CHAPTER 4HEAT

4.1 UNDERSTANDING THERMAL

EQUILIBRIUM

Learning Outcome :

• Explain thermal equilibrium,

• Explain how a liquid-in-glass thermometer works

Cold milkWarm water

How to heat up the milk ?

What changes has occurred ?

Temperature increases

Temperature decreases

Cold milk

Warm water

Faster rate

slower rate

Energy transferred

Net energy transferred (heat transferred) from water to milk.

What happen after sometime ?

Cold milk Warm water

Warms upCool down

No net heat transfer

The two objects are in thermal equilibrium

Thermal equilibrium

Common properties :No net heat transfer between the

objects Objects have the same temperature.

Instrument to measure temperature

Liquid-in-glass thermometer

• volume of liquid increases when it absorbs heat.

• Length of liquid column is directly proportional to the change of temperature.

Principle used :

How to measure temperature ?

The thermometer must be in thermal contact with the object.

Heat transferred occurs between the object and the liquid in thermometer. (From hot object to cold object).

After some time, the objects will reach thermal equilibrium. Both objects have the same temperature.

Therefore thermometer reading is the temperature of the object.

How to calibrate a thermometer ?

Liquid used : mercury can measure -39oC to 357oC

alcohol can measure -115oC to 78oC

Divide equally into 100 divisions.

How to determine temperature if division are not marked ?

0

100 0

, 100oTemperature x C

Worked example

0

100 0

, 100oTemperature x C

The distance between the 0o C and 100oC scales marked on a thermometer is 28.0 cm. What is the temperature of water in container ?

Solution:

24.5100

28.0ocm

x Ccm

= 87.5 o C

EVALUATION

Two objects X and Y are brought into thermal contact. Initially X is hotter than Y. Indicate whether each of the statements is true (T) or False (F).

1. (a) There is transfer of heat energy between X and Y

(b) The temperature of X decreases.

(c) Y gives out more heat energy than it receives.

TT

F

EVALUATION

2. When X and Y have come into thermal equilibrium,

(a) Transfer of heat energy between X and Y

stops.

(b) The final temperature of X is higher than the initial temperature of Y.

(c) Heat energy is exchanged between X and Y at equal rates.

F

F

T

EVALUATION

Which of the following can increase the sensitivity of the thermometer ?A a large bulb B a long stem C a wide bore D a thin bulb wall

bulb stem bore

Diagram shows a mercury thermometer.

Exercise

Mastery Practice 4.1 pg 110

Questions 1, 2, 3