CBSE PHYSICS: LAW OF CONSERVATION OF ENERGY(9th physics):

LAW OF CONSERVATION OF ENERGY

Energy can neither be created nor destroyed, but it

is transformed from one form to another. Alternatively,

whenever energy gets transformed, the total energy

remains unchanged.

It may be shown that in the absence of external frictional force the total mechanical energy of a body remains constant.

Let a body of mass m falls from a point A, which is at a height h from the ground as shown in fig.

Kinetic energy kE = 0

Potential energy Ep = mgh

Total energy E = Ep + Ek = mgh + 0= mgh

Potential energy Ep = mg (h – x)

Total energy E = Ep + Ek = mg (h-x) + mgx = mgh – mgx + mgx= mgh

If the body reaches the position C.

Potential energy Ep = 0

Velocity of the body C is

v

u = 0, a = g, s = h

applying v

kinetic energy Ek =1/2 mv

Total energy at C

E = Ep + Ek

E = 0 + mgh

E = mgh

Watt hour = power in watt x time in hour.

Energy can neither be created nor destroyed, but it

is transformed from one form to another. Alternatively,

whenever energy gets transformed, the total energy

remains unchanged.

**Proof – Freely falling body**It may be shown that in the absence of external frictional force the total mechanical energy of a body remains constant.

Let a body of mass m falls from a point A, which is at a height h from the ground as shown in fig.

**At A,**

Potential energy Ep = mgh

Total energy E = Ep + Ek = mgh + 0= mgh

During the fall, the body is at a position B. The body has moved a distance x from A.

**At B,**

velocity v

^{2}= u^{2}+ 2asapplying, v

Kinetic energy Ek = 1/2 mv^{2}= 0 + 2ax = 2ax^{2}= 1/2 m x 2gx = mgxPotential energy Ep = mg (h – x)

Total energy E = Ep + Ek = mg (h-x) + mgx = mgh – mgx + mgx= mgh

If the body reaches the position C.

**At C,**Potential energy Ep = 0

Velocity of the body C is

v

^{2}= u^{2}+ 2asu = 0, a = g, s = h

applying v

^{2}= 0 + 2gh = 2ghkinetic energy Ek =1/2 mv

^{2}=1/2 m x 2gh= mghTotal energy at C

E = Ep + Ek

E = 0 + mgh

E = mgh

*Thus we have seen that sum of potential and kinetic energy of freely falling body at all points remains same. Under the force of gravity, the mechanical energy of a body remains constant.*__RATE OF DOING WORK (OR) POWER__*Power is defined as the rate of doing work or work done per unit time.*Power =work done/time taken

P = w/t

UNIT OF POWER

The unit of power is J/S known as watt, its symbol is W.

1 watt =1 joule/1 second

1 W = 1 J/ S

Commercial unit of energy is kilo watt hour

We pay electricity bill in terms of unit or kWh. It is a commercial unit of electric energy consumed by the user.

Example : How much energy will be used when a hundred watt bulb is used for 10 hour?

Energy = 100 watt x 10 hour = 1000 w h = 1kw h

I kwh is known as 1 unit.

One kilowatt hour means thousand watt of power is consumed in one hour.

1 kWh = 1 kW x 1 h

= 1000 W x 60 x 60 s

= 1000 Js-1 x 3600 s

= 3.6 x 106 J

1 unit = 1 kilowatt hour = 3.6x10

^{6 }J
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