A clean plate of length 9.8 cm and thickness 0.2 m is in contact with the water . It appears to weigh 3000 dyne . When the plate is greasy and the angle of contact is `180^(@)` , it appears to weigh 6000 dyne . Then the surface tension is

Options

A. 75 dyne/cm
B. 225 dyne/cm
C. 3000 dyne/cm
D. 750 dyne/cm

Answer

A

Explanation

When the plate is clean angle of contact is `theta ` is `0^(@)`
The apparent weight of the plate
` = mg - 2 (l+T)T `
` 3000 = mg - 2 (l+T)T" " ` ……(i)
when the plate is greasy , `theta ` is `180^(@)`
The apparent weight of the plate ,
` = mg + 2 (l+T)T`
` 6000 = mg + 2 (l+T) T " " ` ...(ii)
Subtracting (i) from (ii),
` 4 (l+T)T = 3000 ,`
` 4 (9.8 +0.2) T = 3000`
` T = 3000/4 = 75 ` dyne/cm