Question about Physics and maths

Johnny Lam Cheuk Yin

2007-10-12 1:22 am

1. A gas cooker can supply energy at rate of 15 MJ per hour . How long would it take to increase the temperature of 2 kg of water from 25C to 100C ?
2.A 1200W electric water heater is used to heat up 1.5kg of water . It takes 8 minutes to increase the temperature of the water from 20C to100C
a)calculate the energy absorbed by the water .
b)calculate the energy consumed by the electric heater
c)Explain why there is such a different between the results obtained in part (a) and part (b)

回答 (1)

Audrey Hepburn

2007-10-12 2:03 am

✔ 最佳答案

1. Mass of water, m = 2 kg

Specific heat capacity of water, c = 4200 Jkg-1℃-1

Temperature increase of water, △T = 100 – 25 = 75℃

Power of the cooker, P = E / t = 15 X 106Jh-1 = 4166.67 W

So, By E = mc△T

Pt = mc△T

4166.67t = 2 X 4200 X 75

time, t = 151 s

2. Mass of water, m = 1.5 kg

Specific heat capacity of water, c = 4200 Jkg-1℃-1

Temperature increase of water, △T = 100 – 20 = 80℃

Energy absorbed by the heater, E = mc△T

E = 1.5 X 4200 X 80

E = 504 000 J

b. Energy supplied = Pt

= 1200 X (8 X 60)

= 576 000 J

c. The energy supplied by the heater is greater than the energy absorbed by the water since some energy supplied is dissipated to heat and lost to the surroundings.

參考: Myself~~~

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