Turning Points Flashcards
What are cathode rays?
-During the 19th century, Michael Faraday Heinrich Geissler, William Crookes and Heinrich Hertz studied the effects of applying high voltages across gases at low pressure in discharge tubes
What is a discharge tube?
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What happened to the low pressure gas?
- The low pressure gas emitted light when it was made to conduct electricity by a high potential difference applied between the two electrodes positioned at each end of the tube
- The colour of light emitted was characteristic of the type of gas inside the tube
What are the features of a discharge tube?
- A negative electrode (cathode)
- A positive electrode (anode)
- A low pressure gas within a glass container
- The high voltage between the cathode and anode was achieved using an induction coil
What changes were made to the discharge tube?
- In 1865. Herman Sprengel invited a type of mercury vacuum pump that could achieve much lower gas pressures
- Experiments used this new pump to further reduce the gas pressure inside the discharge tube
- This resulted in the gas remaining dark when a high voltage was applied, but the end of the glass tube, beyond the anode started to glow
What was concluded from the discharge tube?
- Some type of ray was being emitted from the cathode, which traveled towards the anode and struck the end of the tube beyond it
- They called these cathode rays
What are cathode rays?
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What is a paradigm?
Paradigm means the typical pattern or example. In science, a paradigm shift refers to a change in the basic assumptions within the main theories
What is a Geissler tube?
- Late 1860s, glass tubes filled with a gas made to flow
- Large voltage applied to the electrodes either end of the tube, an electrical current passed through the gas
Why does the tube glow?
- The ionisation and subsequent recombination of the molecules in the gas
- These tubes are a form of gas discharge tubes
What did William Crookes and Faraday do do?
- Make discharge tubes of much lower pressure of gas inside and used a vacuum pump to remove the air inside the tubes
- F observed glow appeared part of the way down the tube with dark space near cathode
- In some tubes, glass would glow and this always occurred at the end of the tube near the anode (positive)
- Suggested this glow due to cathode rays, emitted from the cathode of the tube
What happens when there are very few or very many gas molecules?
- The cathode rays are able to travel the length f the tube and reach the end of the glass
- Too many gas molecules, in the tube, cathode rays interact with the gas molecules causing them to glow
What did they think the cathode rays were?
- Some though electrical charged particles
- Some thought a new type of electromagnetic wave which were separate from the current in the gas
How was it settled what cathode rays were?
- Further experiments showed that the path of cathode ray could be altered by bringing a magnet near the discharge tube
- When tube with low pressure, rays were deflected my electric fields
- Suggested cathode rays were negatively charged particles
What is thermionic emission?
The release of electrons (or charge carriers) from a heated source
What are cold discharge tubes?
The early discharge tubes produced cathode rays due to the large electric field between the cathode and anode
Describe thermionic emission
- Using a heated cathode producing cathode rays creates an electron beam
- When a metal filament is heated the conduction electrons in the metal start to move more vigorously and are able to gain enough energy to leave the surface of the metal
What happens during thermionic emission?
- Electrons are produced at the heated cathode and accelerate towards the anode due to the high voltage between the cathode and anode due
- A few electrons pass through the small hole in the anode and travel in a narrow be a towards the screen
- The glass tube contains a vacuum so Rhee are no gas molecules for the electrons to interact with and the tube will not glow
- The heated cathode filament and anode arrangement is sometimes called an electron gun as electrons are fired with a velocity through the hole in the anode
How do you calculate the speed of an electron?
- Work done of each electron accelerated through the potential difference between the anode and cathode =eV, where e is charge on electrons
- Each electron acerbated from vert low initial speed to a greater speed, v as they pass through eh hole int he anode
- The electrons initial speed will be so much less their final speed that treat effectively as zero
- KE as pass through anode equal to 1/2mv^2
What is the energy gained form the work done on each electron?
1/2mv^2 = eV
What happens as you increase the voltage and current?
- Increasing the pd across the anode will increase the speed at which the electrons are moving
- Increasing the current in the cathode filament, makes beam of electrons more intense
- The increase in current leads to an increase in temperature of the cathode, which in term increases the number of electrons that have sufficient energy to escape the surface of the metal
What happened January 1897?
- Measured specific charge of particles in cathode rays
- Value much large than value for hydrogen ions
- This meant that the particles were less massive than hydrogen ion or carried much larger charge
What did J.J Thomson do?
- Used discharge tubes with a combination of electric and magnetic fields to change the path of cathode rays
- By measuring the flection with these different combinations of fields, able to obtain value for specific charge of particles forming the cathode rays
- Could not directly measure speed of electrons because using cold discharge tube
What forces act on the electron?
- The electric field between the parallel plates in the deflection tube deflects the electron beam downwards
- Magnetic field perpendicular to electric field so electron moves in a straight line
What happens when the force acting on the electrons are equal and opposite?
The beam will show no deflection
How do you find out the velocity of the electron?
E=V/d FE=Ee=eV/d FM=Bev FE=FM ev/d = Bev v=V/Bd
How do you deflect the electron beam with a magnetic field only? Why is the speed the same?
- When electric field removed, beam of electron deflected by the magnetic field follow a circular path with radius r
- Magnetic force on each electron provides centripetal force that causes the particles to move in a circular path
- The magnetic force is perpendiculars to the direction of the velocity of the electrons
- So magnetic force does no work on the electrons and their speed stays the same
BeV =mv^2 /r
e/m = v/Br
How do you deflect the electron beam with an electric field only?
- As electrons travel through the electric field they Wille experience a force which will cause them to accelerate
- Amount of particle deflected and electrons enter between the plates moving horizontally so they have no initial vertical velocity
a= 2s /t^2 - t = distance/speed
t= LBd / v
a= 2sV^2 / B^2d^2L^2 - RE = ma
a = eV / md
so:
eV/md = 2sV^2 / B^2d^2L^2
e/m = 2sV / B^2L^2d
What was the significance of Thomson’s experiments?
- Published in 1897
1. The charge would not be separated from the particles, which meant that they were not a new form of electromagnetic wave as had been suggested by Helmholtz
2. The direction of deflection of the particles showed that they were negatively charged
3. The ratio of charge to mass for all the particles is the same. Changing the gas in the discharge tube did not alter the value, so the specific charge was a property of the particles and not the measuring equipment
What did Thomson’s experiment show evidence for?
- That atoms weren’t the smallest part of matter and they could be broken into smaller particles
- Could not measure charge or mass of particle separately
What is the specific charge of an electron and of hydrogen?
1.76 x 10^11 Ckg-1 and for hydrogen ion (proton) 9.6 x 10^7 Ckg-1 so electrons was more than 1800 times greater than the smallest known ion
What was the predicted charge on an electron?
In 1908 Rutherford and Geiger predicted the change on the elections from experiments on helium nuclei and predicted charge half that of helium nuclei so 1.55 x 10^-19 C
What is Stoke’s law?
-Describe drag force acting on spherical droplets falling through a viscous medium, and air seemed viscous to scale of droplet
-Drag force acting on droplets:
FD = 6pi(viscosity)rv
Why did you need stokes law?
Unable to measure radius of falling drops directly experimentally
How can the speed of the droplet be measured?
time to fall through known distance
What happens when particle is falling at terminal speed? How do you work out radius and mass?
Drag = Weight 6pi(viscosity)rv=mg mg is same as 4/4pir^3densityg 18viscoirt/4densityg = r^2 r= sqrt (9viscoisityv/2densityg) -Then calculate mass = r^3 4/3 pi density
What were the problems of using charged clouds of water to find out charge on electron?
- Air currents affected motion of drops
- The drops tended to evaporate in the apparatus
- Electric field wasn’t very uniform
- Stoke’s law was no valid because drops were falling too quickly
How did Millikan improve his experiment?
- Better optics and reduced convection current in the air in apparatus and published data from 58 drops of oil measured over space of 60 days
1. Sprayed oil into body of apparatus using an atomiser to produce a cloud of droplets
2. Small number of droplets fell through pin hole opening in the anode
3. These droplets became negatively charged as stye were ionised by xrays from an x-ray source
4. Using a microscope eyepiece, Millikan able to measure speed of drops as they fell during to gravity
5. Then create an electric field between anode and cathode
6. Drops then stop falling under gravity and even one back up towards the anode if the field was strong enough
What are the forces on an oil drop held stationary in an electric field?
Electric force = with
Qv/d = mg
Q = mgd/v
-Mass used from before with radius
What was the significant of Millikan’s oil drop experiment?
- Charge on each drop would be an integral multiple of the charge on the electron, and common factor gave value for charge on electron
- Published 1.592 ± 0.003 x 10^-19
- Smaller than accepted value as using value for coefficient of viscosity which was too low
- Supported hypothesis that charge quitted in integer multiples of the charge on the electron
