In Thomson’s Experiment For determining , The Potential Difference Between The Cathode And The Anode (in The Accelerating Column) Is The Same As That Between The Deflecting Plates (in The Region Of Crossed Fields). If The Potential Difference Is Doubled, By What Factor Should The Magnetic Field Be Increased To Ensure That The Electron Beam Remains Undeflected?

Why Kaysons ?

Video lectures

Access over 500+ hours of video lectures 24*7, covering complete syllabus for JEE preparation.

Online Support

Practice over 30000+ questions starting from basic level to JEE advance level.

Live Doubt Clearing Session

Ask your doubts live everyday Join our live doubt clearing session conducted by our experts.

National Mock Tests

Give tests to analyze your progress and evaluate where you stand in terms of your JEE preparation.

Organized Learning

Proper planning to complete syllabus is the key to get a decent rank in JEE.

Test Series/Daily assignments

Give tests to analyze your progress and evaluate where you stand in terms of your JEE preparation.



In Thomson’s experiment for determining , the potential difference between the cathode and the anode (in the accelerating column) is the same as that between the deflecting plates (in the region of crossed fields). If the potential difference is doubled, by what factor should the magnetic field be increased to ensure that the electron beam remains undeflected?


Correct option is

The charge to mass ratio of an electron is given by 


Where V is the accelerating voltage between cathode and anode. Let d be the separation between the defecting plates. Now potential difference between the plates equals accelerating voltage V. Hence the electric field between the plates is  


Using (ii) in (i) we get 


If V is charged to V’ = 2VB must be increased to B’ given by 





Obtain de-Broglie wavelength of a neutron of kinetic energy 150 eV. As we have seen in last example, an electron beam of 150 eV energy is suitable for crystal diffraction experiments. Would a neutron beam of the same energy by equally? Explain. The mass of neutron is .


X-rays of wavelength 0.82 Å fall on a metal plate. Find the smallest wavelength associated with the emitted photoelectrons. The work function of the metal is zero.  



Obtain the de-Broglie wavelength associated with thermal neutrons at room temperature (27o C). hence explain why a fast neutron beam needs to be thermalised with the environment before it can be used for neutron diffraction experiments. The mass of neutron is  and the Boltzmann constant . 


Calculate the de-Broglie wavelength of an electronic energy 100 eV.


An electron microscope uses electrons accelerated through 50 kV. Find the de-Broglie wavelength of these electrons. If other factors like numerical aperture etc. are roughly the same, how does the resolving power of an electron microscope compare with that of an optical microscope which uses variable light?


The force exerted by a magnetic field on a charged particle is independent of the


The monoenergetic electron beam electron speed of  is subjected to a magnetic field of  normal to the beam velocity. What is the radius of the circular path traced by the beam? Given e/m  forelectron .


Two parallel plates are placed 5 cm apart in an evacuated tube and a potential difference of 200 V is applied across them? What is the force experienced by an electron at rest in the region between the plates?   


The first scientist to measure the charge to mass ratio of the electron was


The minimum wavelength of X-rays emitted from an X-ray tube operating at a voltage of 104 volts is roughly equal to