Temperature and Thermometer

• Temperature is the degree of hotness or coldness of a body. It measured using a thermometer. The SI unit is Kelvin (K). It is not an energy store and not transfer is possible.
• Heat/ thermal energy is a form of energy that flows from a hotter body to a colder body. It is measured using a joulemeter. The SI unit is a joule. It is an energy store and can e transfered from one store to another
• Scales of measuring temperature:

  1. Degree/ Celsius scale
  2. Kelvin/ Absolute scale
• Celsius Scale: It is based on melting point of pure ice and pure water. These two are called lower and upper fixed point respectively with temperatures of 0°C and 100°C.

  • Lower fixed point: It is determined by placing a thermometer in pure melting ice.
  • Upper fixed point: It is determined by placing a thermometer above boiling water
• To convert degree Celsius to Kelvin add 273. 

  To convert Kelvin to degree Celsius subtract 273. 
• Types of thermometers

  -> Liquid-in-glass thermometer

  ->Minimum and maximum

  ->Doctor's thermometer

  ->Digital thermometer
• Liquid-in-glass thermometer:

  It has the following features

  1. Thin-walled bulb->this is where the liquid is stored. The wall is thin so that heat can easily flow in and out of the bulb
  2. Thick-walled stem-> this makes the thermometer stronger and magnifies the reading
  3. Capillary tube/bore-> it is thin and uniform cross-section area. Uniform cross-section area makes it possible to come up with the scale
  4. Safety bulb-> it gives room for the liquid to expand into increase of overheating. It prevents the cracking of the thermometer
• MERCURY: Strengths

  -> Expands uniformly

  -> Doesnt wet the glass

  -> It is a good thermal conductor

  -> It is opaque hence visible

  -> Very high boiling point
• MERCURY: Weaknesses

  -> It expands less than alcohol

  -> It is very poisonous hence dangerous incase the thermometer breaks

  -> It has a high melting point
• ALCOHOL: Strengths

  -> It has a low-melting point

  -> It expands more times than mercury making a small change noticeable
• ALCOHOL: Weaknesses

  -> It is colorless hence needs dying

  -> It wets the glass
• Doctor's/clinical thermometer

  It is a liquid in glass thermometer that is different from the ordinary laboratory thermometer discussed above by:

  • a short range of scale between 35-42°C
  • an extra thin capillary tube
  • triangular stem
• Thermocouple thermometer

  It consists of 2 wires made of 2 different materials connected together at two junctions, the cold and the hot junction.