1)The \"steam\" above a cup of tea is water droplets condensing after evaporatin

Question

1)The "steam" above a cup of tea is water droplets condensing after evaporating from the tea. What is the temperature of 240 g of tea initially at 98.5°C after 2.45 g evaporates from it? The tea is in an insulated cup, so ignore other methods of heat transfer. (Use the heat of vaporization for water given in OpenStax College Physics.)

    °C

2)Find the net rate of heat transfer by radiation from a skier standing in the shade, given the following. She is completely clothed in white (head to foot, including a ski mask), the clothes have an emissivity of 0.194, a surface area of 1.50 m², and a surface temperature of 10.9°C, and the surroundings are at -20.0°C.



    W

2b)Suppose the skier stays outside in the shade for 4 hours. How many kilocalories of heat would she lose?



    kcal

Explanation / Answer

rate of energy transer H = dQ/dt = Ae (sigma) T^4

where A = area = 4pir^2, e= emissitivity = (1 for black bdoy)

sigma = constant = 5.67*10^-8

T = tempoeratrue in kelvin

emissivity e =0.194

To = 10.9 + 273.15 = 273.15+11 = 284.15oK

Ts = -20 + 273.15 = 253.1 K

heta transfer by radiation: Q = e*A*Sb*(To4 - Ts4 )

H = dQ/dt = 0.194*1.5*5.67*10^-8*(284.15^4 - 253.1^4)

H = 39.85 W

Energy lost = 39.85 *4 *3600 = 573840 J

1kcal =4184J

1J= 1kcal/4184

Energy lost = 573840/4184

Elost = 137.15 k cal

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