1) On the construction site an electric crane is used to lift materials to the 10th floor. A 180 kW motor requires 2.0 minutes to lift a 10500 kg load if bricks 218 m. How efficient is the crane?

Question

2. Burning 1.0 kg of coal releases 33 MJ of energy. This energy is converted into electricity with an efficiency of 18%.Determine the mass of coal that must be used to supply a city with 2.0MWh.

3. It takes a natural gas heater (methane) a considerable amount of time to heat a swimming pool. A 150 kW heater takes 28 hours to heat 110000L (110000kg) of water by 8.00 degrees Celsius. How efficient is it?

No idea how to solve these problems. I know the efficiency formula but have no idea how to go from there.

Answer 1

electrical work in = 180,000 *120 = 21.6 *10^6 Joules

mechanical work out = 10,500*9.81*218 = 22.5 *10^6 Joules

magic? on moon?
let's see ? 10th floor about 9 * 3 meters high or 27 meters, not 218 meters

Answer 2

2.
1 watt hr = 3600 watt seconds or Joules
so
2.0 Mwh = 7200 MJ

33 MJ/kg * m in kg *.18 = 7200 MJ

m = 1212 kg

Answer 3

If you understand 2, I think you can do 3

You need to look up heat capacity of water in Joules/kg deg C

Answer 4

I got 12.88% for the efficiency if I used 27 m for the height. If I use 218 for the height I got 96.1%

I understand how you did 2 but I have doubts about my answer for 3

I got 1.95% for efficiency...

Answer 5

water is 4186 Joules/kg deg C

110,000 kg
8 deg C
so heat out into water = 4186*110,000*8
= 3.68 * 10^9 Joules into water

electricity in = 150,000 * 28 * 3600
= 1.51*10^10 Joules

efficiency = out/in = .368/1.51 = .244
= 24.4 %

Answer 6

Does the heat capacity vary? My book said it is 4181 J

Answer 7

LOL, yes, depends on purity of water etc. Those numbers are the same to me :)

Answer 8

Thank you!

Answer 9

You are welcome.