Ideas to Implementation Flashcards
what is a cathode ray tube?
a vacuum tube wih electrodes at either end. the electrodes are conductive pieces of metal connected in series with some high potential power source
when a potential difference is passed through a cathode tube, what happens?
electrons jump from the cathode(-ve) to the anode(+ve)
the electrons then collide with the gas molecules that are present inside the tube, causing them to absorb and emit the energy that was transfered by the electron from the collision
this energy is seen as light
what is the appearance of light in a cathode ray tube dependant upon?
on both the chemical composition of the gas inside the tube and on the gas pressure
why was their a debate as to the nature of cathode rays in the late 19th century?
experimental observations of cathode rays provided puzzling inconsistencies which led to the emergence of two competeing theories, one supporting that cathode rays were waves and the other supporting that the cathode rays were negatively charged particles
who was it that settled the debate about the behaviour cathode rays and how did he do this?
A man named Thompson settled the debate once he was able to deflect the cathode rays with electric plates. this had been impossible up to this point as vacuum pumps before had not been strong enough and the electric field’s present had not been strong enough.
because emr was known not to be deflected by electric fields, and emr was a wave, this strongly supported that cathode rays were particles
what properties of cathode rays fit the wave theory?
- they travel in straight lines
- produces a shadow when obstructed by large objects
- could pass through thin metal foils without damaging them
what properties of cathode rays fit the particle theory?
- they could be deflected with an electric and magnetic field
- the cathode rays left the cathode at 90 degrees to the surface(instead of propagating out like a wave)
- they travelled considerably slower than light(which is a wave)
- small paddlewheels turned when placed in the path of the rays, indicating that the rays contituents must have momentum and hence mass
what happens to a charge moving through a magnetic field?
it experiences a force
if a charge is moving parallel to a magnetic field, what happens?
it doesnt experience a force
what is the force on a moving charge in a magnetic field equal to?
F = qvBsin(theta)
what do oppositely charged plats create between them?
a uniform electric field running between them from the positive plate to the negative plate
when a potential is made between two oppositely charge plates, what expression gives the magnitude of the field?
E = V/d
where E is electric field strength in Newtons/ coloumb
V is potential difference measured in volts
d is the seperation of the charged plates
what does spacing between electric field lines show?
how strong the electric field is, smaller spaces represent a strong electric field
if a charge moves through an electric field, what will happen to its path?
it will form a parabolic path
what are the field lines like for a positive point charge?
- field lines radiate from the centre of the charge outward, indicating the direction of movement of a positive test charge in the field
- field lines which are closer together show a stronger electric field
- field lines never cross
what are the field lines like for a negative point charge?
- field lines radiate inwards toward the centre of the negative point charge, indicating the movement of a positive test charge in the field
- all of the properties that go with the positive point charge
what is the expression for the magnitude of an electric field at a particular point in that field?
E=F/q
where F is force in newtons
E is electric field strength in Newtons/coloumb
q is charge measured in coloumbs
what did JJ thompson do?
he determined the charge/mass ratio of an electron
what did Thompson use to do this?
Thompson used a large glass vaccum tube containing:
- a cathode, connected to a high voltage DC power source
- anode double slit, connected to a high voltage DC power source
- electric plates
- electromagnets
- fluorescent screen with scale
in his first experiment, what did thompson do?
thompson passed cathode rays through small slots in a cylinder anode making a near parallel beam that his the end of the glass tube in the centre, moving past two opposite electric plates and electromagnets which were perpendicular
first, he varied the strengths of both the electric field supplied by the plates and the magnetic field supplied by the electromagnets so that they would balance eachother out and thus the net force on the cathode rays would be zero and it would resume its straight line trajectory.
by equating the magnetic and electric force equations, thompson developed an expression for the velocity of the cathode rays in terms of E and B, v = E/B
what was thompsons second experiment?
thompson turned off the electric plates and kept the magnetic field applied. the cathode rays would become deflected and follow a circular path. these rays then hit the fluorescent scree at the very end of the apparatus indicating how the electrons were being deflected
the magnetic force provided the centripetal force, causing the rays to curve in a circular arc with a fixed and measurable radius which could be determined b measuring the displacement of the beam
Fcentripetal = qvB (the beams were being fired at 90 degrees to the fields, so sin(theta) would be 1)
what was the final expression for the charge/mass ratio of the electron?
q/m = E/B^2r
what did hertz use to measure and detect radio waves?
- induction coil
- reciever loop
- a spark gap
what did hertz do to produce radio waves?
hertz used an induction coil to produce a high voltage oscillating spark. as these sparks oscillated back and forth across the gap in the induction coil, sparks were also noticed jumping across the air gap in the reciever loop, even when the reciever loop wasnt connected to any power source
what did hertz hypothesise that was causing the sparks in the reciever loop?
hertz suggested the oscillating spark in the induction coil set up changing electric and magnetic fields that propagated as an electromagnetic wave, as postulated by Maxwell. these waves, falling onto the reciever loop, set and electric and magnetic fields inducing a spark
what three things did hertz do which provided proof that the produced radio waves were em waves?
- he was able to reflect the waves of sheets of metal
- he was able to refract the waves with a prism of pitch
- he was able to polarise the waves by reorientating the spark gap of the reciever
therefore, he showed that his waves had the properties of reflection, refraction and polarisation, in common with light and all emr.
what two things did hertz need in order to measure the speed of the waves he had produced?
their wavelength and frequency
how did hertz find the frequency of the waves he produced?
he understood the geometry of the induction coil he was using. by knowing this, he identified the frequency as roughly 100MHz
how did hertz find the wavelength of the waves he produced?
-hertz reflected the waves off of a metal sheet, allowing the radio waves and the reflected waves to produce patterns of interference at a detector some distance away.
this produced a standing wave at the detector, showing points of constructive( antinodes) and destructive interference(nodes). he knew the distance between adjacent nodes was half a wavelength, and so from that he found the speed of the waves as 3x10^8m/s , supporting maxwells equations
how did hertz accidentally discover the photoelectric effect?
when observing the spark gap, hertz wanted to be able to view it more clearly. to do this, hertz enlcosed the reciever loop in a dark box. when he did this, he noticed the spark greatly diminish in size
what did hertz conclude regarding the observation he made with the diminished spark?
he concluded that this was because light or more specifically emr was affecting the size of the induced spark, but he didnt provide any explanation as to why this was happening
what did hertz do to investiage the photoelectric effect further?
he irridated the reciever with different frequencies of emr. he found that the spar was greatest when UV was shone on the reciever, but when a quartz screen obstructed the uv, blocking it, the spark become smaller again
what properties do black bodies have?
- they absorb all emr that falls onto it
- no emr passes through it
- no emr is reflected off them
- emr is emitted at a frequency chracteristic of the body’s temp
what factors effect the type of radiation that is emitted from a black body?
- the material from which it is made
- its shape
- the nature of its surface
- its temp
at all temps, black bodys emit ….
radiation at all wavelengths, but the intensity of each wavelength varies
what was the ultraviolet catastrophe?
classical theory predicted that the radiation emitted by a black body should continuosly increase in intensity as teh wavelength becomes shorter, forming a continuous spectrum with intensities corresponding to an exponential curve
this prediction was obviously incorrect. if it were true, then that would mean that all objects in the universe would be emitting harmful xrays and gamma rays. it also violates the law of conservation of energy
this was the ultraviolet catastrophe
what did planck propose that solved the uv catastrophe?
planck hypothesised that radiation could only occir in small packets of every which he called quanta. this meant that radiation is no emitted or absorbed by a black body continuosly as classical physics said it should, but rather it is emitted or absorbed in little bursts of energy
overall, what did planck propose?
that black body radiation, and indeed all light, is quantised.
what is the expression for the energy contained within a single quanta?
E=hf
where h is plancks constant
f is frequency
E is the energy in joules
how did planck view his proposal of quantised energy?
he thought it was a mathematical trick
what list of ideas did the genius of einstein propose regarding quanta and plancks “mathematical trick”?
- the energy of light is not evenly spread out over the wavefront, but is concentrate in bundles or packets of energy, or photons
- each photon has an energy equal to hf
- photons are the smallest units of light
- intensity of light depends on the number of photons, more photons higher intensity
- all photons of the same frequency have the same energy
- photons have zero rest mass and travel at c in a vacuum regardless of their frequency
how did einstein explain the photoelectric effect?
- to produce the photoelectric effect, the energy contained in the light photons must be equal to, or greater than, the energy required to overcome the forces holding the electrons to the lattice. the energy required to realse the electron from the lattice is called the work function
- if the energy o the photon is greather than the work function, the additional every of the photon, above the work function level, will provide the kinetic energy of the photoelectrons
- a photon can only give up all or none of its energy to one electron, cannot give halfs or fractions, all or nothing principle
what is eintsteins photoelectric equation?
Ek = hf - (work function)
how does einsteins contribution to quantum theory relate to black body radiation?
Eisteins theories came about directly through the work undertaken by planck in explaining the ultraviolet catastrophe and black bodies
how did einsteins work on quantum theory effect physics at the time and assess its impact
- he expanded on plancks ideas and turned what was previously a mathematical trick such as quantum theory into a set if udeas with concrete principles and modelling.
- einsteins with this work could properly explain the photoelectric effect, the ultraviolet catastrophe and the nature of black body radiation
- einsteins work has led to further discoveries including the heat capacity of solids and the compton effect/scattering
- line spectra of atoms and molecules cold now be better explained in terms of einsteins and plancks ideas of the photon and quantised light
- einsteins work opened a door for futher research into quantum ideas. our entire knowledge of physics on the atomic scale today is based on the quantum theory that began with einstein and a few other great individuals
therefore, einsteins made an astounding contribution to quantum theory and its relation to black body radiation
how does the photoelectric effect work?
when light strikes a metallic surface, it is the energy of the photons whichwill activate or energise the electrons in the metal. when a photon strikes a metal, all or none of the energy will be passed on to the electron and then be emitted by a spark
what does the particle model of light state?
light energy travels in photons, small particles or quanta that have mass dependant on their energy
when i increase the frequency of a photon, what happens?
it gains more energy, and these energys can only occur in multiples of plancks constant
when i increase the amplitude of a photon, what happens?
the intensity of the light increases as the number of photons increases
what is the equation for photon energy?
E=hf
where h is plancks constant
where f is the frequency
what is the formula for the speed of a wave?
v = f • λ
the energy of a photon is proportional to…..
its frequency
the speed of a wave is proportional to…..
the waves frequency and wavelength
what is a photocell?
a photocell, or photoresistor, is a light controlled variable resistor where electrons initiating an electric current originiate by photoelectric emission
what does a photocell consist of?
consist of a cathode and anode made from a highly resistive semiconducting material
what are photocells used for?
photocells are used to detect lght as the intensity of the light that is detected is directly proportional to the iniated electric current
how does a photocell work?
when light falls onto the cathode, the released electons are accelerated to the anode by a potential difference between them resulting in a current
what are some of the industrial uses of photocells?
- automatic street lights
- motion detectors
- house alarms
- pollution detectors
- radiation detectors
- light motors
