Thermal Properties and Expansion Notes

Thermal Properties and Expansion

Objectives

• Thermal Expansion
• Specific Heat Capacity
• Investigating Specific Heat Capacity
• Melting & Boiling
• Evaporation

Thermal Expansion

• When a material is heated at constant pressure:
  • Its overall volume increases (it expands).
  • Its density decreases.
• Expansion in 3-D

Why Materials Expand When Heated?

• Molecules start to move around (or vibrate) faster as they gain kinetic energy.
• This causes them to collide with each other more often and push each other apart.

Thermal Expansion in Terms of Particles

• Occurs in solids, liquids, and gases.
• When temperature is increased (at constant pressure):
  • Solids will tend to expand the least.
    • Why? The low energy molecules cannot overcome the intermolecular forces of attraction holding them together.

Expansion of Liquids

• Liquids expand more than solids but less than gases.
  • Why? The molecules have enough energy to partially overcome the intermolecular forces of attraction holding them together.

Expansion of Gases

• Gases expand the most because their high energy molecules have enough energy to completely overcome the intermolecular forces of attraction holding them.

Applications of Thermal Expansion

• Liquid-in-glass thermometer:
  • Consists of a thin glass capillary tube containing a liquid that expands with temperature.
• Temperature-activated switches:
  • Uses a bimetallic strip (made from two types of metal) that expands at different rates and bends by a predictable amount at a given temperature.

Consequences of Thermal Expansion

• Can cause materials to buckle (start to curve) if they get too hot.
  • Examples: Metal railway tracks, road surfaces, bridges.

Preventing Thermal Expansion Damage

• Train tracks often have gaps built in or are tapered to create space for the expansion to happen without causing damage.

Important Reminder

• It is the material that expands, not the molecules.
• As heat is added:
  • The increase in temperature leads to an increase in kinetic energy.
  • Molecules and atoms move more quickly and move apart.

Exam Tip

Thermal Energy vs. Internal Energy

• Thermal energy: Average kinetic energy of the system's constituent particles due to their motion.
• Internal energy: Total potential and kinetic energies in a substance.

Specific Heat Capacity (c)

• Definition: The amount of energy required to raise the temperature of 1 kg of a substance by 1°C.
• The temperature of substances when heated depends on:
  • Mass of substance
  • Type of material
  • Amount of thermal energy transferred

Specific Heat Capacities of Different Substances

• Low specific heat capacity:
  • Heats up and cools down quickly (i.e., it takes less energy to change its temperature).
• High specific heat capacity:
  • Heats up and cools down slowly (i.e., it takes more energy to change its temperature).

Table of Specific Heat Capacity

• Copper Block: $390 \frac{J}{kg \cdot °C}$
• Aluminium Block: $910\frac{J}{kg \cdot °C}$
• Water: $4200\frac{J}{kg \cdot °C}$
• Lower Specific Heat Capacity:
  • Warms up and cools down quickly as it takes much less energy to change its temperature.
• Higher Specific Heat Capacity:
  • Warms up and cools down slowly as it takes much more energy to change its temperature.

Advantages of Water's High Specific Heat Capacity

• Car Radiator:
  • Water in a car's radiator absorbs engine heat, preventing overheating. Its high specific heat enables effective heat transfer, keeping the engine efficient.
• Home Water Boiler:
  • Water retains heat well, providing consistent hot water and reducing energy consumption while maintaining comfort.

Calculating Specific Heat Capacity

• $c = \frac{ΔE}{mΔ\theta}$
  • Where:
    • $ΔE$ = change in thermal energy in Joules
    • $m$ = mass in kilograms
    • $c$ = specific heat capacity in joules per kilograms per degrees Celsius ($\frac{J}{kg \cdot °C}$)
    • $Δ\theta$ = change in temperature, in degrees Celsius

Example

• Water of mass 0.48 kg is increased in temperature by 0.7 °C. The specific heat capacity of water is 4200 J/kg°C. Calculate the amount of thermal energy transferred to the water.

Melting and Boiling

• The melting and boiling points of water are known as fixed points:
  • Ice melts at 0°C, pure water boils at 100°C.

State Change and Temperature

• During a state change (melting and boiling), the temperature of a substance does not change.

Energy During Boiling

• During boiling, the added energy goes into overcoming the intermolecular forces between water molecules, leading to evaporation.
  *Why does temperature of a substance remain constant during a state change? Internal energy does not rise!

Energy During Melting

• During melting, the added energy goes into overcoming the intermolecular forces, and then the solid becomes liquid.
  *Why does temperature of a substance remain constant during a state change? Internal energy does not rise!

Energy Transfer During a State Change

• State change depends on whether energy is being transferred to or away from the system.
• Heating – energy is transferred to the system and the kinetic energy (EK) of the molecules increases.
• Cooling – energy is transferred away from the system (or dissipated to the surroundings), and the EK of the molecules decreases.

Gas Cooling and Condensation

• Gas cools – energy is transferred away from the system, EK decreases until the boiling point is reached.
• At Boiling Point – energy transferred away from the system, its potential energy is reduced
• Particles have insufficient energy to overcome intermolecular forces, only have enough energy to flow past one another.
• The gas has become liquid (Condensation).

Liquid Cooling and Solidification

• Liquid cools – energy is transferred away from the system, EK decreases until the melting point is reached.
• At Melting Point – energy transferred away from the system, its potential energy is reduced
• Particles have insufficient energy to overcome intermolecular forces, only have enough energy to be bound to each other, and can only vibrate around a fixed point.
• The liquid has become solid (Solidification).

Evaporation

• Definition: Liquid to gas
• Happens at any temperature, only from the surface of the liquid.
• Molecules in a liquid have different energies.
• Occurs when more energetic molecules near the surface of the liquid have enough energy to escape.
• Average energy of the liquid decreases, then the liquid cools down.

Factors Affecting Rate of Evaporation

• Temperature of liquid
• Surface area of liquid exposed
• Air movement (wind or a fan)

Evaporation Causes Cooling

• What is the purpose of our bodies sweating?
• In a liquid, for example, the most energetic particles leave, reducing the average EK.
• Object placed near this liquid cools the liquid because the cooler liquid absorbs the thermal energy from the object.
• This process is used in some refrigerators and air conditioning units.

Evaporation vs. Boiling