The energy from the sun is used to produce electricity.

Energy can be defined as the capacity to do work. It exists in many forms such as kinetic, potential, sound, light, heat, electrical, nuclear, and chemical. Any of these forms of energy cannot be created nor destroyed but can be transformed from one form to another. When energy is transformed, nothing goes to waste, thus energy is conserved. How does this work? How is this possible?

Energy Transformation
One form of energy can often be transformed into another form. For example, the chemical energy stored in the food we eat will be eventually turn into heat energy which our body will use to do our day’s work. This means that if we don’t eat, we feel weak and tired.

Another example of energy transformation happens inside a battery. The chemical energy inside a battery can be converted into electrical energy (e.g., in a calculator), sound energy (e.g., in a radio), light energy (e.g., in a flashlight), or mechanical energy (e.g., in a toy car).

The chemical energy in batteries can be converted into many forms.

A wind turbine also follows the same principle. When kinetic energy from the wind passes through a turbine, it will be converted into mechanical energy and then into electrical energy. In a simpler example, the potential energy of a pendulum’s bob is converted into kinetic energy then back again into potential energy as the pendulum swings.

These examples of energy transformation are bounded by an empirical law in physical science—the law of conservation of energy which states that “the total amount of energy in an isolated system remains constant”.

Energy conservation is being shown in a Newton's cradle.

The First Law of Thermodynamics
Thermodynamics is a branch of physical science which studies heat energy and mechanical work in a system. Energy transformation and conservation are under the first law of thermodynamics. This law states that energy can be converted from one form to another but it cannot be created nor destroyed. It further says that energy is consumed in any process involving a thermodynamic system and its surroundings.

When we say that energy is always conserved, this means that the total amount of energy in any process or reaction never changes. In other words, the total energy in a system always remains the same whenever energy changes from one form to another even if the energy loses its ability to do work.

Escaping Energy
The first law asserts that the internal energy of a system is conserved and that heat and work are forms of energy transfer. A mixture of chemical compounds is an example of a system with internal energy. When a reaction occurs in the system, work is done and heat may form in the surrounding environment. In such cases, the first law of thermodynamics can be represented by the equation:

∆U = q + w

where,
∆U is the change in the internal energy of the system;
q is the amount of heat transferred to the system; and
w is the amount of work done in the system.

The total amount of energy in a chemical reaction remains the same.

When heat is produced in a system, where does it go? What will happen to it? The heat will escape into the surrounding environment. There is no thermodynamic system that is so perfect that no energy will escape from it. Whatever the system is, there will always be some energy escaping into the surroundings.

This phenomenon could be the only flaw of the first law of thermodynamics. To correct this flaw, physicists came up with the second and third law of thermodynamics. However, discussing these will require two new different stories. So for the mean time, just stick to what you have learn and remember to always practice energy conservation.