Thermometers Thermometric Properties You will be able to

Thermometers/ Thermometric Properties

You will be able to: • Appreciate how a physical property that varies with temperature may be used for the measurement of temperature, and state examples of such properties • Describe and explain the structure and action of liquid-in-glass thermometers

MEASUREMENT OF TEMPERATURE: Remember! Temperature is: – ‘the average kinetic energy per particle’. – not all particles have the same amount of thermal energy • the hotter the material the faster, on average, the particles move.

TEMPERATURE SCALES Kelvin, Celsius and Fahrenheit The Celsius Scale • In 1742, a Swedish astronomer named Anders Celsius selected the fixed point as follows: » most commonly used scale » numbers were chosen using the melting and boiling points of water (0 and 100 respectively). Ice point is the temperature of pure melting ice at standard atmospheric pressure, and is assigned a value of 0 °C. 10/15/2021

Fahrenheit • least used scale. • designed so the boiling and melting points of water are exactly 180 degrees apart.

The Kelvin (thermodynamic) scale • understand that there is an absolute scale of temperature that does not depend on the property of any particular substance (i. e. thermodynamic scale and the concept of absolute zero) • convert temperatures measured in kelvin to degrees Celsius and recall that T / K = T / °C + 273. 15 – scale that is used by most scientists. – one Kelvin is equivalent to one degree Celsius. • T (K) = T (°C) + 273. 15 – The 0 on the Kelvin scale is absolute zero. • Absolute zero (− 273 C) – point of which particles have no kinetic energy. » technically impossible according to the laws of atomic physics. » at this temperature the particles would have the minimum energy possible.

Exercise: • 1. Liquid oxygen boils at normal pressure at -182. 96 °C. What is this temperature in Kelvin?

2. In many ideal gas problems, room temperature is considered to be at 300 K to make calculations easier. What is this temperature in Celsius?

THERMOMETRIC PROPERTIES • Physical properties that vary with temperature: – Examples: • • • volume length gas pressure electrical resistance electromotive force (voltage)

Volume of a liquid • When a liquid is heated, it expands. – expansion is related to the temperature of the liquid. • volume of the liquid is calibrated with the temperature using fixed points. 100° scale 0° • using the existing scale, temperature obtained by reading the volume of liquid.

Liquid-in-Glass Thermometers Two common liquids used in thermometers: – Mercury: • freezes at -39°C • boils at 357°C. • low specific heat capacity – (not much energy needed to change the temperature) • expands uniformly when heated. • doesn’t stick to the glass.

– Alcohol: • freezes at -115°C • boils at 78°C – suitable for low range temperature. • ideal for measuring atmospheric temperature. • expands uniformly when heated.

Length of Solid • bimetallic strip • consist of two strips of different metals vexample: iron and brass. vtemperature rises: • each metal expands a different amount • causes the bar to bend – greater expansion of brass compared steel makes the brass curve round on the outside of the steel. – motion of the bar causes a pointer to move up or down a scale

Gas Pressure Constant volume gas thermometer – contains a gas – exert a pressure on the wall of its container – at certain temperature, the gas exerts a certain pressure – temperature rises and pressure increases. – another temperature = another pressure. – change in pressure indicates a change in temperature. – by calibrating using known lower and upper fixed points • temperature scale may be obtained

Electrical Resistance 1. Resistance thermometers – made of metals like copper, nickel, or platinum. • platinum extremely accurate! • used to check accuracy of all othermometers – temperature change causes a change in the electrical resistance • (Resistance of a metal increases with temperature increase) – used in a circuit • an ammeter used to give a read-out in temperature. 2. Thermistors – resistance decreases as its temperature increases.

Electromotive Force • Thermocouple thermometer – very common electrical thermometer – used widely in industry. – made from two different metalsconsists of two wires of different metals joined together at the ends to form to junctions. – junctions are placed in different temperature – If the junctions are at different temperatures, a small electromotive forces or e. m. f. (voltage) is produced.

Advantages and Disadvantages of Thermistors and Thermocouples • compare the relative advantages and disadvantages of thermistor and thermocouple thermometers as previously calibrated instruments Thermocouple Thermistor Temperature Range (typical) -200 to +1750 °C -100 to +325 °C Response Time Fast 0. 10 to 10 seconds Fast 0. 12 to 10 seconds Accuracy (typical) 5 to 5. 0 °C 0. 05 to 1. 5 °C

Fixed Points • to calibrate a thermometer, fixed points must be chosen. – for a liquid in glass thermometer, these points are the melting and boiling points of the liquid.

Responsiveness • Demonstrate understanding of sensitivity, range and linearity • In understanding the features of thermometer a few consideration should be taken: – Responsiveness – this measures how quickly thermometer can respond changes in temperature. » a thermometer with a thicker glass round the bulb is less responsive as it takes longer for the liquid to reach the temperature of the surrounding.

Sensitivity – this measures the amount of change in thermometric property per unit change in temperature. • How can you increase the sensitivity of a liquid in glass thermometer? – – Thinner capillary tube glass bulb thinner wall larger bulb different liquid » Alcohol expands more than mercury, so it will move the a greater distance for the same temperature rise.

Range – this denotes the minimum and maximum temperatures that thermometer can measure. • Mercury is liquid between – 39 ᵒC and 350 ᵒC. • Alchol is liquid between -115°C and 78°C So a mercury thermometer has a greater range but is less sensitive.

Linearity – is the uniform expansion of the liquid to temperature which give the reading. • Mercury expands at a steady rate as it is heated.

Exercises 1. To mark a temperature scale on a thermometer, fixed points are needed. Which is a fixed point? A. the bottom end of thermometer tube B. the top end of thermometer tube C. the temperature of pure melting ice D. the temperature of pure warm water

2. The top of the mercury thread in a mercury-in-glass thermometer reaches point X at 0 °C and point Z at 100 °C. Where might it be at a temperature below the ice-point? A. point W B. point X C. point Y D. point Z 25 10/15/2021

3. A thermometer with no scale is taped to a ruler as shown. When placed in steam, the mercury level rises to 22 cm. When placed in pure melting ice, the mercury level falls to 2 cm. Which temperature is shown by the mercury level in the diagram? A. B. C. D. 10/15/2021 6 °C 8 °C 30 °C 40 °C 26

4. The diagram shows a mercury-in-glass thermometer. The distance between the – 10 °C and the 110 °C markings is 25 cm. At which temperature is the end of the mercury thread 15 cm from the – 10 °C mark? A. B. C. D. 10/15/2021 50 °C 62 °C 72 °C 28