1) Stephanie is boiling water in a pot with diameter 20cm. The base of the pot s

Question

1) Stephanie is boiling water in a pot with diameter 20cm.The base of the pot she is using has a thick stainless steel base of 1cm. Stainless steel has a thermal conductivity of ~17Wm-1K-1. If the flame is 400°C and the water is 25°C, at what rate is energy flowing from the flame to the water? What type of heat flow is this?

b) How long will it take to get the water to 100°C (Assuming no heat flows out of the water)

2) A person’s skin has emissivity e = 0.90 and area of 1.1 m 2 exposed to the environment. What is the rate (in watt) at which thermal radiation is absorbed by the skin if the environment is at 306K (33°C) and the skin is at 302K (29°C)?

Explanation / Answer

Thickness of base of pot, x=1cm = 0.01 m.

Diameter of base of pot, d=20cm

Radius of base of pot, r=d/2=10cm = 0.10m.

Area of base of pot, A=r2 = 3.14*(0.10)2 = 0.0314 m2 .

Thermal conductivity of stainless steel, K=17 Wm-1 K-1

Temperature of flame, 1=400oC

Temperature of water, 2=25oC

As

            Rate of energy flow(q)=KA(1 - 2) / x

                                                    q =17* 0.0314 * (375)/0.01

                                                     q=20017.5 W

So, Rate of energy flow is 20017.5 W

Type of heat flow = conduction.

Part b,

Heat required to raise the temperature of water from 25oC to 100oC is

            Q1=mCdT=1*4.184*(75)=313.8 J

Time required to raise the temperature of water from 25oC to 100oC is

            T=Q1/q   =313.8/200017.5 =1.569*10-3   s.

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