theory Flashcards
assumptions of kinetic theory of gasses (4)
- collisions are elastic
- no forces between molecules
- collisions are instantaneous
- molecules are in random motion
the minimum frequency in the photoelectric effect
threshold frequency
how does the photon model of electromagnetic radiation takes account for photoelectric effect (3)
- discrete packets of energy
- one electron can only absorb one photon
- emission only works if photon energy is above the work function
define acoustic impedance (2)
- speed x density
- speed of ultrasound in medium
explain why dark lines occur (4)
- photon is absorbed by the electron and electron becomes excited
- photon energy is the difference between two energy levels
- photon energy relates to a single frequency
- electron becomes de-excited and emits photon in any direction
how is ultra sound produced? (3)
- piezoelectric crystal
- ultra sound waves causes vibration
- vibrations produces emf
define de broglie wavelength
- wavelength associated with a moving particle
what is meant by r.m.s. (root mean squared) of an alternating voltage? (2)
- constant voltage
- that produces power equal to the mean power by an alternating voltage
in x-ray state the meaning of sharpness & contrast
sharpness: ease with edges that can be distinguished
contrast: difference of blackening
explain the existence of discrete energy levels (3)
- energy of photon has corresponding frequency
- change in electron energy level emits a photon
- photon energy is the difference in energy levels
why does a mercury thermometer not provide a direct measurement of thermodynamic temperature (2)
- depends on properties of a real substance
- 0°c isn’t absolute zero
explain how surface temperature of a star can be determined from the wavelength spectrum? (3)
- determine wavelength of peak intensity
- wave length of peak intensity from an object of a known temperature determined
- use wein’s displacement law
explain why the satellite is geostationary (2)
- orbit isn’t polar
- orbits from west to east
what is the type off rectification that converts a.c to pulsing d.c
full wave retrification
how is x-rays produced (2)
- electrons are decelerated/stopped on impact
- KE loss = x-ray photons
explain how there is a point between the earth and the moon where resultant gravitational field strength is equal to zero
- same magnitude
- opposite direction
what happens when there’s no insulation in turns? (3)
- induced emf creates current
- current causes energy dissipation
- temperature increases
how can positrons enter the body in PET scan
- introduction of tracer
- with B+ emmiter
wein’s displacement law
wavelength is inversely proportional to thermodynamic temperature
define redshift
wavelength looks greater due to star moving away from the observer
how to determine if a star has undergone redshift (2)
- examining the lines in the spectrum
- and compare with known spectrum
why is kinetic energy = internal energy (2)
no forces between the molecules
potential energy = 0
when a piston is moved so ideal gas expands, why does the temperature decrease? (3)
- speed decreases
- rms is directly proportional to thermodynamic temperature
- KE decrease so temperature decreases
state the effect of giving rise to maximum amplitude
resonance
[motion is produced by oscillators]
at any given frequency, amplitude of oscillations of the trolley is constant, explain how this indicates that there are resistive forces? (2)
- oscillators constantly supply energy
- without loss of energy amplitude would continuously increase
outline the use of ultrasound to obtain diagnostic information about internal body structures (5)
- ultrasound pulse
- reflected at boundaries
- generated and detected by quartz crystal
- time delay between generation and detection gives information about depth
- intensity of reflected wave gives information about nature of boundary
state what is meant by capacitance of a parallel capacitor (2)
- charge/potential difference
- charge is on one plate, potential is between the plates
explain why in a hall voltage, electrons do not continuously accumulate on one side (3)
- accumulating electrons creates electric field
- electric force opposes magnetic force
- accumulation stops when electric force is equal to magnetic force
suggest why hall probes are usually made using a semiconductor instead of metal
number density of charge carriers is much smaller so hall voltage is bigger
a metal ring is suspended from a fix point by means of a thread
its displaced at distance d and released to oscillate
poles of a magnet is now placed near the ring so it hangs midway
the ring is displaced again at d but have fewer oscillations
explain why (4)
- ring cuts magnetic flux and induces emf
- induced emf induces current
- current produces magnetic field
- that causes resistive force
state how in and x-ray tube the intensity and hardness is controlled (2)
- intensity: vary filament current/pd across the filament
- hardness: vary accelerating pd
explain why maximum kinetic energy of an electron is independent of photon intensity in photo electric effect (3)
- frequency determines energy of photon
- intensity determines number of photons
- kinetic energy depends on the energy of one photon
state two situations in which a charged particle in a magnetic field does not experience a force (2)
- charge is not moving
- charge is parallel to magnetic field
[coil attached to a battery]
when the switch is closed a magnetic field is set up around each coil in the spring
by reference to these magnetic fields explain why there is a change in length and state whether it extends or contracts (4)
- magnetic field around coil is circular
- current in coil interacts with magnetic field
- force is normal to both coil and magnetic field
- force between coils are attractive so contract
[coil attached to a battery and has a small mass hanging]
when the switch is closed the small mass on the free end of the spring is now made to oscillate vertically
use the principal of emf induction to explain why small fluctuations in the current in the spring is found to occur (3)
- coil cuts through magnetic flux
- emf induced
- changing emf causes changing currents
state the purpose of CT scanning
to produce a 3D image of the body
outline the principles of CT scanning (5)
- x-ray
- scan in sections
- scan in different angles
- one section is 2D
- combine images from different sections
what is a photon (2)
quantum energy of electromagnetic radiation
a sinusodial alternating current in a primary coil gives rise to a varying magnetic flux linking in the secondary coil
use faraday’s law to explain why the output from the transformer is an electromotive force (emf) that’s alternating (3)
- magnitude of emf varies as rate of change of flux changes
- direction also change
- flux’s continuously increasing and decreasing so polarity of change emf keeps switching
state what is meant by the mass defect of a nucleus (2)
difference between mass of nucleus and nucleons when infinitely separated
state what is meant by resonance (2)
oscillations at maximum amplitude
driving frequency=natural frequency
state three functions of a capacitor
any points from:
- smoothing
- temporary power supply
- oscillator
- blocking dc
- surge protection
- time delaying
- tuning
state what is meant by radioactive decay (2)
- spontaneous emission of (ionizing) radiation
- emission from unstable nucleus
state hubbles law (2)
speed is directly proportional to distance
where speed is speed of recession of galaxy and distance of galaxy away from observer
explain how hubble’s law and the idea of the expanding universe lead to the big bang theory of the origin of the universe
- parts of universe are moving away from each other
- more distant objects are moving faster
- matter must have been closer together in the past
state what is meant by the internal energy of a system (2)
sum of potential and kinetic energy
of random motion
what happens to the internal energy when string is stretched at constant temperature within its elastic limit (3)
- work done on the system
- no thermal transferred
- internal energy = work done
what happens to the internal energy of a sample of water when it evaporates from a rain puddle in a hot day (3)
- thermal transferred to water
- work done by water
- more thermal transfer than work done so internal energy increase
explain how the detection of gamma ray photons is used to produce an image of the tissue being determined (2)
- arrival time of photons are processed
- image built up of tracer concentration in the tissue
state what happens when a charged capacitor is moved to another circuit (2)
- charges are transferred
- transfers because the pd is not equal
OR - transfer stops when pd is equal
what is the unit of magnetic flux
Weber (Wb)
state what is meant by a tracer
substance with radioactive nuclei introduced to the body
