# An electron moving through a magnetic field [closed]

An electron, that has been accelerated from rest by a potential difference of $$250\ \rm V$$, enters a region of magnetic field of strength $$0.12\ \rm T$$. Show that speed of the electron after acceleration is $$9.4 \times 10^6\ \rm m s^{–1}.$$

For this problem I first tried to find the velocity by rewriting the equation for the force an electron will experience when moving through a magnetic field B. $$F=qvB$$ $$v=\frac{F}{qB}$$ I then attempted to find an equation which I could rewrite to give me $$F$$ and then substitute it into the equation for velocity. However, I can only find $$E=\frac{F}{q}$$ for an electric field. I know the charge of an electron, but with the information given I cant find the electric field strength.

Any ideas as to which equation I could use?

## closed as off-topic by AccidentalFourierTransform, user191954, John Rennie, ZeroTheHero, user197851 Dec 3 '18 at 14:28

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You need to observe that magnetic field never does any work on a moving charge so it can't accelerate or decelerate the electron. All the kinetic energy that the electron must have gained must come from the initial accelerating potential. So as the the electron is accelerated through a potential say V we can denote its kinetic energy as $$\ \rm eV$$. Now also we know that $$KE=\frac{1}{2}mv^2$$ by equating the two you can obtain the velocity of the accelerated electron.
• Thanks for the answer. Where do you get $eV$ from? From which expression of kinetic energy is that from? – Pablo Dec 2 '18 at 11:22
• @Pablo eV is the customary unit of energy in particle physics. It's used because it's got such a nice direct relation to the experimental setup: A particle with the charge of 1e (electrons, protons, deuterons, etc.) gains/looses exactly 1eV when it's accelerated by an electric field of 1V. I.e. You want 42eV electrons? Just put 42 volts on your electron gun. You want 100keV X-Rays, apply 100kV to your X-Ray emitter (ok, not precisely because of conversion losses, but you get the idea...). In your question, the potential is 250V, you have electrons, so 250eV. Simple, isn't it? – cmaster Dec 2 '18 at 12:47