Question
Rectangular film of soap solution is 6 cm long and 4 cm wide. To make its size $8 cm \times 5 cm$, how much work would have to be done against surface tension? If surface tension of soap solution is $40 \times 10^{-3} N / m$
✓
Answer
$
\begin{aligned}
A & =2 \times 6 \times 10^{-2} \times 4 \times 10^{-2} \\
& =48 \times 10^{-4} m^2
\end{aligned}
$
$
\begin{aligned}
A+\Delta A & =2 \times 8 \times 10^{-2} \times 5 \times 10^{-2} \\
& =80 \times 10^{-4} m^2 \\
\therefore \quad \Delta A & =(A+\Delta A)-(A) \\
& =80 \times 10^{-4}=48 \times 10^{-4} \\
& =32 \times 10^{-4} m^2
\end{aligned}
$
We know that $W = T \Delta A$
$\begin{array}{l}=40 \times 10^{-3} \times 32 \times 10^{-4} \\ =1280 \times 10^{-7} \\ =1.2 \times 10^{-4} \text { Joule }\end{array}$
\begin{aligned}
A & =2 \times 6 \times 10^{-2} \times 4 \times 10^{-2} \\
& =48 \times 10^{-4} m^2
\end{aligned}
$
$
\begin{aligned}
A+\Delta A & =2 \times 8 \times 10^{-2} \times 5 \times 10^{-2} \\
& =80 \times 10^{-4} m^2 \\
\therefore \quad \Delta A & =(A+\Delta A)-(A) \\
& =80 \times 10^{-4}=48 \times 10^{-4} \\
& =32 \times 10^{-4} m^2
\end{aligned}
$
We know that $W = T \Delta A$
$\begin{array}{l}=40 \times 10^{-3} \times 32 \times 10^{-4} \\ =1280 \times 10^{-7} \\ =1.2 \times 10^{-4} \text { Joule }\end{array}$
Need a full question paper?
Generate a complete, print-ready paper with questions like this in minutes — across 16+ boards, with answer keys.
Explore more
Similar questions
Objects at rest on the earth's surface move in circular paths with a period of 24 hours. Are they in orbit in the same sense that an earth's satellite is in orbit? Why not? What would the length of the day have to be to put such objects in a true orbit?
A small drop of water of surface tension T is squeezed between two clean glass plates so that a thin layer of thickness d and area A is formed between them. If the angle of contact is zero, what is the force required to pull the plates apart.
A double slit $S_1-S_2$ is illuminated by a coherent light of wavelength $\boldsymbol{\lambda}$. The slits are separated by a distance d . A plane mirror is placed in front of the double slit at a distance $D_1$ from it and a screen $\sum$ is placed behind the double slit at a distance $D_2$ from it (figure 17- $E_2$ ). The screen $E$ receives only the light reflected by the mirror. Find the fringewidth of the interference pattern on the screen.
→Air is thrown on a sail attached to a boat from an electric fan placed on the boat. Will the boat start moving?
How is the gravitational force between two point masses affected when they are dipped in water keeping the separation between them the same?
In a Callender's compensated constant pressure air thermometer, the volume of the bulb is 1800 cc. When the bulb is kept immersed in a vessel, 200 cc of mercury has to be poured out. Calculate the temperature of the vessel.
Find the average force needed to accelerate a car weighing 500kg from rest to 72km/h in a distance of 25m.
Can you ever have a situation in which a light ray goes undeviated through a prism?
When the filament of a bulb is given an energy of 32 watt then its temperature becomes 2000 K. Find that energy given to the filament when its temperature becomes 3000 K. Here the filament acts, as black-body.
If a constant potential difference is applied across a bulb, the current slightly decreases as time passes and then becomes constant. Explain.