WORK AND ENERGY

 Work 

• When a force changes the position or produces motion in a body, we can say work is done.
• Also we can say work is done, when a moving body stops or starts from rest or change its shape after applying force.
• Work done is equal to the product of force exerted on the body and the distance. Work = Force x Distance 
• Its SI unit is joule (J).
• It is a scalar quantity because it has only magnitude and no direction.
• One joule work is done when a force of 1 N displaces the object by 1 m. 
• Positive, Negative and zero work - When work is done in the direction of force, it is called positive work and and when work is done in opposite direction of force it is called negative work and work is zero when there is no displacement after applying force.

Energy -

•  Ability to do work is called energy.
• Eergy is a scalar quantity.  
• Its SI unit is joule (J). 1 kJ = 1000 J

There are some forms of energy: l

• Kinetic energy - It is possessed by a body by virtue of its motion. 
• Kinetic energy is denoted by K.E   K.E = 1/2 mv^2, where m is mass and v is velocity of the object. Example: Moving bus or car, etc.
• Potential energ y - It is possessed by a body of its position. Potential energy is denoted by P.E. P.E = mgh,  Where m is mass of the object,  g is the acceleration due to gravity and h is the height of the the object from the ground. 
• Example: Stretched rubber (elastic P.E), water at a heigh (gravitational P.E) in the tank or dam, etc.
• Mechanical energy- Sum of potential energy and Kinetic energy is called mechanical energy. Mechanical energy = P.E + K.E = meh +1/2mv^2
• Light, Heat, Sound and Chemical energy are the transformation energy.

Power

• The rate of doing work is called power.
• Power = Work done/Time taken
• Power is a scalar quantity. 
• Its SI unit is watt (W).
• It is said to be one watt when the rate of consumption of energy is 1 J/s. Larger units of power are:
• 1 kW = 1000 W
• 1 MW = 10^6 W
• 1 hp = 746 W

Mathematical relation between kWh and joule:

1 kWh = 1 kw x 1 hr

Or 1 kWh = 1000w x 3600s.   ( 1 hr = 60min = 60x60 sec)

Or 1kwh = 3.6x10^6 joule 

Comercial unit of energy - The commercial unit of energy is kilowatt hour (kWh).

• One kilowatt hour is the amount of electrical energy consumed when an electrical appliance with a power rating of 1 kilowatt is used for 1 hour.
•  1 kWh is equal to 3.6 × 10^6 J of energy.
•  1 kilowatt hour of electrical energy is known as 1 unit.

Law of conservation of energy

According to the law of conversion of energy- Energy can neither be created nor destroyed, it can only be converted from one form to the other.

Therefor, total energy after and before the transformation remains same.

Example: Electric energy to light energy, electric energy to sound energy, light energy to electric energy.