big Bang would change wavelength as the universe expanded and is consistent with the current average temperature of the universe.

critical density The critical density is the density of the universe that would create a flat universe. It is approximately 5 × _10–26 kg m–3.

dark matter Dark matter makes up the majority of the mass in the universe and its nature is unknown. It is called dark matter because it does not emit light. The mathematics of orbital motion allow us to calculate how much matter must be contained within a galaxy in order for the outer stars to remain in orbit. The stars and their associated planes account for a maximum of 10% of the matter that must be there. The unexplained 90% is dark matter.

eclipsing binary starsA spectroscopic binary is a pair of stars whose existence can be deduced from analysis of variations in the brightness of the light received. Over time the wavelengths show a periodic ‘dip’ in brightness.

electron degeneracy pressure The quantum mechanical process that allows white dwarf stars to be stable and avoid further gravitational collapse.

elementary (fundamental) particles in the early universe At the very high temperatures of the early universe, only elementary (fundamental) particles could exist. Over time, expansion gave rise to cooling, eventually resulting in temperatures at which light nuclei could be stable, followed by temperatures at which atoms could be formed.

equilibrium of a starA star’s size is stable because it is in equilibrium between radiation ‘pressure’ (the tendency of the hot central mass to expand outwards into the surrounding vacuum of space) and gravitational ‘pressure’, the inward pull of gravity.

flat universe



A flat universe is one that continues to expand forever. The force of gravity will continually slow down the rate of recession of the galaxies. The density of the universe is an exact amount so that it takes an infinite amount of time to bring the expansion to a halt. This happens if the density of the universe were equal to the critical density.

fusion Fusion is main energy source in stars.

galactic cluster A galactic cluster is a number of galaxies (e.g. 103) that are located relatively close to one another.

galactic supercluster A galactic supercluster is a large number of galactic clusters that are located relatively close to one another. Typically the galaxies are arranged together in bands.

Hertzsprung–Russell diagram (and main-sequence stars)Hertzsprung–Russell diagrams are plots of data from different stars with each point representing a different star. Their axes have different possible labels, but whichever one is chosen the diagram will be equivalent. The vertical axis is a logarithmic plot of the luminosity or the absolute magnitude of the star. The horizontal axis is the spectral class of the star or a (decreasing) logarithmic plot of it temperature. The majority of stars are found on a line that runs from top left to bottom right called the main sequence.

light-year A unit of distance to measure the distance travelled by light in a vacuum during one year. It is equal to 9.46 × 1015 m.

luminosity The luminosity of a star is the total power radiated in W.

MACHOs Massive astronomical compact halo objects (MACHOs) are one possible theoretical explanation for dark particles.

mass of star, effect on the end product of nuclear fusion and changes in nucleosynthesisStars on the main sequence are fusing hydrogen nuclei to create helium nuclei. A star’s mass affects the end product of nuclear fusion. Small-mass stars go through a red giant phase in which helium is fused, creating carbon and oxygen. This process ends in a planetary nebula with the remnant being a white dwarf. Larger-mass stars end up as a red supergiant in which the elements up to iron can be fused. This process ends in a supernova with the remnant being a neutron star or a black hole.

mass–luminosity relationshipThe luminosity of a star on the main sequence is related to its mass by: L is proportional to mn

L is the luminosity in W, m is the mass in kg, n is a number between 3 and 4.

neutron degeneracy pressureThe quantum mechanical process that allows neutron stars to be stable and avoid further gravitational collapse.

neutron starA large-mass star ends its red giant phase in a supernova. The remnant in the centre could be visible as a neutron star if its mass is small enough.



Newton’s model of the universeFrom observations of the stars, Newton’s model of the universe is that it is infinite, uniform, and static.

Olbers’ paradox Olbers’ paradox is that the night sky is dark, but if one accepts Newton’s model of the universe, then the night sky should be bright. In simple terms, whatever direction you look in, you should eventually come across a star. A more mathematical analysis shows that more distant stars will appear dimmer, but this effect is cancelled out by the increased likelihood of observing stars at larger distances.

open universe An open universe is one that continues to expand forever. The force of gravity will slow down the rate of recession of the galaxies, but it is not strong enough to bring the expansion to a halt. This would happen if the density of the universe were low.

Oppenheimer–Volkoff limitThe Oppenheimer–Volkoff limit is the maximum size for a neutron star to be able to exist (between 2 and 3 solar masses). Below this mass, neutron degeneracy pressure allows the neutron star to be stable. Above this mass, further gravitational collapse must take place into a black hole.

parsecThe parsec is an astronomical measurement of distance equal to 3.26 light-years. It is the distance to a star that has a parallax angle of one second of arc

planetary nebula A low-mass star ends its red giant phase by ejecting the other layers of the star. The remains of the outer layers are visible as a planetary nebula. The remnant in the centre is a white dwarf star.

pulsars Pulsars are cosmic sources of very weak radio wave energy that pulsate at a very rapid and precise frequency. They have been theoretically linked to rotating neutron stars.

red giants and supergiants Red giant stars are large in size and comparatively cool and so red in colour. They are one of the later possible stages for a star where the source of energy is the fusion of some elements other than hydrogen. Red supergiants are even larger.

redshift and recession speed of galaxies Redshift is due to the expansion of the universe. The recession speed of galaxies can be approximated from the redshift: Δλ/λ = v/cΔλ is the difference between the observed wavelength and the emitted wavelength of EM radiation in m, λ is the wavelength of EM radiation as emitted by the source in Hz, v is the recession speed of the source in m s–1, c is the speed of EM waves in m s–1.

redshift Light from distance stars and galaxies that are receding will show a Doppler shift towards longer wavelengths. This is known as redshift.

spectral class Stars are classified into different categories depending on their different spectra of light emitted. The seven main spectral classes (in descending temperature) are O, B, A , F,



G, K and M.

spectroscopic binary stars A spectroscopic binary is a pair of stars whose existence can be deduced from analysis of variations in the spectrum of the light received. Over time the wavelengths show a periodic shift of splitting of frequency.

spectroscopic parallaxIt is possible to estimate a star’s luminosity from the relative intensities of different frequencies in its spectrum. The luminosity and the apparent brightness can be used to estimate the stellar distance. This process is known as spectroscopic parallax. The method of spectroscopic parallax is limited to measuring stellar distances less than about 10 Mpc.

standard candles‘Standard candle’ is a term used to describe a source of known luminosity used to make comparisons and estimate other sources of luminosity that are at equivalent distances; for example, if the luminosity of one star in a galaxy is known, then the luminosities of all the other stars in that galaxy can be estimated.

Stefan–Boltzmann lawThe equation for the total power radiated in black-body radiation is: P = σAT4

P is the total power radiated by the black body in W, σ is the Stefan–Boltzmann constant (5.67 × 10–8 W m–2 K–4), A is the surface area in m–2, T is the absolute temperature of the black body in K.

stellar clusterA stellar cluster is a group of stars that are in close relative proximity to one another.

stellar parallax Stellar parallax is the apparent movement (when compared to the ‘fixed’ background of stars) of nearby stars over the course of a year. It is a result of the movement of the Earth during the year. The measured amplitude of the angle variation is called the parallax angle for the star and can be used to calculate its distance.

supernovaA supernova is the catastrophic final end of a red giant star – most of the mass of the star is ejected in a violent explosion.

white dwarfs White dwarf stars are small in size and comparatively hot so white in colour. They are one of the final possible stages for a star being a hot remnant that is cooling down without any fusion reactions taking place.

Wien’s displacement law Wien’s displacement law relates the wavelength at which the intensity of radiation is a maximum to the temperature of the black body:λmax = 2.90x10-3/Tλmax is the wavelength at which the intensity of radiation is a maximum in m, T is the absolute temperature of the black body in K.

WIMPs Weakly Interacting Massive Particles (WIMPs) are one possible theoretical explanation for dark particles.



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