A.) A styrofoam cup is so light and it is such a good insulator, that its mass a

Question

A.) A styrofoam cup is so light and it is such a good insulator, that its mass and heat capacity may often be neglected in measurements like this one, thus making the analysis much simplier. If 150 Grams of water at room temp, 20 C, is placed in a styrofoam cup, how long will it take to raise the temperature of the water to 30 C? Assume the power supply a current of 2 amperes at voltage of 10 V. Show all work.

b.) IN part A, if the water is replaced by 150 grams of ethylene glycol whose specific heat is 0.57 kcal/kg/C, what will the time be?

Explanation / Answer

Part A)

Power = Enegry/time and also P = IV,

Thus IV = Q/t

IV = mc(delta T)/t

(2)(10) = (.150)(4186)/t

t = 31.4 sec

Part B)

Same idea, new specific heat

.57 kcal converts to 2384.88 J/kgK

(2)(10) = (.150)(2384.88)/t

t = 17.9 sec

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