WORK AND ENERGY
IMPORTANT TERMS
Work: In physics, work is defined as an activity where a force acting on a body produces displacement in it.
Energy: Energy is the capacity of a physical system to do work.
Kinetic Energy: The kinetic energy of an object is the energy that it possesses due to its motion.
Potential Energy: The potential energy of an object is the energy because of its position or configuration, rather than its motion.
Power: It is the rate of doing work.
Joule: It is SI unit of work as well as energy. It is denoted by J. One Joule = 1Nm.
Watt: SI unit of power is watt. It is denoted by W.
Work
Work is done when force acting on a body produces displacement in it.
- The work done on an object is the product of the force applied and the displacement.
- Work done = force x displacement
W = F X s
- Work has only magnitude and no direction.
- The unit of work is Joule (J).
- If F = 1 Newton and s= 1 metre then the work done= 1Nm or 1 joule (J).
- Thus 1 joule is the amount of work done when a force of 1 Newton displaces an object by 1 metre.
- The work done by a force may be positive or negative.
- It is positive if the force and displacement are in the same direction.
- It is negative if the force and displacement are in opposite direction.
- It is zero if there is no displacement.
- It is zero if there is no force.
- It is zero if the force is perpendicular to the displacement.
Energy
An object having a capability to do work is said to possess energy. The object which does the work loses energy and the object on which the work is done gains energy. Thus energy of an object is its capacity for doing work.
- The SI unit of energy is the same as that of work that is joule (J).
- 1 joule is the energy required to do 1 joule of work.
- Larger unit of energy is kilo joule and is denoted by kJ. 1 KJ = 1000 Joule.
- There are different forms of energy i.e heat energy, light energy, electrical energy, chemical energy, mechanical energy (potential energy + kinetic energy) etc.
Kinetic Energy
The kinetic energy of an object is the energy possessed by the object due to its motion.
- All moving objects possess kinetic energy. A falling mango, a speeding car, a flying aeroplane, flowing water, blowing wind, a running athlete etc. possess kinetic energy.
- The kinetic energy of an object depends upon its speed or velocity. An object moving faster has more kinetic energy than an object moving slower.
- The kinetic energy (Ek) possessed by an object of mass m and moving with uniform velocity v is equal to mv2/2.
- Kinetic energy of a moving object increases with increase of mass and velocity of the object.
Potential Energy
The potential energy possessed by the object is the energy present in it by virtue of its position or configuration.
- Examples of potential energy :
A stretched rubber band possesses some energy because of its position. A stretched bow possesses energy because of its position of stretched string. When a stone is kept at a height, it possesses some energy because of its height.
- The potential energy (Ep) is equal to the work done over the object. Therefore, Ep = mgh, Where, ‘h’ is height, ‘m’ is mass and ‘g’ is acceleration due to gravitiy.
- The potential energy of an object depends upon the mass and height (position) of the object and not upon the path.
Transformation of energy
§ The conversion of energy from one form into another form is called transformation of energy.
§ When energy is converted from one form into another, the total energy always remains the same.
Mechanical Energy
The sum of kinetic energy and potential energy of an object is its total mechanical energy.
The law of conservation of energy
- The law of conservation of energy states that energy can only be converted from one form into another, it can neither be created nor destroyed.
- The total energy before and after the transformation remains the same.
- The law of conservation of energy is valid in all situations and for all kinds of transformations.
An Example of The law of conservation of energy
- Let an object of mass, m be made to fall freely from a height, h.
- At the start, the potential energy= mgh and kinetic energy = zero because its velocity is zero. The total energy of the object is thus mgh.
- As it falls, its potential energy will change into kinetic energy. If v is the velocity of the object at a given instant, the kinetic energy would be (mv2)/2.
- As the fall of the object continues, the potential energy would decrease while the kinetic energy would increase.
- When the object is about to reach the ground, h = 0 and v will be the highest. Therefore, the kinetic energy would be the largest and potential energy the least.
- The sum of the potential energy and kinetic energy of the object would be the same at all points.
mgh + (mv2)/2 = Constant at every point
- During the free fall of the object, the decrease in potential energy, at any point in its path, appears as an equal amount of increase in kinetic energy.
- A continual transformation of gravitational potential energy into kinetic energy occurs during free fall.
Power
Rate of doing work is called power.
- If W is the work done in time t, then Power = Work done/time taken = W/t
- The unit of power is watt (W) named after James Watt, the inventor of steam engine. 1 watt is the power of an agent which does work at the rate of 1 joule per second.
- 1 watt = 1 joule/second
1 W = 1 J/s
- The bigger unit of power is kilo watt and is written as KW.
- kilowatt = 1000 watts
1 KW = 1000 W= 1000 J/s
Commercial unit of energy
Since joule is very small thus, large quantity of energy is expressed in kilowatt hour and is written as KWh.
- The commercial unit of energy is kilowatt hour (KWh).
- 1 kilowatt hour is the energy used in one hour at the rate of 1 kilowatt (1000 Js).
1 KWh = 1 KW x 1 h
= 1000 W x 1 h
= 1000 W x 3600 s
= 3600000 J
- 1 KWh = 3.6 x 106 J
- The electrical energy used in homes and industries are expressed kilowatt hour. The electrical energy used during a month is expressed in ‘units’. Here 1 unit means 1 kilowatt hour.
Interesting Facts
- The word energy comes from the Greek word energeia.
- Food contains chemical energy which is used by living organisms such as animals to grow and reproduce.
- Wind farms contain large numbers of wind turbines which are used to transform wind energy into electrical energy.
- Plants use energy from sunlight during an important process called photosynthesis.
- During chemical reactions, chemical energy is often transformed into light or heat.
- Stretched rubber bands and compressed springs are examples of elastic potential energy.
