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Discovery of electron Flashcards

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1
Q

Explain main features of discharge tube

A

evacuated glass tube (only gas present that’s meant to be)
pd applied across tube causes e- to jump from cathode -> anode

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2
Q

Why must tube be at low pressure?

A

if tube isn’t then e- don’t gain enough KE to produce ionisation as too many gas particles present

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3
Q

Why does tube emit light?

A

When e- hit other end - release energy as visible light

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4
Q

Impact of cathode rays

A

taught us _ about electrons
- have properties such as mass, momentum, energy
- charge
- properties

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5
Q

What are cathode rays

A

beam of electrons
most commonly produced in discharge tubes
name since beams appear to originate at cathode
used in older screens

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6
Q

What is thermionic emission

A

metal releases electrons when heated

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7
Q

What materials are used for thermionic emission

A

transition metals or there carbides/borides

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8
Q

Uses of thermionic emission

A

Thermionic emitted electrons used in electron guns.

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9
Q

How does an electron gun work?

A

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.

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10
Q

What is specific charge

A

measure of charge per unit mass

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11
Q

What was Thomson’s experiment?

A

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.

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12
Q

What did Millikan notice about his results?

A

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

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13
Q

How did Millikan use the e field in order to calc sQ?

A

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

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14
Q

What is Millikans’ experiment?

A

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

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15
Q

What is Stokes’ Law

A

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.

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16
Q

Method by Millikans’s experiment?

A
  • 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.
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17
Q

Why did M perform experiment with no electric field?

A

see motion under weight of drop & viscous force

calc radius of particles using their velocity & stokes law

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18
Q

How did he calculate the charge?

A

turned on electric field and adjusted so droplets held still

equate weight to electric field to calculate charge on each droplet

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19
Q

How did he conclude the charge on electron?

A

Each droplet’s charge had a common factor of 1.6×10-19C, which is the charge on one electron.

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20
Q

How did M charge the oil droplets?

A

used x-rays to produce the Q
there were also electrically charged brass plates

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21
Q

Summaries Millikan’s oil drop experiment

A
  • 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
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22
Q

Difference between electric magnetic fields in terms of motion of particle?

A

Electric: parabolic motion in same plane - force parallel to field
Magnetic: circular motion - force perpendicular to field so curvature different firection to plate

23
Q

How did the pressure of glass tube get reduced?

A

used vacuum pump

24
Q

What did they discover abt low pressure gases in tubes?

A

conduct elect + emit light of characteristic colour

25
Q

Observations of discharge tube?

A

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)

26
Q
A

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

27
Q

What is glowing gas near cathode due to?

A

photons emitted
as some of +ve ions & e- produced by ionisation recombine so e- fall to ground state

28
Q

What do we need in order for emission of light from discharge tube?

A

large pd - accel voltage - pull e- out of some of gas atoms - ionised

29
Q

What is glowing gas at anode/+ve column due to?

A

de-excitation of excited gas atoms

30
Q

Why are photons emitted in discharge tube?

A

process of recombination + de-excitation of e-, +ve ions, excited gas atoms result in emission of visible & UV light photons

31
Q

Why are free e- emitted in discharge tube?

A

positive ions created near cathode are attracted onto cathode surface - so e- emitted from cathode

32
Q

Describe how charged particles are responsible for conduction in gas are produced in glass tube

A

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

33
Q

Why is light emitted in glass tube?

A

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

34
Q

Why is there no glow in tube until low pressure?

A

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

35
Q

What are the cathode rays?

A

charged particles moving towards negative electrode

36
Q

What does specific Q of charge particles depend on?

A

choice of gas in tube
as MASS of ion depends on gas type

37
Q

Evidence of cathode rays? (radiation from direction of cathode)

A

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

38
Q

How was the particulate nature proved of cathode ray particles?

A

blocked by metals - produced shadow on phosphorus screen

39
Q

What is thermionic emission?

A

when heat metal and free e- gain sufficient KE to leave metal surface - they’re emitted by thermionic emitter

40
Q

What materials is used for thermionic emitter?

A

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

41
Q

How does thermionic emission form e- beam?

A

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

42
Q

What does the equation equating ke and eV assume?

A

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

43
Q

how can e- beam deflected?

A

e or mg field

44
Q

What is the significance of Thomson’s results for particles in cathode rays when compared to sQ of H ion?

A

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!]

45
Q

how did he calc sQ

A

had to then switch off e or mg field in order to measure the resulting deviation of the beam - circular or parabolic

46
Q

How is the electron beam made visible?

A

use fine beam tube
made visible by collisions bet e- & helium gas in tube
He small amt gas - most e- unaffected by its presence

47
Q

What do we need in order for the diameter/radius of the e- beam to be measured?

A

sufficiently strong mg field
initial direction of beam right angles to mg field lines
so beam is complete circle

48
Q

Why does the gas glow? in gas tube

A

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

49
Q

Why is it actually a spiral rather than circle in fine beam tube?

A

e- in beam collide w He atoms so e- slow down (lose KE)
slower e- have reduced curvature since B, m, e are constant

50
Q

Why does the beam start to dim?

A

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

51
Q

What did Millikan notice about his resultss?

A

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

52
Q

Forces acting on drop when held stationary by e field?

A

initially drag = 0, initial accel = g
as gains speed, drag increases so Fres + accel decreases to 0

53
Q

Forces acting on droplet when electric f on and it moves vertically?

A

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
54
Q

An oil droplet of Q 8x10^-19 is stationary in e field, it splits in half what happens to it’s motion?

A

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

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