Basic concepts of mechanical engineering Part 2 ( work, power, energy, heat)

In Part 1 we have seen about Prime movers, Force and Mass, and Pressure.
If you haven't read it then I will suggest you take a look at it!!!

In this Part 2, we will learn about
4. Work
5. Power
6. Energy
7. Heat

4. work

 work is said to be done when a force moves through a distance. If any part of the limit of a system undergoes a displacement under the action of a pressure and/or the work done W is the product of the force (Pressure × area) and the distance it moves in the direction of the force.

    Hence,

              Work = Force × Distance moved into the direction of a force.

• If the work is done by the system on surrounding, e.g. when a fluid expands pushing a piston outwards, the work is said to be positive. 

           ∴ Work output of the system = +W

• If the work is done on the system by surrounding, e.g. when a force is applied to a piston to compress a fluid, the work is said to be negative.

            ∴ Work output of the system = -W

• Unit of Work (W) = Unit of Force Unit of displacement

                                         = N × m

                                         = N⋅m or Joule

5.Power

Power is defined as the rate of doing work OR The power is work done per unit time.
Mathematically,
Power = work done/time
           = Joule/second (units)

In SI unit Joule/second is called Watt(W)
Watt is a very small unit, so recommended larger units are Kilowatt (kW), Megawatt(MW), etc.

• The power available at the engine shaft is called Brake Power(B.P.) and the power developed by the engine is called Indicated Power(I.P.)

6. Energy

"Energy", a word derived from the Greek word "Energia", means a capacity for doing work.

• The unit of energy is the unit of work i.e. Joule.

Another important unit of energy is Kilowatt hour(kWh) which is derived from the unit of Power kilowatt.

Forms of Energy:-

The different forms of energy are;

1. Mechanical Energy
2. Chemical Energy
3. Thermal or Heat Energy
4. Electrical Energy
5. Nuclear Energy

It is possible to convert one form of energy into another form of energy. The heat engine is a device which converts heat energy into mechanical energy.

   Energy can neither be created nor be destroyed but the total amount of energy remains constant before and after the transformation. This is called the law of conservation of energy.

High and Low-Grade energy

The second law of thermodynamics prohibits the complete conversion of heat into work. Sources of energy may be divided into two groups viz.

a.) High-grade energy:

 The energy that can be completely converted (neglecting losses) into the work.

Examples: Mechanical work, Electrical energy, Water power, Wind and tidal power, a Kinetic energy of jet.

b.) Low-grade energy:

  Only a certain portion of energy that can be converted into a mechanical work (shaft power), that energy is called low-grade energy.

Examples: Thermal or heat energy, Heat derived from combustion of fuels, Heat of nuclear fission.

Types of energy:

Energy may be classified as 1) Stored energy 2) Energy in transition

   The stored energy of a substance may be in the form of mechanical energy, internal energy, nuclear energy etc.

   Energy in transition is the energy transferred as a result of potential difference. This energy may be in the forms of heat energy, work energy etc.

Types of Mechanical Energy:

   There are two types of mechanical energy

a) Potential energy b) Kinetic energy

a) Potential energy:

the energy which a body possesses by virtue of its elevation or position is known as its potential energy.

Examples: Water stored at a higher level in a dam

Potential energy, P.E. = w ⋅ h

                          ∴P.E. = mgh

where w = weight of body in N, h = Height in meter, m = mass of body in kg, g = Gravitational acceleration = 9.81 m/s²

b) Kinetic energy:

The energy which a body possesses by virtue of its motion is known as its kinetic energy.

Example: Jet of water coming out from a nozzle.

     Kinetic energy (K.E.) = 1/2×(mV²N⋅m)
where m = mass of body in kg, v = velocity of body in m/s.

7.Heat

     when two bodies at different temperatures are brought into contact there are observable changes in some of their properties and changes continue till the two don't attain the same temperature if contact is maintained. Thus, there is some kind of energy interaction at the boundary which causes change in temperatures. This form of energy interaction is called Heat.

Definition of Heat:

'Heat may be defined as the energy interaction at the system boundary which occurs due to temperature difference only.'

when heat is removed from a body or supplied to it, there are some changes found to happen such as (a) change of temperature, (b) change of volume, (c) change of state (solid to liquid, liquid to gas, etc.), (d) change of physical properties, etc.

Positive and negative heat

      In general, the heat transferred to the system is considered as positive heat while the heat transferred from the system is considered as negative heat.

     Mass of the substance, specific heat and temperature difference are the factors on which the heat transfer rate depends.

Comparision of work and heat:

similarities:

1. Both are path functions
2. Both are boundary phenomenon
3. Both are associated with a process, not a state
4. Systems possess energy, but not work or heat

Dissimilarities:

1. In heat transfer, a temperature difference is required.
2. In a stable system there cannot be work transfer, however, there is no restriction for the transfer of heat.
3. Heat is low-grade energy while work is high-grade energy.

In next part 3, we will see about Temperature and scale, Units of heat, specific heat capacity and thermodynamics