9th Class Science Work and energy Power

         Power

The rate of doing work is called power.

\[(P)=\frac{\text{work}\,\text{done}\,\text{(W)}}{\text{time}\,\text{taken}\,\text{(t)}}=\,\,\text{or}\,\,P=\frac{W}{t}\]

• Power is expressed in watt (W). When a body performs work at the rate of 1 J per second, its power is 1 W. Power is a scalar quantity.
• Horse power is another unit of power.

1 hp = 746 W

= 0.746 kW

           Commercial Unit of Energy

The unit joule is too small and hence is inconvenient to express large quantities of energy. So, we use a bigger unit of energy called kilowatt hour (kW h).

1 kW h = 1 kW \[\times \] 1 h

= 1000 W \[\times \] 3600 s

= 3600000 J

1 kW = 3.6 \[\times \] 10⁶ J

          Which one of the following is the commercial unit of energy?

(a) Joules

(b) Watt

(c) KW

(d) KWh

(e) None of these

Answer: (d)

         One kilowatt hour is equal to:

(a) \[36\times {{10}^{6\,}}J\]

(b) \[136\times {{10}^{6}}J\]

(c) \[3.6\times {{10}^{6}}J\]

(d) \[1.36\times {{10}^{6}}J\]

(e) None of these

Answer: (c)

         One horse power is equal to:

(a) 746 watt

(b) 74.6 watt

(c) 7.46 watt

(d) 746 kw

(e) None of these

Answer: (a)

       Work done by a string when a stone is tied to it and whriled in a circle is:

(a) Positive

(b) Negative

(c) Zero

(d) All of these

(e) None of these

Answer: (c)

• Work is said to be done if the force applied on the object cause displacement.
• The capacity to do the work is called energy.
• The energy possessed by the object by the virtue of its motion is called kinetic energy.
• The energy possessed by the body by the virtue of its shape and position is called potential energy.
• The commercial unit of power is KWh.