Heat & Thermodynamics

Introduction: Internal Energy, Temperature, and Heat

In Physics, it's crucial to distinguish between three related concepts: internal energy, temperature, and heat. All matter is made of particles (atoms and molecules) that are in constant, random motion.

• Internal Energy (U): This is the **total** energy of all the particles within an object. It is the sum of the random kinetic energies (due to motion) and potential energies (due to intermolecular forces) of the particles. Its SI unit is the **Joule (J)**.
• Temperature (T): This is a measure of the **average** kinetic energy of the particles in a substance. It tells us how hot or cold an object is, but not how much total energy it contains. It is measured in **degrees Celsius (°C)** or **Kelvin (K)**.
• Heat (Q): This is the energy transferred from a hotter object to a colder object due to a temperature difference. Heat is energy in transit; an object contains internal energy, it does not contain 'heat'. The SI unit for heat is also the **Joule (J)**.

Temperature Scales: The Kelvin scale is the absolute temperature scale. **Absolute zero (0 K)** is the lowest possible temperature, where particles have minimum internal energy.

The conversion is: Kelvin (K) = Celsius (°C) + 273

An important exam point: A *change* in temperature of 1 K is equal to a *change* of 1°C.

Heat Transfer Mechanisms

Heat energy moves from a region of higher temperature to a region of lower temperature through three processes:

1. Conduction: This is the primary method of heat transfer in solids. When one end of a solid is heated, its particles gain kinetic energy and vibrate more vigorously. They collide with neighbouring particles, transferring energy along the object.

• Conductors: Materials that transfer heat well, like metals (copper, aluminum), have free-moving electrons that also carry kinetic energy, making them excellent conductors. This is why a metal *karahi* heats up quickly and evenly for cooking.
• Insulators: Materials like wood, plastic, and air are poor conductors. Air trapped in materials like wool or fiberglass makes them excellent insulators, used in winter clothing and building insulation.

1. Convection: This is the primary method of heat transfer in fluids (liquids and gases). When a part of a fluid is heated, it expands, becomes less dense, and rises. Cooler, denser fluid sinks to take its place, gets heated, and rises in turn. This continuous circulation is called a convection current.

• Practical Applications: Convection is responsible for weather patterns. For instance, the **sea breeze** experienced along Karachi's coastline is a large-scale convection current. During the day, the land heats up faster than the sea, causing hot air to rise over the land and cooler, denser air to move in from the sea to replace it.

1. Radiation: This is the transfer of heat through electromagnetic waves, specifically infrared (IR) radiation. It does not require a medium and can travel through a vacuum. This is how the Earth receives heat from the Sun.

• Surface Properties: The rate of absorption and emission of thermal radiation depends on the surface's colour and texture.
• Dark, matt surfaces (like charcoal) are good absorbers and good emitters of radiation.
• Light, shiny surfaces (like polished metal) are poor absorbers and poor emitters, but are good reflectors of radiation.
• Application: This principle is why people in hot parts of Pakistan, like Punjab and Sindh, often wear light-coloured cotton clothing in summer. The light colours reflect the sun's radiation, helping to keep the person cooler.

Quantifying Heat: Specific Heat Capacity

The specific heat capacity (c) of a substance is the amount of energy required to raise the temperature of 1 kg of the substance by 1°C (or 1 K). Its unit is Joules per kilogram per degree Celsius (J/kg°C) or Joules per kilogram per Kelvin (J/kgK).

The formula to calculate the heat energy transferred is:

Q = mcΔT

Where:

• Q = heat energy transferred (in Joules, J)
• m = mass of the substance (in kg)
• c = specific heat capacity of the substance (in J/kg°C)
• ΔT = change in temperature (in °C or K)

Water has a very high specific heat capacity (approx. 4200 J/kg°C). This means it takes a lot of energy to heat water, and it releases a lot of energy as it cools. This property is vital for life and has major climatic effects. The Arabian Sea absorbs huge amounts of heat in summer and releases it slowly in winter, moderating the climate of Karachi and the Makran coast, preventing the extreme temperature swings seen in inland cities like Jacobabad.

Change of State and Latent Heat

When a substance changes state (e.g., ice melting to water), the energy supplied does not increase the temperature. Instead, it is used to break the bonds holding the particles together, increasing their potential energy. The temperature remains constant during the entire phase change. This 'hidden' heat is called latent heat.

The specific latent heat (L) of a substance is the energy required to change the state of 1 kg of the substance at a constant temperature. Its unit is Joules per kilogram (J/kg).

Q = mL

Where:

• Q = heat energy transferred (in Joules, J)
• m = mass of the substance changing state (in kg)
• L = specific latent heat (in J/kg)

There are two types:

1. Specific Latent Heat of Fusion (L_f): Energy needed to change 1 kg of a substance from solid to liquid at its melting point.
2. Specific Latent Heat of Vaporization (L_v): Energy needed to change 1 kg of a substance from liquid to gas at its boiling point.

Common Exam Trap: When solving a problem that involves both a temperature change and a change of state (e.g., heating ice from -10°C to steam at 110°C), you must calculate the energy for each stage separately. Use **Q=mcΔT** for temperature changes and **Q=mL** for melting and boiling, then add them all together.