Useful for class 12

1. CROSS FIELD AND ITS APPLICATION IN
DETERMINATION OF SPECIFIC CHARGE

2. HELLO!
1)SADEEP KHANAL
2)SHRISTI TAMANG
3)SHRADDHA POKHAREL
4)SANYAM KOTHARI
2

3. WE ARE REALLY THANKFUL TO OUR
PHYSICS TEACHER MR.SUSHIL UPRETI SIR
AND HOD PHYSICS MR.DEVENDRA KHADKA
FOR THEIR CONTINUOUS HELP AND
SUPPORT.
3
ACKNOWLEDGEMENT

4. CONTENTS;
✘ ELECTRIC FIELD AND MAGNETIC FIELD
✘ CROSS FIELD
✘ SPECIFIC CHARGE
✘ J.J. THOMSON EXPERIMENT
✘ CONCLUSION
✘ REFERENCES
4

5. ELECTRIC FIELD
Electric field is defined
as the electric force per
unit charge. The direction
of the field is taken to be the
direction of the force it
would exert on a positive
test charge. The electric
field is radially outward from
a positive charge and
radially in toward a negative
point charge.
5

6. MAGNETIC FIELD
A magnetic field is a vector
field that describes the
magnetic influence on
moving electric charges,
electric currents, and
magnetic materials. A moving
charge in a magnetic field
experiences a force
perpendicular to its own velocity
and to the magnetic field.
6

7. CROSS FIELD
★ The uniform electric and
magnetic fields acting
mutually perpendicular to
each other is called cross field.
★ The magnitude and direction
of both fields are so adjusted
that the deflection produced
by one field on charged
particle is cancelled by the
deflection produced by the
other.
7

8. DETERMINATION OF VELOCITY IN CROSS FIELD
Consider a beam of electron is
moving with the velocity v in the
electric field ‘E’ and magnetic field ‘B’.
The force on electron due to electric
field is Fₑ = eE .
8
The force on electron due to magnetic field is Fₘ =Bev
When the beam is undeflected, then
magnetic force (Fₘ) = electric force (Fₑ)
Bev = eE
or, v = E/ B .

9. SPECIFIC CHARGE
★ The ratio of charge of electron to its mass is called as
specific charge.
★ It is denoted by e .
★ It can be determined by using JJ Thomson experiment.
9
m

10. J.J. THOMSON EXPERIMENT
DETERMINATION OF SPECIFIC CHARGE
10

11. J.J THOMSON EXPERIMENT
11
★ The set up consists of a discharge tube with fluorescent
screen. There are two electrodes: cathode ‘C’ and anode
‘A’.
★ A uniform electric field ‘E’ is applied in downward
direction through the two horizontal metal plates P and
Q.
★ A uniform magnetic field ‘B’ is applied in inward
direction and is perpendicular to the electric field.
★ When the high voltage is applied between cathode and
anode, a narrow beam of cathode rays is emitted from
cathode to anode. This beam is allowed to pass between
two plates ‘P’ and ‘Q’ and then strikes the fluorescent
screen and luminous spot is seen. When both electric
and magnetic fields are not applied, the beam of
cathode rays moves straight and strikes at the centre 'O'
of the screen.

12. 12

13. DETERMINATION OF SPECIFIC CHARGE
13
When sufficient amount of potential (V) is applied between the two electrodes electrons
emitted from the cathode accelerate with velocity ‘v’ then,
When the charge particle moves under the action of electric field strength (E) it moves
towards the +ve plate and finally incident at point O, at fluorescent screen. In case of applying
magnetic field only, the charge particle deviate and finally incident .
If both magnetic and electric field are applied and their magnitude and direction is adjusted
so that the charged particles move without deviation. At this condition charge particle
incident at point O at the fluorescent screen,
Then,
We have,

14. 14
Where, B is magnetic field strength. In this case the direction of magnetic field is
perpendicular to the direction of the motion of the charged particle.
From equation (i) and (ii),
We get,
Hence, the specific charge at the electron is determined if the values of E, B and V are known.
The experimentally obtained specific charge of the electron is 1.7 ✕ 10¹¹ C/kg.

15. CONCLUSION
15
In conclusion, this presentation helped us build our skills and
expand our knowledge. We hope it also taught you something
new about these topics. We would like to thank Sushil Sir and our
classmates for their patience during our presentation.

16. REFERENCES
16
https://tyrocity.com
https://www.askmattrab.com
https://byjuys.com
Pioneer Physics Book-XII
https://youtube.com

17. THANKS!
Any questions?
17