A cation of charge q travelling with a velocity v enters a magnetic field B at right angles (θ = 90°) to the field and experiences a magnetic force F_{M} which is the vector product, below. Draw a diagram to describe the direction of the force felt by a cation. What would be the direction of the force vector for an anion?

**q (v x B)= F _{M} **

In a practical mass spectrometer the magnetic sector is not straight but the north and south poles are both curved round into an arc of a circle (typically 90°). The ion follows a circular path of radius r whilst inside this field. The centripetal force F_{C} required to balance this magnetic force is **F _{C} = mv^{2 }/ r.**

**mv ^{2 }/ r = q (v x B**

Draw a vector diagram of the various vectors and forces experienced by the positive ions during its time inside the circular magnetic field.

**Solution**

For a negative ion the force vector would point in the opposite direction.