Q1. An intrepid backpacker has found herself camping outside overnight. Unfortun

Question

Q1. An intrepid backpacker has found herself camping outside overnight. Unfortunately, the forecast indicates that the temperature will drop to 0C, and the backpacker doesn’t have a tent. To make matters worse, winds are predicated to increase to 30 km/hr during the early morning. The backpacker’s body should maintain a core body temperature of 37C. Assume that the backpacker’s exposed skin and outer clothing is at a temperature of 34C, and has a surface area of 1.50 m2.

(a) Air convection plays a major part in heat loss, and poses a serious danger to the backpacker.

i) Calculate the rate of heat loss due to the convection of air when the air is still during the night

(coefficient of air convection heat transfer is 10.0 Js1 m2 K1).

ii) Calculate the rate of heat loss due to air convection when the night gets windy (coefficient of air

convection heat transfer is 35.0 Js1 m2 K1).

(b) Fortunately, the backpacker has a goose down sleeping bag, which prevents the movement of air directly on the backpacker’s body. Using the sleeping bag, the major mechanism for heat loss is now conduction through the sleeping bag. Calculate the heat lost through the sleeping bag. The thermal conductivity of goose down is 0.02 W m1 K1, and the thickness of the sleeping bag is 1 cm. The sleeping bag is quite close fitting, so you can assume the surface area is essentially the same as the surface area of the backpacker’s body.

(c) A ‘Space Blanket’ is a light, thin blanket made from reflective materials, and often included in many emergency or first aid kits. Discuss how the blanket retains heat in the body and why and when it would be useful.

Explanation / Answer

a) Air convection:

(i) Rate of heat loss can be given as:

Q = hA(T)

h = 10 Js-1m-2K-1

A = 1.50 m2

T = (34+273)k- (0+273)k = 34 k

i.e. Q= (10)(1.50)(34) = 510 J/s

(ii) dQ= hA(T)

h = 35 Js-1m-2K-1

Q= (35)(1.50)(34) = 1785 J/s

B) Conduction

Heat loss due to conduction :

Q = (KA/t) [ T]

K = 0.02 Wm-1k-1

A = 1.50 m2

t = 1 cm = 0.01 m

T = 34 k

Q = (0.02×1.50/0.01)× (34) = 102 J/s

C) As heat can not easily passed through blanket . It acts as somewhat insulator. Hence body gets ward due to its own heat , ( heat generated by body ) since it not passed through blanket easily.

Related Questions

Navigate

• Browse (All)
• Browse Q
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.