Materials Spec Points

Question 1

Occurrence of upthrust when a cylinder is submerged in water

Answer 1

Bottom of cylinder is deeper down in the fluid meaning h is larger than it is for the top of the cylinder. This means that the pressure at the bottom of the cylinder will also be larger. As the two faces of the cylinder have an equal area, and P=F/A, the force experienced by the bottom of the cylinder is larger than thee top and it is pushed upwards. The force pushing the cylinder upwards is upthrust.

Question 2

Archimedes’ principle

Answer 2

Upthrust experienced by an object is equal to the weight of the fluid it displaced

pVg
density x volume of displaced fluid x gravitational field strenght

Question 3

What is viscous drag force

Answer 3

Resistive force experienced by an object in a fluid

Question 4

What conditions apply for stokes’ law to be used

Answer 4

• the object is small and spherical
• the object moves at a low speed with laminar flow

Question 5

What is laminar flow

Answer 5

The particles in a fluid move by following smooth paths with little to no mixing between adjacent layers of the fluid

Absence of turbulent flow

Question 6

What is turbulent flow

Answer 6

Particles in the fluid mix between layers and form separate currents, often described as chaotic

Question 7

Stokes’ law equation

Answer 7

F = 6 π ηrv

η - viscosity of fluid
r - radius of object
v - terminal velocity of object
F - viscous drag force

Question 8

What is viscosity

Answer 8

A measure of how resistant a fluid is to deformation

A fluid’s viscosity is determined by the internal frictional forces that occur between adjacent layers of the fluid

(Low viscosity - water, high viscosity - honey)

Question 9

How temperature affects viscosity

Answer 9

Liquids (most) - as temperature increases, the viscosity of a liquid decreases

Gases
As temperature increases, the viscosity of gas increases

Question 10

CP04: use a falling-ball method to determine the viscosity of a liquid
Variables and equipment

Answer 10

IV - weight of ball bearing, Ws
DV - terminal velocity, Vterm
CVs - fluid being tested, temperature

Equipment
Long measuring cylinder
Viscous liquid to be tested (washing up liquid)
Stand and clamp
Metre rule
Rubber bands
Steel ball bearings of different weights
Digital scales
Vernier calipers
Digital stopwatch
Magnet

Question 11

CP04: use a falling-ball method to determine the viscosity of a liquid
Method

Answer 11

1. Weigh the balls, measure their radius using Vernier callipers and calculate their density
2. Place three rubber bands around the tube. The highest should be far enough below the surface of the liquid to ensure the ball is travelling at terminal velocity when it reaches this band. The remaining two bands should be 10-15cm apart so that time can be measured accurately.
3. Release the ball and wait until it reaches the first rubber band. Start the timer at the first band, then use the lap timer to find the time to fall d1 and also d2
   (If lap timing is not available, two stopwatches operated by different ppl should be used.
   If the ball is still accelerating as it passes the markers, they need to be moved downwards until the ball has reached terminal velocity before passing the first mark)
4. Measure and record the distances d1 and d2 between the highest and lowest bands.
5. Repeat at leat three times for balls of this diameter and three times for each different diameter.
6. Ball bearings are removed from the bottom of the tube using the magnet against the outside wall of the measuring cylinder.

Question 12

CP04: use a falling-ball method to determine the viscosity of a liquid
Analysis

Answer 12

• terminal velocity is used in this investigation since at terminal velocity the forces in each direction are balanced
  Ws = Fd + U
  Ws , weight of sphere
  Fd, the drag force (N)
  U, upthrust (N)

Weight of sphere is found using volume, density and gravitational force
W(s) = V(s)p(s)g
W(s) = 4/3 π r^3 p(s)g

Stokes law
F = 6 π ηrv
Upthrust equals the weight of the displaced fluid
The volume of displaced fluid is the same as the volume of the sphere
The weight of the fluid is found from volume, density and gravitational force as above
U = 4/3 π r^3 p(f) g

Sub into eq1
4/3 π r^3 p(s)g = 6 π ηrv + 4/3 π r^3 p(f) g
Rearrange to make viscosity the subject of the equation

Question 13

CP04: use a falling-ball method to determine the viscosity of a liquid
Systematic and random errors

Answer 13

Systematic
- ruler must be clamped vertically and close to the tube to avoid parallax errors in measurement
- ball bearing must reach terminal velocity before the first marker

Random
- cylinder must have a large diameter compared to the ball bearing to avoid the possibility of turbulent flow
- ball must fall in the centre of the tube to avoid pressure differences caused by being too close to the wall which will affect the velocity

Question 14

CP04: use a falling-ball method to determine the viscosity of a liquid
Safety

Answer 14

• measuring cylinders are not stable and should be clamped into a position at the top and bottom
• spillages will be slippery and must be cleaned up immediately
• avoid getting fluids in the eyes

Question 15

Hookes law equation

Answer 15

F=kx

Question 16

Stress equation

Answer 16

Stress = force/area

Question 17

Strain equation

Answer 17

Strain = change in length / original length

Question 18

What is the young modulus

Answer 18

A value that describes the stiffness of a material
Up to the limit of proportionality, for a material which obeys Hooke’s law, stress is proportional to strain, therefore the value of stress over strain is constant. This is the young modulus

Question 19

Limit of proportionality

Answer 19

The point after which Hooke’s law is no longer obeyed

Question 20

Elastic limit

Answer 20

Just after the limit of proportionality, if you increase the force applied beyond this, the material will deform plastically

Question 21

Yield point

Answer 21

Point at which the material begins to stretch without an increase in load

Question 22

Elastic deformation

Answer 22

Where a material returns to its original shape once the force applied is removed. This is because all the work done is stored as elastic strain energy

Question 23

Plastic deformation

Answer 23

Where a material’s shape is changed permanently. This is because work is done to move atoms apart, so energy is not only stored as elastic strain energy but also dissipated as heat.

Question 24

Difference between stress-strain graphs and force-extension graphs

Answer 24

Stress-strain : behaviour of a material
force-extension : behaviour of a specific object

Question 25

Ductile materials on a stress-strain graph

Answer 25

Can undergo a large amount of plastic deformation before fracturing

(On graph, after reaching elastic limit still goes on for a while)

Question 26

Brittle materials on a stress-strain graph

Answer 26

Where a material undergoes little to no plastic deformation before fracturing at a low strain

(High gradient on graph)

Question 27

Plastic materials on a stress-strain graph

Answer 27

Where a material will experience a large amount of extension as the load is increased

(Low gradient)

Question 28

What is breaking stress of a material

Answer 28

Value of stress at which the material will break apart, this value will depend on the conditions of the material e.g. its temperature.

Question 29

Core practical five: determine the young modulus of a material
Aim and variables

Answer 29

To measure the Young Modulus of a metal in the form of a wire. Using a clamped horizontal wire over a pulley. Reference marker needed on the wire
IV - load
DV - extension

Question 30

Core practical five: determine the young modulus of a material
Method

Answer 30

1. Measure the original length of the wire using a metre ruler and mark this reference point with tape.
2. Measure the diameter of the wire with micrometer screw gauge or digital calipers
3. Measure or record the mass or weight used for the extension e.g. 300g
4. Record initial reading on the ruler where the reference point is
5. Add mass and record the new scale reading from the metre ruler.
6. Record final reading from the new position of the reference point on the ruler.
7. Add another mass and repeat method.

Question 31

Core practical five: determine the young modulus of a material
Reducing uncertainty

Answer 31

• take pairs of readings of the diameter right angles to each other, to ensure the wire is circular
• six to ten readings altogether is enough to get an average value
• remove the load and check the wire returns to its original limit after each reading.
• take several readings with different loads and find average.
  Use a vernier scale to measure the extension of the wire.

Question 32

Core practical five: determine the young modulus of a material
Analysis

Answer 32

Find extension x from final and initial readings
Plot graph of force against extension and draw line of best fit
Determine the gradient
Calculate the cross-sectional area of the wire
Calculate the young modulus (gradient x length/area)

Question 33

Core practical five: determine the young modulus of a material
Safety considerations

Answer 33

• safety glasses should be worn in case of the wire snapping
• protect feet and the floor from falling weights by cushioning the area underneath the weights.

Question 34

What is elastic strain energy

Answer 34

Energy stored when Work is done on a material to stretch or compress it.

Question 35

How to find elastic strain energy on a force extension graph

Answer 35

Area under graph
Or
1/2 F x