Question
Read the passage given below and answer the following questions from (i) to (v).
It is well known fact that liquids assume theshape of the container. Why is it then smalldrops of mercury form spherical bead insteadof spreading on the surface. Why do particlesof soil at the bottom of river remain separatedbut they stick together when taken out? Whydoes a liquid rise (or fall) in a thin capillary assoon as the capillary touches the surface ofthe liquid? All these phenomena are causeddue to the characteristic property of liquids,called surface tension. A molecule in the bulkof liquid experiences equal intermolecularforces from all sides. The molecule, thereforedoes not experience any net force. But for themolecule on the surface of liquid, net attractiveforce is towards the interior of the liquid, due to the molecules below it. Since thereare no molecules above it.Liquids tend to minimize their surface area.The molecules on the surface experience a netdownward force and have more energy than the molecules in the bulk, which do notexperience any net force. Therefore, liquids tendto have minimum number of molecules at theirsurface. If surface of the liquid is increased bypulling a molecule from the bulk, attractiveforces will have to be overcome. This willrequire expenditure of energy. The energyrequired to increase the surface area of theliquid by one unit is defined as surface energy.Its dimensions are Jm. Surface tension isdefined as the force acting per unit lengthperpendicular to the line drawn on the surfaceof liquid. It is denoted by Greek letter γ(Gamma). It has dimensions of kg $s^{–2}$ and in SIunit it is expressed as $Nm^{–1}.$
The lowest energystate of the liquid will be when surface area isminimum. Liquid tends to rise (or fall) in the capillarybecause of surface tension. Liquids wet thethings because they spread across their surfacesas thin film. Moist soil grains are pulled togetherbecause surface area of thin film of water isreduced. It is surface tension which givesstretching property to the surface of a liquid.On flat surface, droplets are slightly flattenedby the effect of gravity; but in the gravity freeenvironments drops are perfectly spherical. Viscosity is a measure of resistance toflow which arises due to the internal frictionbetween layers of fluid as they slip past oneanother while liquid flows. Strongintermolecular forces between molecules holdthem together and resist movement of layerspast one another.
When a liquid flows over a fixed surface,the layer of molecules in the immediate contactof surface is stationary. The velocity of upperlayers increases as the distance of layers fromthe fixed layer increases. This type of flow inwhich there is a regular gradation of velocityin passing from one layer to the next is calledlaminar flow.‘$ η’$ is proportionality constant and is calledcoefficient of viscosity. Viscosity coefficientis the force when velocity gradient is unity andthe area of contact is unit area. Thus ‘$ η’$ ismeasure of viscosity. SI unit of viscositycoefficient is $1$ newton second per square metre $\left( N s m ^{-2}\right)=$ pascal second (Pa s $\left.=1 g cm ^{-1} s^{-1}\right)$. Incgs system the unit of coefficient of viscosity ispoise (named after great scientist Jean LouisePoiseuille). 1 poise $=1 g cm ^{-1} S^{-1}=10^{-1} kg m ^{-1} S^{-1}$ Greater the viscosity, the more slowly theliquid flows. Hydrogen bonding and van derWaals forces are strong enough to cause highviscosity. Glass is an extremely viscous liquid.It is so viscous that many of its propertiesresemble solids.Viscosity of liquids decreases as thetemperature rises because at high temperaturemolecules have high kinetic energy and canovercome the intermolecular forces to slip pastone another between the layers.
It is well known fact that liquids assume theshape of the container. Why is it then smalldrops of mercury form spherical bead insteadof spreading on the surface. Why do particlesof soil at the bottom of river remain separatedbut they stick together when taken out? Whydoes a liquid rise (or fall) in a thin capillary assoon as the capillary touches the surface ofthe liquid? All these phenomena are causeddue to the characteristic property of liquids,called surface tension. A molecule in the bulkof liquid experiences equal intermolecularforces from all sides. The molecule, thereforedoes not experience any net force. But for themolecule on the surface of liquid, net attractiveforce is towards the interior of the liquid, due to the molecules below it. Since thereare no molecules above it.Liquids tend to minimize their surface area.The molecules on the surface experience a netdownward force and have more energy than the molecules in the bulk, which do notexperience any net force. Therefore, liquids tendto have minimum number of molecules at theirsurface. If surface of the liquid is increased bypulling a molecule from the bulk, attractiveforces will have to be overcome. This willrequire expenditure of energy. The energyrequired to increase the surface area of theliquid by one unit is defined as surface energy.Its dimensions are Jm. Surface tension isdefined as the force acting per unit lengthperpendicular to the line drawn on the surfaceof liquid. It is denoted by Greek letter γ(Gamma). It has dimensions of kg $s^{–2}$ and in SIunit it is expressed as $Nm^{–1}.$
The lowest energystate of the liquid will be when surface area isminimum. Liquid tends to rise (or fall) in the capillarybecause of surface tension. Liquids wet thethings because they spread across their surfacesas thin film. Moist soil grains are pulled togetherbecause surface area of thin film of water isreduced. It is surface tension which givesstretching property to the surface of a liquid.On flat surface, droplets are slightly flattenedby the effect of gravity; but in the gravity freeenvironments drops are perfectly spherical. Viscosity is a measure of resistance toflow which arises due to the internal frictionbetween layers of fluid as they slip past oneanother while liquid flows. Strongintermolecular forces between molecules holdthem together and resist movement of layerspast one another.
When a liquid flows over a fixed surface,the layer of molecules in the immediate contactof surface is stationary. The velocity of upperlayers increases as the distance of layers fromthe fixed layer increases. This type of flow inwhich there is a regular gradation of velocityin passing from one layer to the next is calledlaminar flow.‘$ η’$ is proportionality constant and is calledcoefficient of viscosity. Viscosity coefficientis the force when velocity gradient is unity andthe area of contact is unit area. Thus ‘$ η’$ ismeasure of viscosity. SI unit of viscositycoefficient is $1$ newton second per square metre $\left( N s m ^{-2}\right)=$ pascal second (Pa s $\left.=1 g cm ^{-1} s^{-1}\right)$. Incgs system the unit of coefficient of viscosity ispoise (named after great scientist Jean LouisePoiseuille). 1 poise $=1 g cm ^{-1} S^{-1}=10^{-1} kg m ^{-1} S^{-1}$ Greater the viscosity, the more slowly theliquid flows. Hydrogen bonding and van derWaals forces are strong enough to cause highviscosity. Glass is an extremely viscous liquid.It is so viscous that many of its propertiesresemble solids.Viscosity of liquids decreases as thetemperature rises because at high temperaturemolecules have high kinetic energy and canovercome the intermolecular forces to slip pastone another between the layers.
- The dimension of surface energy is:
- $Jm^{–2}$
- $Jm^2$
- $Kjm^{–2}$
- $Kjm^2$
- 1 poise =
- $1cmskg^{-1}$
- $1gcm^{–1}s^{–1}$
- $1gcms^–1$
- $1gcm^{–1}s$
- Which of the following is most viscous liquid?
- Glass
- Water
- Mercury
- Kerosene
- Surface Tension denoteed by greek letter...
- $\in$
- $\zeta$
- $\delta$
- $\gamma$
- Flow in which there is a regular gradation of velocity in passing from one layer to the next is called:
- Turbulent flow
- Shear flow
- Streamline flow
- laminar flow.
