Thursday , December 8 2016
Revision Notes

Temperature

Teperature:

It is the degree of hotness or coldness. As it refers to how hot or cold the object is .

Heat:

Heat refers to the amount of thermal energy that is being transferred from a hotter to a cooler region.

Constructing A Thermometric Scale:

  1. Choose a thermometric substance.
  • Mercury or alcohol
  1. Select two fixed points.
  • Ice point – as the upper fixed point.
  • Steam point – as the lower fixed point.
  1. Divide the temperature range between the two fixed points into equal divisions.

 

Ice point (lower fixed point):

This is the temperature of pure melting ice at a pressure of one standard atmosphere. It is assigned a value of 0°C

 

Steam point (upper fixed point)

This is the temperature of steam from water boiling at a pressure of one standard atmosphere. It is assighned a value of  100 °C

 

Centigrade Scale:

A Temperature scale that defines the freezing points of water as 0 °Cand the boiling  points of water as 100 °C

Determination Of Ice Point :

  • Immerse the bulb of the thermometer into a funnel containing pure melting ice.
  • The mercury level in the stem should be just above the ice.

Determination Of Steam Point:

Insert the thermometer into the apparatus. The bulb of the thermometer should be above the boiling water.

Manometer is used to ensure pressure inside the apparatus is same as the atmospheric pressure outside.

 

Measuring tempereature using centigrade scale:

Centigrade Scale Temperature

Advantages And Disadavantages Of Using Mercury As A Thermometric Liquid:

Advantages And Disadavantages Of Using Mercury As A Thermometric Liquid

Advantages And Disadvantages Of Using Alcohol As A Thermometric Liquid:

 

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Responsiveness:

This measures how quickly the thermometer can register changes in temperature.

Sensitivity:

This measure the amount of changes in the thermometric property (e.g. length of mercury column) per unit change in temperature.

Range:

This denotes the minimum and maximum temperature that the thermometer can measure.

 

Laboratory Thermometer:

Laboratory Thermometer

Mercury is contained in a thin walled glass bulb:

Thin wall of the glass bulballows conduction of heat quickly through the glass(poor conductor) to the liquid.

 

The bulb is made small to contain a small amount of liquid:

A smal amount of liquid responds more quickly to changes in temperature.

The bore of the capillary tube is fine and uniform:

The fine tube allows a noticeable movement of the liquid column for a small change in temperature. (good sensitivity).

Uniform tube ensures even expansion of liquid.

The walls of the long tube above the bulb are made thick:

This acts as a magnifying glass for easy reading of the mercury thread in the stem.

The size of the thermometer is relatively small:

The small size allows it to be portable and cheap to produce.

 

Clinical Thermometer:

Clinical Thermometer

The scale is limited to a small range of between 34 to 42:

Since the normal body temperature is 36.9 the short range allows for greater accuracy and the stem can be made responsibally short

Narrow constriction:

It prevents mercury from flowing back into the bulb by breaking the mercury thread at the constriction.

 

Thermocouple Thermometer:

It consists of two wires made of different metals such as iron and copper. The ends of wires are joined together to form two junctions.

When two junctions are at different temperature e.g. one is hot and the other is cold. An emf is produced.

The greater the change in temperature the greater the emf is produced across the two junctions.

Advantages:

  • Large temperature range (-200 to 1500 ).
  • Very responsive to rapidly changingn temperature due to its low thermal capacity, due to its small mass and metals are good conductors of heat.
  • Output is elctrical signal therefore can be connected to a suitable electrical equipment for checking rapid or sudden temperaturechanges.

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