2102 ‫ مايو‬، ‫الفصل الدراسي الثانى‬
‫الفرقة األولى‬
‫جامعة بنـها‬
‫كلية العلوم‬
‫ ساعتان‬: ‫الزمن‬
Heat : ‫مادة االمتحان‬
Heat
4 A- At what temperature do the Fahrenheit and Celsius scales give the
same reading ?
Solution
Using the formula
C /100 = F-32 / 180
Here C = F so
C /100 = F- 32 / 180
Or
18F = 10F – 320
F = - 40o #
B- Define Latent Heat (3 types) , units and equations?
The latent heat of fusion of a substance is the amount of heat required to
convert unit mass of the solid into the liquid without a change in
temperature".
Q = mLf
The latent heat of vaporization of a substance is the amount of heat
required to convert unit mass of the liquid into the vapor without a change in
temperature
Q = mLv
5 A- two hundred thousand joules of heat is removed from 25kg block
of ice initially at -5oC. What is its final temperature? (cice =
2110J/kg oC).
solution
Q = 200000 = 2 × 105 J
T1 = -5 oC
Q = mc

m= 25kg
T2 = ?
T = mc (T2-T1)
-2 x 105 = 25 x 2110 x [T2-(-5)]
T2 = 8.8oC
B- write the fundamental law of heat conduction, and define the thermal
conduction?
dQ
dT
H



kA
the fundamental law of heat conduction is
dt
dx
thermal conduction is the rate of heat flow by conduction per unit area
per unit temperature gradient.
6 A- Drive the Newton law of Cooling?
It states that the rate of heat loss of a body is proportional to the
difference of temperature between the body and the surroundings. Also, the
loss of heat depends upon the area of the exposed surface. The law is
dQ
α  A (T  Ts )
dt
dQ/dt = -k A (T-Ts)
where Q is the thermal energy transfer in joules, k is the heat transfer
coefficient, t is temperature of the object, and Ts is the temperature of the
surroundings. The negative sign is due to the heat lost.
Consider a body of mass m, specific heat c, and at temperature T,
suppose the temperature falls by a small amount dT. Then the amount of heat
lost is
dQ = m c dT
so the rate loss of heat is
dQ /dt = mc dT/ dt
from eqs. (3) and (4)
me dT/ dt = - k (T-Ts)
or
dT / dt = kA/ me (T- Ts)
or
dT/ dt = - r (T- Ts)
where r is appositive constant characteristic of the system, which must be in
units of 1/ time, and is sometimes expressed in terms of a time constant r=1/
to. By rearranging the last equation we get.
B- Define the Heat capacity and Specific Heat and there units?
Since different substances have different capacities for heat, as indicated
above, they can be distinguished by a quantity known as heat capacity of a
body.
Heat capacity The heat capacity of a body is the quantity of heat
required to raise the temperature of the whole of the body through one
degree. If m is the mass of the body and c its specific heat, its heat capacity
is, C = mc
Specific heat : is defined as the quantity of heat required to raise the
temperature of one gram of substance through 1 oC.The specific heat of a
substance is not constant and it is different at different temperatures.
Ordinary, the specific heat determined is the mean specific heat. From eq.
(1), the mean specific heat c is given by
C = Q/ m

T