Physics A Level
Chapter 19: Thermal physics 19.5 Thermometers
Physics A Level
Chapter 19: Thermal physics 19.5 Thermometers
A thermometer is any device that can be used to measure temperature. Each type of thermometer makes use of some physical property of a material that changes with temperature. The most familiar is the length of a column of liquid in a tube, which gets longer as the temperature increases because the liquid expands.
This is how a liquid-in-glass thermometer works; it depends on a change in density of a liquid. Other physical properties that can be used as the basis of thermometers include:
- the resistance of an electrical resistor or thermistor
- the e.m.f. (voltage) produced by a thermocouple
- the colour of an electrically heated wire
- the volume of a fixed mass of gas at constant pressure.
In each case, the thermometer must be calibrated at two or more known temperatures (such as the melting and boiling points of water, which correspond to $0{\,^ \circ }C$ and $100{\,^ \circ }C$), and the scale between divided into equal divisions. There is no guarantee that two thermometers will agree with each other except at these fixed points.
Questions
5) a: Convert each of the following temperatures from the Celsius scale to the thermodynamic scale: 0
$0{\,^ \circ }C,\,20{\,^ \circ }C,\,120{\,^ \circ }C,\,500{\,^ \circ }C,\, - 23{\,^ \circ }C,\, - 200{\,^ \circ }C$.
b: Convert each of the following temperatures from the thermodynamic scale to the Celsius scale: 0
$K,\,20K,\,K,\,100K,\,300K,\,373K,\,500K$.
6) The electrical resistance of a pure copper wire is mostly due to the vibrations of the copper atoms.
Table 19.1 shows how the resistance of a length of copper wire is found to change as it is heated. Copy the table and add a column showing the temperatures in K. Draw a graph to show these data. (Start the temperature scale of your graph at $0 K$.) Explain why you might expect the resistance of copper to be zero at this temperature.
| Temperature / $0{\,^ \circ }C$ | Resistance / $\Omega $ |
| 10 | 3120 |
| 50 | 3600 |
| 75 | 3900 |
| 100 | 4200 |
| 150 | 4800 |
| 220 | 5640 |
| 260 | 6120 |
increases with temperature at a fairly steady rate. However, for a thermistor, the resistance changes rapidly over a relatively narrow range of temperatures. A small change in temperature results in a large change in resistance, so a thermometer based on a thermistor will be sensitive over that range of temperatures.
A thermocouple is another electrical device which can be used as the sensor of a thermometer. Figure 19.13 shows the principle. Wires of two different metals, X and Y, are required. A length of metal X has a length of metal Y soldered to it at each end. This produces two junctions, which are the important parts of the thermocouple. If the two junctions are at different temperatures, an e.m.f. will be produced between the two free ends of the thermocouple, and can be measured using a voltmeter. The greater the difference in temperatures, the greater the voltage produced; however, this e.m.f. may not vary linearly with temperature, i.e., a graph of e.m.f. against temperature is not usually a straight line.
Electrical thermometers can measure across a great range of temperatures, from $0 K$ to hundreds or even thousands of kelvin.
Table 19.2 compares resistance and thermocouple thermometers.
temperature (as shown in the calibration graph) and on the metals chosen
| Feature | Resistance thermometer | Thermocouple thermometer |
| robustness | very robust | robust |
| range | thermistor: narrow range resistance wire: wide range |
can be very wide |
| size | larger than thermocouple; has greater thermal capacity therefore slower acting | smaller than resistance thermometers; has smaller thermal capacity so quicker acting and can measure temperature at a point |
| sensitivity | thermistor: high sensitivity over narrow range resistance wire: less sensitive |
can be sensitive if appropriate metals chosen |
| linearity | thermistor: fairly linear over narrow range resistance wire: good linearity |
non-linear so requires calibration |
| remote operation | long conducting wires allow the operator to be at a distance from the thermometer | long conducting wires allow the operator to be at a distance from the thermometer |
Question
7) Give one word for each of the following:
a: adding a scale to a thermometer
b: all the temperatures, from lowest to highest, which a thermometer can measure
c: the extent to which equal rises in temperature give equal changes in the thermometer’s output
d: how big a change in output is produced by a given change in temperature.