Modern physics Flashcards
3 properties of an electron
extremely small mass
has a negative charge
located in orbit around the nucleus of an atom
thermionic emission
the emission of electrons from the surface of a hot metal
electrons were originally called
cathode rays
what are cathode rays
electrons emitted from a cathode
example of electrons emitted from a cathode
an old television
who suggested the name electron
G J Stoney, irish hysicist
who and when was the charge measured on the electron
milikan in 1909 with his oil drop experiment
in a cathode ray tube what does the cathode do
electrons are emitted from the cathode by thermionic emission
in a cathode ray tube what does the control grid do
by being kept more or less negative (o to -50V) you can control the amount of electrons passing through ie. brightness control
in a cathode ray tube what does the focusing anode do
converges the electrons into a narrow beam so that a well defined image is seen on the screen
in a cathode ray tube what does the accelerating anode do?
has a potential usually over 1000V. accelerates the electrons to a very high speed
in a cathode ray tube what does the flourescent screen do?
when the very fast electrons hit the screen, their kinetic energy is converted to light
what chemical is used on the screen
zinc sulfide
Y-plates
a pair of charged horizontal metal plates
what do the y-plates do?
the electric fields of these plates can deflect the electrons vertically
X-plates
a pair of charged vertical metal plates
what do the x-plates do?
the electric fields of these plates can deflect the electrons horizontally
2 parts of the deflecting system
x plates and y plates
what does the coating of graphite inside of the tube do?
conducts the electrons from the screen back to the accelerating anode. The passage of electrons between cathode and anode is maintained
2 uses of a cathode ray tube
an old television
a cathode ray oscilloscope
2 medical uses of a cathode ray tube
ECG
EEG
what does ECG stand for?
electrocardiogram
what does EEG stand for?
electroencephalogram
what does an ECG do?
displays electrical signals in the heart
what does an EEG do?
displays small varying electrical signals in the brain
kinetic energy of an electron
1/2 m v squared
work done to accelerate an electron
q V
how do you find the speed of an electron accelerated
let the gain in kinetic energy equal to the work done to accelerate
how to find the velocity of an electron in a circular orbit in a magnetic field
let the force of magnetic field equal the centipetal force
force of magnetic field on electron
B q v
centripetal force
r
set up of demonstration to show photoelectric emission
high frequency light falling on a negatively charged plate on a gold leaf electroscope
what happens in the demonstration to show photoelectric emission
gold leaf electroscope deflection decreases
in the demonstration to show photoelectric emission, why does the deflection of the gold leaf electroscope decrease
electrons are being emitted from the negatively charged metal plate due to the incident light
** definition of photoelectric emission **
the emission of electrons from a metal surface using light of a suitable frequency
what do you use to change the frequency of the incident light
using special filters
photoelectric emission AKA
photoelectric effect
demonstration to show photoelectric emission. what do low frequencies result in?
no electrons, the deflection remains constat
below what number is there no loss in deflection
the threshold frequency
graph of number of electrons emitted and frequency of incident light
straight line graph started at the x-axis and threshold frequency
graph of number of emitted electrons and frequency of incident light, what’s on x-axis
frequency of incident light
graph of number of emitted electrons and frequency of incident light, what’s on y-axis
number of emitted electrons
what does the kinetic energy of emmitted elecgrons due to photoelectric emisiion depend on?
the frequency of the incident light (Hz)
to show the effect of intensity of incident light experiment. describe the set-up
a wire anode and cathode in a photocell (vacuum inside) connected to a milliammeter. bulb connect to battery and rheostat shining on anode
to show the effect of intensity of incident light experiment. the brighter the light
the more current recorded
to show the effect of intensity of incident light experiment. the number of electrons emitted is proportion to
the intensity of the incident light
rheostat
variable resistor
to show the effect of intensity of incident light experiment how can you vary the amount of light
the rheostst
what do you measure photocurrent with
an ammeter
to show the effect of intensity of incident light experiment. graph x-axis
photocurrent (mA)
to show the effect of intensity of incident light experiment. graph y-axis
intensity of incident light
to show the effect of intensity of incident light experiment describe the graph
straight line through the origin
what is photocurrent an indication of?
the number of emitted electrons
if you wanted to be very precise what could you use to measure the intensity of light
light meters
to show the effect of intensity of incident light experiment. the brighter the light
the more current recorded
to show the effect of intensity of incident light experiment. the number of electrons emitted is proportion to
the intensity of the incident light
rheostat
variable resistor
to show the effect of intensity of incident light experiment how can you vary the amount of light
the rheostst
what do you measure photocurrent with
an ammeter
to show the effect of intensity of incident light experiment. graph x-axis
photocurrent (mA)
to show the effect of intensity of incident light experiment. graph y-axis
intensity of incident light
to show the effect of intensity of incident light experiment describe the graph
straight line through the origin
what is photocurrent an indication of?
the number of emitted electrons
to show the effect of intensity of incident light experiment. describe the set-up
a wire anode and cathode in a photocell (vacuum inside) connected to a milliammeter. bulb connect to battery and rheostat shining on anode
to show the effect of intensity of incident light experiment. the brighter the light
the more current recorded
to show the effect of intensity of incident light experiment. the number of electrons emitted is proportion to
the intensity of the incident light
2 things that happens to the energy acquired by the electron
used to escape from the metal surface and the rest is used as kinetic energy
what does the number of electrons emitted depend on?
the number of photons ie. the intensity of the incident light
energy of incident photon =
work function + max. kinetic energy of emitted electron
to show the effect of intensity of incident light experiment. graph x-axis
photocurrent (mA)
to show the effect of intensity of incident light experiment. graph y-axis
intensity of incident light
to show the effect of intensity of incident light experiment describe the graph
straight line through the origin
what is photocurrent an indication of?
the number of emitted electrons
what did planck propose?
that electromagnetic radiation consisted of discrete quantities of energy called a quantum or a photon
what year did planck make his proposal
1902
what was the classical idea of electromagnetic radiation
that it was a continuous wave
energy of the quantum is proportional to
the frequency of the radiation
E =
hf
E
energy of the photon
photoelectric emission experiment: why does the leaf not collapse when zinc covered by a piece of glass
uv light cannot pass through ordinary glass and photoelectric emission does not occurs
f
frequency of the radiation
when a photon collides with an electron at or just within the surface of a metal
it may transfer its energy to the electron
3 applications of photoelectric effect
burglar alarms
smoke alarms
reading barcodes
what does it mean to be an all or none process
either the electron gets all the energy of the photon or none at all
energy of the photon is proportional to what
the frequency of radiation
2 things that happens to the energy acquired by the electron
used to escape from the metal surface and the
what did planck propose?
that electromagnetic radiation consisted of discrete quantities of energy called a quantum or a photon
what year did planck make his proposal
1902
what was the classical idea of electromagnetic radiation
that it was a continuous wave
energy of the quantum is proportional to
the frequency of the radiation
what can x-rays also have to help the cooling process
A liquid cooling system
E
energy of the photon
h
planck’s constant
f
frequency of the radiation
when a photon collides with an electron at or just within the surface of a metal
it may transfer its energy to the electron
what type of process is the transfer of energy
an all or none process
what does it mean to be an all or none process
either the electron gets all the energy of the photon or none at all
energy of the photon is proportional to what
the frequency of radiation
2 things that happens to the energy acquired by the electron
used to escape from the metal surface and the rest is used as
why is it good that there isn’t much gas inside the glass in the x-ray
electrons are less probable to hit a gas atom on the way
Φ
work function
threshold frequency is related to
work function (Φ)
threshold frequency equation
Φ = h fo
fo
threshold frequency
other way of writing hf, when equal to
Φ + 1/2 m v squared
hc
—— = Φ + 1/2 m v squared
λ
unit of energy in atomic physics
the electron volt
1 ev =
1.6 x 10 -19 joules
can you use electron volts in calculations
no, must convert to joules
photoelectric emission experiment: why does the leaf collapse when it is a negatively charged piece of metal
the uv light causes photoelectric emissions, ie electrons leave the surface of the metal, therefore the negative charge reduces and it collapses
photoelectric emission experiment: why does the leaf not collapse when the zinc is illuminated with green light
the frequency of the green light is lower than that of uv radiation and it is below the threshold frequency of zinc and photoemission cannot occur
photoelectric emission experiment: why does the leaf not collapse when the electroscope is charged positively
electrons emitted will be attracted back to the zinc plate, and if they could escape it would become even more positive and it would diverge more
3 applications of photoelectric effect
burglar alarms
smoke alarms
reading barcodes
3 steps in how x-rays are made
electrons emitted from the cathode by thermionic effect
accelerated to very high speed by high voltage
they hit the tungsten target and electrons are emitted
what does a lead shield do when making x - rays
protects the user from the harmful effect of x - rays
atomic view; 3 steps to making an x - ray
- high speed electrons hit the tungsten anode and some are energetic enough to penetrate
- outer electro falls into a lower energy level due to missing electron
- excess energy emitted as a photon - an x - ray
99% of energy of the electrons
becomes heat energy when hits the tungsten
does tungsten melt
no, has a high melting point
what conducts the heat to the outside when making x - rays
the tungsten is set in copper which conducts the heat to the cooling fins outside
why does the heat escape off the fins into the air
the cooling fins have a large surface area
what can x-rays also have to help the cooling process
A liquid cooling system
who and when was the x-ray discovered
1895 by the German Physicist, W Rontgen
what does the intensity of an x-ray beam refer to?
the number of x-rays in the beam
what does the number of x-rays required depend on?
the surface area of what is to be examined
intensity depends on (3)
number of electrons crossing the tube
this depends on temperature of the heating coil
this depends on the current from the low voltage battery
what does the penetrating power of the x-rays refer to?
the ability of the x-rays to pass through different thicknesses of material or different densities of material
penetrating power of x-rays depends on what (2)
speed of electrons crossing the tube
which depends on the value of high voltage
2 uses of x-rays
damaged tissue in lungs is more dense, absorbs more x-rays, and shows up as a dense could on x-ray
can locate flaws in metal eg.crack in the body of an airplane
what’s special about the gas in the x-ray tube
very low pressure inside, very little gas inside
why is it good that there isn’t much gas inside the glass in the x-ray
electrons are less probable to hit a gas atom on the way
what would gas molecule collisions do to the electrons
would slow them down
relationship between x-rays and photoelectric emission
it is said that one is the reverse of the other
what goes into making x-rays
electrons
what goes out of making x-rays
photons
when electrons hit the tungsten target when making x-rays
photons are emitted
photoelectric effect what goes in
photon
photoelectric effect what comes out
electrons
photoelectric effect, what happens when photons fall on a metal
electrons emitted
speed of a photon
c, speed of light
we assume that all energy
is given to the electron (even though it isn’t)
speed of an x-ray
same as speed of light when it’s in a vacuum
what does it mean that x-rays are regarded as ionising radiation
they ionise the molecules of a material that they pass through
are x-rays deflected in electric or magnetic fields
no
what does it mean that x-rays aren’t deflected in electric or magnetic fields
they are neutral
how are x-ray photographs possible
they cause a chemical reaction with they film in a similar way that light does
1 danger of x-rays
excessive doses of x-rays could lead to certain forms of cancer
what is a photon
a discrete quantity of electromagnetic radiation
what is an x-ray
a photon of electromagnetic radiation of a very high frequency
who discovered radioactivity
Becquerel
when did Becquerel discover radioactivity?
1896
intensity of radiation
number of emissions per second
the intensity of radiation is proportional to
the mass of the radioactive sample
3 types of radiation
alpha
beta
gamma
emission of radiation
spontaneous
cannot be controlled
definition of radioactivity
the spontaneous disintegration of the unstable nucleus of the unstable nucleus of an atom with he emission of a B or y radiation
alpha charge
positively charged
beta charge
negatively charged
gamma charge
neutral, no charge
alpha
a
beta
B
gamma
y
what does the radiation do to the substance it passes through
it ionises it
splits it into positive and negative ions
are all molecules encountered ionised
no
not all
what happens to the ray or particle after each ionisation
it loses me kinetic energy
how far does a good ioniser travel
not far
how far does a bad ioniser travel
ver far
how are penetration and ionisation related
penetration and ionisation are inversely proportional
a is a
particle
B is a
particle
y is a
ray
ionisation of a
very good ionising power
penetration of a
very little penetration
how do u stop an alpha particle
a thick sheet of paper
how far do alpha particles travel
only a few centimetres in air
ionisation of B
much less ionising than a particles
penetration of B particle
100 times more penetrating power than a particles
what can B particles pass through
up to 5mm thickness of aluminium
ionisation of y rays
very little ionisation
penetrating power of y rays
very penetrating
what does it take to stop y rays
several cm of lead
what do alpha particles consists of?
2 protons and 2 neutrons
what is the alpha particle like
the nucleus of a helium atom
what does a beta particle consist of?
an electron
how is a beta particle formed?
a fast moving electron that comes from the nucleus of an atom
a neutron decays into a proton and an electrons and the electron is the B-particle
symbol for a particle
4
He
2
charge on an alpha particle
++
symbol for a B particle
0
B
-1
what is a gamma ray
a photon of electromagnetic radiation from the nucleus of an atom
how do you calculate how many a and B particles
- take the smaller atomic mass away from the larger atomic mass and divide by 4
- that’s how many alpha particles
- if it is lower than the atomic number still after taking away the 2 protons per a particle. that’s how many B particles were emitted
how does the atomic number get bigger when you take away B particles
misusing a minus
how do you construct a radioactive decay series
you put a new element in after you take away each alpha or beta particle
how do you demonstrate ionisation
charge a gold leaf electroscope bring a radioactive source near it ionises the air above it some ions attracted to cap neutralised less deflection
demonstration to show penetration
bring the radioactive source near the geiger muller tube, you will notice an increase in count rate
distance for a particle count to drop
10cm
what to put between a particles and g m t for count to drop
a piece of paper
what happens when you take the b radiation source away
no difference
what to put between B particles and g m t for count to drop and why
sheets of aluminium. B particles cannot penetrate aluminium
what to put between y rays and g m t for count to drop
a few cm thick lead
who first performed Rutherford’s experiment?
Geiger and Mardsen
what is the set up of Rutherford’s experiment?
a particles bombard a very thin piece of gold foil
most a particles in Rutherford’s experiment?
go straight through gold foil
some a particles in Rutherford’s experiment?
deflected through a small angle
very few a particles in Rutherford’s experiment?
deflected through angles greater than 90º
where was Rutherford’s experiment performed and why
in a vacuum because a particles have a very short range in air
what detected the alpha particles?
a fluorescent screen
how did the fluorescent screen detect the alpha particles
they cause scintillations
what are scintillations
small flashes of light
why would an a particle be deflected through a small angle
if it is repulsed by the positively charged nucleus
why would an a particle be deflected through an angle greater than 90º
if the a particle was hiding straight for the nucleus
why is there a very small chance of being deflected through an angle greater than 90º
the nucleus is very small, very small probability of going straight for it
3 conclusions of Rutherford’s experiment
- the nucleus is very small
- electrons orbit the central nucleus
- atom is mostly made of empty space
4 features of the Bohr model of the atom
- electrons only move in allowed orbits
- when electrons move to a higher energy level, the atom is in an excited state
- when the electron returns to its original position it emits a photon
- energy emitted by an electron:
hf = E1-E2
what acts as the cathode in the geiger muller tube
a metal cylinder
what acts as the anode in a geiger muller tube
a thin wire along the centre
what does the voltage applied across the geiger muller tube depend on? 2
the density and pressure of the gas in the tube
what is usually the gas in the geiger muller tube
argon
what is in the geiger muller tube as well as argon?
a small amount of bromine
function of bromine in geiger muller tube
a quenching agent
how does the particle or ray enter the geiger muller tube
through a micra end window
what happens as the particle or ray enters the geiger muller tube
it ionises an argon gas molecule
what does the amount of molecules ionised depend on?
depends on the radiation
what happens to the negative ions in a geiger muller tube
they are rapidly accelerated towards the wire anode
what causes further ionisation in a geiger muller tube
high speed electrons
what is it called when high speed electrons causes further ionisation in a geiger muller tube 2
gas amplication or avalanche
in an avalanche, one electron can make how many electrons?
1 x 10 o the power of 8
when the very large number of electrons in the geiger muller tube, reach the anode what happens
they pass through the external circuit as a fairly sizeable current
what happens each time a current flows through a resistor
a potential drop occurs
what counts the potential drops in a geiger muller tube
a counter (scaler or ratemeter)
what can the counter in a geiger muller tube
can count the number of particles that enter the tube
what is there a risk of when positive ions hit the cathode of a geiger muller tube and why is that bad
risk of electrons being knocked off the cathode and would register as a count on the scaler
what does bromine do in a geiger muller tube
stops the electrons being emitted (quenching agent )
what does a scaler give
total number of particles that entered the tube
what does a ratemeter give
reading in counts per second
law of radioactive decay
the rate of decay of a radioactive isotope is proportional to the number of atoms of the isotope present
A is proportional to
N
A =
λN
N
number of atoms present in the sample
λ
the decay constant
unit of the decay constant
s to the minus one
half life
the time required for half the nuclei in a radioactive sample to decay
symbol of half life
T and a small half beside it
unit of half-life
second (s)
connection between decaying constant and the half life
1 ln2
T - = —–
2 λ
if they give half life in hours for a maths question?
get seconds by dividing by 60x60
if they want you to find amount left after a certain time but they give you the decaying constant?
find the half life first and then count out the hours
if they give the half life in years
divide by 365x25x60x60
how do you writ an equating to represent the decay of an isotope
original element -> B/a partices + new element
rate of decay unit
bequerel
Bq
1 bequerel =
1 second
nuclear fusion definition
the combining of two small nuclei to form a larger nucleus with a loss in mass and a release in energy
does nuclear fusion involve atoms and why
no because that would be chemical bonding. its nuclei
example of nuclear fusion
2 2
H + H —>
1 1
4
He + energy
2
2
H is also known as
1
deuterium
deuterium is an
isotope of hydrogen
2 problems with nuclear fusion
nuclei have positive charges and it is difficult to overcome the force of repulsion between them
controlling such high temperatures has not yet been achieved
whats the speed problem with nuclear fusion
the very high speeds would require very high temperature, as hot as the sun
what is a hydrogen bomb
a fusion reaction that has gone out of control
1 advantage of fusion
very little radioactive waste to dispose of
what is the required fuel for fusion and why is it good
deuterium, can be extracted from water, there would be no fuel shortage
E = (einstein)
mc squared
m
change in mass
c
speed of light
c =
3 x 10 to the power of eight m/s
definition of nuclear fission
a nucleus is bombarded with a neutron, it splits into two nuclei of approximately equal size, two or three neutrons are emitted and the resulting loss of mass is released as energy
2 main isotopes in natural uranium
u 238 and u 235
percentage of u 238 in natural uranium
99.3% of world supply
percentage of u 235 in natural uranium
0.7% of world supply
if you increase the percentage of u 235
you get enriched uranium
chain reaction
where one fission reaction causes another reaction which causes another reaction and so on
a fission reaction in uranium is likely to
be followed by another one
when do chain reactions not occur
when the hydrogen escapes through the surface of the sample
how do you achieve the escaping of electrons?
keeping the samle size below a certain size, the critical size
in a nuclear reactor
a chain reaction happens but the reaction is controlled
in a nuclear bomb
a chain reaction happens but it is purposely let go out of control
2 isotopes used in nuclear bombs
u235 or plutonium
in a nuclear reactor what are the fuel rods made of 2
natural uranium or enriched uranium
in a nuclear reactor what is the moderator usually made of
grahite or heavy water
heavy water
D2O
function of the moderator in a nuclear reactor
the neutrons are travelling too fast, but the moderator slows them down to a suitable speed for further fission
what are the control rods in a nuclear reactor made of 2
boron or cadmium
what can the control rods do
absorb the emitted neutrons
function of the control rods
prevent further fission and it getting out of control
how do you regulate the energy output in a nuclear reactor
by raising and lowering the control rods
what does the coolant do in a nuclear reactor
pumps carbon dioxide around the system that carries the heat to the heat exchange unit
where does the coolant in a nuclear reactor absorb the heat from?
very high kinetic energy creates temperature rises in a nuclear reactor
what happens in the heat exchange unit in a nuclear reactor
the heat carried by the CO2 gas is used to boil water
what generates the electricity in the heat exchange unit in a nuclear reactor
steam from boiling water is used to turn a turbine which creates electricity
what do conventional power stations do?
boil water to make steam using fossil fuels
what is the purpose of the shielding in a nuclear reactor
a thick concrete safety shield gives protection from neutrons and gamma rays to the outside world
impact on miners, mining for uranium
the workers are exposed to radiation from materials like radon which can cause lung cancer
what can an accident at a nuclear reactor do
release radioactive chemicals into the atmosphere
why do used nuclear rods have to be carefully stored
because they are radioactive
3 uses of radioisotopes
medical imaging
food irradiation
smoke detectors
explain how medical imaging is a use of radioisotopes
radioactive chemicals with very short half lives are inserted into the body to trace blood flow or to help obtain an image of a particular organ
explain how medical therapy is a use of radioisotopes
radiation from certain radioactive chemicals can be used to kill cancerous cells
explain how food irradiation is a use of radioisotopes
if food is placed in a sealed container and bombarded with radiation , any harmful bacteria is killed and it stays fresh for a very long time
explain how agriculture is a use of radioisotopes
the passage of chemicals through a plant can be traced by making the chemicals radioactive beforehand
explain how radiocarbon dating is a use of radioisotopes
by examining the amount of C-14 in old samples of dead matter, their age can be determined
explain how smoke detectors are a use of radioisotopes
they contain radioactive material that ionises the gap between the two electrodes, allowing current to flow. if smoke gets between it cuts it off and triggers the alarm
explain how industry has a use of radioisotopes
manufacturing of paper: thickness can be checked
leaks in underground pipes can be detected
4 factors that the harmful effects of radiation of on the human body depend on
nature of the radiation
the part of the body being exposed
how active the radioactive material is
the dose that the body reiceves
a particle source outside the body
no damage, cannot penetrate skin
B particle on the body
most radiation absorbed by surface tissue, a few mm of aluminium could protect you
y rays on body
present greatest external radiation as they can penetrate deep into the body
3 most susceptible parts of the body to harm by radiation
reproductive organs, blood forming organs eg. liver, and eyes to a smaller extent
less vulnerable organs 4
hands, forearms, feet and ankles
cosmic radiation
from outerspace, bombards the earth and we are constantly exposed to it
what rock gives off radiation
granite has a minute traces of uranium which decays to radon gas
2 things you can do to prevent radiation if your house is built above granite
proper ventilation so that the gas can escape and good insulation to stop gas entering
how to measure background radiation
use a geiger muller counter and a ratemeter and take a count each minute for 20 minutes. it will vary a lot but get an average value
4 ways to protect yourself from radiation
correctly dispose of radioactive waste
wear protective clothing (gloves, glasses)
do not eat or drink near radioactive material
measure radiation levels near you to ensure your safety
