Short notes of chapter - 3 energy of class 7

*Energy:

•energy is the ability or capacity to do work.

• SI unit of energy is joule.

•symbol is J.

•CGS unit of energy is erg.

•another unit of energy is calorie.

Symbol is cal.

*A body is said to possess an energy of one joule if one joule work is done to bring the body in that state.

•Relationship between SI and CGS unit of energy

1J = 1N × 1m

     = 1 kgm/s² × 1m ( F =mass × acceleration

                                     F = 1kg × 1m/s²

                                         = 1kgm/s²

                                          =1N        )

     = 1×1000 g×100cm/s² × 100cm (1kg= 1000g ,                                                                   1m= 100cm )

    = 10⁵g.cm/s² × 10² cm

    = 10⁵ dyne × 10² cm

   = 10⁵+² dyne.cm

  = 10⁷ erg

1 J = 10⁷ erg

1 erg = 1/10⁷ J

          = 10-⁷ J

•Relationship between 1J and 1 calorie

1cal = 4.186 J

1 cal ≈ 4.2 J

1J =0.24 cal

•Relationship between kilo calorie and calorie

1 kcal = 1000 cal

1 cal = 1/1000 kcal

         = 10-³ kcal

* Work:

•Work  is said to be done, when a force is applied on the body and the body gets displaced or moved.

•work done= force applied × distance moved in the direction of force.

 work = force × displacement

• SI unit of work is J.

1 joule of work is said to be done when a force of 1 Newton moves a body by a distance of 1 metre in the direction of force.

• CGS unit of work  is erg.

1 erg of work is said to be done when a force of 1 dyne moves a body by a distance of 1 cm in the direction of force.

• SI unit of force is Newton.

•CGS unit of force is dyne.

1 N = 10⁵ dyne.