chapter 10 temperature

7/28/2019 Chapter 10 Temperature

1/12

We intuitively know what hot

and cold is, but our senses can

be fooled.

For example, if we touch a metal

ice tray and a plastic cup insidethe refrigerator, the ice tray feels

colder than the plastic cup even

though both items are at the

same temperature.

In order to study thermal physics,

we must have an objective and

reliable way of measuring

temperature.

10Chapter

temperature


2/12

Chapter 10: Temperature


3/12

TemperatureTemperatureis a physical property of a body that is related

to its degree of hotness.

Thermal Energyis a form of kinetic energy related to themotion of the atoms and molecules of a body.

Other than kinetic energy, thermal energy can also be in the

form of radiation from hot objects.



4/12

The particles in a body with a higher temperature will move

faster than the particles in a body with lower temperature.

This is known as the kinetic theory of matter. The theoryassumes that all matter consists of particles that are in

continuous motion.

Temperature at the Microscopic Level

Matter is made of small particles consisting of atoms ormolecules.



5/12

A Laboratory Thermometer

Liquid-in-glass type of thermometer

Thermal expansion of the mercury results in an increase in the

length of the liquid mercury thread in the capillary.

Length of the mercury thread thermometric property



6/12

Mercury is chosen because:

its temperature range (from 39 C to 357 C) is suitable for

laboratory use;

it can be easily seen (it is opaque); its volume changes almost uniformly with temperature;

it does not stick to glass.

Defined as the change in its thermometric

property per degree change in temperature.

The wall of the thermometer bulb is thin so that heat exchange between the

bulb and surrounding can occur quickly (fast response time).

The bulb has a large volume so that the mercury thread changes by a

large amount with a small change in temperature (increased sensitivity of

the thermometer).



7/12

Calibration

Calibration is the process of establishing the relationship between a

measuring instrument and the physical quantity it is measuring.

For the laboratory thermometer, calibration relates the length of

the mercury thread to the temperature of the bulb.

Material A has a thermometricproperty that varies linearly

with temperature. This will

simplify calibration as it only

requires two fixed points for a

complete calibration.

Material A and B are suitable for use as thermometers.



8/12

The Celsius Scale

In the Celsius scale, the lower fixed point is the temperature of

melting pure ice and the upper fixed point is the temperature wherepure water and steam are in equilibrium.

The fixed points will be the same under given conditions (e.g. at one

standard atmospheric pressure).


Setup to determine the lower and upper fixed points


9/12

Clinical ThermometerA clinical thermometer is designed to measure body temperature

when its bulb is inserted into the mouth of a person.

A constriction is specially built into the capillary tube.

When the thermometer is removed from the mouth, the lower temperature outside

the body causes the mercury to contract. The mercury thread breaks at the

constriction allowing the observer to read the maximum temperature without

worrying about the thread moving due to the environments lower temperature.



10/12Chapter 10: Temperature


11/12

Advantages of Thermal Expansion

Bimetallic Strip

The bimetallic strip is arranged so

that it will bend upwards to close theelectrical circuit when heated,

thereby activating the electric bell.

The bimetallic strip is arranged sothat it will bend downwards to open

the electrical circuit when it gets too

hot, thereby regulating the

temperature of the oven.


The bimetallic strip is made of two pieces of different metals, such as brass andsteel, riveted together. When heated, brass will expand more than steel causing

the bimetallic strip to bend.


12/12

A 1.0 m long water pipe made of steel will expand by1.3 105 m for every degree Celsius rise in temperature.

Imagine a 10 km long water pipe undergoing a temperature

change of 10 C. The change in the length of the pipe would

amount to 1.3 m!

Such an expansion could cause severe damages to the pipe.


Disadvantages of Thermal Expansion

chapter 10 temperature

Documents