Charge to mass ratio of electrons

Physical Chemistry
All India Engineering Entrance
Contact: positronAT@gmail.com
Aranisha Raha
Master of Science-Chemistry

Charge to Mass Ratio of
Electron
Atomic Structure
Part: 1/14
5th April’2018
Aranisha Raha
Master of Science-Chemistry

Contents
• Introduction
• Thomson’s Cathode Ray Discharge Tube
Experiment
• Inference of Thomson’s Experiment
• Millikan’s Oil Drop Experiment
• Correlation Between Thomson’s Experiment &
Millikan’s Oil Drop Experiment
Aranisha Raha Contact: positronAT@gmail.com

• In 1878, William Crookes discovered cathode ray by studying the conduction of electricity through gases
in a discharge tube filled with a gas at a very low pressure(0.01 mm of Hg) & connecting the electrodes to
a source of high voltage( approx. 104 v).
• Experimental Findings:
1. These rays are composed of material particles & possess momentum as they are capable of rotating a
light paddle wheel when placed in their path.
2. These rays constitute of –vely charged particles as they are deflected towards the +ve plate when
electric field is applied.
3. These rays produce heating effect.
4. These rays ionise the gas through which they pass.
5. These also affect the photographic plates termed as Fogging.
6. The characteristics of cathode rays are independent of the material of the cathode and nature of the gas
used in discharge tube.
Introduction

Thomson’s Cathode Ray Discharge Tube Experiment
Electrical Field
Magnetic Field
A
B
C
Electrical Field
• J.J. Thomson was
able to determine
the ratio of
electronic charge to
mass of the electron
by using a cathode
ray discharge tube.
• He applied electrical
and magnetic field
perpendicular to
each other as well as
the path of the
electrons.

Electrical Field
Magnetic Field
A
B
C
Electrical Field
• When the magnetic
field along turned
on, electron strikes
the cathode ray tube
at point C.

Electrical Field
Magnetic Field
A
B
C
Electrical Field
• When the electric
field along turned
on, electron strikes
the cathode ray tube
at point A.

Electrical Field
Magnetic Field
A
B
C
Electrical Field
• When the both
electric field and
magnetic field are off
or when they are
balanced to cancel
each other's effects,
the electrons strike
at point B

Inference of the Thomson’s Experiment
Aranisha Raha Contact: positron@gmail.com
The Calculated ratio of charge to mass of electrons (e/m)
e/m of electron =
Charge of the cathode ray particle
Mass of the cathode ray particle
= 1.76 X 108 C g-1 1.76 X 1011 C kg-1
=

Millikan’s Oil Drop Experiment
• Millikan with his Oil Drop Experiment, determined the charge
of the electron.
• By comparing applied electric force with changes in motion of
the oil drops he was able to determine the electric charge on
each drop .
• The charge of the electron = -1.602 X 10-19 C = - 4.8 X 10-10 esu
• This is the minimum measurable quantity of –ve charge.
• That is why electronic charge is considered to be the
fundamental unit of electricity and is called one unit.
• The quantity of electrical charge carried by all –vely charged
particles is an integral multiple of this charge.

• From Thomson’s experiment:
e/m of electron = 1.76 X 108 C g-1
From Millikan’s Oil Drop Experiment
The charge of the electron = -1.602 X 10-19 C
• By combining this two result, we can get the mass(m) of electron:
m
e
e/m
= =
1.602 X 10-19 C
1.76 X 108 C g-1
= 9.11 X 10-28 g 9.11 X 10-31 kg= 0.000548 amu=
• Therefore an electron may be defined as a subatomic particle having a unit –ve
charge(1.602X10-19 C) and negligible mass (9.11X10-31 kg).
Correlation of Thomson’s Experiment & Millikan’s Oil Drop Experiment

Keynotes
An electron may be defined as a subatomic particle
having a unit –ve charge (1.602X10-19 C) and
negligible mass (9.11X10-31 kg).

Thank You
Tanumoy Das
reachdastanumoy1987@gmail.com
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Charge to mass ratio of electrons

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