JEE Main & Advanced Physics Work, Energy, Power & Collision / कार्य, ऊर्जा और शक्ति Energy

The energy of a body is defined as its capacity for doing work.

(1) Since energy of a body is the total quantity of work done, therefore it is a scalar quantity.

(2) Dimension: \[[M{{L}^{2}}{{T}^{-2}}]\] it is same as that of work or torque.

(3) Units : Joule [S.I.], erg [C.G.S.]

Practical units : electron volt (eV), Kilowatt hour (KWh), Calories (cal)

Relation between different units:    

1 Joule = \[{{10}^{7}}\] erg

1 eV  = \[1.6\times {{10}^{-19}}\] Joule

1 kWh = \[3.6\times {{10}^{6}}\] Joule

1 calorie = \[4.18\, Joule\]

(4) Mass energy equivalence : Einstein's special theory of relativity shows that material particle itself is a form of energy.

The relation between the mass of a particle m and its equivalent energy is given as

\[E=m{{c}^{2}}\] where c = velocity of light in vacuum.

If \[m=1\,amu=1.67\times {{10}^{-27}}\,kg\]

then \[E=931\,MeV=1.5\times {{10}^{-10}}\,Joule\].

If \[m=1kg\] then \[E=9\times {{10}^{16}}\,Joule\]

Examples : (i) Annihilation of matter when an electron \[({{e}^{-}})\] and a positron \[({{e}^{+}})\] combine with each other, they annihilate or destroy each other. The masses of electron and positron are converted into energy. This energy is released in the form of\[\gamma \]-rays.

\[{{e}^{-}}+{{e}^{+}}\to \gamma +\gamma \]

Each  \[\gamma \] photon has energy = 0.51 MeV.

Here two \[\gamma \] photons  are emitted instead of one \[\gamma \] photon to conserve the linear momentum.

(ii) Pair production :  This process is the reverse of annihilation of matter. In this case, a photon \[(\gamma )\] having energy equal to 1.02 MeV interacts with a nucleus and give rise to electron \[({{e}^{-}})\]and positron \[({{e}^{+}})\]. Thus energy is converted into matter.

(iii) Nuclear bomb : When the nucleus is split up due to mass defect (The difference in the mass of nucleons and the nucleus), energy is released in the form of \[\gamma \]-radiations  and  heat.

(5) Various forms of energy

(i) Mechanical energy (Kinetic and Potential)        

(ii) Chemical energy                 

(iii) Electrical energy

(iv) Magnetic energy                                 

(v) Nuclear energy                    

(vi) Sound energy

(vii) Light energy                                                       

(viii) Heat energy

(6) Transformation of energy : Conversion of energy from one form to another is possible through various devices and processes.    

Various devices for energy conversion from one form to another