Measurement of Temperature: Temperature Scales, Scale Conversion

A candle flame is hot and is said to be at a high temperature. Ice, on the other hand, is cold and is said to be at low temperatures. Our sense of touch is a simple way to know how hot or cold a body is. However, this temperature sense is somewhat approximation and unreliable. Moreover, it is not always safe to touch a hot body. What we need is a reliable and practicable method to determine the relative hotness and coldness of bodies.

Table of Contents:

Measurement of Temperature
Thermometer
Temperature Scale
Scale Conversions
Absolute Zero Temperature
Practice Questions of Measurement of Temperature
FAQs of Measurement of Temperature

Measurement of Temperature

If we want to measure the accurate temperature, a reference body is required and a certain characteristic of that body which changes with temperature. The change in that characteristic may be taken as an indication of a change of temperature. That selected characteristic is known as a thermodynamic property.

Thermometer is a device to measure temperature.

There are many types of thermometers. The major ones employ the characteristics of expansion and contraction of substance according to the temperature, employ the electrical characteristics (electrical resistance) of substance according to temperature, or employ characteristics between temperature and heat radiation energy emitted from the surface of the substance.

You may also want to read about Ohm's Law.

Many physical properties depend on temperature. For example, volume increases with temperature for most substances. This property is the basis for the common alcohol thermometer, the old mercury thermometer, and the bimetallic strip as shown.

Thermometer:

The mercury thermometer is the oldest form of a temperature sensor used to measure the temperature.
The expansion of mercury due to heat is what tells us the temperature.
If we calculate the expansion of mercury per degree change in temperature; then we can calibrate the thermometers with a scale based on the expansion. This is how a thermometer works.
The temperature sensors are so calibrated that the numbers on the thermometers represent the temperature of the body.

Type of Thermometers

Liquid thermometer: These thermometers use the expansion property of liquids when they come in contact with the body at higher temperature whose temperature needs to be measured. The most common liquid thermometer that we use is mercury thermometer.
Digital thermometer: These thermometers uses thermistor (a type of resistor) for temperature measurement.
Non-contact thermometer (Infrared): These thermometers uses optical sensors for temperature measurement. For ex. Infrared thermometers detect the infrared radiation emitted by the body whose temperature needs to be measured.

Temperature Scale

Thermometers are calibrated so that a numerical value may be assigned to a given

temperature. For the definition of any standard scale, two fixed reference points are needed.

Typical temperature scales take the ice point and the steam point of water as two convenient fixed points also known as the freezing and boiling points. These two points are the temperatures at which pure water freezes and boils under standard pressure.
There are more precise treatments of standard points for defining temperatures. One standard point is the triple point of water which has been defined to be 273.16 K. Other standard point is obtained by means of a thermometer which does not depend upon the material used in making the thermometer, a constant-volume gas thermometer was chosen to measure the boiling point of water. This method is based upon the ideal gas law.

Thermometers measure temperature according to well-defined scales of measurement. The three most common temperature scales are the

Kelvin: The Kelvin scale is the temperature scale that is commonly used in science. It is an absolute temperature scale defined to have 0 K at the lowest possible temperature, called absolute zero. The freezing and boiling points of water are 273.15 K and 373.15 K, respectively.
Celcius: the freezing point of water at 0ºC and the boiling point at 100ºC. Its unit is the degree Celsius(ºC).
Fahrenheit: the freezing point of water is at 32ºF and the boiling point is at 212ºF. The unit of temperature on this scale is the degree Fahrenheit(ºF)

Absolute Zero Temperature

Absolute temperature also called zero Kelvins equals
- 273.15^oC or - 459.67^oF is the lowest temperature possible according to the laws of thermodynamics.
At this temperature, according to kinetic theory the kinetic energy of the particles ceases to zero or we can also say that the particles will have the minimum vibrational motion.
This temperature is more of a concept since we haven’t been able to achieve absolute zero.
It is believed that as we reach absolute zero temperature or below that (i.e. negative), the efficiency of the engines might increase although it may have some other consequences too.

Scale conversions:

In making such scales, boiling and freezing points of water are taken as reference points.

Or,

Celsius() to Kelvin(K):

Celsius() to Fahrenheit():

Fahrenheit() to Kelvin(K):

Practice problems

Question.1 Convert to .

Answer:

Question.2 Convert to

Answer.

Question.3 A new temperature scale N needs to be defined. The boiling point and freezing point on this scale corresponds to 100°N and 400°N respectively. What will be the reading on this scale corresponding to 60°C ?

Answer.

Question.4 At what temperature are the Celsius and Fahrenheit equal?

Answer. (B)

Relation between Celsius and Fahrenheit scale,

Let be the common temperature, then,