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you can enjoy the air conditioner, refrigerator, and another heat machines. Let’s go to li rary to explore more.
Reading Room
A. Thermodynamics System
In thermodynamics, an amount of matters that we observe is called
system
. And everything arround it is called environtment. If there is no heat transfer into or out of the system, the system said to be thermally
isolated system. When you learn about heat and temperature, you find that heat is
generated by a change of temperature. Heat transfers from higher temperature to the lower temperature. So, heat is a form of energy
transfer.
B. The First Law of Thermodynamics
If a gas with constant volume is heated, the temperature will increase. So the molecules of gas move faster than before. It cause more collution
between molecules and wall. The collutions cause the pressure of gas increase, also the kinetic energy. It means the internal energy also
incrrease. In order to raise gas temperature, an amount of heat Q is needed. If
an amount of heat is added to the system, the heat will be used to do work. But there is some of heat is used to raise internal energy of the
system. If an amount of heat is given to the system, the internal energy of the system will increase.
The heat given to the system is expressed by equation Q =
∆U + W Q : heat transferred to or from the system J
∆U : change of internal energy J W : work done by system J
Real power does not hit
hard , but straight to
the point
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The equation above is the formulation of first law of thermodynamics. We have to follow some rules to apply first law of thermodynaics. The
rules are 1.
If
heat transferred to
the system, value of Q is
positive
+Q. If heat
transferred from
the system, value of Q is
negative
-Q. 2.
If the
system do work , value of W is
positive +W. If the
system accept work
, value of W is negative
-W. You can also find work done by system using curve. Work done is equal to
under the process curve on a P-V diagram. P
V V
1
V
2
Now let’s go to al o a d take a look at application of first law of
thermodynamics.
Balcony A worker with weight 65 kg shovels coal for 3 hours. During the shoveling,
the worker did work at average rate of 20 W and loss heat to the environtment at average rate of 480 W. How much fat will the worker
lose? Energy value of fat E
f
is 9.3 kcalg.
Solution
Listing the given value, then converting power to owrk and heat. Given: W = Pt = 20 W3h3600s = 2.16 x 10
5
J Q = - 480 W3 h3600 s = -5.18 x 10
6
J Q is negative because heat is lost
E
f
= 9.3 kcalg = 9.3 x 10
3
kcalkg = 9.3 x 10
3
kcalkg4186 Jkcal
10
= 3.89 x 10
7
Jkg Find: mass of fat burned
Answer from the first law of thermodynamics, Q = ∆U + W, we have
∆U = Q – W = - 5.18 x 10
6
J – 2.16 x 10
6
J = - 5.4 x 10
6
J The mass of fat loss is
m =
=
= 0.14 kg
follow up exercise
How much fat will be lost if the worker were playing basketball, doing work at rate of 120 W and generating heat at rate of 600 W?
Hint : the answer is 0.2 Kg Living Room
Lapindo and First Law of Thermodynamics
The example of The first Law of Thermodynamics is
mud spray in sidoarjo. The mud inside the earth
blows out because it was at a higher pressure than
the atmosphere. So it did positive work on its
surroundings. So its internal energy decreased negative.
∆U = Q – W. Because the work is
positive, and ∆U is negative, so the Q is zero. The reduction in internal
e e g ill ause ud’s s oke.
Reading Room
C. Thermodynamics Processes for an Ideal Gas
