Question

In Millikan’s oil drop experiment, an oil drop carrying a charge q falls with a terminal velocity v0 when there is no electric field between the plates. An electric field E is applied to keep it stationary. What additional charge should the oil drop acquire so that it begins to move upwards with a velocity 2v0 in the same electric field?     

Solution

Correct option is

2q

When the drop is stationary, we have  

        

  

let Q be the additional charge acquired by the drop so that its charge now is q’ = q + Q. When it moves upwards with a velocity v = 2v0, then we have

        

               

    

                = 3q.  

Thus, 

          

              

              .

SIMILAR QUESTIONS

Q1

A proton projected with a velocity v describes a circle of radius r in a uniform magnetic field. With what velocity should an alpha-particle be projected so that it describes a circle of the same radius r in the same magnetic field?   

Q2

A proton projected with kinetic energy K describes a circle of radius r in a uniform magnetic field. With what kinetic energy should an -particle be projected in the same magnetic field so that it describes a circle of radius 2r?

Q3

A proton and an -particle projected with equal linear momenta describe circular paths of radii r1 and r2 respectively in a uniform magnetic field. The ratio  is

Q4

An -particle and a deuteron projected with equal kinetic energies describe circular paths of radii r1 and r2 respectively in a uniform magnetic field. The ratio  is  

Q5

In a cyclotron the speed of a charged particle is increased by

Q6

A monoenergetic electron beam with 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 .

Q7

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?    

Q8

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? 

Q9

In Millikan’s oil drop experiment, a charged oil drop of mass  is held stationary between two parallel plates 6 mm apart by applying a potential difference of 1200 V between them. How many excess electrons does the oil drop carry? Take g = 10 ms–2.

Q10

A metal whose work function is 3.31 eV is illuminated by light ofwavelength . What is the threshold frequency for photoelectric emission? Take