Physics – Temperature, Unit 8
8.1 Temperature and Its Measurement
How are temperature and heat related?
Temperature refers to how hot or cold an object is.
Heat refers to the amount of thermal energy that is being transferred from a hotter to a
colder region.
How do we measure temperature?
A thermometer makes use of substances that have physical properties that vary
continuously with temperature. This substances are called thermometric substances.
List of physical properties that change with temperature – (Tested for class test 2012)
* Volume of a fixed mass of liquid (eg. Mercury-in-glass thermometer)
* Electrical Voltage or Electromotive force (e.m.f.) (eg. Thermocouple)
* Electrical resistance of a piece of metal (eg. Resistance thermometer)
* Pressure of a fixed mass of gas at constant volume (eg. Constant-volume gas
thermometer)
What makes a good thermometer?
1. An easy-to-read scale
2. Safe to use
3. Responsive to temperature changes
4. Sensitive to small temperature changes
5. Able to measure a wide range of temperatures
Constructing a temperature scale
Step 1 – Choosing appropriate substance
Step 2 – Choose 2 fixed points
Step 3 – Set up the scale
8.2 Common Temperature Scales
The Centigrade scale (Celsius Scale)
Based on :
1. Ice Point (lower fixed point) : Temperature of pure melting ice at one atmospheric
pressure. (0’c)
2. Steam Point (upper fixed point) : Temperature of steam from water boiling at one
atmospheric pressure. (100’c)
Calibrating a Thermormeter –
Determining the Ice point :
1. Immerse the bulb and the lower part of the stem of the thermometer into a funnel
containing pure melting ice. (Crushed ice advised to ensure good contact between ice and
thermometer).
2. The mercury level in the stem should be just above the surface of the ice.
3. When mercury level in stem remains constant after some time, a mark l0 is made at that
point on the stem. (Ice point, 0’c)
Determining the Steam point (tested for Class test 2012):
1. Insert thermometer into apparatus. The bulb of thermometer should be ABOVE boiling
water. The stem of the thermometer should protrude above the top of the apparatus.
2. Manometer is included to ensure that the pressure inside apparatus is equal to the
atmospheric pressure outside.
3. When mercury level in stem remains constant after some time, a mark l100 is made at that
point on the stem. (Steam point, 100’c)
* Calculating temperature on a Centigrade scale Important Formula
Physical property that changes continuously with temperature is the volume of a fixed mass
of mercury.
𝜃°𝑐 =
∫𝜃 − ∫0
∫100 − ∫0
× 100
Where 𝜃°𝑐 is the unknown temperature,
∫𝜃 = length of mercury column at 𝜃°𝑐,
∫0 = length of mercury column at 0°𝑐,
∫100 = length of mercury column at 100°𝑐
General equation for the Centigrade scale Important Formula
𝜃°𝑐 =
𝑋𝜃 − 𝑋0
× 100
𝑋100 − 𝑋0
Where 𝑋 can be any physical properties.
The Kelvin scale
The unit for this scale, as well as the SI unit of temperature, is kelvin (K).
To convert degree Celsius to Kelvin :
* 𝜃°𝑐 = 𝐾 − 273
8.3 Thermocouple Thermometer
What is a Thermocouple?
A thermocouple consists of two types of wires made of different metals. The ends of wires
are joined together to form two junctions.
How does a thermocouple thermometer work? Important formula
A small voltage (electromotive force, e.m.f.) is produced when there is a difference in
temperatures at the two junctions.
The greater the difference in temperature, the greater the voltage produced across the ends
of the two junctions.
𝜀 = ∆𝜃 ,
Where 𝜀 = 𝑒. 𝑚. 𝑓. produced,
∆𝜃 = temperature difference between the reference junction and the probe.
Advantages of a Thermocouple thermometer Tested for Class Test 2012:
- Robust, compact, fairly accurate, and able to measure a range of -200°𝑐 − 1500°𝑐 (able to
choose suitable types of metals).
- Thermocouple thermometer can be used to measure temperatures at a point (wire
junctions are small).
- Very responsive (due to small mass and properties of metal)
- It can check rapid or sudden temperature changes (when connected to a suitable electrical
equipment).