Heat capacity facts for kids

Heat capacity is a way to measure how much heat different materials can store when their temperature changes. Think of it like a sponge for heat!

Materials with a high heat capacity need a lot of heat to be added before their temperature goes up even a little bit. Water is a good example of this. Materials with a low heat capacity need only a small amount of heat to make their temperature change a lot.

When we talk about the heat capacity of just one gram (or another unit of mass) of a material, we call it specific heat capacity. So, the total heat capacity of something is its mass multiplied by its specific heat capacity.

In building design, the heat capacity of a building is often called its thermal mass. This is important for keeping buildings warm in winter and cool in summer.

Measuring Heat Capacity

The standard unit for heat capacity is joule per kelvin (J/K). This is the same as joule per degree Celsius (J/°C).

We can usually measure heat capacity by following these steps:

• Start with an object at a known, steady temperature.
• Add a measured amount of heat energy to it.
• Wait for the object's temperature to become steady again.
• Measure how much its temperature changed.

This method works pretty well for many solid objects. However, it's not as accurate for gases.

What is Negative Heat Capacity?

Most things you encounter have a positive heat capacity. This means they get hotter when you add heat and colder when they lose heat.

But some special systems can have a negative heat capacity. This sounds strange, but it means they get colder when they absorb more energy, or hotter when they lose energy!

These systems are usually not in a perfect thermal equilibrium. Examples include huge objects like stars and galaxies, or very tiny groups of atoms called nano-scale clusters.

For example, a black hole is thought to have a negative heat capacity. If a black hole absorbs more mass and energy, it actually gets colder. On the other hand, if it gives off energy (through something called Hawking radiation), it gets hotter and hotter until it might disappear!

Thermal Balance

The Second Law of Thermodynamics tells us that when two things with different temperatures touch, heat will always flow from the hotter one to the cooler one. If they reach the same temperature, they are in thermal equilibrium.

This balance is stable if both things have a positive heat capacity. When heat moves from the hotter object to the colder one, the hotter object cools down and the colder one warms up. This makes their temperatures get closer, leading to a stable balance.

However, for systems with negative heat capacities, the process is different. If heat flows from a hotter system to a colder one, the hotter system actually gets even hotter, and the colder one gets even colder! This means they move further away from balance, making the equilibrium unstable.

See also

In Spanish: Capacidad calorífica para niños