Thermal expansion is the increase in dimensions of a material when heated, while thermal contraction is the decrease when cooled. This occurs because particles vibrate more vigorously when heated, pushing farther apart.

Demonstrating Thermal Expansion

Ball and Ring Experiment

• A metal ball that just passes through a metal ring at room temperature is heated.
• When heated, the ball expands and no longer passes through the ring.
• After cooling, it contracts and passes through again.

Bar and Gap Experiment

• A metal bar that fits into a gap at room temperature is heated.
• Upon heating, the bar expands and no longer fits into the gap.

Coefficient of Linear Expansivity

The fractional increase in length per degree Celsius rise in temperature is called the coefficient of linear expansivity (α). The formula is:

$\Delta L = L_0 \times \alpha \times \Delta T$

Where:

• $\Delta L$ = increase in length
• $L_0$ = original length
• $\alpha$ = coefficient of linear expansivity (°C⁻¹)
• $\Delta T$ = change in temperature

Example: An iron rail has a length of 50 m at 20°C. What will be its length at 40°C? (α for iron = 0.000012 °C⁻¹)

$\Delta T = 40 - 20 = 20°C$

$\Delta L = 50 \times 0.000012 \times 20 = 0.012 m$

$L = 50 + 0.012 = 50.012 m$

Applications of Thermal Expansion

• Railway tracks: Gaps are left between rail segments to allow for expansion on hot days.
• Bridges: Expansion joints absorb thermal expansion and contraction.
• Bimetallic strips in thermostats: Two metals with different expansion rates bend with temperature changes, controlling electrical circuits.
• Iron tyres on wooden wheels: Tyres are heated before fitting so they shrink tightly as they cool.

Heat is the transfer of energy due to temperature difference. It is a form of energy measured in joules (J).

Temperature is the degree of hotness or coldness—a measure of the average kinetic energy of particles.

Difference Between Heat and Temperature

Specific Heat Capacity

The heat energy required to raise the temperature of 1 kg of a substance by 1°C is called its specific heat capacity (c). The formula is:

$Q = mc\Delta T$

Where:

• $Q$ = heat energy (J)
• $m$ = mass (kg)
• $c$ = specific heat capacity (J/kg°C)
• $\Delta T$ = change in temperature (°C)

Example: How much heat is needed to raise the temperature of 2 kg of water from 25°C to 85°C? (c for water = 4200 J/kg°C)

$\Delta T = 85 - 25 = 60°C$

$Q = 2 \times 4200 \times 60 = 504,000 J$

Heat can be transferred in three ways: conduction, convection, and radiation.

Conduction

Conduction is the transfer of heat through a material without the material itself moving. It occurs mainly in solids, especially metals, where free electrons help transfer energy.

• Good conductors: Metals like copper, aluminum, and iron allow heat to pass easily.
• Bad conductors (insulators): Materials like wood, glass, and air are poor conductors.

Example: A cooking pot has a metal bottom (good conductor) to transfer heat quickly to food, while its handle is made of plastic or wood (insulator) to prevent burns.

Convection

Convection is the transfer of heat through fluids (liquids and gases) by the movement of fluid particles. When a fluid is heated, it becomes less dense and rises, while cooler fluid sinks, creating convection currents.

Example: In a pot of water being heated on a stove, warm water rises while cooler water sinks, creating循环 (circulation) that eventually heats all the water.

Radiation

Radiation is the transfer of heat in the form of electromagnetic waves, without requiring a medium. All objects emit thermal radiation.

Example: The Sun's heat reaches Earth through radiation. A coal fire warms people nearby through radiation.

Water vapour is water in gaseous form, present in the atmosphere. Humidity is a measure of the amount of water vapour in the air.

Types of Humidity

• Absolute humidity: The actual amount of water vapour in a given volume of air (measured in g/m³).
• Relative humidity: The ratio of the amount of water vapour present in the air to the maximum amount the air can hold at that temperature, expressed as a percentage.

$Relative\ humidity = \frac{\text{Actual vapour density}}{\text{Saturation vapour density}} \times 100\%$

Measuring Humidity

A hygrometer is an instrument used to measure humidity. It helps in weather forecasting and understanding weather patterns.

Example: On a humid day in Tanzania, during the rainy season, the relative humidity can exceed 80%, making the air feel uncomfortable as sweat evaporates slowly.

In everyday life in Tanzania, understanding heat and temperature is essential for activities like cooking using charcoal stoves (where conduction transfers heat from the fire to the cooking pot), weather monitoring for farming decisions where farmers use thermometers and hygrometers to track temperature and humidity for crop planning, and in healthcare where thermometers are used to measure body temperature to diagnose fever. Additionally, construction engineers in Tanzania must account for thermal expansion when building bridges and roads, ensuring proper expansion joints are installed to prevent damage during hot seasons.