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Chapter2

2.2  Heat and Energy Transfer

Classwork 3

Q1 (P30 of textbook)
What is the energy consumed by a 2-kW heater in 30 minutes.

P = 2000W
t = 30x60 = 1800 s
Energy = Power x Time = 2000 x 1800 = 3600000 J
      

Q2.(P30 of textbook)
An immersed heater rated at 50W heats water in a polystyrene cup. How long does it take to transfer 15000J of energy to the water? 

t = 15000 / 50 = 300 (s)


Q3 Assume each unit of electricity (kWh) cost $0.8, Find the energy consumed in J and kWh and the cost for the following

a)    Switch on a light of 100W for 10 hours.
b)    Switch on a air conditioner of 2000W for 8 hours.
       Calculate Q in Joule ONLY
c)    Use a oven of 900W for 5 minutes
       Calculate Q in Joule ONLY
d)    Run a computer of 250W for 30 days.




a)    Switch on a light of 100W for 10 hours.
       P = 100 W = 0.1 kW
       t = 10 hours = 36000 s
      i)  Q = 100 x 36000 = 3600000 J
      ii) Q = 0.1 kW x 10 hours = 1 kW h
      iii) It costs $ 0.8

b)    Switch on a air conditioner of 2000W for 8 hours.
       Calculate Q in Joule ONLY
       Energy = 2000 x (8 x 3600) = 57600000 J  
                   = 57.6 MJ


c)    Use a oven of 900W for 5 minutes
       Calculate Q in Joule ONLY
       Energy = 900 x 5 x 60 = 270000 J

d)    Run a computer of 250W for 30 days.
         P    = 250W = 0.25kW
         t     = 30 days  = 30 x 24 = 720 hours
               = 720 x 3600 = 2592000 s
        i.     Q = P x t = 250 x 2592000 = 648000000    (J)
        ii.    Q = P x t = 0.25 x 720 = 180 kWh
        iii.   It costs 180 x 0.8 = $ 144




S3B. Lab on next lesson (Day 8 Nov 27)
        Test on Dec 11 (Day 8) - Chapter 2


S3C Lab on next Day 4 (Dec 4)
       (Test on Dec 11)


In electricity bill, 1 unit (一度) = 1kWh

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