A beam of electrons is moving with constant velocity in a region having electric and magnetic fields of strength $ 20 V m^{-1} $ and 0.5 T at right angles to the direction of motion of the electrons. What is the velocity of the electrons?
- $ 8\, ms^{-1}$
- $ 5.5 \,ms^{-1}$
- $ 20\, ms^{-1}$
- $ 40 \,ms^{-1}$
The Correct Option is D
Solution and Explanation
(B) = 0.5 T.
The force on electron in a magnetic field = evB
Force on electron on an electric field = eE
Since the electron is moving with constant
velocity, therefore the resultant force on electron
is zero.
i.e $ eE = ev B \Rightarrow v= E/B = 20/0.5 = 40 \,ms^{-1}$
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Concepts Used:
Moving Charges and Magnetism
Moving charges generate an electric field and the rate of flow of charge is known as current. This is the basic concept in Electrostatics. Another important concept related to moving electric charges is the magnetic effect of current. Magnetism is caused by the current.
Magnetism:
- The relationship between a Moving Charge and Magnetism is that Magnetism is produced by the movement of charges.
- And Magnetism is a property that is displayed by Magnets and produced by moving charges, which results in objects being attracted or pushed away.
Magnetic Field:
Region in space around a magnet where the Magnet has its Magnetic effect is called the Magnetic field of the Magnet. Let us suppose that there is a point charge q (moving with a velocity v and, located at r at a given time t) in presence of both the electric field E (r) and the magnetic field B (r). The force on an electric charge q due to both of them can be written as,
F = q [ E (r) + v × B (r)] ≡ EElectric +Fmagnetic
This force was based on the extensive experiments of Ampere and others. It is called the Lorentz force.