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

## 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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