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
