Law of Conservation of Energy: Overview, Questions, Preparation

In an isolated system, the conservation law states that physical properties cannot change. These quantities are said to be conserved (constant). In classical physics, examples of conserved quantities are energy, momentum, angular momentum, mass and electric charge. In particle physics, the law applies to subatomic particles. If some of the forces involved are non-conservative, the energy transforms into other forms such as heat, light, and sound.

It states that it may transform energy from one form to another, but the total energy remains constant. Energy can be neither created nor destroyed. In an isolated system, the sum of all forms of energy remains constant. The law is valid in all situations for all kinds of transformations.

The total energy (E) of an isolated system is equal to the sum of potential energy (V) and kinetic energy(T).

E = T + V

The principle of conservation of energy cannot be proven. The concept of conservation of energy and transformation of energy into various forms links together various branches of physics, chemistry and life sciences

Energy that can change forms is heat and work. When heat is applied to the closed system, the system works by increasing its volume.

W = P_exΔV

P_ex is the external pressure, and ΔV is the volume change.

Example: Piston

When heat is added to the cylinder, the pressure increases. The piston then releases the pressure inside the cylinder, which increases the volume of the cylinder.

The sum of heat and work is the change in the internal Energy, ΔU

q+w = ΔU

In an isolated system, q=−w

Therefore, ΔU=0

Example: object falling from rest

• When an object of mass m falls freely from a height, h, the potential energy is mgh, and kinetic energy is zero because its velocity is zero. Thus the total energy of the object is mgh.
• As the object drops, the potential energy converts into kinetic energy. If v is the object's velocity at a given instant, the kinetic energy would be ½mv²
• As the object continues to drop, the potential energy decreases while increasing the kinetic energy.
• The velocity of the object is high as it reaches the ground, h= 0
• As the object is at rest, the kinetic energy is high, and the potential energy is low.
• However, the sum of the object's potential energy and kinetic energy would be the same at all points. That is, potential energy + kinetic energy = constant

mgh + ½mv² = constant

Law of Conservation of Energy for Class 11

This chapter provides an overview of different types and energy, work-energy theorem, potential energy and kinetic energy. It introduces the topic Law of Conservation of Energy. But, there is no detailed explanation of how the law is useful and applied for various principles. The weightage of the topic is less than 3 marks as it describes only the definition.