Phy.(15mark)

chi ming

2009-04-26 4:20 am

The iron is automatically awitched off when the ironing plate is heated to200℃.When the 'jet steam' button is pressed once , 20 g of water at 28℃ will be pumped onto the heated plate at 200℃and the water is converted into steam at 100℃ instantly.
given: specific heat capacity of water=4200 Jkg^-1℃^-1 , specific latent heat of vaporization of water=2.2610^6 J kg^-1
a)
1. estimate the energy required to produce 20g of steam from water at28℃
2.Given that the heat capacity of the ironing plate is 637J℃^-1.Estimate the temperature of the plate just after producing 20 g of steam.
b) the iron cannot convert 40 g of water at 28℃ to steam when the button has been pressed twice quickly. Explain why.

回答 (1)

老爺子

2009-04-26 4:51 am

✔ 最佳答案

a)1.
Energy required to produce 20 g of steam from water at 28oC
= Energy needed for 20 g (28oC water → 100oC water) + (100oC water → 100oC steam)
= (20/1000)(4200)(100 - 28) + (20/1000)(2.26 x 106)
= 51248 J

a)2.
Temperature drop of the plate
= 51248/637
= 80.5oC

Temperature of the plate after producing 20 g of steam
= 200 - 80.5
= 119.5oC

b)
If 40 g of 28oC is converted to steam, final temperature of the plate
= 200 - 2(80.5)
= 39oC

It is impossible because this temperature is far below the boiling point of water.

Therefore, the iron cannot convert 40 g of water at 28oC to steam.
=

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