What is a functional material?
A structural material is chosen because it is strong. A functional material is chosen because it does something: it gives a magnetic push, makes sound, bends light, makes electricity or senses heat.
Most of these materials turn one kind of energy or signal into another. For example, light into electricity, or electricity into sound.
Electromagnetic materials
These materials react to electric current and magnetic fields.
- Conductors such as copper let current flow easily, so we make wires from them.
- Insulators such as rubber and plastic stop current, so we cover wires with them.
- Soft magnetic materials (soft iron, silicon steel) become strong magnets when current flows and lose it when it stops. Used in electromagnets, transformer cores and relays.
- Hard magnetic materials (steel alloys, ferrite) stay magnetised for a long time. Used in permanent magnets, speakers and fridge magnets.
In the 3D, an iron core inside a coil lifts many more clips than air does.
Acoustic and optical materials
Acoustic materials deal with sound. A piezoelectric material (quartz, some ceramics) shakes when a voltage is applied, and also produces a voltage when squeezed. That is how buzzers, microphones and the "click" of a gas lighter work. Foam and cork are acoustic materials too: they soak up sound.
Optical materials deal with light. Glass lets light pass and bends it; lenses use this. Thin glass fibre guides light along its length (optical fibre), carrying internet data. Special crystals in LCD screens let light pass or block it when a small voltage changes them.
Energy-conversion materials
These change energy from one form to another.
- Solar cell (silicon): light to electricity.
- Thermoelectric materials: heat difference to electricity (and the reverse, as in small coolers).
- Piezoelectric materials: pressure to electricity.
- Battery materials (lithium compounds): chemical energy to electricity, and back when charging.
A typical silicon solar cell turns roughly one fifth of the light into electricity. The rest becomes heat.
Sensor materials
A sensor material changes one of its properties when its surroundings change. We measure that change as an electrical signal.
- Thermistor: resistance falls as temperature rises. Used in digital thermometers and fridges.
- LDR (light-dependent resistor): resistance falls in bright light. Used in street lights that switch on at dusk.
- Strain gauge: a thin metal pattern whose resistance changes when it is stretched. Used in weighing machines.
- Piezo crystal: gives a voltage when pressed or vibrated. Used in knock sensors.
Choosing a functional material
Ask three questions: What input do I have (current, light, heat, pressure)? What output do I need (force, sound, electricity, a signal)? Will it work long enough and cheaply enough? Then pick the material that gives that conversion.
Key formulas and definitions
- Soft magnetic: loses magnetism when current stops; hard magnetic: keeps it
- Piezo: pressure ↔ voltage
- Snell's law: sin(i) / sin(r) = n (n is about 1.5 for glass)
- Solar cell efficiency = electrical output ÷ light input × 100%
- Thermistor (NTC): temperature up, resistance down
Worked examples
1. A solar panel gets 1000 W of sunlight and has 20% efficiency. How much electrical power does it give?
Output = 0.20 × 1000 = 200 W. The other 800 W becomes heat.
2. Light hits glass (n = 1.5) at 30°. Find the angle inside the glass. (sin 30° = 0.5)
sin r = sin i / n = 0.5 / 1.5 = 0.333. So r ≈ 19.5°. The light bends towards the normal.
3. Why is soft iron used in an electromagnet rather than hard steel?
Soft iron becomes strongly magnetic when current flows and loses its magnetism when the current is switched off, so the magnet can be switched on and off. Hard steel would stay magnetised.
4. An LDR has high resistance in the dark and low resistance in light. How can it switch on a street lamp at night?
In the dark the resistance is high, so the circuit detects a high value and turns the lamp on. In daylight the low resistance turns it off.
5. A thermistor reads 10 kΩ at 25 °C and about 1 kΩ at 100 °C. Is it a good sensor for temperature? Why?
Yes. A change of 75 °C changes resistance by ten times, which is easy to measure, so even small temperature changes give clear signals.
6. A piezo buzzer is driven at 2000 Hz. How many times does the disc shake in 3 seconds?
2000 × 3 = 6000 times.
Common mistakes
- Mixing up soft and hard magnetic materials. Soft means "easy to magnetise and demagnetise", not "easy to bend".
- Thinking a solar cell gives out as much energy as it takes in. About 20% becomes electricity; the rest is heat.
- Saying light bends away from the normal when entering glass. It bends towards the normal because glass slows light.
- Calling every sensor a thermistor. A thermistor senses temperature; an LDR senses light; a strain gauge senses stretch.