Two identical point charges are fixed at and on the -plane

PHYSICS · Multi-part · A Level · Physics · Fields and their consequences · Fields and their consequences

Problem

Two identical point charges

q=+5\,\mu\text{C}

are fixed at

A(-4\,\text{cm},\,0)

and

B(4\,\text{cm},\,0)

on the

xy

-plane. A third charge

Q

of unknown sign is placed at

C(0,\,4\,\text{cm})

. The magnitude of the net force on

Q

is observed to be

0.225\,\text{N}

and its direction is exactly along the negative

y

-axis. (i) Determine the sign of the charge

Q

and give a reason based on the observed direction of the net force. (ii) Calculate the magnitude of

Q

. (iii) If the charge

Q

were moved to the point

C'(0,\,-4\,\text{cm})

, predict qualitatively (without calculation) the new direction of the net force on

Q

and justify your answer.