Physics

Problem

Point charges

+5\,\mu\text{C}

(at

A

) and

-20\,\mu\text{C}

(at

B

) are separated by

AB = 40\,\text{cm}

in air. Along the straight line

AB

, measured from

A

, at what position will a small positive test charge experience zero net electrostatic force? a)

13.3\,\text{cm}

from

A

towards

B

13.3\,\text{cm}

on the side of

A

away from

B

40\,\text{cm}

on the side of

A

away from

B

d) No finite position exists

Intuitive explanation

The key is recognizing that a point of zero net force occurs where the electric forces from both charges cancel—this happens when the two force vectors point in opposite directions with equal magnitude. Since the negative charge at $B$ is stronger, the balance point must lie closer to it, on the side of the weaker positive charge. Set up the force-balance equation by equating the magnitudes of Coulomb forces at an unknown distance $x$ from $A$ , then solve for where they're equal.

Intuitive explanation

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