Physics questions

2009-03-28 6:49 am

1. Tommy is preparing a cup of 250ml 85 degree Celsius coffee.
(Given: Mass of 1 ml of coffee/ milk = 1g.
Specific heat capacity of milk = 4800 J /kg/C
Specific heat capacity of coffee = 4200 J/kg/C
Specific heat capacity of copper = 370 J/kg/C)
(a) When he adds 50ml 5 degree Celsius milk to the coffee, what is the final temperature of the coffee?
(b) Then he stirs the coffee with a 100 g copper teaspoon at 20 degree Celsius. What is the temperature of the coffee when he stirs the coffee?
(c) Some people prefer to add suger before milk. Can you suggest a reason for that? Why don't they do the other way round?

2. An electric heater has two settings: 'Low' and 'High'. The power output of the heater is 1400 W at the 'Low' setting and 2200 W at the 'High' setting. The heater is uesd to cook an egg. The water and the heater is operated at the 'High' setting. The temperature of the water is recorded every 30 s.
(a) After the water boils, the heater is turned to the 'Low' setting and the water still boils afterwards. A student argues that this will lengthem the time required to cook the egg. Do you agree? Explain your answer.

回答 (1)

天同

2009-03-28 8:27 am

✔ 最佳答案

1.(a) Let t be the final temperature
Heat gained by milk = (50/1000) x4800x(t-5) J
Heat lost by coffee = (250/1000) x 4200 x (85-t) J
Since heat lost by coffee = heat gained by milk
(50/1000) x4800x(t-5) = (250/1000) x 4200 x (85-t)
solve for t'
(b) Let t be the final temperature of the coffee.
Heat agined by copper spoon = (100/1000) x 370 x (t'-20) J
Heat lost by coffee = (250/1000) x 4200 x (t-t') J
hence, (100/1000) x 370 x (t'-20) = (250/1000) x 4200 x (t-t')
solve for t', given that t is found in (a)
(c) More sugar could be dissolved in hot coffee than in cold. If milk is added to the coffee before sugar, the temperature of the coffee will be lowered by the milk, thus making less sugar could be dissolved.
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2. No. The temperature of the water will no longer rise even the heater is turned to 'High' setting. It is because during boiling, all heat energy supplied by the heater is absorbed by the water to prvide for the necessary latent heat of evaporation. The temperature will not go beyond 100 C during boiling. Hence, the egg will not be cooked faster.

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