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# 3. Work and Energy

#### Energy & its Unit

Definition : Energy is the ability to do work. The amount of energy possessed by a body is equal to the amount of work it can do when its energy is released. Thus, energy is defined as the capacity of doing work. Energy is a scalar quantity and it exists in various forms.
Units of energy :The units of energy are the same as that of work. In SI system, the unit of energy is joule (J). In CGS system, the unit of energy is erg.
1 Joule = 107 ergs
Other units of energy in common use are watt-hour and kilowatt hour.
1 watt-hour = 1 watt × 1 hour
= 1 watt × 60 × 60 sec
= 3600 J
1 kilowatt-hour (kWh) = 3.6 × 106 Joule
Heat energy is usually measured in calorie or kilocalorie such that
1 calorie = 4.18 J
A very small unit of energy is electron volt(eV).
1 eV = 1.6 × 10–19 J

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#### NTSE Physics (Class X)

• Electricity
• Magnetic Effect of Curent
• Light Reflection
• Light Refraction
• Human Eye & Colourful World
• Source of Energy
• Motion

#### NTSE Physics (Class IX)

• Force & laws of motion
• Work and Energy
• Gravitation
• Sound

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