Thermal Properties of Matter Class 11 Notes, Newton's Law of Cooling and Important Definitions

First off, let us clarify a common confusion: temperature and heat are not the same thing. They are related, yes, but they tell us different stories.

Temperature is a measure of how energetically particles are moving in a material. The faster they jiggle and bounce, the higher the temperature. Heat, meanwhile, is energy in motion—it flows from a hotter object to a cooler one. Think of heat as what’s transferred, and temperature as the level things rise to.

Here is a real-life example: a cup of boiling tea feels scorching, but a lukewarm swimming pool might contain more heat overall. Why? Because there is more water, and therefore more particles moving around with energy.

You might also come across terms like internal energy, thermal energy, and enthalpy. These describe what's going on inside materials—the total of all those little atomic movements and interactions.

How do we know how hot something is? That is where temperature measurement comes in. Most of us are familiar with simple thermometers that use mercury or alcohol to expand and contract, indicating temperature on a scale.

But science and engineering do not stop there. They lean on more precise tools. Thermocouples—tiny metal junctions—convert heat into voltage, which sounds a bit like sci-fi but is very real. Then there are infrared sensors, which "see" heat being radiated by objects. Ever tried one of those forehead thermometers? Same idea.

Turns out, calibration is really important. Two thermometers may give different readings, especially if they have not been fine-tuned. And in lab work, even a one-degree mistake can mess up an entire result. Precision matters more than we often realise.

This is an important chapter for JEE Mains exams and students must be well prepared to answer both theoritical and practical questions related to the concepts covered in Thermal Properties of Matter chapter.

Have you noticed that some things heat up faster than others? That is tied to specific heat capacity. It is a term to explain how much energy it takes to warm something up.

The specific heat of water is high, which is why it takes forever to boil a big pot. But it is also why oceans help stabilise Earth’s climate. Metals like iron or aluminium? They heat up in seconds and cool just as fast.

This property pops up in all kinds of design decisions—from building materials to cooking pans to climate models and thermal regulation systems.

Want to know exactly how much heat a reaction gives off? Enter calorimetry. It’s a technique for tracking heat changes during physical or chemical transformations.

Picture this: you drop a hot piece of metal into a cup of cool water. The metal cools down, the water heats up. With a calorimeter, you can measure that exchange and calculate how much energy was involved.

Along the way, you will learn about concepts like heat capacity (total heat storage ability), latent heat (heat absorbed or released during phase changes, like when ice melts), and thermal equilibrium, when two systems stop exchanging heat.

Believe it or not, calorimetry is the science behind food calories. That label on your granola bar? Calorimetry made it possible.

Heat does not just sit around; rather, it moves. And it tends to travel in the following three familiar ways:

1. Conduction – This happens in solids for example a metal spoon heating up in a hot pot.
   
2. Convection – It happens in fluids. Think swirling water in a boiling pot.
3. Radiation – For radiation, no medium is required. That is how the Sun heats Earth across empty space.
   

Each type plays a role in real life. Ever wonder why your house stays warm with thermal insulation? Or how microwaves heat food without touching it? Yup—heat transfer in action.

Thermal conductivity, thermal diffusivity, and thermal inertia are all factors that affect how efficiently heat moves through materials.

Here is something intuitive: hot things cool off when left alone. But there is actually math behind that. Newton’s law of cooling says the rate of cooling depends on the difference between the object’s temperature and its surroundings.

Let us say you bake a pie and leave it on the counter. It cools quickly at first, then more slowly as it approaches room temperature. That curve? That is Newton at work.

This principle finds use in everything from estimating time of death in forensics to designing cooling systems in electronics.

Materials do not just heat up, but they expand. This is thermal expansion, and it explains why gaps are left in bridges and why lids loosen when run under hot water.

There are three ways of expansion:

• Linear – It is an expansion along a single axis, like a rod getting slightly longer when heated
• Area – This expansion is across a two-dimensional surface, such as a metal sheet growing wider and longer
• Volumetric – This expansion happens in all directions, which increases the overall volume of a substance, like air in a hot balloon

Every substance has its own expansion rate, which is known as the coefficient of thermal expansion. This is an important number for engineers when designing anything that faces temperature changes.

And when expansion happens unevenly or is restricted, it can cause thermal stress, which may lead to warping or cracking and designers have to plan for this very carefully.