Turning Points Flashcards
What is a Geissler tube?
They are tubes that are filled with Gas that could be made to glow when a large voltage was applied to the electrodes at either end.
How does a Geissler tube work?
When a voltage is applied, the molecules in the gas are ionised and are therefore able to carry a current through the tube.
What did Michael Faraday observe in Geissler tubes?
The glow appeared half way down the the tube but was dark near the negative cathode
What was suggested as the reason to a glow half way down the Geissler tube?
Cathode rays emitted from the cathode. When there are many gas molecules the cathode rays interact with the gas molecules causing them to glow.
How can the path of a cathode ray be altered?
By bringing a magnet near to the discharge tube
By applying an electric field across the discharge tube
What were cathode rays concluded to be?
Negatively charged particles
Why does the gas glow?
positive ions in the gas recombine with electrons and visible light and UV photons are emitted causing the gas to glow
Describe Crooke’s apparatus
When a large potential difference is applied, the paddle is seen to rotate and move along the rail. This is because the charged particles collide with the paddles and transfer momentum to the wheelW
What is the conclusion from Crooke’s experiment?
The constituents of a cathode ray have mass
What is thermionic emssion?
When a metal filament is heated conduction electrons have more kinetic energy so can more easily escape the surface
What is the electron volt?
For each electron accelerated through the potential difference, the work done is equal to one electron volt
What is one electron volt equal to?
The kinetic energy
What is electron deflection tube?
It is made of an electron gun wich fires a cathode ray through an evacuated tube. The path of the ray is viewed on a screen that is parallel to the ray. An electric field is produced between the top and bottom plates and a Helmholtz coil produces a magnetic field parallel to the screen
How can the deflection of a cathode ray be controlled in an electron deflection tube?
By adjusting the strength of the electric / magnetic fields
What happens when you remove the electric field in an electron deflection tube?
The electrons will be deflected by the magnetic field and will follow a circular path with radius r.
What happens when you remove the magnetic field in an electron deflection tube?
The electrons will be deflected by the electric field and they experience a force which causes them to accelerate. (Horizontal velocity remains the same but they will accelerate in the vertical direction towards the positive pate)
What did Weichert and Thomson discover and what was concluded?
The specific charge of “electrons” was 1800 times greater than that of hydrogen and they concluded that “electrons” must have a much lower mass or much greater charge
What did Helmholtz suggest about cathode rays?
That they were a new form of electromagnetic waves
What did “electron” deflection show about the charge?
They are negatively charged as they attracted to the positive plate
What did the discovery of specific charge show?
That atoms weren’t the smallest part of matter
What did Rutherford and Geiger predict about that charge on the electron?
That it is half the charge of a helium nucleus
Describe Millikan’s oil drop experiment
Spray oil into the apparatus using an atomiser.
A small number of droplets fall through a hole in the anode and are negatively charged as they are ionised by X-rays.
By using an eyepiece, you can measure the speed of droplets as they fell due to gravity.
The strength of the electric field could be altered to move upwards or downwards and at different speeds.
what force will the droplet in Millikan’s experiment experience?
resistive force upwards (The viscous drag force) and its weight downwards.
What is the viscous force (drag) equal to?
The weight
What is a limitation of light microscopes?
Light with shorter wavelengths increases resolving power but that still limits them to 200nm
Why was is suggested that electrons could be used for microscopes instead of light?
Because they have a much smaller wavelength so it would have a higher resolving power.
How do TEMs (Transmission electron microscopes) work?
An electron gun is used to produce a beam of electrons at a fixed speed.
The condenser lens is used to form the electrons in a parallel beam.
The scattered beam then passes through the objective lens, forming a magnified, inverted image of the sample.
The electrons from the immediate image are then focused using the magnifier lens to form a final magnified image on the screen.
Describe young’s double slit experiment
He used a single slit and a colour filter to produce coherent, monochromatic light. This light was the incident on the double slit and the interference pattern was observed
What does fringe spacing depend on?
The slit separation (inverse)
Wavelength (direct)
distance between slit and screen (direct)
Whose theory does Youngs slit experiment support and whose does it disprove?
Supports Huygens that light is a wave
Disproves Newtons theory that light is made of corpuscles (particles)
What was Newtons theory of light?
That light was formed of corpuscles
What was Huygens wrong about in his theory of light?
He believed it was a longitudinal wave
He believed it required a medium to travel through and that every point on a wavefront is a point source to secondary wavelets which spread out to form the next wavefront
How does Huygens theory explain reflection?
As the whole wavefront will not reach the surface at once, wavelets spread away from the surface once they reach it and re-join others to form the
How does Huygens theory explain refraction?
It is assumed that light travels slower in denser mediums therefore when it enters a denser medium it would slow down so bend towards the normal
What Phenomenon proved that light is a transverse wave?
Polarisation
Describe Fizeau’s experiment
He used a lens to focus light from a source onto the edge of a toothed wheel. In one rotation light would flash as many times in a rotation as there are teeth on the wheel. The wheel is rotated with measurable angular velocity so that the speed of light could be measured
Which theory did Fizeau’s experiment support?
Huygens theory that light is a wave
How does an STM (scanning tunnelling microscope) work?
A slightly positively charged probe is close to (~1nm) a negatively charged specimen so a small p.d is produced between (forbidden space) due to quantum tunnelling electrons move across the gap causing a tunnelling current.
Does the tunnelling current increase or decrease the likelihood of tunnelling?
Increase
What does the permittivity of free space determine?
The strength of an electric field
What does the permeability of free space determine?
The strength of an magnetic field
How are electromagnetic waves formed?
By accelerating charged particles. These produce electric field and magnetic field oscillations
Describe electron diffraction
A beam of electrons with a uniform speed is incident on a thin metal foil. As the electrons pass through the gaps between the atoms in the foil, they diffract. This produces a pattern of rings on the fluorescent screen.
What did electron diffraction prove?
That all particles have a wavelength
What is black body radiation?
Dark, matt surfaces that absorb and emit radiation better than light, shiny surfaces
What is a perfect black body?
A theoretical object that absorbs all radiation incident (and is the best possible emitter of radiation)
What assumption did Max Planck make?
That the energy of electromagnetic radiation was quantised
What could Newtons theory of light explain?
Reflection, refraction and dispersion
How did Newtons theory explain reflection?
when the corpuscles collide with the surface, the component of the velocity perpendicular to the surface to change direction while the component parallel to the surface stays the same
How did Newtons theory explain refraction?
as the corpuscles approach a denser medium, short range forces of attraction cause the component of velocity perpendicular to the surface to increase while the parallel stays the same therefore light will bend towards the normal
According to newton does light travel in denser or less dense mediums?
Denser mediums
What could Huygens theory of light explain?
Reflection, refraction, diffraction, interference and later dispersion
What could neither theory of light explain?
Polarisation
What is an inertial frame of reference?
A frame of reference not undergoing acceleration
What is the principle of relativity?
All the laws of physics are the same for any observer in an inertial reference frame (moving with constant relative velocity)
What is the principle of constancy of the speed of light?
The speed of light is invariant and will always travel at the same speed in a vacuum
What is proper time?
The time interval measured by the observer who is at rest relative to the event that is being timed
What is length contraction?
Consequence of special relativity so it only occurs in inertial reference frames and causes the length of objects moving at high speeds to appear shorter to an external observer
What is the rest mass of an object?
The mass measured in the reference frame of the object being weighed
What happens when an object is moving at a higher relative speed to the observer?
It’s mass increases
It requires more energy to increase the speed
What is equation for kinetic energy?
Ek=mc²-m0c²
E=E0+Ek
How did Hertz discover radio waves?
By using an apparatus which allowed high voltage sparks to jump across a gap of air as this leads to the production of radio wave. These could be detected by a dipole receiver or a loop of wire with a gap. By placing a metal sheet in front of the apparatus the radio waves are reflected back onto themselves causing stationary waves to be formed. By using the detectors you can find the distance between adjacent nodes in order to find the wavelength and using the frequency, also their speed.
How is a dipole receiver made? And what does it detect?
By placing a second set of charged plates parallel to those forming the high voltage sparks
It detects the electric field
How is an alternating magnetic field detected in the loop of wire with a gap?
The field will enter the loop causing a change in magnetic flux, inducing a potential difference which will cause a spark to cross the gap in the wire
How did Herz know that radio waves were electromagnetic waves?
because his calculated value was the same as Maxwell’s predictions
How did Hertz show that the radio waves produced were polarised?
Because the signal varied from a maximum to a minimum value after a rotation of 90
Why is the resolving power of a TEM limited?
As electrons pass through the sample they will slow down casing their wavelength to increase so resolving power will decrease
Electrons travel at a range of speeds so are diffracted by different amounts which causes the image to be blurred
What happens to the resolving power of a TEM when the accelerating voltage of the electron gun is increased?
It increases because the speed of the electrons increases
What is the Michelson-Morley experiment?
They designed an apparatus known as the interferometer to measure the absolute speed of earth through the æther. This consisted of a partially reflective surface (beam splitter) a glass block (compensating plate) and two mirror. The partially reflective surface would reflect some light while allowing some to pass creating two beams of light moving perpendicular to each other which travels towards the two mirrors which are set up the same distance from the beam splitter. After being reflected on the mirrors the two beams of light return to a detector and the interference pattern they form can be recorded.
What is time dilation?
The consequence of special relativity meaning that it only occurs in inertial reference frames and causes time to run at different speeds depending on the motion of the observer.
Will proper time be shorter or longer than the time measures by an external observer?
Shorter
What decay provides experimental evidence for time dilation?
Muon decay
How is muon decay detected?
Place one detector at a high altitude and one much lower down to measure the change in muon count rate. You will need to know the distance between the two plates and the final velocity
What is proper length?
The length measured by an observer who is as rest relative to the object
What represents proper time in an equation?
t0
What symbol shows proper length?
L0
What did Einstein prove in special relativity?
Mass and energy are interchangeable and are related by E=mc2
when does the classical calculation not apply?
When objects are moving at relativistic speeds of over 1/10th of the speed of light as the mass changes significantly
What was Berzotti’s experiment?
It involved a particle accelerator which could emit electrons at varying kinetic energies, two detectors connected to an oscilloscope and an aluminium plate connected to a temperature sensor.
electrons were released in pulses and the time between the two detectors could be calculated byr measuring the distnace between the peaks displayed
The electrons are directed at the aluminium target and when they collide with it, their kinetic energy is transferred to the target in the form of heat. The change in temperature is measured so that the kinetic energy cord be directly measured.
According to special relativity, can an object reach the speed of light? Why?
No
Because as it speed tends toward the speed of light, its mass trends towards infinity therefore its energy tends ot infinity and it is not possible to have an infinite amount of energy
