有關physics的計數問題F.3

2008-12-21 7:10 am

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(ANS IN ENGLISH PLZ..)

1. A piece of aluminium of mass 0.04kg is heated in a Bunsen flame for 2 minutes. The aluminium block is transferred to a cup of water of mass 0.2kg. The temperature of the water is found to increase from 25℃ to 55℃. What is the temperature of the Bunsen flame?
(Specific heat capacity of aluminium = 900Jkg-1℃-1)
(Specific heat capacity of water = 4200Jkg-1℃-1)
PS:Jkg-1℃-1 ge -1應該系右上角,但系唔知點解整唔到^^"

2.A student performed an experiment to measure the specific heat capacity of water using the apparatus. The power of the heater is 1500W and the mass of water is 1 kg. The heater is switched on for 3 minutes and then switched off.
Explain why the temperature continues to increase a few time after the heater is switiched off.

回答 (1)

Uncle Michael

2008-12-21 8:53 am

✔ 最佳答案

1.
Temperature of the Bunsen flame = ToC

Assume that the aluminium block is at ToC when it is heated in the Bunsen flame for 2 minutes.
When the aluminium block is put into water, the final temperature of the block is 55oC.

When the aluminium block is put into water:
Heat loss from the aluminium block
= m c ΔT
= 0.04 x 900 x (T - 55) J
= 36 x (T - 55) J

Heat gain by water
= m c ΔT
= 0.2 x 4200 x (55 - 25) J
= 25200 J

Assume that there is no heat loss to the surroundings.
Heat loss from the aluminium block = Heat gain by water
36 x (T - 55) = 25200
T - 55 = 700
T = 755

The temperature of the Bunsen flame = 755oC

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2.
When the heater is switched on, current passes through the heater and heat is produced to increase the temperature of the heater. Thus, the temperature of the heater is much higher than that of water. This leads to a heat flow from the heater to water, and this would heat up the water and lower the temperature of heater.

When the heater switched off after 3 minutes, the heater is still at a higher temperature than water. Therefore, heat is continuously flowing from the heater to the water for some time until they have equal temperatures.
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