IMathAS-libretexts

Two identical spheres, sphere $\displaystyle {A}$ and sphere $\displaystyle {B}$ , are suspended from insulating threads. The charge on sphere $\displaystyle {A}$ has the same sign and twice the magnitude as the charge on sphere $\displaystyle {B}$ . The spheres are at rest. A student draws a free-body diagram to represent the forces acting on the spheres. Which of the following describes a change that should be made to the diagram to make it correct?

The student has not included the third-law pair for the electrostatic force arrows. An arrow should be added to sphere $\displaystyle {A}$ with equal length and opposite direction to $\displaystyle {F}_{{{E},{A}}}$ and an arrow should be added to sphere $\displaystyle {B}$ with equal length and opposite direction to $\displaystyle {F}_{{{E},{B}}}$ .
The length of the electrostatic force arrows the student has drawn does not reflect the difference in the magnitude of the charge on spheres $\displaystyle {A}$ and $\displaystyle {B}$ . The length of $\displaystyle {F}_{{{E},{A}}}$ should be drawn with twice the length of $\displaystyle {F}_{{{E},{B}}}$ .
The direction of the electrostatic force arrows the student has drawn indicates that the spheres have opposite charges. $\displaystyle {F}_{{{E},{A}}}$ and $\displaystyle {F}_{{{E},{B}}}$ should be directed toward each other instead of away from each other.
The force diagram the student has drawn indicates a net horizontal force on each sphere. $\displaystyle {F}_{{{E},{A}}}$ should be drawn with length equal to the horizontal component of $\displaystyle {F}_{{{T},{A}}}$ , and $\displaystyle {F}_{{{E},{B}}}$ should be drawn with length equal to the horizontal component of $\displaystyle {F}_{{{T},{B}}}$ .

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