1.5 Solids under stress

Question 1

What is Hookes law?

Answer 1

Hooke’s law states that the tension in a spring or wire is proportional to its extension from its natural length, provided the extension is not too great.

Question 2

What is the equation for Hookes law?

Answer 2

F=kx

F = force/tension in N
k = Spring constant in Nm−1
x = extension in m

Question 3

What does a force extension graph look like?

Answer 3

• Starts with straight diagonal line where force is proportional to extension.
• Reaches elastic limit where force is no longer proportional to extension and starts to slope downwards.

Question 4

How do you find the spring constant on a force extension graph?

Answer 4

The spring constant is equal to the gradient of the line on a force-extension graph.

Question 5

How do you find the work done on a force extension graph?

Answer 5

The total work is equal to the area under the graph.
W= Fx/2

Question 6

What is the equation for energy stored in a spring?

Answer 6

Combining the equation for work done W= Fx/2 with the equation for Hooke’s law, F=kx, will give this equation for the energy stored in a spring: W=kx^2/2

Question 7

What is the equation for Youngs modulus?

Answer 7

E= σ/ε
Youngs modulus = stress/strain

Question 8

What is stress?

Answer 8

Stress, σ, is defined as the force per unit cross-sectional area:

σ= F/A
​
Stress = Force/cross sectional area

Question 9

What is strain?

Answer 9

Strain, ε, is defined as the extension per unit length due to an applied stress:

ε=Δl / l
​
Strain = extension / length

Question 10

What are the three main solid classifications?

Answer 10

-crystalline
-amorphous
-polymeric

Question 11

What is a crystal?

Answer 11

-A crystal is a solid in which atoms are arranged in a regular array.
-There is a long-range order within crystal structures

Question 12

What is a polycrystalline?

Answer 12

-A polycrystalline solid consists of many crystals, known as a grain, usually arranged randomly.
-Metals and many ionic minerals such as salt are polycrystalline.

Question 13

What is an amorphorous solid?

Answer 13

• In an amorphous solid, there is no regular order; the atoms are arranged randomly.
  -There is no long-range order in the way atoms are arranged, though there may be ordered clusters of atoms.
  -Include solids such as glass or brick

Question 14

What is a polymeric solid?

Answer 14

• A polymeric solid consists of long chain-like molecules.
• Examples include rubber, wool and synthetic polymers such as nylon and polythene.

Question 15

What does a stress strain graph look like?

Answer 15

-Initially stress and strain are proportional; the material obeys Hooke’s law.
-Then the limit of proportionality is reached , where the metal no longer obeys Hooke’s law.
- The elastic limit of the material is then reached ; this corresponds to the maximum stress that can be applied where the material will return to its original length.
- Up to this point the material experiences elastic strain.
- Any extension above this point will cause plastic strain; the atoms in the material will begin to move past each other and the material will be permanently deformed
-Then the yield point is reached – a small increase in the load on the material will cause a large extension. This means that the material is ductile.
- The line continues quite horizontally for a while until it reaches its breaking point and the material breaks.

Question 16

What is an edge dislocation?

Answer 16

As a metal crystal forms, edge dislocations will frequently appear where a plane of atoms will not be complete before the next plane

Question 17

How does plastic deformation take place?

Answer 17

-When a force is applied to the metal, the bonds between the atoms will stretch. If the stress is below the elastic limit, when the force is no longer applied, the bonds will return to their original length.
-However, if the force applied is large enough, the bonds around the edge dislocation are stretched further and will snap, causing the dislocation to move. As long as the force is applied, this process will repeat until the dislocation moves to the edge of the crystal or grain.
-This will make the crystal longer as a result of the apparent movement of one plane of atoms past another. As metals contain many edge dislocations, this can result in a large increase in strain with only a little stress.

Question 18

How are metals strengthened?

Answer 18

In order to strengthen metals, impurities are introduced to stop this process. For example, steel contains approximately 99% iron atoms and a comparatively small number of carbon atoms. These atoms create a barrier to dislocation movement.