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Physics Quiz Question Solutions
1. Q. Why are we subtracting the electric field of the point charge from the electric field of the
wire? Is it because the mid-point is to the right of the wire and left of the point charge?
It is given that both of the point charge and the long wire are positively charged. The electric field
vectors due to the positive charge will have the direction that points away from that charge. Hence, in
the diagram, at the half-way, the electric field due to the wire E
w
is a vector pointing left, away from the
wire. Similarly, the electric field due to point charge E
p
is a vector pointing left, away from the point
charge. Since E
w
is pointing right and E
p
is pointing left, the two vectors E
w
and E
p
are opposite
directions. To find the resultant vector of two with opposite direction, we subtract the vector pointing
left (E
p
) from the vector pointing right (E
w
).
It is not because the mid-point is to the right of the wire and left of the point charge; rather, the
direction of the vectors E
w
and E
p
(determined by the rule that the electric field vectors due to the
negative/positive charge will have the direction that points towards/away from that charge) will
determine if we should subtract or add electric fields.