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NCERT Class XII Chapter
Electrostatic Potential and Capacitance
Questions With Solutions

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Question : 28
Total: 37
Show that the force on each plate of a parallel plate capacitor has magnitude equal to
1
2
 QE , where Q is the charge on the capacitor, and
→
E
 is the magnitude of electric field between the plates. Explain the origin of the factor
1
2
Solution:  
Magnitude of electric field between the plates of charged capacitor is E =
σ
ε0
However, magnitude of electric field of one plate on the other plate of charged capacitor is E1 =
σ
2ε0
 . So, force on the one plate of charged capacitor due to the other is
F = QE1 = Q .
σ
2ε0
 =
1
2
 × Q ×
σ
ε0
 or F =
1
2
 QE
The factor
1
2
 is because the electric field of one plate on the other plate of charged capacitor is
1
2
 of the resultant electric field E between the plates of charged capacitor.
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