Question

A beam of cathode rays moves from left to right in a plane of paper and it enters into a uniform magnetic field acting perpendicular to the plane of the paper and inwards. Now, the cathode rays are deflected:
(A) Downwards
(B) Upwards
(C) In a direction perpendicular to the plane of the paper and inwards
(D) In a direction perpendicular to the plane of the paper and outwards

Answer 1

Hint: In order to find the correct answer for the above question, we will be using the concept of the effect of the magnetic field on a moving charge. We will also use Fleming's left hand rule and the concept of Lorenz force.

Complete step by step answer:
First of all, whenever we talk about a charge moving with velocity in a particular direction, there is a magnetic field created and the direction of this magnetic field can be given with the help of the Right Hand Thumb Rule. Remember that a moving charge represents a current.
Now, if a charge is moving with a certain velocity in a particular direction in the effect of some magnetic field, then the particle experiences a force and the magnitude of this force is given by the Lorenz force equation and the force is said to be Lorenz force. The direction of this force can be determined by Fleming's left hand rule.

Fleming's left hand rule states that the direction of velocity, the direction of the magnetic field and the direction of force experienced due to this magnetic field are perpendicular to each other. In the figure, the thumb represents the direction of current, the first finger shows the direction of the magnetic field and lastly the second finger shows the direction of force experienced by the moving charges.
The main purpose of this solution is to find the direction of the force acting on the electrons present in the cathode ray. Simply using Fleming's left hand rule, we can conclude that the direction of the force experienced by the cathode ray charges will be in the upward direction and the ray will be deflected upwards.
Therefore, the correct answer is option (B).

Note:
Keep in mind that the name Fleming’s Left Hand Rule itself suggests that it should be performed with the help of the left hand and not the right hand. Cathode ray is a group of electrons which are emitted from cathode through various techniques and then concentrated into a beam.