Notes on Work, Energy and Power NCERT Class 9 for CBSE

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Table of Contents

  1. Work
  2. Energy
  3. Power

Work (w)

Work is said to be done when a force is applied on a body and the body displaces from its initial position.

   w = F x s

Types of Work (Positive, Negative, Zero)

  • Positive Work Done: When the displacement is in the direction of the force applied. Ex: A boy pulls an object towards himself.
  • Negative Work Done: If the displacement is in the opposite direction of force applied. Ex: When you’re pushing an object along the floor
  • Zero Work Done: If the displacement is perpendicular to the force applied, or either of the two is zero. Ex: a man pushes a wall

Energy (E)

The ability or capacity to do work is called Energy.

Note: Both Work and Energy are scalar quantity and have the same SI, CGS units, as mentioned below

  • Scalar quantity
  • SI Unit: J
  • CGS Unit: Erg
1 kJ 103 J
1 cal 4.2 J
1 Erg 10-7 J
1 ev 1.6 x 10-19 J

Energy Transformation

  • Heat Engine: Heat energy to Mechanical energy
  • Solar Cell: Solar energy to Electrical energy
  • Microphone: Sound(acoustical) energy to Electrical energy
  • Loudspeaker: Electrical energy to Sound energy
  • Battery: Chemical energy to Electrical energy
  • Candle: Chemical energy to Light energy
  • Electric Motor: Electrical Energy to Mechanical Energy
  • Sitar: Mechanical Energy to Sound Energy
  • Electric Bulb: Electrical energy to Light Energy

Types of Energy

1. Kinetic Energy

The energy possessed by a body due to its motion, is called its kinetic energy.

   KE = (mv2) / 2 = p2 / 2m
  • Ex: A collision of pool balls

Work Energy Theorem: w = change in kinetic energy

Potential Energy (U)

The energy possessed by a body due to change in its position or shape, is called potential energy.

  • Ex: A coiled spring, Water behind a dam.

The sum of the potential energy and Kinetic energy of an object is known as mechanical energy.

Gravitational Potential Energy

When an object is raised through a certain height above the ground, its energy increases. The energy is called Gravitational Potential Energy.

   U = mgh

Various forms of Energies

  • Internal Energy: The energy in a system arising from the relative positions and interactions of its parts.
  • Heat Energy: Energy possessed by a body due to its temperature, is called heat energy.
  • Electrical Energy: The energy which is caused by the movement of the electrons from one place to another.
  • Chemical Energy: The energy absorbed or released during a chemical reaction is called chemical energy.
  • Nuclear Energy: The energy absorbed or released during a nuclear reaction is called Nuclear Energy.

Power (P)

The rate of doing work by a body is called power.

   P = w / t
  • SI Unit: W (Watt)
  • Scalar Unit
  • 1 kw = 3.6 x 106 J = 3.6 megajoule (mJ)
  • 1 HP (Horse Power) = 746 W

Energy vs Power

Energy is the amount of work done whereas Power is the rate of doing work.

Law of Conservation of Energy

Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.

  • The total energy of an isolated system remains constant, it is said to be conserved over time.

Energy Resources

1. Renewable Energy Resources

The energy resources which are naturally replenished on a human timescale.

  • Ex: Solar, Wind, Hydroelectric, Biomass, Geothermal, etc.

2. Non-renewable Energy Resources

The resource that does not renew themselves at a sufficient rate for sustainable economic extraction in meaningful human time-frames.

  • Ex: Petroleum, Coal, Natural gas, Nuclear Energy, etc.

Conventional Sources of Energy

  1. Fossil Fuel: A natural fuel such as coal or gas, formed in the geological past from the remains of living organisms.
  2. Thermal Power Plant: A power plant in which heat energy is converted to electric power.
  3. Hydro-power Plant: Power plants which converts the potential energy of falling water into electricity. Electricity thus produced is called hydro electricity.
  4. Wind Energy: The energy possessed by the wind is due to its high speed. Wind turbines converts wind energy into electrical energy.

Wind vs Air: Moving air is called wind.

Non-Conventional Sources of Energy

  1. Geothermal Energy: The heat from interior of the earth under certain favourable conditions that are created by the natural processes. This is known as geothermal energy.
  2. Solar Energy: It is radiant light and heat from the Sun that is harnessed using a range of ever-evolving technologies such as solar heating, photovoltaics, solar thermal energy, solar architecture, molten salt power plants and artificial photosynthesis.

Solar Cell (photovoltaic cell)

A solar cell is an electrical device that converts the energy of light directly into electricity by the photovoltaic effect, which is a physical and chemical phenomenon.

  • Made from semiconductors like Si, Ga, Ge, etc.

Tidal Energy

Tidal power or tidal energy is a form of hydropower that converts the energy obtained from tides into useful forms of power, mainly electricity.

  • Tide: The rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun, and the rotation of Earth.
  • High Tide: The natural tide at its highest level for a particular tidal cycle at a certain place.
  • Low Tide: The tide at its lowest level for a particular tidal cycle at a certain place.


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