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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