Turning Points- Discovery Of Electron Flashcards
What is a discharge tube?
A sealed glass tube held at low pressure, with a cathode and anode at each end.
When the electrodes are connected to a supply,
the gas in the tube will glow and conduct electricity
What is a cathode and anode?
A negatively and positively charged electrode
How does a discharge tube glow?
-High pd across discharge tube pulls electrons off gas atoms, causing ionisation and producing positive ions and electrons
-Positive gas ions are accelerated towards the cathode and produce more electrons when they collide with it
-Electrons are accelerated towards the anode and collide with gas atoms causing excitation. Atoms then quickly de-excite and release photons of light
What does the glow in a discharge tube look like?
Glow is brightest at cathode as gas ions and electrons recombine and emit photons of light
No glow in middle as electrons are gaining kinetic energy so cannot cause excitation yet
Glow at anode as electrons cause excitation which follows with de-excitation
What did Thomson determine from the discharge tube glowing?
The glow was called the cathode ray which he determined had a mass, a negative charge, same properties no matter the gas, very large charge to mass ratio. They were named electrons
What are cathode rays?
Beams of electrons observed in discharge tubes
What is thermionic emission?
The emission of electrons from a metal due to heating it, the electrons gain sufficient energy to escape from the metal surface and are emitted
How is a filament heated for thermionic emission?
A current flows through it
What is an electron gun and how does it work?
A pair of electrodes that produce a thin and fast moving beam of electrons held at low pressure.
Electrons are emitted from the hot cathode by thermionic emission and are then accelerated through a gap by the anode
How much energy do electrons from an electron gun have?
Work done on electrons in E field W = QV = eV
so eV = 1/2mv^2
What are three methods to determine the specific charge of an electron?
Using magnetic fields
Using magnetic and electric fields (crossed fields)
Using electric fields
What is a fine beam tube?
Apparatus containing a low pressure gas and uniform magnetic field passing through
What is the method for specific charge of an electron using a B field?
-Electrons from electron gun enter magnetic field which exerts a magnetic force perpendicular to the electron’s direction of motion. This magnetic force acts as a centripetal force causing circular motion.
-As electrons move through fine beam tube, they collide with gas atoms causing excitation and de-excitation so photons of light are released so r can be measured.
-centripetal and magnetic force equated, v subbed out for rearrangement of eV = 1/2mv^2
What is the method for specific charge of an electron using crossed fields?
-Electrons from electron gun enter perpendicular B and E fields which exert equal and opposite forces on the electrons
-Magnetic and electric force are equated giving E = Bv, E is known from Vplates/ d and v is known from eV=1/2mv^2, rearrange for e/m
What was significant about Thomson’s calculation of the specific charge of the electron?
Previously highest was H+ ion, 10^ 8 Ckg^-1
Electron’s was 10^11 Ckg^-1, so very high
He determined that it had a smaller mass than H+ ions which were also the lightest known thing, so he determined that a knew particle was discovered and was smaller than the atom
What was the apparatus for Millikan’s experiment?
An atomizer containing oil, an X-ray source directed at container with oil droplets, charged plates, a microscope/eyepiece between plates
How does Millikan’s experiment work?
-An atomizer sprays tiny droplets of oil which are negatively charged due to friction/ x-rays.
-The droplets fall into a region between two charged plates, an E field where they experience a force due to charge, pd changed until they are stationary
-To find mass, pd is switched off and droplets fall and has its weight and a drag force up determined by Stokes’ law
-Terminal velocity of droplet can be calculated using distance and time, at which point drag force and weight are equal
What equations are used for Millikan’s experiment?
-When droplets are stationary, EQ = mg, so QVp/d = mg
-Stokes’ law F=6πηrv is drag force when falling
-Terminal v, 6πηrv = mg, using density m = 4/3πr^3, sub in and rearrange for r squared
-Equate QVp/d to mg which is the drag force, mg with m subbed out for ρV
What was significant about Millikan’s experiment?
He showed that the charge of all oil droplets was a multiple of integer 1.5/1.5 x 10^-10C, showing that charge was quantised and finding the smallest unit of charge, that held by an electron
