Grade : 9
Measuring Temperature
Temperature, T/K, is a measure of how hot or cold something is. It is also defined as a measure
of the average kinetic energy of molecules. The more kinetic energy molecules have, i.e. the faster they
are vibrating, the higher the temperature.
There are two main units of temperature, degrees celsius (oC) and kelvin (K) - the Kelvin
regarded as the basic SI unit. The liquid-in-glass thermometers, digital thermometers and thermocouple
thermometer are common instruments used to measure temperature. Thermometers may use any of the
following properties that change (usually increase) with an increase in temperature as a means to measure
temperature: volume of liquid, resistance, electromotive force or the volume of a gas.
In scientific work we normally use the Celsius scale (sometimes called the centigrade scale) to
measure temperature with a mercury-in-glass thermometer. It follows that the zero point on the Kelvin
scale is equal to -273 oC on the Celsius scale of temperature. In experiments, the degree Celsius ( oC)
scale is used for all temperature measurements and if temperature is needed in K, then it is calculated.
(See Conversion). One division, interval or the change of one degree Celsius is equal to one division
or the change of one kelvin.
1 oC = 1 K
The Mercury-in-Glass Laboratory Thermometer
Smallest Division: 1 oC
Range: -10 oC to 110 oC
handle
Suitable for work in the laboratory, laboratory thermometers are based on the
or thermometric change in volume of a liquid with temperature; the liquid being mercury (and
bore in some cases alcohol). Mercury is the only metal that is liquid at room temperature and is
normally chosen because of its high conductivity. The thermometer consists of a glass tube
with a narrow channel running through the centre called the capillary or bore. At the
bottom is a small bulb filled with liquid mercury. When heated, the mercury fills up the
narrow bore inside the stem. Above the mercury is a vacuum, which allows for free
scale movement of the mercury. The thermometer can be made to respond more quickly because
of having:
- walls of the bulb being thin;
- capillary tube that has a narrow bore.
Mercury thermometers cannot measure temperatures over 340 oC (mercury boil at that
temperature).
Using a Laboratory Thermometer
Place the bulb of the thermometer in the system.
Ensure that the bulb is completely immersed in the liquid and is not in contact with the sides
of the container.
Stir the system (with a stirrer – not with the thermometer itself) to ensure even distribution
of heat.
Leave the thermometer immersed for enough time to ensure equilibrium.
Take the reading at eye level to avoid parallax.
The Thermocouple Thermometer
Smallest Division: 1 oC
Range: -200 oC to 1500 oC
A thermocouple thermometer is a device which uses electromotive force (e.m.f.) produced
across metal junctions to measure temperature difference. Electromotive force is the difference in electric
potential (potential difference or voltage) which causes current to flow in a circuit. The amount of current
depends on the temperature of both substances. A thermocouple consists of two wires made of different
metals joined together to form two junctions. When two junctions are placed at different temperatures,
then an electromotive force will be set up. The greater the temperature difference, the greater the
electromotive force – the greater the flow of current.
The following are advantages of using a thermocouple:
- It can measure varying temperatures quickly;
- It can measure a wide range of temperature (-200 oC to1500 oC).
Clinical Thermometer
Smallest Division: 0.1 oC
Range: 34 °C to 43 oC
Clinical thermometers are used to measure body temperature. They can be of two different
physical forms: mercury-in-glass and electronic digital.
The Mercury-in-Glass Clinical Thermometer
The Electronic Digital Clinical Thermometer
Table Comparing the Types of Thermometers
Laboratory Clinical Thermometer Thermocouple
Thermometer Thermometer
Uses - measure - measure body - measure
chemicals/substances in temperature materials/substances in
the laboratory industries that has very
high or very low
temperatures
Range -20 – 110 oC 35 – 43 oC -200 – 1500 oC
Design - straight, narrow bore - narrow bore; mostly - materials of different
straight but with types produces an
- bulb containing constriction (prevents electromotive force (emf).
mercury/alcohol with backflow of mercury);
thin walls (mercury-in-
glass thermometer); - bulb containing
mercury/alcohol with
- many of the newer thin walls (mercury-in-
types are digital (see glass thermometer);
diagram above).
- rarely seen in digital
form.
Thermometric - mercury and alcohol; - mercury and alcohol; - voltage that produces an
Property electric charge (emf) for electric charge (emf) for emf that is directly
the digital ones. the digital ones. proportional to
temperature.
Activity – Project
Utilize everyday materials to make a model or a diorama of a laboratory thermometer. Marks will be
awarded for originality, presentation and practicality. (10 marks)
ADDITIONAL READING & COMPREHENSION
1. Are there any differences between temperature and heat? Cite at least three reasons for your answer.
2. Even though the same amount of oil and water were heated from similar burners in container made
from the same materials, the temperature of the oil increased faster than that of the water. Explain the
reason that happened.
HINT: Utilize the terms specific heat capacity. Cite the differences between specific heat capacity and
heat capacity.
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