Internal energy refers to the total energy contained within a material, including both kinetic and potential energy of its particles. Changes in temperature or state of matter are influenced by alterations in this internal energy. The amount of energy needed for such changes is determined by the material’s characteristics, specifically its ‘heat capacity’—the energy required to change its temperature—and ‘latent heat’—the energy absorbed or released during a change in its state.
Daily Life Examples
Internal energy is a concept that can be observed in various aspects of daily life. Here are some examples:
- Boiling Water:
- When you heat water on a stove, the internal energy of the water increases.
- As the water absorbs heat, its internal energy rises, leading to a phase change from liquid to vapor (steam).
- Melting Ice:
- Similarly, when you apply heat to ice, its internal energy increases.
- The ice absorbs energy, causing the internal energy to rise, resulting in a phase change from solid to liquid.
- Hot Beverage Cooling:
- When you pour a hot beverage into a cup and leave it on the table, it gradually cools down.
- This cooling is a result of the transfer of internal energy from the hot beverage to the surrounding air.
- Rubbing Hands Together:
- When you rub your hands together, the friction generates heat.
- This increase in internal energy is felt as warmth in your hands.
- Inflation of a Tire:
- When you inflate a tire, the air molecules inside the tire gain kinetic energy.
- The internal energy of the air increases, causing an increase in pressure and volume inside the tire.
- Refrigeration:
- In a refrigerator, the internal energy of the air inside is reduced, causing the temperature to drop.
- This results in the cooling of the contents as their internal energy decreases.
When a material is heated or cooled, its particles undergo two key changes. Firstly, chemical bonds may form, break, or stretch, altering the material’s chemical energy storage. Simultaneously, the material heats up or cools down as particles gain or lose speed, affecting its thermal energy storage.
Mathematical Form
The change in internal energy (ΔU) of a system is related to the heat (Q) added to or removed from the system and the work (W) done by or on the system. The equation is given by the first law of thermodynamics:
ΔU=Q−W
Here:
- ΔU is the change in internal energy of the system.
- Q is the heat added to the system (positive when heat is added, negative when heat is removed).
- W is the work done by the system on its surroundings (positive when work is done by the system, negative when work is done on the system).
Crux Points
- The internal energy is the total kinetic and potential energy of all particles in the system.
- Adding energy to raise the temperature results in particles speeding up, gaining kinetic energy.
- Melting or boiling a substance requires energy to break bonds, increasing potential energy.
- Conservation of energy means added energy is distributed between the chemical store and thermal store of internal energy.
- The effect of energy transfer (breaking bonds, stretching bonds, or increasing particle speed) depends on temperature and material state.
