THERMAL EQUILIBRIUM
Lesson #4
Objectives
Define thermal equilibrium
Solve sample problems involving thermal equilibrium
Apply thermal equilibrium in daily life
Toon Tales
What’s the title
of the lesson
again, Sheldon?
Mrs. Puff,
can we go
back to the
1st slide?
Quiet now children!So before we begin, eyes on the board please.Study the illustration.
A thermometer is placed on the forehead of a kid with a fever.
98 °F
After 30 seconds
The thermometer reads 98 °F
The thermometer was not removed until the time reached 5 minutes.
101 °F
After 5 minutes
The thermometer now reads 101 °F
Why did the temperature change after some time?
Easy! Because they’re not yet in thermal equilibrium!
Bu..Bu..But..I, I can’t
understand! Why?!
What!? Remember?
When two substances having different temperatures are introduced or kept together, heat flows from a substance at higher temperature to a substance at lower temperature.
To make it more correct, heat continues to be transferred until their
temperatures are equalized. When this stage is achieved, only we can say that we can say that the substances are
in thermal equilibrium.
Oohh.. So that explains why the liquid inside the thermometer rises as time
passes by.
Yes! That is why we need to wait for a few minutes before
taking the reading of the thermometer.
Of course. With that, the kid’s body and the thermometer will reach the same temperature
Hey! You keep on finishing my sentences! Why are you
late anyway?
So what? I already understand the lesson.
You… you!!!!! Darn!You’ll pay for this!!!You’ll see! One day, I
will beat you!! Whatever.
Timmy.
Holy!! My water is now getting warmer!
Help me Timmy!!!
Don’t worry doggy, we’ll get you some ice for you later Really?
Thanks
He he he… They
don’t know I have
fairy godparents!
But I wonder how
many ice will make the juice
cold again?
COSMO!!! WANDA!!!Where are you guys!?!?
But it seems that you lost your magic powers..
Timmy needs help…
..and equipped only with the knowledge of thermal equilibrium.
How are you going to help him?
First you need to consider the condition of Doggy’s drink.
Ti = 15 °Cm = 200 g
How much ice would be enough to cool down the temperature of the water? (mass)
Tf = 10 °C
The mixture of ice and water would eventually reach thermal equilibrium.
However, they must be in a closed system so that heat will completely transfer from one body to another.
Or simply, it must follow the Law of Conservation of Energy
ΔE = 0 No heat will flow out of the system
Q + Q = 0 Qlost + Qgained = 0
Qgained = - Qlost
(where Q = mcΔT)negative denotes heat released
Can you solve the problem of Timmy?
Do you now have what it takes?
Well then, let us see if you can use your skills in problem solving.
Or do you wish to give up?
Ti = 15 °Cm = 200 g Ti = 0 °C
m = ?
Tf = 10 °C
MIXED
Qgained = - Qlost
mcΔT = - (mcΔT) mici(Tf-Tii) = - [mwcw(Tf-Tiw)]
“i” subscript for ice “w” subscript for water
A 200 g sample of water at 353 K is mixed with 200 g of water at
283 K. What is the final temperature of the
mixture?
Test Yourself
3. A 100-g Aluminum block at 373 K is placed in 100 g of 281 K water. The final temperature of
the liquid is 298 K. What is the specific heat
of aluminum?
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