Chapter 11 - Materials

Question 1

What is the definition of the density of a substance?

Answer 1

Mass per unit volume

Question 2

What is the equation for density? Give the units.

Answer 2

density = mass/volume
Kgm^-3

Question 3

How could you measure the density of a regular solid?

Answer 3

1. ) Measure its mass using a balance
2. ) Measure its dimensions using a vernier calipers or a micrometer and calculate its volume using the appropriate equation.
3. ) Then calculate density by density =mass/volume

Question 4

How could you measure the density of a liquid?

Answer 4

1. ) Measure the mass using a volume of an empty measuring cylinder and measure the volume of the liquid directly. (use as much liquid as possible to reduce percentage error)
2. ) measure mass of the cylinder and liquid to enable the mass of the liquid to be calculated.
3. ) measure density from mass/volume

Question 5

How could you measure the density of an irregular solid?

Answer 5

1. ) measure the mass of the object
2. ) immerse the object on a thread in liquid in a eureka can, collect volume of water dispersed in a measuring cylinder - this is the volume of the object.
3. ) calculate density of the object from it’s mass/volume

Question 6

What is an alloy?

Answer 6

Solid mixture of two or more metals

Question 7

How do you work out the density of an alloy?

Answer 7

• Find mass of each metal in alloy using mass = volume x density
• use equation density = mass/volume

Question 8

What is Hooke’s law?

Answer 8

Hooke’s law states that the force needed to stretch a spring is directly proportional to the extension of the spring from its natural length - up to the limit of proportionality

F = k x e 
force = spring constant x extension

Question 9

What is the elastic limit?

Answer 9

Point where spring does not return to original length after force applied is removed

Question 10

Whats the equation for work done to stretch a spring?

Answer 10

= 1/2 x change in length x force

Question 11

Using equation for work done to stretch a spring, find the equation for elastic potential energy stored in a stretched spring?

Answer 11

F = ke
so, Ep = 1/2 x k x e^2

Question 12

What is the limit of proportionality?

Answer 12

Point to which a material obeys Hooke’s law

Question 13

What is elastic deformation?

Answer 13

When a material to regains it’s shape after it has been deformed and forces are released.

Question 14

What is plastic deformation?

Answer 14

When a material is permanently stretched after forces are removed. Work is done to separate atoms

Question 15

What are the forces that stretch a material called?

Answer 15

Tensile forces

Question 16

What are the forces that squash a material called?

Answer 16

Compressive forces

Question 17

What is tensile stress? give the equation the equation and give the units.

Answer 17

Force applied divided by the cross sectional area
stress = force/area
units - Nm^-2 or Pa

Question 18

What is tensile strain? Give the equation and the units

Answer 18

Change in length (extension) divided by the original length of the material. strain caused by stress.

strain = extension/ length

No units as it’s a ratio

Question 19

What is breaking stress?

Answer 19

The stress at which the atoms in the material are separated completely, and the material breaks

Question 20

What is the ultimate tensile stress?

Answer 20

Maximum stress a material can withstand

Question 21

Describe what happens, in terms of energy transfers, when a spring is stretched vertically down then released?

Answer 21

Elastic strain energy is stored in the spring as it’s stretched down. When released the elastic strain energy is transferred to kinetic energy and gravitational potential energy (as it gains height). The spring then begins to compress and the kinetic energy is transferred back to stored elastic strain energy.
change in kinetic energy = change in potential energy

Question 22

How is plastic deformation ideas used in transport design?

Answer 22

Dissipation of energy is used to design safer vehicles. Crumple zones deform plastically in crashes - so some of the cars kinetic energy goes into changing the shape of the vehicles’s body, so less is transferred to the people inside

Question 23

What is young modulus?
What are the units?

Answer 23

Measure of how stiff a a material is.

Young modulus = tensile stress/ tensile strain
= F/A x L/E
Units - Nm^-2

Question 24

How do you find young modulus from a stress - strain graph?

Answer 24

Gradient - stress/strain

Question 25

What is the area under a a stress - strain graph?

Answer 25

Strain energy per unit volume

Question 26

What is the yield point on a stress- strain graph?

Answer 26

Point at which a material suddenly starts to stretch without any extra load

Question 27

What is the difference between force-extension graphs and stress-strain graphs?

Answer 27

Force-extension graphs depend on the dimensions of material. Stress-strain graphs describe the general behaviour of a material, because stress and strain are independent of the dimensions.

Question 28

Describe the stress-strain graph of a brittle material

Answer 28

Doesn't curve. Straight until it reaches a point where the material fractures

Question 29

What would a typical stress-strain graph look like?

Answer 29

Question 30

Give some examples of brittle materials?

Answer 30

Ceramics or chocolate bars

Question 31

What is a brittle fracture?

Answer 31

When stress is applied to a brittle material any cracks at the material's surface get bigger and bigger until the material breaks completely.

Question 32

Why are brittle materials brittle?

Answer 32

As they have a rigid structure that allow cracks to be able to grow.

Question 33

Describe an experiment that tests young modulus

Answer 33

1. ) Find the cross-sectional area of the wire. Use a micrometer and measure the diameter of the wire in several places. Then use formula for area of a circle for finding cross sectional area. 2. ) Clamp the wire to the bench so you can hang weights off one end 3. ) Measure the distance between the fixed end of the wire and the marker - this is the un-stretched length 4. ) Then if you increase the weight, the wire stretches and the marker moves 5. ) Increase the weight in steps, recording the marker reading each time - the extension is the difference between this reading and the un-stretched length. 6. ) You can use your results from this experiment to calculate the stress and strain of the wire and plot a stress-strain curve

Question 34

What does area between loading and unloading graph on force-extension graph represent?

Answer 34

Work done to permanetly deform the material and thermal energy store