An electron in a uniform magnetic field has a velocity . It experiences a force . If , we have to calculate the magnetic field.

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The force on an electron, charge e, velocity , in magnetic field is given by the equation

Data of the problem are

,

As , we will write the magnetic field as

We thus have the vector equation

We will use the properties of the cross- product

and

and rewrite equation as

and

It may be noted that the -equation is consistent with the -equation, as it also gives

Therefore, the magnetic field is