Efficiency

1
Efficiency
Definition Efficiency useful output x 100
input

• Example
• A crane operates at 15 kW and lifts a 700 kg
  weight 50 m in 2 minutes. Find the efficiency.

Power Energy / time
Input
P x t
1.8 x 106 J
Ei
15000W x 120s
Ouput
PE
Eo
mgh
700 x 9.8 x 50
3.43 x 105 J
Efficiency
Eff 3.43 x 105 x 1001.8 x 106
19
2

• A 6.5 hp forklift is 15 efficient when lifting a
  220 kg box. How long does it take to lift the
  box 1.5m? (1 hp 746 W)

Ouput
Eo
mgh
220 x 9.8 x 1.5
3234 J
Efficiency
0.15
Input
21560 J
Ei
P x t
Ei
21560 (6.5 x 746) x t
t 4.5s
3

• An 800 kg, 100 hp car is 65 efficient at
  transferring energy to the wheels. How long does
  it take to go from 0 ? 100 km/h?

Efficiency
t 6.4s
4

• How efficient is Mr. Davies kettle at heating
  water?(900W) It heated 300mL of water from 23C
  to 49C in 1 minute.

Input
P x t
54000 J
Ei
900 x 60
Ouput
Q
Eo
mc?T
0.3 x 4200 x (49-23)
32800 J
Efficiency
Eff 32800 x 10054000
61
5
Homework

• Energy Conservation
• Textbook Page 243
• Questions 41 44
• Efficiency
• Textbook Page 244
• Questions 45 47