Step-by-Step Procedure for Measuring Viscosity in the Lab
The viscosity is a measure of the resistance of a fluid to flow. It defines the friction within a moving fluid. A fluid with large viscosity resists motion because it gives it a lot of internal friction due to its molecular structure. A fluid with low viscosity flows easily because when it is in motion, its molecular structure results in very little friction. For example, let’s take a funnel. Water flows very fast through a pipe, as it has very little flow resistance or very little viscosity. That is to say, it's not very thick. On the other hand, it may take a little longer to run honey through a funnel. This is because it has greater flow resistance, more viscosity, and is thicker in nature.
What is the Coefficient of Viscosity?
The quantitative value of the viscosity i.e degree to which a fluid resists flowing under an applied force is called the coefficient of viscosity. There are two types of coefficient of viscosity.
Dynamic Viscosity: Dynamic viscosity(η) normally called viscosity is the ratio between the shearing stress (F/A) to the velocity gradient \[(dv_{x}/dz)\] in a fluid.
\[\eta = \frac{\frac{F}{A}}{\frac{dv_{x}}{dz}}\]
A common form of this equation is known as Newton's equation which says the resulting shear of a fluid is directly proportional to the force applied and inversely proportional to its viscosity.
\[\frac{F}{A} = \eta \left ( \frac{dv_{x}}{dz} \right ) \Leftrightarrow F = \frac{mdv}{dt}\]
The SI unit of dynamic viscosity is pascal second and the common unit: dyne second per square centimeter(\[dyne - s/m^{2}\]).
Kinematic viscosity: Kinematic viscosity(ν) is the ratio between the viscosity of a fluid to its density. Kinematic viscosity is a measure of a fluid's resistive flux under gravity influence.
\[V = \frac{\eta }{\rho}\]
Units: SI unit: square meter per second (\[m^{2}/s\]). Common unit: square centimeter per second (\[cm^{2}/s\]).
Factors Affecting Viscosity
Temperature: Temperature is one of the key factors influencing viscosity. When the temperature decreases, viscosity gets higher.
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Chemical Composition: The viscosity of liquids generally depends on their molecule’s size, shape, and chemical nature. It is greater with smaller molecules than with larger; with elongated molecules than with spherical ones. Normally large quantities of dissolved solids increase the viscosity.
Colloid Systems: The lyophilic colloid solution has typically a fairly high viscosity
Suspended Material: Suspended particles cause the viscosity to increase.
Viscosity Experiment to Determine the Coefficient of Viscosity
Aim:
Determine the viscosity coefficient of a given viscous liquid by measuring the terminal velocity of a given spherical organism (by Stokes method).
Material Required:
A half-meter high transparent viscous liquid, one steel ball 5 cm broad glass cylindrical jar with millimeter graduations along with its height, screw gauge, clamp withstand, stop clock/watch, thermometer.
Theory
Terminal velocity: Terminal velocity is the maximum velocity attained by the object falling through a fluid. The acceleration of the object becomes zero when the summation of drag force and buoyancy equals the gravity, this makes the acceleration zero.
The formula for the terminal velocity:
\[V = \frac{2r^{2}(\rho - \sigma )g}{9 \eta} \]
Where,
v-terminal velocity
r-radius of the spherical body
g-acceleration due to gravity
ρ-density of the spherical body
σ-density of the liquid
η-coefficient of viscosity
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Knowing ρ, σ, r, and calculating v, we can find the coefficient of the viscosity.
Procedure:
Clean the glass jar, and fill it with transparent viscous liquid.
Verify that the vertical scale is clearly visible along with the height of the jar. Note its slightest count.
Test the tight spring stopwatch. Find the least count and (if any) zero error.
Find and note the screw gauge's least count and zero error.
Determine the mean ball radius.
Drop the ball in the liquid, gently. It falls down with accelerated velocity in the liquid for about one-third of the liquid's height. Then, uniform terminal velocity falls.
When the ball hits a suitable division (20 cm; 25 cm; ..........) start the stopwatch. Note its downfall.
Just when the ball hits the lowest convenient division (45 cm), stop the stopwatch.
Find and note the falling distance and the time the ball has taken.
Repeat steps 6 to 9 more than two times.
Note, and record the liquid temperature.
Record your remarks as given ahead here.
Observations:
Least count of vertical scale = 1 mm
Least count of stopwatch = …….. s
Zero error of stopwatch = ……. s
Pitch of screw gauge (p) = 1mm
No.of divisions on the circular scale = 100
Least count of the screw gauge (LC) = 1/100 = 0.01 mm
Zero error of the screw gauge (e) = …… mm
Zero correction of the screw gauge (c) =….. Mm
For the diameter of the spherical ball:
Along one direction, \[D_{1}\] = ….. mm
In the perpendicular direction, \[D_{2}\] = …….. Mm
For the terminal velocity of the spherical ball
Distance fallen, S = …… cm
Time took,
\[t_{1}\] = …….. S
\[t_{2}\] = …….. S
\[t_{3}\] = …….. S
Result
The coefficient of viscosity of the liquid at a temperature (T℃) is ______
Note
In gases, the viscosity coefficient increases with an increase in the temperature.
In the case of the liquid, the coefficient of viscosity decreases with an increase in the temperature.
Types of Viscosity
Dynamic viscosity is defined as the tangential force per unit area necessary to move a fluid in one horizontal plane with respect to another plane at a velocity of unit value while the fluid's molecules retain a unit distance apart.
Kinematic Viscosity
Kinematic viscosity is a type of viscosity calculated by dividing the fluid mass density by the dynamic fluid, viscosity, or absolute fluid viscosity. It's also known as momentum diffusivity from time to time. Kinematic viscosity is measured in terms of time and fluid area. When no external force is applied except gravity, kinematic viscosity is the measurement of a fluid's inherent resistance to flow. This is a force-independent quantity that is the ratio of dynamic viscosity to density. The kinematic viscosity of a fluid may be calculated by dividing its absolute viscosity by its mass density.
Application of Viscosity
The distinctive attribute of a liquid is viscosity, which is undifferentiated from the frictional force. The following are a few of the numerous applications of viscosity:
High-thickness liquids are used in painting.
Viscosity is considered while arranging food items such as dosas and chapatis.
Pen ink is made up of liquids with a high viscosity.
Paints, varnishes, and similar home items have their viscosity carefully controlled so that they may be applied easily and uniformly with a brush roller.
Gum is made up of very sticky substances that cling to objects inexorably.
The thickness of family unit items like paints and stains is directed in such a way that applying paint over the brush is straightforward.
The viscosity of fluids affects blood circulation in arteries and veins.
The oil drop experiment was used by Millikan to calculate the charge of an electron. He calculated the charge using his understanding of viscosity.
Brake fluid transmits force via the braking system, and if it had a different viscosity, it would not function correctly.
FAQs on How to Determine the Coefficient of Viscosity of a Viscous Liquid
1. What is the coefficient of viscosity and how is it determined in a laboratory experiment?
The coefficient of viscosity quantifies the internal friction within a fluid that resists flow. In the laboratory, it is typically determined by measuring the terminal velocity of a small spherical ball falling through a viscous liquid. Using Stokes' law, the coefficient is calculated from the measured radius and densities of the ball and liquid, gravitational acceleration, and observed terminal velocity.
2. How does dynamic viscosity differ from kinematic viscosity, and what are their SI units?
Dynamic viscosity measures a fluid's resistance to shear flow and is defined as the ratio of shear stress to shear rate. Its SI unit is the pascal-second (Pa·s). Kinematic viscosity is the ratio of dynamic viscosity to density, indicating a fluid's resistive flow under gravity, with the SI unit of square meter per second (m²/s).
3. Why is it important for the liquid used in the viscosity experiment to be transparent?
The liquid must be transparent so that the movement of the falling ball can be observed clearly. This visibility allows for precise timing as the ball crosses marked positions, ensuring accurate calculation of terminal velocity and, consequently, the coefficient of viscosity.
4. What precautions should be taken to obtain accurate results while determining the coefficient of viscosity in a physics experiment?
- Ensure the glass jar and measuring instruments are clean and free from contaminants.
- Measure and account for least count and zero error in all instruments.
- Release the ball gently to avoid any initial velocity.
- Conduct multiple trials and use the average value for accuracy.
- Monitor and record the temperature, as viscosity is temperature dependent.
- Use a liquid of uniform composition and no bubbles or suspended particles.
5. How does temperature affect the viscosity of liquids and gases?
As temperature increases, the viscosity of liquids decreases because the intermolecular bonds weaken, making flow easier. Conversely, the viscosity of gases increases with temperature, as increased molecular motion results in more frequent collisions and greater resistance to flow.
6. What is terminal velocity, and why is its measurement crucial in viscosity determination experiments?
Terminal velocity is the constant speed reached by a spherical object falling through a viscous fluid when the force of gravity is balanced by drag and buoyant forces. Accurately measuring terminal velocity allows calculation of the coefficient of viscosity using Stokes' law.
7. In what ways does viscosity play a role in real-world applications and everyday life?
- Automotive fluids: Correct viscosity of engine oils ensures proper lubrication and prevents wear.
- Food industry: Consistency of sauces and syrups relies on viscosity control.
- Medicine: Blood viscosity affects circulation and intravenous injections.
- Manufacturing: Lubricants of proper viscosity prevent machinery malfunction.
- Paints and coatings: Viscosity ensures even application and surface finish.
8. What are the main sources of error in the experimental determination of viscosity, and how can they be minimized?
- Unsteady temperature can change viscosity; maintain a constant temperature.
- Incorrect ball release may impart extra velocity; drop gently.
- Parallax error in timing and position; align eye to marking.
- Presence of air bubbles or impurities disrupts flow; use filtered liquids.
- Instrument zero error or improper calibration; check and correct before starting.
9. How does the size of the spherical ball affect the outcome of the viscosity experiment?
The radius of the ball directly impacts the drag force. According to Stokes' law, terminal velocity increases with the square of the ball's radius. Using too large a ball may cause turbulent flow, violating the experiment's assumptions; too small a ball may not be measurable with available instruments. Optimal size ensures laminar flow and accurate results.
10. Why does the coefficient of viscosity decrease for liquids but increase for gases with rising temperature?
In liquids, higher temperature reduces molecular attraction, making them flow more easily, so viscosity decreases. In gases, increased temperature raises molecular speed and collision frequency, leading to higher internal friction and increased viscosity.
11. What are common misconceptions about the viscosity experiment and how should they be addressed for CBSE 2025-26 exams?
- Misconception: Heavier balls always result in better accuracy. Correction: The ball must be small enough for laminar flow (Reynolds number <1) and not too heavy to avoid terminal velocity being reached too quickly.
- Misconception: Temperature changes have little effect. Correction: Always monitor and record temperature, as accuracy depends greatly on it.
- Misconception: Only one trial is sufficient. Correction: Multiple readings and averaging improve reliability.
