Discovery of electron Flashcards
(54 cards)
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
