ENERGY, CLASS9

Forms of Energy

  • Mechanical Energy

  • Heat Energy

  • Light Energy

  • Chemical Energy

  • Sound Energy

  • Magnetic Energy

  • Electrical Energy

  • Atomic Energy

Kinetic Energy

  • Defined as the energy of a body due to its motion.

  • Formula: ( KE = \frac{1}{2}mv^2 )

  • Examples:

    • A speeding car

    • A rolling stone

    • A running athlete

    • A flying aircraft

Derivation of Kinetic Energy

  • Consider an object of mass ( m ) moving with initial velocity ( u ) and covered distance ( s ) under constant force ( F ).

  • Work done: ( W = F \times s )

  • Work done causes a change in velocity from ( u ) to ( v ); using acceleration ( a ):

    • Relation: ( v^2 - u^2 = 2as )

    • ( F = ma )

  • If starting from rest (( u = 0 )), kinetic energy becomes ( KE = W ) which leads to ( KE = \frac{1}{2}mv^2 ).

Important Points on Kinetic Energy

  • Doubling mass doubles kinetic energy.

  • Halving mass halves kinetic energy.

  • Doubling velocity quadruples kinetic energy.

  • Halving velocity results in a quarter of kinetic energy.

  • Heavy, fast objects have more kinetic energy than lighter, slower ones.

Potential Energy

  • Defined as energy possessed by a body due to its position or configuration.

Potential Energy Types

  1. Potential Energy due to Position

    • Example: Stone on top of a building or water in a reservoir (gravitational potential energy).

  2. Elastic Potential Energy

    • Caused by compression, stretching, bending, or twisting of an object.

Examples of Potential Energy

  • Water in a dam (position)

  • Stone on a mountain (position)

  • Stretched/compressed spring (shape)

  • Wound spring of a watch (shape)

  • Stretched bow and arrow (shape)

Gravitational Potential Energy

  • Energy increases when an object is raised against gravity:

  • Formula: ( PE = mgh )

    • Work done on the object against gravity.

Energy Transformation

  • Electric motors convert Electrical Energy to Mechanical Energy;

  • Generators convert Mechanical Energy to Electrical Energy;

  • Electric iron, heater, bulb convert Electrical Energy to Heat Energy;

  • Solar cells convert Light Energy to Electrical Energy;

  • Burning LPG converts Chemical Energy to Heat and Light Energy;

  • Car Engine transforms Chemical Energy to Heat and then to Kinetic Energy;

  • Steam Engine: Heat Energy to Kinetic Energy.

Fundamental Concepts of Energy

  • Energy is the capacity of a body to perform work.

  • SI unit of energy: Joule (J)

Conservation of Energy

  • Energy cannot be created or destroyed; it can only transform from one form to another while total energy remains constant.