My book states that :

When only electric field is applied, the electrons deviate from their path and hit the cathode ray tube at point A. Similarly when only magnetic field is applied , the electron strikes the cathode ray tube at point C. enter image description here

I tried to apply the Fleming's Left hand rule used to find the direction of force on a current carrying wire where the direction of current is taken opposite to the direction of flow of electrons.

The rule states that :

Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

Applying this rule I found that the electrons should hit the cathode ray tube at A in a magnetic field . But in the book it is given that they strike it at C.

Please explain me where I went wrong . Thank you.

  • 6
    $\begingroup$ I'm voting to close this question as off-topic because it's not about chemistry. $\endgroup$
    – Mithoron
    Apr 14, 2019 at 16:47
  • $\begingroup$ Ok so should I ask it in Physics SE. $\endgroup$ Apr 14, 2019 at 17:04
  • $\begingroup$ @Chemist I don't see any physical chemistry in here. If the question had been how to make an electron beam, OK. Or how the flourescent screen works. $\endgroup$
    – Karl
    Apr 14, 2019 at 17:27
  • $\begingroup$ Cross posted on Physics SE $\endgroup$
    – Alchimista
    Apr 15, 2019 at 8:10

1 Answer 1


1) for individual charges Lorentz force F =q[v*B], so curl your right hand from velocity vector of positive charge towards magnetic field so that your thumb points at the force acting on the charge. Here we have considered positive charge

2)It is better use this mnemonic to find out the direction of force to avoid confusion

Stretch the thumb, forefinger and middle finger of your right hand such that they are mutually perpendicular

  • fore finger denote direction of electron moving

  • middle finger denotes the direction of magnetic field (north to south on outside the magnet)

  • finally force direction can be found out using your the direction where your thumb is pointing

The book is correct

Tip- If current is mentioned then Use your left hand, to avoid twisting your hand in weird ways! .

  • $\begingroup$ Ok with the right hand rule you stated I am getting it right. But when using the left hand rule I pointed the direction of current opposite to the direction of cathode ray and I am still getting the opposite answer that they should hit at A. $\endgroup$ Apr 14, 2019 at 17:27
  • $\begingroup$ Chemistry and physics cannot be separated by a boundary. Everything is grey beyond textbooks. We should stop teaching atomic structure because that is also physics. $\endgroup$
    – AChem
    Apr 14, 2019 at 17:27
  • $\begingroup$ I want to use my LEFT hand for finding the deflection of cathode ray and even after using the direction of current as opposite to that of cathode ray I am not getting the right answer. $\endgroup$ Apr 14, 2019 at 17:30
  • $\begingroup$ @AshokSharma You are using N-->S for the magnetic field lines, right? ... and you are right, and your book is wrong. If they wanted the beam to hit in C, they should have but the magnet the other way round. ;-) $\endgroup$
    – Karl
    Apr 14, 2019 at 17:37
  • $\begingroup$ @Chemist In the picture in your answer you have shown the green line pointing north to south as middle finger . But in the left hand the middle finger shows the direction of current and not of magnetic field.Thanks for your efforts $\endgroup$ Apr 14, 2019 at 17:53

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