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Physics Magnetic Field (Circular Path)

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(edited 7 years ago)
Original post by JF-17
In a demonstration, electrons with 200eV of kinetic energy are going round in a 12cm diameter circle. Calculate the magnetic flux density. Give your answer to 2 significant figures.


For this question you literally just need to plug in some numbers but lets derive the answer.
The Lorentz force is given by
F=qvB F=qvB
The centripetal force for the electrons is
F=mv2/rF=mv^2/r
Equating these two forces since it is the magnetic field which provides the force on the electrons so that they follow the circular path.

qvB=mv2/r qvB=mv^2/r
B=mv/qr B= mv/qr

So you use this formula to calculate the magnetic field, using the mass and charge of an electron
Reply 2
Original post by langlitz
For this question you literally just need to plug in some numbers but lets derive the answer.
The Lorentz force is given by
F=qvB F=qvB
The centripetal force for the electrons is
F=mv2/rF=mv^2/r
Equating these two forces since it is the magnetic field which provides the force on the electrons so that they follow the circular path.

qvB=mv2/r qvB=mv^2/r
B=mv/qr B= mv/qr

So you use this formula to calculate the magnetic field, using the mass and charge of an electron

but what are we supposed to do with the value for kinetic energy they give us? and how do we get the speed to use in that equation?

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