## Power

Category : 9th Class

### 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.

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