3.12 - Turning Points in Physics Flashcards
How are electron beams formed in a discharge tube?
Cathode connected to -ve of a DC power supply and anode connected to +ve. A low pressure gas is inside this tube. The high potential difference applied pulls electrons from the gas atoms, forming ion and electron pairs. Positive gas ions are accelerated towards the cathode, releasing even more electrons. These electrons accelerate along the tube, colliding with gas ions, where they become excited. They quickly de-excite, releasing photons of light. The brightest glow is at the cathode, where the gas ions and electrons recombine and emit light photons.
What did the production of cathode rays show?
mass, negative charge, same properties no matter what gas was used, very large charge to mass ratio
What is the work done on an electron accelerated through a potential difference?
W=eV
How to electron guns work?
Potential difference accelerates elecrons released from the hot cathode (thermionic emission). They accelerate towards the anode, which has a small gap. Electrons pass through this, forming a narrow electron beam travelling beyond the anode.
What happens to electrons when accelerated through a pd and what equation does this lead to?
the kinetic energy is equal to the work done on the electron by the electric field
1/2mv^2=eV
How does Tomson’s crossed fields determine the specific charge of an electron?
Magnetic and electric fields perpendicular to one another, deflecting electrons in opposite directions. Electrons accelerated using e- gun, entering apparatus perpendicular to both fields. electric field deflects e- up, and magnetic field deflects e- down. For the e- beam to pass straight through, electric force = - magnetic force.
What is the significance of determining the electron SC?
Showed that elctron SC is constant
Beginning of atomic physics: showing electrons have mass, energy, momentum
E/Me was 1800x hydrogen (proton) SC; either electron mass is much smaller or charge is much larger; 1899: estimated e around 10^-19 C, so same charge as proton but much smaller mass.
What is the purpose of Millikan’s oil-drop experiment?
To calculate the charge of an electron
What apparatus did Millikan use?
Two metal plates with a high d.c. voltage put between them - creates an electric field between the plates. Above the plates, oil droplets are sprayed from an atomiser, where some fall through a hole in the upper plate. A gamma source is directed at the oil droplets that fall in between the plates, in order to ionise them (giving them charge).
What happens to oil droplets when the electric field is then turned on?
They move upwards, pulled by the electric field, with a new terminal velocity
What are the forces on an oil droplet when the electric field is off? Where does its terminal velocity go?
Force of weight downwards and equal magnitude of drag upwards; terminal velocity downwards
What are the forces on an oil droplet when the electric field is on? Where does the terminal velocity go?
Forces of weight + drag downwards and equal magnitude of electric force upwards; terminal velocity upwards
What dies Stoke’s law give us?
The magnitude of force on a spherical object due to viscous drag
What is the equation to find terminal velocity when the field is off?
mg=6πrη(v1)
What is the equation to find terminal velocity when the field is on?
mg + 6πrη(v2) = qV/d
What is so significant about Millikan’s findings?
The smallest value of q was 1.6x10-19C and all the others were multiples of this; he found that CHARGE IS QUANTISED
What did Huygens believe light could be fully explained by?
light as a wave
What is Huygens’ Theorum?
Every point on a wavefront can be assumed to be a point source; The waves from these interfere to produce the wavefront as it travels.
What did Newton believe about light?
That light is explained by thinking of it being made up of many tiny particles, called corpuscles.
What was Young’s experiment and what did it show?
Double-slit experiment.
Showing that light interferes, which is a wave property, just as water waves do in a ripple tank.
What does it mean to say that electromagnetic waves are self-sustaining?
Oscillating B field produces ELECTRIC field and the oscillating E field produces the MAGNETIC field; they sustain each other - couldn’t exist on their own
What did Maxwell show?
That electric and magnetic forces were different manifestations of the same force
What did Maxwell’s formula show?
The speed of electromagnetic radiation in a vacuum
What did Hertz do/show?
He produces radio waves using a spark transmitter; about 1m away, a spark was produced in the gap between two poles of a loop antenna
What did Hertz do in his later experiment? How did this compare to Maxwell’s findings?
Produces standing waves by reflecting the radio waves off a metal sheet, allowing him to measure the speed of the waves.
Agreed with Maxwell’s findings.
How did Fizeau measure the speed of light?
A ray of light passed through one of the slots on a rotating wheel, travels around 8.6km to a mirror and is reflected back. If the wheel is rotating at a a certain speed, then the time it takes for the light to travel to a distant mirror and back is equal to the time it takes the wheel to rotate by one slot.
How could you study the spectrum for different temperatures?
Put a cube in an oven, raise it to a certain temperature. Any radiation emitted from the inside walls of the cube exits through the hole to a spectrometer, which separates the different wavelengths. You can measure the intensity of these with a detector.
How did the oven cube results abide with the current wave theory?
They agreed well and low temperatures, but differed greatly at higher temperatures.
What did the wave theory at the time of the UV catastrophe predict?
That at lower temperatures, huge amounts of small wavelength radiation would be produced
What did Planck assume when producing the new wave theory?
Atoms can only have discrete amounts of energy and so the photons they emit can also only have discrete amounts of energy
What did Einstein say about light?
It is not a continuous wave but a swarm of discrete packets of energy; the energy is proportional to their frequency. Emission occurs if an individual photon gives sufficient energy to an individual electron.
How did Maxwell prove his theory?
By producing an equation for EM wave speed, combined with experiments
