e/m- Determination of Specific charge of an electron

1.  DETERMINATION OF SPECIFIC CHARGE (e/m) Of ELECTRON
BY THOMSON’S METHOD.

2.  Contents
Introduction
History
Principle
Equipment Description
Cathode Ray Tube
Formulae
Conclusion

3.  Introduction
• Objective: Determination of the specific charge of the electron (e/m) by Thomson’s method.
• Measuring separately the electric charge (e) and the rest mass (m) of an electron is a difficult
task because both quantities are extremely small.
• Mullikan oil drop experiment enabled us to calculate electron charge but mass of the electron
can’t be calculated directly.
• Mass of electron is calculated by determining the e/m ratio .
• In this experiment we observe the motion of free electrons in a vacuum tube. From their
response to electric and magnetic fields , the ratio of charge to mass for the electron can be
determined.

4.  History
• During the late 19th century, it was impossible to determine very small charge and mass of particle
like electron.
• Even imagination of electrons as an individual part was not clear.
• J. J. Thomson first determined the specific charge (charge to mass ratio-e/m) of the electron in
1897.
• J. J. Thomson was able to determine the e/m ratio by using the cathode ray discharge tube ,
applying electric and magnetic fields perpendicular to each other as well as to the path of
electrons.
• In 1896, J. J. Thomson performed a series of revolutionary experiments with cathode ray tubes and
demonstrated some interesting things about the atom.

5. • In the presence of electrical or magnetic field, the conduct of cathode rays are similar to that
expected from negatively charged particles.
• Electric field produced by a short length of deflection plates and the magnetic field is applied
from outside over the same length.
• Cathode rays originating from a heated cathode(electron gun) were actually a stream of small
negatively charged particles which is evident from their behaviour as discussed above.
• This suggests that the cathode rays consist of negatively charged particles, called electrons
• He named these particles as “corpuscles” but we now know them as electrons.
• He Provided the first evidence that atoms were in fact not fundamental building blocks of
matter.
• He measured the charge to mass ratio of the cathode rays by observing how much they were
deflected by magnetic field and how much energy they carried.

6. According to Thomson ,the amount of deviation of the particles from their path in the
presence of electrical or magnetic field depends upon:
1.The magnitude of the negative charge on the particle,
• greater the magnitude of the charge on the particle, greater is the interaction with the electric
or magnetic field and thus greater is the deflection.
2. The mass of the particle — heaver the particle, lesser the deflection.
3. The strength of the electrical or magnetic field —
• the deflection of electrons from its original path increases with the increase in the voltage
across the electrodes ,or the strength of the magnetic field.
 Measurement of the specific charge of cathode rays for different metals made him conclude
that the particles that constituted cathode rays form a part of all the atoms in the universe.
 For his work J. J. Thomson received the Nobel Prize in Physics in 1906, “in recognition of
the great merits of his theoretical and experimental investigations on the conduction of
electricity by gases”.

7. 
PROPERTIES OF CATHODE RAYS
 In 1878,William Crookes discovered cathode ray by
studying the conduction of electricity through gases in
a discharge tube filled with a gas at very low pressure
(0.01 mm Hg) and huge voltage of order 10^4 .
 1.Cathode rays are beam of fast moving electrons.
 2. These rays are composed of material particles and
posses momentum as they are capable of rotating a
light paddle when placed in their path.
 3.These rays produce heating effect.
 4.These rays ionize the gas through which they pass.
 5.These also effect the photographic plates termed as
fogging.
 6.The characterstics of cathode rays are independent of the
material of the cathode and nature of gas used in discharge
tube.

8.  Principle
• If an electron is subjected to electric and magnetic fields it experiences forces.
• By adjusting magnitude and directions of field the net force on electron is made zero that is beam of electrons
travel without any deflection.
• If both magnetic and electric field are applied and their magnitude and direction is adjusted so that
the charged particles move without deviation.
• Here Electron goes undeflected in presence of both electric and magnetic field if resultant force on the
electron is zero. i.e electric force= magnetic force.
• EQ=Q v B ,
• V=E/B

9. EQUIPMENT DESCRIPTION
 Equipment used in the experiment include the following:
 Cathode ray tube (CRT),CRT power supply , wooden frame, deflection magnetometer,
 Wooden bench, bar magnets , stop watch.
1. CATHODE RAY TUBE:

10. Cathode ray tube is the main component of this experiment. It has three components namely
1.ElectronGun 2.Deflecting plates 3.Fluoroscent screen
The cathode ray tube is filled with helium at very low pressure. You need the low pressure in
the tube so that the passage of the beam of electrons is not impeded very much by Collision
with air molecules

11.  1.ELECTRON GUN:
 The electron gun section of the cathode ray tube provides a sharply focused, electron beam directed
towards the fluorescent-coated screen. This starts from thermally heated cathode, emitting the
electrons which is called as thermo ionic emission.
 The whole assembly of cathode, control grid, focusing anode, and accelerating electrode are called
the electron gun.
 The control grid controls the brightness of the spot on the screen.
 By controlling the number of electrons by the anode , hence the focusing anode ensures that electrons
leaving the cathode in slightly different directions are focused down to a narrow beam and all gather
at the same spot on the screen.
 The control grid is given negative potential with respect to cathode. This grid controls the number of
electrons in beam, going to the screen.
 The momentum of the electrons determines the intensity, or brightness, of the light emitted from the
fluorescent screen due to the electron bombardment.
 The function of these anodes(focusing and accelerating) is to concentrate and focus the beam on the
screen and to accelerate the speed of electrons.

12.  DEFLECTION PLATES:The deflection system in CRT consists of two pairs
of parallel plates, referred to as the vertical and horizontal deflection plates.
Vertical plate moves electron beam upward and downward, and horizontal
plates moves electron beam from one side to another.
FLOUROSCENE:The light which is emitted through phosphorous excitation
is called fluorescence. When the electron beam stop, the phosphorous crystal
regain their original position and release a quantum of light energy which is
called phosphorescence or persistence.
AQUADAG: The Aquadag is the aqueous solution of graphite which is
connected to the secondary of the anode. The Aquadag collects the secondary
emitted electrons which are necessary for keeping the CRT screen in the state
of electrical equilibrium.

14.  FORMULA:- Formula deduced from this experiment to calculate e/m ratio is as follows
e/m = V py ( l+ p/2 )/ t A2
B = B0 (T0
2 / T2 )Sin φ
Where e – charge of an electron
m – mass of an electron
V – voltage applied to deflection plates
y – displacement of the spot
t – separation between the deflecting plates
p – length of deflecting plates
l – distance between the end of deflecting plates and screen
B – magnetic induction
B0 – horizontal component of earth’s magnetic field
T – Time period of oscillation of deflection magnetometer needle in the presence of two bar magnets.
T0 – period of oscillation of deflection magnetometer needle in the presence of earth’s magnetic field.
Φ – inclination of the magnetic needle to the earth’s magnetic field
A – area under the curve plotted between x and [ B ( L- x ) ]
x - is the distance of the screen from the aperture
L – distance of electron gun exit to the screen.

15.  IMPORTANT CONCEPTUAL QUESTIONS
 Why CRT tube is placed along north south direction?
 This is because the earths magnetic field is along north-south
and when CRT tube is placed in the same direction then the
motion of electron beam is not affected by earth’s magnetic
field.
 How magnetic field is applied in this experiment?
 Magnetic field is applied using bar magnets or pair of
Helmholtz coils.
But in this experiment we use bar magnets to produce
magnetic field.
 Why the magnetic field of bar magnets can’t be measured by
gaussmeter?
 charged plates cannot be measured using a gaussmeter as the
magnetic field produced by bar magnets is quite small.
Therefore magnetic field is calculated using deflection
magnetometer.

16.  CONCLUSION:
ERROR ANALYSIS:
 Therefore we can calculate mass of the electron by
using above e/m value.
 By substituting all known values in the formula , the
value Standard value of e/m was found to be
 JJ Thomson found that the value of e/m for these cathode
rays remains the same for all cathodes ,anodes , gas present,
and applied voltages.