Temperature
Temperature is defined as the degree of hotness or coldness of a body. The S.I unit of temperature is Kelvin. The Kelvin scale is called the absolute or thermodynamic scale. Other common scales of temperature are centigrade or Celsius (0C) and Fahrenheit (0F).
0 0C = (0 + 273) K
The need to have two reference or fixed points of temperature called the upper and lower fixed points gave rise to different temperature scales.
Upper fixed point: This is the temperature of steam from pure water boiling at standard atmospheric pressure. On the Celsius scale, it has the value of 1000C.
Lower fixed point: This is the temperature of pure melting ice at standard atmospheric pressure. On the Celsius scale, it has the value of 00C.
The interval between the upper and lower fixed points is called the fundamental interval.
The lowest possible temperature that can be achieved theoretically is called the absolute zero i.e 0 K or
-273 0C.
Task for students: State four differences between heat and temperature.
Types of thermometers
There are different types of thermometers depending on the temperature to be measured.
Liquid in glass thermometers: Examples of these types of thermometers are mercury in glass thermometers, Six's minimum and maximum thermometer used by Agriculturists to measure the highest and lowest temperature of the day. They contain liquids (mercury or alcohol) as their thermometric substances.
A typical and common liquid in glass thermometer is the clinical thermometer. The clinical thermometer is used in the clinics or hospitals for measuring human body temperature.
Features of the clinical thermometer:
- The thick dark bulb houses the mercury.
- The fine bore is to increase the sensitivity of the thermometer - small temperature change causes large change in the length of mercury column.
- The kink prevents mercury from flowing back into the bulb when taking measurements.
- The thick glass stem acts as a a lens that magnifies the mercury thread for visibility.
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thermometer cannot do this because the glass expands with mercury. This makes the readings not to be extremely accurate.
Thermoelectric thermometer: This works on the principle of thermocouples in which two dissimilar metals
are placed at hot and cold junctions.
Resistance thermometers: These thermometers work based on the principle that the resistance of a metallic conductor increases with an increase in temperature.
Resistance thermometers are useful in the accurate measurement of very low and very high temperatures. An infrared pyrometer, for example, uses infrared radiation from hot matter to measure temperature.
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Advantages of mercury over water in thermometers
1. Mercury has a boiling point of 3570C so it can measure higher temperatures than alcohol which boils at 780C.
2. Mercury is not easily vaporized like alcohol.
3. Mercury does not wet glass while alcohol does.
4. Mercury is opaque so it is easily seen but alcohol has to be coloured before it can be used.
5. Mercury respond more rapidly to temperature change because it has a greater conductivity than alcohol.
Advantages of alcohol over mercury
1. Alcohol expands about six times as mercury.
2. Alcohol is cheaper than mercury.
3. Alcohol can measure very low temperatures. It freezes at -1150C while mercury freezes at -390C.
3. Mercury does not wet glass while alcohol does.
4. Mercury is opaque so it is easily seen but alcohol has to be coloured before it can be used.
5. Mercury respond more rapidly to temperature change because it has a greater conductivity than alcohol.
Advantages of alcohol over mercury
1. Alcohol expands about six times as mercury.
2. Alcohol is cheaper than mercury.
3. Alcohol can measure very low temperatures. It freezes at -1150C while mercury freezes at -390C.
Homework
1. Derive a formula that can be used to convert temperature from Celsius to Fahrenheit.
2. When the fixed points of a Celsius thermometer are verified, it read 0.5 at the melting point of ice and
99.2 at the boiling point of water at normal pressure. What is the correct temperature when it reads
150C and at what temperature will the reading be exactly correct?
3. State three reasons why water cannot be used as a thermometric substance.
4. Why is it impossible to measure the boiling point of water with a clinical thermometer?
I am expecting your answers soon. Paste your answer in the comment box or on Facebook.
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ReplyDeleteDifferences between heat and temperature
ReplyDeleteHeat Temperature
It is the amount of thermal energy that flows from hotter object to a colder object.
It is the degree of hotness or coldness of an object.
It is the total energy of all the molecular motion or sum of all kinetic energies inside the object.
It is the measure of average heat or thermal energy of molecules in a substance or an object.
The S.I Unit is Joules.
The S.I Unit is Kelvin, Celsius or Fahrenheit
It is the process function which is defined as the internal energy interchanged which is neither work nor due to flow of matter.
It is a state function which is defined as the inverse variation of entropy to changes to internal energy
Conversion from Celsius to Fahrenheit
Celsius to Fahrenheit
F= 9/5 (C+32)
Reasons why water is not a thermometric substance
1. Water has higher freezing point and lower boiling point than other thermometric substances.
2. Water has high specific heat capacity so that it does not take up negligible temperature of hot body being measured.
3. It is transparent making the reading of the scales of thermometer more difficult.
Reasons why a clinical temperature cannot measure the temperature of boiling water
A clinical thermometer cant measure high temperature because it is designed to measure temperatures a handful of degrees at either side of normal body temperature (98-99). If you heat liquid in thermometer, it would expand to top of capillary tube, finally bursting its way through and break the glass.
The boiling point of the water used to indicate the reading in the thermometer is different in both of them.