Temperature and Thermometers
The Temperature of an object is a measure of the hotness or coldness of that object. An alternative way to think of temperature is to say that “the temperature of an object is a number – on some manmade scale – that indicates the hotness of the object”. ‘Hotness’ in turn is a measure of the kinetic energy of the molecules of the material. Note: You must use the term ‘hotness’.*
The SI unit of temperature is the Kelvin (K)*
Relationship between degrees Celsius and Kelvin*:
A Thermometric Property is any physical property that changes measurably with temperature. Note: You must use the term ‘measurably’.
Examples of thermometric properties:
Length of a column of liquid, e.g. mercury and alcohol thermometers, •
Electrical resistance (see chapter 23),
Colour (colour ‘strips’ are sometimes used by nurses and placed against a person’s forehead), •
Emf of a thermocouple (emf is a fancy word word for ‘voltage’), •
Volume of gas at constant pressure,
Pressure of a gas at constant volume.
Disagreement between thermometers
Two different types of thermometer will give slightly different readings at the same temperature.* This means we need to agree on one particular thermometer to have as a standard.
Temperature gauge in a car
Clinical thermometer; Two Main Features:
Short range because temperature of the human body doesn’t vary much either side of normal body temperature (37.60 C). A kink to prevent the liquid falling back down when the thermometer is removed from the mouth and held vertically.
Experiment: Using a thermometric property to measure temperature* 1.
Note the value of the thermometric property in melting ice (00 C) and also in boiling water (1000 C). 2.
Plot these two points on a graph and draw a straight line joining them. 3.
For an unknown temperature, note the thermometric property and use the graph to calculate the corresponding temperature.
Mandatory Experiment: To plot a calibration curve for a thermometer using a mercury thermometer as a standard.
Leaving Cert Physics Syllabus
Depth of Treatment
Concept of temperature
Measure of hotness or coldness of a body. The S.I. unit of temperature is the Kelvin.
Celsius scale is the practical scale of temperature.
T /0C = T /K – 273.15
A physical property that changes measurably with temperature.
Demonstration of some thermometric properties: •
length of liquid column, e.g. length of mercury column
emf of thermocouple
Pressure of a gas at constant volume
Volume of a gas at constant pressure
Thermometers measure temperature.
Two thermometers do not necessarily give the same reading at the same temperature. The need for standard thermometers – use any commercial laboratory thermometer as a school standard.
Graduate two thermometers at ice and steam points. Compare values obtained for an unknown temperature, using a straight-line graph between the reference points.
Practical thermometers, e.g. •
temperature gauge in a car.
* You must use the term ‘hotness’
So why can’t we say that “the temperature of an object is a measure of how hot or cold an object is”? ‘Hot’ is a vague term. Does it refer specifically to temperature, or to the amount of heat in the object? After all, a litre of water at 1000 Celsius has twice as much heat as half a litre of water at 1000 Celsius. ‘Hotness’ is the physicists way of overcoming this potential confusion.
*The SI unit of temperature is the Kelvin (K)
SI stands for ‘Standard Internationale’ (French). Basically it’s a system of units (incorporating the metric system) which connects up all the main physical quantities, and which has been agreed by all scientists (but not...
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