A level physics question

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Fig. 3.1 shows a simple representation of a hydrogen iodide molecule. It consists of two ions
1
1H+ and 127
53 I −, held together by electric forces.
H+ I
–
Fig. 3.1
(a) (i) Draw on Fig. 3.1 a minimum of five lines to show the electric field pattern between the
ions. [2]
(ii) The charge on each ion has a magnitude e of 1.6 × 10−19C. The ions are to be treated
as point charges 5.0 × 10−10m apart. Calculate the magnitude of the resultant electric
field strength E at the mid-point between the ions.

For part ii, i got E of H to be +2.3x10^10 and E of I to be -2.3x10^10 why is the resultant magnitude 4.6x10^10 and not 0NC^-1?

Reply 1

lordaxil

Remember that electric fields are VECTOR quantities (i.e. they have a direction). Also, field lines can only start or end on charges. Both e-fields from the H+ and I- ions are pointing from the ions to the mid-point. So, to correctly combine the two fields, you need to subtract them, and hence the magnitude of the resultant is twice the individual magnitude.