Bukhovtsev-et-al-Problems-in-Elementary-Physics

70 PROBLEMS
What quantity of heat will be liberated by the refrigerator
in a room during this time if the heat capacity of the refrigerator
can be neglected?
303. Will the temperature in a room drop if the door of a
working refrigerator is opened?
304. It is easiest to heat premises by means of electric appliances.
Is this method the most advantageous from the energetic
point of view?
305. Equal quantities of salt are dissolved in two identical
vessels filled with water. In one case the salt is one large crystal
and in the other-powder.
In which case will the temperature of the solution be higher
after the salt is completely dissolved, if in both
cases the salt
and the water originally had the same temperatures?
306. It is known that if water is heated or cooled with certain
care it can retain its liquid state at temperatures below
O°C and higher than + 100°C.
A calorimeter with a heat capacity of q = 400 cal/deg contains
m, = 1 kg of water cooled to t 1
= -10°C. Next m. = 100g
of water overheated to /2 = + 120°C is added to it.
What is the temperature in the calorimeter?
307. An incandescent lamp consuming P = 54 watts is immersed
into a transparent calorimeter containing'V = 650 em! of
water. During ~ = 3 minutes the water is heated by t = 3.4°C.
What part Q of the energy consumed by the lamp passes out
of the calorimeter in the form of radiant energy? .
308. The area of a brick wall facing a street is A = 12 m S
and its thickness d = 1 metre. The temperature of the outside
air T o=-15°C and that of the air in the room T= + 15°C.
What amount of heat is lost from the room in 24 hours? The
coefficient of thermal conductivity of brick is A= 0.003 cal/ern X
X s-deg,
309. A wall consists of two adjoining panels made of different
materials. The coefficients of thermal conductivity and the
thicknesses of the panels are At' d 1
and A 2 , d 2
, respectively
(Fig. 123). The temperatures of the external surfaces of the
wall are T 1 and To (where To> T.) and are kept constant.
Find the temperature T" on the plane between the panels.
.
310. Assuming in Problem 309 that the panels have the same
thickness d, determine the
of the wall.
coefficient of thermal conductivity