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