MODERN PHYSICS Flashcards
thermionic emission
the emission of electrons from the surface of a hot metal
The Photoelectric effect
the emission of electrons from a metal, caused by the incidence of electromagnetic radiation of suitable frequency
the threshold frequency
the threshold frequency of a metal is the minimum frequency of electromagnetic radiation which will cause photoemission from that metal
pair production
creation of a subatomic particle and its antiparticle from another form of energy
pair annihilation
the transformation of a particle and its anti-particle into 2 photons of electromagnetic energy
the electron volt
the amount of energy gained or lost by a single electron when it moves through a potential difference of 1 volt
radioactivity
the emission of either particles or electromagnetic radiation from the nucleus of an atom
X-rays
high energy electromagnetic radiation
the law of radioactive decay
states that the number of disintegrations per second is proportional to the number of nuclei present
activity = λN
the half-life
the half-life of an element is the time taken for half of the nuclei in any given sample to decay.
OR
the half-life of an element is the time taken for the activity of any given sample to decrease to half its original value
one becquerel
defined as the disintegration of one nucleus per second
nuclear fission
the breaking up of a large nucleus into two smaller nuclei of similar size with the release of energy
nuclear fusion
occurs when two small atomic nuclei join together to form a larger nucleus, accompanied by the release of large amounts of energy
properties of a cathode ray
- consist of electrons travelling at high speed
- they travel in straight lines from the cathode to the anode.
- they cause certain substances to fluoresce when struck.
- they have kinetic energy
- they can be deflected by electric and magnetic fields
properties of X-rays
- they are electromagnetic radiation, of very high energy.
- they pass through many materials, such as skin and muscle tissue.
- they will not pass through more dense material (bone).
- they affect photographic film
atomic number
the number of protons in the nucleus
mass number
tells us the total number of protons and neutrons in the nucleus.
isotopes
atoms of the same element which have different numbers of neutrons and therefore different mass numbers
α- particles
these are the nucleus of a helium atom (2 protons and 2 electrons). They are relatively big, and have a positive charge. They have very good ionizing ability, but poor penetrating ability (a few cm in air).
The emission of α-particles reduces the atomic number by 2, and the mass number by 4.
β-particles
these are electrons. They are formed when a neutron divides into the nucleus to become a proton and an electron. They have good ionizing ability and reasonably good penetrating power (30-40cm in air, a few mm of aluminium).
The emission of β-particles increases the atomic number by 1.
γ-rays
this is high energy electromagnetic radiation. it is released because the protons and neutrons rearrange themselves inside the nucleus. It is poor at ionizing, but has a very good penetrating power (few cm of lead).
The emission of γ-rays has no effect on the mass and atomic numbers.
Robert Milikan is usually associated with what physical quantity?
the charge on an electron
properties of an electron
negative charge negligible mass orbits nucleus no internal structure deflected by electric / magnetic field
irishman who gave the electron its name in the 19th century
George Stoney
ways of deflecting a beam of electrons
by means of an electric field and magnetic field
how are electrons accelerated in a cathode ray tube
by the high positive voltage at the anode
what happens to the energy of the electron when it hits the screen of the CRT
it gets converted to light
applications of the photoelectric effect
sound track in film
photography
burglar alarm
photon
a packet of electromagnetic radiation
why was the quantum theory of light revolutionary
light has a particle nature as well as a wave nature
explain why the leaves of the golf leaf electroscope collapse when zinc metal is placed near it
photoelectric emission occurs (emissions get emitted from the surface of the metal)
the leaves become uncharged and therefore collapse
explain what is meant by “zinc has a threshold frequency of 1.04 x 10^15 Hz”
below this frequency electromagnetic radiation / photons will not cause emission of electrons (from the zinc).
what property of light controls the photocell
intensity
how do xrays differ from light rays
xrays penetrate matter / cause ionization, light rays dont
who discovered xrays
Wilhelm Rontgen in 1895
how are xrays produced
accelerated electrons strike a metal target causing electrons in the target to rise to a high orbital level. When these electrons fall back down to a lower level they emit the energy as xrays
justify the statement “xray production may be considered as the inverse of the photoelectric effect”
xray: electrons in, electromagnetic radiation is emitted.
photoelectric: electromagnetic radiation in and electrons are emitted
rutherford had bombarded gold foil with alpha-particles, what conclusion did he form about the structure of the atom?
the atom was mostly empty space with a dense, positively-charged core and with negatively-charged electrons in orbit around it
describe how an emission line spectrum is produced
when the gas is heated the electrons in the gas move up to a higher orbital level and as they fall back down, they emit electromagnetic radiation of a specific frequency
examples of radioisotopes
iodine
caesium
radon
carbon 14
what is measured by the becquerel
rate of decay
activity of a radioactive substance
uses of radioactive isotopes
battery of heart pacemakers. irradiation of food killing cancer cells measuring thickness smoke detectors
instrument used to detect radiation / alpha particles / measure the activity of a sample
geiger muller tube
principle of geiger muller tube
incoming radiation causes ionisation of the gas
what is ionsiation
occurs when a neutral atom loses or gains an electron
material in which fission occurs
uranium
plutonium
parts of a nuclear fission reactor
fuel rods, control rods, shielding, moderator, coolant
what is the function of the moderator in a fission reactor
to slow down fast neutrons to facilitate fission
e.g. graphite / heavy water
role of neutrons in nuclear reactor
to cause nuclear fission / to initiate reaction
in a nuclear reactor how can fission be controlled or stopped
dropping the control rods absorbs the neutrons and prevents further fission
type of material control rods are made of
boron, steel, silver
explain how the heat exchanger operates
energy from reactor transfers to liquid in heat exchanger to drive a turbine
why is it necessary to use a heat exchanger
the material in the reactor is radioactive / allows the core to reach a higher temperature
positive and negative environmental impacts of fission reactors
positive - no CO2 emissions / no greenhouse gases / no gases to result in acid rain
negative - radioactive waste / potential for major accidents
application of fission
generating electrical energy
bombs
source of the suns energy
nuclear fusion
why are large temperatures required for fusion to occur
nuclei are positively charged so enormous energy is required to overcome the very large repulsion
advantages of fusion over fission
hydrogen fuel from the sea is plentiful - uranium for fission is scarce.
no radioactive waste with fusion - fission results in radioactive waste
Work function
Φ the minimum energy needed to remove the loosest electron from the surface of that material
Isotopes
Atoms of an element that have the same number of protons but different numbers of neutrons
Activity (A)
The activity of a radioactive substance is the number of nucleus of that substance decaying per second
The mole
A mole of any substance is the amount of that substance that contains as many particles as there are atoms in exactly 12grams 12
6 C.
This number is 6.02x10^23 and is called Avogadro number
