Core Practical 4: Falling ball viscometer

Question 1

CP4 Equipment List

Answer 1

• Measuring cylinder
• Liquid to be tested
• Stand and clamp
• Metre ruler
• Rubber bands (fiducial markers)
• Steel ball bearing of diff weights
• Stopwatch
• Micrometre
• Magnet

Question 2

CP4 Method

Answer 2

1. Weigh and measure radius to calculate density
2. 3 rubber bands around tube; Highest should be below surface of liquid, ensuring ball is at terminal velocity when first band reached
3. Release ball
4. At 1st band, start timer then use lap timer to 2nd - 3rd band
5. Measure, record distances between rubber bands
6. Repeat at least three times for balls of this diameter and three times for each different diameter
7. Ball bearings are removed from the bottom of the tube using the magnet against the outside wall of the measuring cylinder

Question 3

Why can you not use a light gate in CP4?

Answer 3

Liquid will block beam and difficult to get ball to fall directly through the beam.

The liquid is coloured and the low intensity of the light transmitted will make the timing
unreliable. It is likely that the ball will not fall through the relatively narrow beam of the light
gate

Question 4

Potential Errors for CP4

Answer 4

Systematic
- Ruler must be clamped vertically straight
Random
- Cylinder should have large diameter compared to ball to ensure laminar flow
- Ball must fall in centre to avoid pressure differences caused by being too close to wall; affects velocity

Question 5

Safety measures CP4

Answer 5

Mop spilled liquid
- Easier to slip on floor; mop any spills
Goggles
- Avoids splashes in the eye
Clamp measuring cylinder
- Cylinder not stable; clamp so doesn’t fall

Question 6

How to increase accuracy of CP4?

Answer 6

Smaller balls and viscous fluids
- Will take longer for ball to fall; more accurate timing
Increase values of readings
Video analysis
- More accurate timing