Assume there are three changes with mass m1,m2,m3 and charge q1,q2,q3
There are attached to each other with strings and hanged on top of ceiling. All the strings are the same length.
The following simulation calculate static equilibrium positions.

For example:
[img]http://www.phy.ntnu.edu.tw/ntnujava/index.php?action=dlattach;topic=1161.0;attach=1163;image[/img]
is the static equilibrium positions when there is no electric field.

Which one of the following is the final static equilibrium positions after electic field E has been turned on.

You can find out the answer from the following simulation. Click "[b]Turn on E[/b]" !
You can also try out with different mass and charge combinations.

Assume the mass from the top to the bottom are $m_1,m_2,m_3$, the tension of the string are $T_1,T_2,T_3$.
The condition for equilibrium are
1. $T_{3x}= q_3 E$ and $T_{3y}= m_3 g$
2. $T_{2x}= (q_2+q_3) E$ and $T_{2y}= (m_3+m_2) g$
3. $T_{1x}= (q_1+q_2+q_3) E$ and $T_{1y}= (m_3+m_2+m_1) g$

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