A-level Physics - Materials

Materials

What is stress

Force per unit area acting on a material. It describes how concentrated the force is.

Why does increasing area reduce stress

Because the same force is spread over a larger surface, reducing the force per unit area.

What is strain

The ratio of extension to original length. It shows how much a material deforms relative to its size.

Why is strain dimensionless

It is a ratio of two lengths, so units cancel.

What is tensile stress

Stress caused by forces pulling the material apart.

What is compressive stress

Stress caused by forces pushing the material together.

Why do longer wires stretch more

More atoms are involved, so more bonds are stretched, increasing total extension.

Why do thinner wires stretch more

Smaller area means higher stress for the same force, leading to greater extension.

What does Hooke’s Law state

Extension is directly proportional to applied force, provided the limit of proportionality is not exceeded.

What does “directly proportional” mean in this context

A straight-line graph through the origin where doubling force doubles extension.

What is the limit of proportionality

The point where the force-extension graph stops being linear and proportionality breaks down.

What causes deviation from Hooke’s Law

Changes in atomic structure where bonds no longer behave elastically.

What is elastic deformation

Temporary deformation where the material returns to its original shape when the force is removed.

What is plastic deformation

Permanent deformation due to rearrangement of atoms or dislocations in the material.

What happens at a microscopic level during elastic deformation

Atomic bonds stretch but return to their original positions when force is removed.

What happens at a microscopic level during plastic deformation

Atoms slip past each other, causing permanent structural change.

What does Young modulus represent

A measure of stiffness — how resistant a material is to being deformed.

Why is Young modulus useful

It allows comparison of materials independent of their shape or size.

What does a high Young modulus indicate

The material requires a large stress to produce a small strain (very stiff).

What does a low Young modulus indicate

The material deforms easily under small stress (flexible).

Why is steel stiffer than rubber

Steel has stronger atomic bonding, requiring more force to stretch bonds.

How do engineers use Young modulus

To select materials that will not deform excessively under load.

What does the gradient represent

Stiffness (spring constant).

What does a steeper gradient mean physically

More force is needed for the same extension → stiffer material.

Why is the graph initially linear

Because Hooke’s Law is obeyed.

What happens after the linear region

The graph curves as the material begins to deform plastically.

What happens if the load is removed in the elastic region

The graph retraces back to the origin.

What happens if the load is removed in the plastic region

The material does not return to its original length.

What does the gradient represent

Young modulus (stiffness).

What is the elastic region

The section where deformation is reversible.

What is the yield point

The point where permanent deformation begins.

What is ultimate tensile strength (UTS)

The maximum stress the material can withstand before weakening.

What happens after UTS

The material undergoes necking and eventually fractures.

What is necking

A local reduction in cross-sectional area before breaking.

Why does stress increase during necking

Area decreases, increasing stress for the same force.

What is elastic potential energy

Energy stored when a material is deformed elastically.

Where is energy shown on a graph

Area under the force–extension graph.

Why is this important

It shows how much energy a material can store and release.

What happens to energy in plastic deformation

Some energy is dissipated as heat and not recovered.

What is density

Mass per unit volume.

Why do denser materials feel heavier

More mass is packed into the same space.

Why is density important in materials science

It affects weight, strength, and suitability for applications.

Why is measuring diameter important

It determines cross-sectional area, which affects stress calculations.

Why use a micrometer instead of a ruler

Greater precision for small measurements.

Why measure diameter in multiple places

To account for irregularities in the wire.

Why use a long wire

Increases extension, reducing percentage uncertainty.

Why add masses gradually

Prevents sudden failure and improves accuracy.

What is a systematic error

An error that consistently shifts results in one direction.

Give an example of systematic error

Zero error in a measuring instrument.

What is a random error

Unpredictable variations in readings.

How can random errors be reduced

Repeat measurements and calculate an average.

Why is percentage uncertainty important

It shows the reliability of measurements.

What is a brittle material

Breaks with little or no plastic deformation.

Why are brittle materials dangerous

They fail suddenly without warning.

What is a ductile material

Can undergo large plastic deformation before breaking.

Why are metals ductile

Layers of atoms can slide over each other.

What is a strong material

Can withstand high stress before failure.

What is a stiff material

Resists deformation.

Difference between strong and stiff

Strong relates to breaking, stiff relates to stretching.

Can a material be strong but flexible

Yes, e.g. some polymers.

Can a material be stiff but weak

Yes, e.g. glass.

Why is steel widely used

It combines strength, stiffness, and ductility.

Why are suspension bridge cables made of steel

They need to be strong and ductile.

Why are crash helmets stiff

To resist deformation and protect the head.

Why are springs used in vehicles

To absorb and release energy.

Why do buildings use thick columns

To reduce stress and prevent failure.

Why is precise terminology important

Mark schemes award marks for correct physics language.

Why describe graphs carefully

Many marks come from interpretation, not calculation.

What should you do if unsure

Describe what you see in the graph logically.

Why show working even with equation sheet

Method marks are awarded even if final answer is wrong.