CHAPTER 7 WORK, ENERGY AND POWER The formula for power is:
Power P = W
___ t
P = power (watts), W = work ( J) and t = time (s)
The unit of power is the watt, named after James Watt (Fig. 7.7); 1 W can also be written as 1 J s−1.
Power efficiency
Power efficiency is the fraction or percentage of power you get out compared to what you put in. You can never get 100 per cent efficiency as there is always some energy converted to heat, sound, light or other forms but decreasing the loss makes energy cheaper to use.
% Efficiency = Power output Efficiency can never exceed 100 per cent. QUESTIONS AND ANSWERS
5. A weightlifter can raise 40 kg a height of 1.5 m 20 times in one minute. Calculate his average power.
Solution
First find the work done for one lift and then for 20 lifts.
W = Fs = mgh W = (40)(9.8)(1.5) = 588 J 20 times = 20 × 588 = 1176 J P = W
___ t = 1176
You may now complete Exercise 7B (page 70).
_______ 60 = 196 W
__________________ Power input ë 100
STS
Each electrical device has a power rating on it. (For example, a kettle = 2 kW or light bulb = 120 W.) This tells you the rate at which electrical energy is converted into other forms (120 W means 120 joules per second converted).
