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NCERT Class XII Chapter
Electric Charges and Fields
Questions With Solutions

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Question : 3
Total: 34
Check that the ratio
Ke2
Gmemp
 is dimensionless. Look up a table of Physical Constants and determine the value of this ratio. What does the ratio signify?
Solution:  
The ratio of electrostatic force to the gravitational force between an electron and a proton separated by a distance r from each other is
Felec
Fgrav
 =
Ke.∕r2
Gmemp∕r2
 =
Ke2
Gmemp
In terms of dimensions
So, [
Ke2
Gmemp
] = [
Felec
Fgrav
] =
MLT−2
MLT−2
 = 1
Thus, the ratio
Ke2
Gmemp
 is dimensionless
Ke2
Gmemp
 =
9×109×(1.6×10−19)2
6.67×10−11×9.1×10−31‌Kg×1.67×10−27‌Kg

= 0.23 × 1040 = 2.3 × 1039
This ratio signifies that electrostatic forces are 1039 times stronger than gravitational forces
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