Physical Properties of Materials
Physical Properties
Physical properties are characteristics that can be observed or measured without altering the material's chemical composition.
- Physical properties are intrinsic to the material and remain constant regardless of the sample size or shape.
- They tend to be the characteristics of materials that can be identified through non-destructive testing.
- The key physical properties include:
- Density
- Thermal Conductivity
- Thermal Expansion
- Melting Point
- Electrical Resistivity
- Electrical Conductivity
Density
Density
The mass per unit volume of a material (usually measured in kg/m³ or g/cm³). Its a measure of mass divided by volume (ρ = m/V)
Significance:
- Weight Considerations: High-density materials like steel are used in structures requiring strength, while low-density materials like aluminum are preferred in aerospace for weight reduction.
- Buoyancy: Density determines whether an object will float or sink in a fluid.
Aluminium's low density makes it an ideal material for an aircraft.
Thermal Conductivity
Thermal Conductivity
A measure of how quickly heat passes through a material when there is a temperature difference across it. It is measured in watts per metre per degree Celsius (W/m·°C).
- Think of thermal conductivity like a highway for heat.
- A high-conductivity material is like a wide, fast highway, allowing heat to travel quickly, while a low-conductivity material is like a narrow, slow road, restricting heat flow.
Significance:
- Insulation: Materials with low thermal conductivity, like fiberglass, are used as insulators.
- Heat Dissipation: High thermal conductivity materials, such as copper, are used in heat sinks to dissipate heat in electronics.
- Cookware: Needs materials with high thermal conductivity to heat food evenly.
- Building insulation: Uses materials with low thermal conductivity to keep heat in or out.
Thermal Conductivity Experiment
- Collect 4 rods or strips of different materials (e.g. metal, plastic, wood, ceramic), all the same length.
- Apply a small dab of Vaseline to the top end of each rod.
- Place a washer or coin on top of the Vaseline on each rod.
- Stand all rods vertically in a beaker of hot water, with the bottom ends submerged and top ends above water.
- Start a timer as soon as the rods are placed in the water.
- Observe and record which washer slides off first.
- Compare results to see which material conducts heat fastest (the one where the washer slides off first).
Thermal Expansion
Thermal Expansion
A measure of the degree of increase in dimensions (length, area or volume)
Significance:
- Structural Integrity: Bridges and railways use expansion joints to accommodate thermal expansion and prevent structural damage.
- Material Compatibility: In electronics, materials with similar rates of expansion when heated are used to prevent stress and failure.
Melting Point
Melting Point
The temperature at which a material changes from a solid to a liquid.
Significance:
- Manufacturing Processes: Materials with high melting points, like tungsten, are used in applications requiring high-temperature resistance.
- Product Design: The melting point influences material selection for products exposed to heat, such as cookware.
Electronics - solder has a low melting point so it can melt and join components, then solidify again.
Electrical Resistivity & Conductivity
Electrical Resistivity
Measures how much a material resists the flow of electricity.
Electrical Conductivity
Electrical conductivity is a measure of how easily electricity flows through a material. It is the inverse of resistivity
Significance:
- Conductors: Materials like copper have low resistivity and high conductivity, making them ideal for electrical wiring.
- Insulators: Materials like rubber have high resistivity, preventing the flow of electricity.
Don’t confuse resistivity with resistance:
- Resistivity is a material’s intrinsic property - it stays the same no matter the size or shape.
- Resistance depends on the material’s resistivity, but also on its length and thickness - longer and thinner wires have more resistance.
Application
SpaceX’s Starlink Satellites
- SpaceX’s Starlink satellites are built using materials chosen for their lightweight (low density) to reduce launch costs, high thermal conductivity to manage temperature extremes in orbit, and minimal thermal expansion to ensure stability.
- Components must also withstand high melting points and maintain electrical conductivity for efficient onboard power systems.
- These combined physical properties ensure long-term performance in harsh space environments.
State one physical property that is relevant to the design of a reusable bamboo coffee cup. (1 mark)
Solution
Answers may demonstrate:
Award [1] for stating one of the listed terms below that would be used for physical property that is relevant to the design of this reusable coffee cup up to [1 max]
- Thermal conductivity
- Density
