Nanomaterials
One nanometre is one billionth of a metre (10⁻⁹ m). A nanomaterial has at least one size between about 1 and 100 nm. A hair is about 80 000 nm wide.
Why do small pieces act differently? A cube of side L has surface-to-volume ratio 6 ÷ L. Cut the cube into n × n × n small cubes and the total volume stays the same but the total surface becomes n times bigger. At the nano size, a large share of the atoms sit on the surface, so the material reacts faster, melts at a lower temperature or changes colour. Gold nano particles can look red, not yellow.
Uses: medicine delivery, strong light composites, filters, solar cells, sunscreens. Safety studies are still needed because tiny particles can enter the body easily. More in Nanoparticles.
Superconductors
In a normal metal wire, moving electrons keep bumping into atoms. This resistance turns some electrical energy into heat (P = I²R). In a superconductor, below a critical temperature Tc, the resistance drops to exactly zero. A current started in a closed superconducting ring can go round for years without a battery.
A second sign is the Meissner effect: a superconductor pushes magnetic field lines out of itself. This makes a magnet levitate (float) above it.
Old superconductors need liquid helium (about 4 K). Newer ones work in liquid nitrogen (77 K), which is cheaper. Uses: strong magnets for MRI scanners, particle accelerators and maglev trains, and lossless power cables. Today the main problem is the cooling needed.
Functional materials
A functional material does a useful job in response to a change around it. Examples:
- Shape-memory alloy (nitinol): bent when cold, returns to its remembered shape when warmed. Used in spectacle frames and in tiny medical stents.
- Piezoelectric crystal: makes a voltage when pressed (gas lighter, microphone) and moves when a voltage is applied (buzzer).
- Thermochromic material: changes colour with temperature (battery tester, mug that shows hot).
- Semiconductors: conduct a little; the amount can be controlled (chips, LEDs, solar cells).
More examples are in Smart and modern materials.
Try it
Sugar race: drop a sugar cube and the same amount of powdered sugar into two cups of warm water. The powder dissolves first because it has more surface. This is the same idea as nano pieces.
Heat a spring: a paper clip springs back less than a shape-memory wire, but you can see how a warm spoon changes a thermochromic sticker.
In the 3D: on Superconductor, move the temperature slider across the dip and watch the magnet rise and fall.
Key formulas and definitions
- 1 nm = 10⁻⁹ m
- Cube surface-to-volume ratio = 6 ÷ L
- Cutting a cube into n × n × n pieces multiplies total surface by n
- Power loss in a wire: P = I²R (zero when R = 0)
- Superconductor: R = 0 and magnetic field pushed out below T_c
Worked examples
1. A cube of side 1 cm is cut into 1 mm cubes. How many cubes? By what factor does the total surface grow?
Each side is cut into 10, so 10 × 10 × 10 = 1 000 cubes. The surface grows by a factor of n = 10.
2. A wire has resistance 0.5 Ω and carries 4 A. How much power is lost as heat? What if it becomes a superconductor?
P = I²R = 16 × 0.5 = 8 W. As a superconductor R = 0, so P = 0.
3. How many 50 nm particles in a line fit across 1 µm?
1 µm = 1 000 nm. 1 000 ÷ 50 = 20 particles.
Common mistakes
- Thinking nano particles are a different substance. It is the same material with a much larger surface share.
- Saying a superconductor has very small resistance. Below T_c it is exactly zero.
- Thinking a superconductor works at room temperature. Common ones need strong cooling.
- Saying a levitating magnet is held up by air. It is held by the magnetic field the superconductor pushes out.