Question Details

A positive, singly ionized atom of mass number 𝐴M is accelerated from rest by the voltage 192 V. Thereafter, it enters a rectangular region of width 𝑀 with magnetic field  B β†’ 0 = 0.1 k ^ Tesla, as shown in the figure. The ion finally hits a detector at the distance π‘₯ below its starting trajectory.

[Given: Mass of neutron/proton = (5/3) Γ— 10βˆ’27 kg, charge of the electron = 1.6 Γ— 10βˆ’19 C.]

Which of the following option(s) is(are) correct?

Options

A

The value of π‘₯ for 𝐻+ ion is 4 cm

B

The value of π‘₯ for an ion with 𝐴M = 144 is 48 cm.

C

For detecting ions with 1 ≀ 𝐴M ≀ 196, the minimum height (π‘₯1 βˆ’ π‘₯0) of the detector is 55 cm.

D

The minimum width 𝑀 of the region of the magnetic field for detecting ions with 𝐴M = 196 is 56 cm.

Show Answer

Correct Answer :

Option A

The value of π‘₯ for 𝐻+ ion is 4 cm

Option B

The value of π‘₯ for an ion with 𝐴M = 144 is 48 cm.

The value of π‘₯ for 𝐻+ ion is 4 cm; The value of π‘₯ for an ion with 𝐴M = 144 is 48 cm.

Solution :

1. Relation between acceleration potential and kinetic energy:
Let the mass of the singly ionized atom be m and its charge be q=e.
The ion starts from rest and is accelerated through a potential difference V=192 V. The kinetic energy gained by the ion is:
K = q V = 1 2 m v 2
where v is the velocity of the ion when entering the magnetic field.
The momentum p=mv is therefore given by:
p = 2 m q V

2. Motion in the magnetic field:
As shown in the image, the ion enters the rectangular magnetic field region of width w perpendicularly (at 90∘ to the boundary).
The magnetic field is given as B→0=0.1k^ T (pointing out of the page).
The magnetic force F→=q(v→×B→) acts perpendicular to both velocity and the field. For a positive ion traveling in the +y direction into a field along the +z direction, the cross product j^×k^=i^ points downwards (in the +x direction).
This constant perpendicular force deflects the ion into a circular arc of radius R, where:
R = m v q B = 2 m q V q B = 1 B 2 m V q

3. Calculating the deflection distance:
From the geometry in the diagram, the ion turns by 180∘ and exits back through the left boundary, hitting the detector.
The vertical deflection x below its entry level is equal to the diameter of the circular path:
x = 2 R = 2 B 2 m V q

4. Substituting the numerical values:
Let the mass number of the ion be AM. The mass of the ion is:
m = A M Γ— 5 3 Γ— 10 βˆ’ 27 kg
Using q=1.6Γ—10βˆ’19 C, V=192 V, and B=0.1 T:
x = 2 0.1 2 Γ— A M Γ— 5 3 Γ— 10 βˆ’ 27 Γ— 192 1.6 Γ— 10 βˆ’ 19
Simplifying the term inside the square root:
2 Γ— 5 3 Γ— 192 1.6 = 10 3 Γ— 192 1.6 = 640 1.6 = 400
For the powers of 10:
10 βˆ’ 27 10 βˆ’ 19 = 10 βˆ’ 8
Thus:
x = 20 400 Γ— 10 βˆ’ 8 Γ— A M = 20 Γ— 20 Γ— 10 βˆ’ 4 A M = 0.04 A M m
Converting this to centimeters:
x = 4 AM cm

5. Verifying the correct options:
- For H+ ion, the mass number is AM=1:
x = 4 1 = 4 cm
This makes the first option correct.
- For an ion with AM=144:
x = 4 144 = 4 Γ— 12 = 48 cm
This makes the second option correct.

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