Page 121 - Physics - XI

Sources of Error

1. Surrounding temperature may not be constant.
2. Lid may not be airtight.

VIVA VOCE

Q1. On what factors does the rate of loss of heat of a body depend?
Ans. It depends upon

(i) nature and extent of the body
(ii) temperature of the body
(iii) temperature of surroundings.

Q2. Name the diff erent modes of transmission of heat.
Ans. Diff erent modes of transmission of heat are conduction, convection, and radiation.
Q3. What is radiation?
Ans. It is the mode of transmission of heat in which heat travels from one place to another place without
an intervening medium.

Q4. State Newton's law of cooling.
Ans. The rate of cooling of a body is directly proportional to the diff erence in temperature of the body
and its surroundings provided the temperature diff erence is not too large.

Q5. Explain the law which replaces Newton's law of cooling if the temperature diff erence between
hot body and surroundings is large.
Ans. It is the Stefan's law. It states that amount of heat energy radiated per unit area per second by a body
is directly proportional to the fourth power of its absolute temperature.

Q6. How is Newton's law of cooling diff erent from Stefan's law of heat radiation? (NCERT)
Ans. According to Stefan's law of heat radiation, "The total amount of radiant energy emitted by unit area
of perfectly black body in one second is directly proportional to the fourth power of its absolute
temperature."
i.e. E ∝ θ 4

While, according to Newton's law of cooling, the rate of cooling of a body is directly proportional
to the temperature diff erence between the body and its surroundings provided that the temperature
diff erence is small.

d
i.e. ( )
dt 0

Q7. Find the specifi c heat of a liquid/solid by using Newton's law of cooling apparatus. (NCERT)
dQ
Ans. According to Newton's law of cooling, ( and we know that for a body of mass m,

)
dt 0
specifi c heat s, temperature θ kept in surroundings of temperature θ 0
dQ d
Therefore, rate of cooling ms . So, by this formula, the value of specifi c heat (s) can be
calculated. dt dt

119