Chapter 12 Answer Key: Power, Work and Eneregy

12.1 Power

1.  10 000 W

2.  5.4 MJ

3.  30 s  4.  89 500 W or 89.5 kW

12.2 Power & Change in Energy

1.  1300 W

2.  368 W

3.  (i)  7840 J  (ii)  1120 W

4.  23 700 W

5.  62 000 W

6.  0.95 MW

7.  22.2 MW

8.  17.2 kW

12.3 Power & Accelerated Motion

1.  0.0371 m/s

2.  2.28 m/s

3.  (i)  11 400 W (ii)  45 700 W

4.  (i)  8500 N  (ii)  2.28 x 105 W

5.  27.4 m/s

6.  7.7 m/s

7.  152 000 W

8.  9.3°

12.4.1 Large Tree Evaporation

1.  (i)  98 000 J  (ii)  2.27 W

12.4.2  Human’s Radiated Heat

1.  2.2 x 106 J

12.4.3 Power Output of the Sun

1.  3.8 x 1026 W

12.4.4  Power from Wind

1.  (i)  0.785 m²      (ii)  9.46 m/s     (iii)  421 Rotors (iv)  9.18 m/s  (v)  45.4 kW

2.  (i)  The curve should flatten out due to decreased air density

     (ii)  The curve should become steeper climb due to increased air density

     (iii)  The ratio of 600 to 300 Watts is roughly the same as the ratio of (10 m/s)3 to (8 m/s)³

   Alternatively, on can look at the ratio of  P/v³ which equals 0.6 for both speeds.

     (iv)  0.997 m2 or 1 m2

12.4.5 Power from Ocean and River Currents

1.  13.8 W at 0.25 m/s to 1.73 kW at 1.25 m/s

2.  120 kW maximum power, approximately 4200 turbines to replace 50 MW

3.  67 kW for water compared to 41 kW from a hurricane