Discovery of electron Flashcards
Explain main features of discharge tube
evacuated glass tube (only gas present that’s meant to be)
pd applied across tube causes e- to jump from cathode -> anode
Why must tube be at low pressure?
if tube isn’t then e- don’t gain enough KE to produce ionisation as too many gas particles present
Why does tube emit light?
When e- hit other end - release energy as visible light
Impact of cathode rays
taught us _ about electrons
- have properties such as mass, momentum, energy
- charge
- properties
What are cathode rays
beam of electrons
most commonly produced in discharge tubes
name since beams appear to originate at cathode
used in older screens
What is thermionic emission
metal releases electrons when heated
What materials are used for thermionic emission
transition metals or there carbides/borides
Uses of thermionic emission
Thermionic emitted electrons used in electron guns.
How does an electron gun work?
An electric field is used to accelerate particles to a high velocity.
By using a small hole in the anode, a narrow electron beam (cathode ray) can be produced.
What is specific charge
measure of charge per unit mass
What was Thomson’s experiment?
measured sc without knowing charge/mass.
He performed an experiment which made use of an electric field & measured deflection of e-
The deflection allowed him to show that the electrons had a negative charge & a finite mass.
What did Millikan notice about his results?
Millikan measured mass by turning off e fields and calculated Q using mgd/V
found that charge was always n1.6x10^-19 so electric charge quantised in whole # multiples
so concluded quantised charge of electron and then n = electrons in droplet
How did Millikan use the e field in order to calc sQ?
Find charge:
varied magnitude of e field so droplets stationary (depends on droplet Q)
using N1 equate weight = electrical f
OR when at terminal v
Find radius -> mass:
If no e field then fall at terminal v (accelerates downwards but rate decreases over time)
weight = drag/viscous f using Stokes’ law
What is Millikans’ experiment?
oil drop - discovered the charge
& combined his results (terminal speed of drop) w Thomson’s sc of e- => discover mass
investigated motion of falling oil drop with/out e field
What is Stokes’ Law
gives the viscous force on a sphere in a fluid.
F = 6πηrv
F is the viscous force.
η is the viscosity of the fluid.
r is the radius of the sphere.
v is the velocity of the sphere.
Method by Millikans’s experiment?
- used an atomiser to create a mist of charged oil droplets which fell into a viewing chamber.
-applied pd across viewing chamber to create an electric field. - study motion of these charged particles in a variable electric field.
Why did M perform experiment with no electric field?
see motion under weight of drop & viscous force
calc radius of particles using their velocity & stokes law
How did he calculate the charge?
turned on electric field and adjusted so droplets held still
equate weight to electric field to calculate charge on each droplet
How did he conclude the charge on electron?
Each droplet’s charge had a common factor of 1.6×10-19C, which is the charge on one electron.
How did M charge the oil droplets?
used x-rays to produce the Q
there were also electrically charged brass plates
Summaries Millikan’s oil drop experiment
- create mist of fine charged oil droplets using an atomiser
- allow droplets to fall into viewing chamber
- observe motion - calc r w eqns of motion + stokes law
- apply electric field across chamber until particles hold still
- equate force due to electric field to weight to find Q
Difference between electric magnetic fields in terms of motion of particle?
Electric: parabolic motion in same plane - force parallel to field
Magnetic: circular motion - force perpendicular to field so curvature different firection to plate
How did the pressure of glass tube get reduced?
used vacuum pump
What did they discover abt low pressure gases in tubes?
conduct elect + emit light of characteristic colour
Observations of discharge tube?
changing gas pressure causes glow of light to come from different paths along tube
if bring magnet near tube then glowing gas near anode distorted -> shows charged particles move through gas when it conducts elect (needs to be uniform mf) (stationary p unaffected)
high ef causes gas atoms to become ionised
+ve ions accelerate towards cathode + collide -> releases electrons (successive ionisations)
e- from cathode accelerate towards anode by ef
e- collide w more gas atoms which ionise
glowing gas near cathode
some of e- pulled out of gas atoms don’t recombine & attracted to anode so move away from cathode = ‘cathode rays’
e- move towards anode & cause excitation by collision of gas atoms in tube
glowing gas at anode
What is glowing gas near cathode due to?
photons emitted
as some of +ve ions & e- produced by ionisation recombine so e- fall to ground state
What do we need in order for emission of light from discharge tube?
large pd - accel voltage - pull e- out of some of gas atoms - ionised
What is glowing gas at anode/+ve column due to?
de-excitation of excited gas atoms
Why are photons emitted in discharge tube?
process of recombination + de-excitation of e-, +ve ions, excited gas atoms result in emission of visible & UV light photons
Why are free e- emitted in discharge tube?
positive ions created near cathode are attracted onto cathode surface - so e- emitted from cathode
Describe how charged particles are responsible for conduction in gas are produced in glass tube
positive ions hit cathode causing e- release
ionisation by collision occurs
e- pulled out of gas atoms so become +ve ions
conduction due to e- + positive ions
Why is light emitted in glass tube?
ions + e- moving in opposite directions collide w each other - recombine + emit photons
e- excite gas atoms by collision & photons emitted when de-excitation occurs
Why is there no glow in tube until low pressure?
particles (uncharged atoms/ions/e-) gas need to be widely spaced
so + ions not stopped by gas atoms
ions/electrons are accelerated & gain enough KE to cause excitation
What are the cathode rays?
charged particles moving towards negative electrode
What does specific Q of charge particles depend on?
choice of gas in tube
as MASS of ion depends on gas type
Evidence of cathode rays? (radiation from direction of cathode)
rotated paddle wheel placed in their path - impart force so momentum - means particulate!
placing magnet near tube stopped paddle wheel rotating - deflected cathode rays away -> charged
How was the particulate nature proved of cathode ray particles?
blocked by metals - produced shadow on phosphorus screen
What is thermionic emission?
when heat metal and free e- gain sufficient KE to leave metal surface - they’re emitted by thermionic emitter
What materials is used for thermionic emitter?
metal = wire filament heated by passing electric current through it - placed as one end of evacuated glass tube (cathode)
metal place - anode at other end
How does thermionic emission form e- beam?
e- emitted from filament are attracted to anode
(connect high pd bet anode & cathode)
no gas m to stop/scatter e- or absorb their ke in collisions - e- accelerated to anode where pass through small holder to form narrow beam
What does the equation equating ke and eV assume?
each e- starts from filament w negligible KE in comparison to work done on it by accelerating pd V
speed of e- in beam is less than speed of light in free space so non-relativistic formula for KE applies
how can e- beam deflected?
e or mg field
What is the significance of Thomson’s results for particles in cathode rays when compared to sQ of H ion?
magn of sQ much greater than H ion so if charges similar then cathode rays particles
before H was known to have largest sQ but showed this was 1860x bigger than H
couldn’y conclude if smaller mass as didn’t know e- Q (this was measured by Millikan
[we now know that mass much smaller than H ion so smaller than an atom!]
how did he calc sQ
had to then switch off e or mg field in order to measure the resulting deviation of the beam - circular or parabolic
How is the electron beam made visible?
use fine beam tube
made visible by collisions bet e- & helium gas in tube
He small amt gas - most e- unaffected by its presence
What do we need in order for the diameter/radius of the e- beam to be measured?
sufficiently strong mg field
initial direction of beam right angles to mg field lines
so beam is complete circle
Why does the gas glow? in gas tube
e- in beam collide w atoms of gas (usually He)
e- in atom jump up to higher energy level
e- fall back to lower energy level - release photons of em radiation - BLUE light
Why is it actually a spiral rather than circle in fine beam tube?
e- in beam collide w He atoms so e- slow down (lose KE)
slower e- have reduced curvature since B, m, e are constant
Why does the beam start to dim?
intensity decreases - colour doesn’t change! same energy given off but less e- causing photons to be released
not enough e- w high enough velocities to cause successful ionisations - less excitations - less cascading
What did Millikan notice about his resultss?
Millikan measured mass by turning off e fields and calculated Q using mgd/V
found that charge was always n1.6x10^-19 so electric charge quantised in whole # multiples
so concluded quantised charge of electron and then n = electrons in droplet
Forces acting on drop when held stationary by e field?
initially drag = 0, initial accel = g
as gains speed, drag increases so Fres + accel decreases to 0
Forces acting on droplet when electric f on and it moves vertically?
moves at constant speed that depends on weight, drag f, electric f
- if electric f > weight, moves up & drag down
- if electric f < weight or electric f, downwards then falls & drag up
An oil droplet of Q 8x10^-19 is stationary in e field, it splits in half what happens to it’s motion?
5 electrons - can’t split equally as charge quantised
So splits into 2 and 3 e- per 1/2 mass
for 2e:
mass has halved but charge 2/5 less than original so grav f > elect f so accelerates downwards
for 3e:
mass halved but charge 3/5 of original
elect f > grav f so accelerates upwards
