S3 Physics‎ > ‎

### Chapter2

 2.2  Heat and Energy TransferClasswork 3Q1 (P30 of textbook)What is the energy consumed by a 2-kW heater in 30 minutes.P = 2000Wt = 30x60 = 1800 sEnergy = 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 followinga)    Switch on a light of 100W for 10 hours.b)    Switch on a air conditioner of 2000W for 8 hours.       Calculate Q in Joule ONLYc)    Use a oven of 900W for 5 minutes       Calculate Q in Joule ONLYd)    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 \$ 0.8b)    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 MJc)    Use a oven of 900W for 5 minutes       Calculate Q in Joule ONLY       Energy = 900 x 5 x 60 = 270000 Jd)    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 = \$ 144S3B. Lab on next lesson (Day 8 Nov 27)        Test on Dec 11 (Day 8) - Chapter 2S3C Lab on next Day 4 (Dec 4)       (Test on Dec 11)https://www.clponline.com.hk/MyBusiness/CustomerService/TariffOverview/GeneralServiceTariff/Pages/Default.aspx?lang=enIn electricity bill, 1 unit (一度) = 1kWh