Temperature

How are temperature and heat related?

Temperature is a measure of degree of hotness of a body. Heat refers to the amount of thermal energy that is being transferred from a hotter to a colder region.

How do we measure temperature?

Our hands are able to tell us how hot or cold an object is by touch. However, we know from experience that our sense of touch is a poor estimate of temperature. As with any other physical quantity, we have to give a numerical value to temperature before we can measure it. Any instrument which is used to measure temperature can be called a thermometer and we need to invent a scale for the thermometer.
We need an instrument, the thermometer, to measure temperature accurately. The thermometer makes use of certain substances, such as mercury, to measure temperature. Thrse substances have physical properties that vary continuously with temperature, and are called thermometric sunstances. There are several types of thermometers. The choice of what to use will depend on the range of temperatures to be measured, the accuracy required and the physical conditions of the matter concerned. Exampel of physical properties used for designing thermometers are:
1. the expansion of a column of a liquid in a capillary tube
2. the electrical resistance of a plantinum wire
3. the voltage of a thermocouple
4. the expansion of a bimetallic strip
5. the pressure of a gas at constant rate

So 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

Scales

In order for thermometers to give us a temperature reading, they must be marked with a standard temperature scale. How is this standard temperature scale derived?

Step 1: Choose an appropriate substance

Choosing a suitable thermometric substance and its physical property, e.g. a column of mercury. The volume of mercury is able to vary continously with changes in temperature.

Step 2: Choose two fixed points

Choose two standard degrees of hotness or coldness, which are easily obtainable and reproducible. We shall call them fixed points. it is common to choose the temperatures of pure melting ice and steam at one atmospheric pressure as fixed points. These two fixed points are called the lower and upper fixed points respectively. Record the values of the physical property of the substances at these two fixed points.

Step 3: Set up the scale

Divide the temperature range between the two fixed points into a fixed number of equal parts to obtain a scale. For example, on the Centigade (Celsius) scale, there are 100 equal parts or degrees between the lower and upper fixed points. by doing this, we assume that the physical property varies continously with temperature. This means that when temperature changes, the physical property must change evenly and continously.

The Centrigrade Scale

The centrigrade scale, or Celsius scale, is a common temperature scale. it is based on a simple experiment procedures that determine the two fixed points, called the ice point and steam point.

1. ice point (lower fixed point): This is the temperature of pure melting ice at one atmospheric pressure. It is assigned a value of 0 degrees celsius.

2. steam point (upper fixed point): This is the temperature of steam from water boiling at one atmospheric pressure. It is assigned a value of 100 degrees celsius.

To determine the ice point,

- Immerse the bulb at and the lower part of the stem of the thermometer into a funnel containing pure melting ice

-To ensure good conduct between the bulb and the ice, crushed ice should be used. The mercury level in the stem should be just above the surface of the ice.

- When the mercury level in the stem remains steady after some time, a mark is made at the point of the stem. This mark corresponds to the ice point and is assigned a value of 0 degrees celsius.

To determine the steam point,

- insert the thermometer into the apparatus. The bulb of the thermometer should be above the boiling water. The stem of the thermometer should protude above the top of the apparatus.

- the manometer is included to ensure that the pressure inside the apparatus is the same as the atmospheric pressure outside

- when the mercury level in the stem remains steady after some time, a mark is made at that point of the stem. This mark corresponds to that steam point and is assigned a value of 100 degrees celsius.

Note: FOR THE CENTRIGRADE SCALE, THE INTERVAL BETWEEN THE ICE POINT AND THE STEAM POINT IS DIVIDED BY 100 EQUAL PARTS. EACH MARK ON THE THERMOMETER IS A MEASURE OF 1 DEGREES CELSIUS.

Calculating temperature on a Centrigrade Scale

In a mercury thermometer the physical peroperty that changes continously with temperature is the volume of a fixed mass of mercury. We can measure th echanges in a volume of mercury, by measuringh the change of th elength of a mercury column or thread. This is only possible if the cross-sectional area of the column or thread in a thermometer is uniform. If we know the values l0 (height of mercury level at 0degrees celsius) and l100 (height of mercury level at 100 degrees celsius) of a mercury in glass thermometer, we can easily determine the temperature of any unknown body with that particular thermometer.

We must first find the height of the mercury level l0 when the thermometer is placed in temperature. Since the volume of mercury varies continously with temperature, we know that the height of mercury level is directly proportional to the temperature.

We can then find the value of unknown using the equation:

Unknown Degrees Celsius = l uknown- l 0/ l100 - l0 x 100

The Kelvin Scale

The Kelvin scale, which is named after the Englishman, Lord Kelvin. it is based on the theory that there is a lowest possible temperature that exists in the universe- the absolute zero. It is the temperature at which all possible thermal energy has been transferred away from the body. Hence, the Kelvin scale is also called the Absolute scale.

The unit for this scale is the kelvin (k). This is the Si unit for temperature. One unit of kelvin is the same size as one unit of degree Celsius, but the kelvin starts its scale at 0K- the absolute zero.

Thermocouple Thermometer

What is a thermocouple?

A thermocouple consists of two types of wires made of different metals such as copper and iron. The ends of the wires are joined together to form two junctions. The temperature is then calculated using the readings of a voltmeter.

How does a thermocouple thermometer work?

If the two junctions are at different temperatures, a small voltage ( or electromotive force e.m.f) is produced. The greater he difference in temperature, the greater the voltage produced across the ends of the two junctions. if one junction is kept at a fixed temperature, such as 0 degrees celsius, then the other junction can be used as a tiny probe to measure other temperatures.
The defining equation of the thermocouple thermometer is given by :

E= *U
where E= e.m.f produced,
* U is the temperature difference between the reference junction and the probe.

For practical use, especially in measuring high temperatures such as the temperature of lava, the cold junction is removed. This produces a single junction thermocouple. An error of a few degrees in every one thousand degrees may occur but it is often negligible.

The advantages of a thermocouple thermometer are:

1. It is robust, compact, fairly accurate and able to measure a very large temperature range of -200 degrees celsius to 1500 degrees celsius by choosing suitable types of metals for Wire A and B
2. As the wires junctions are very small, the thermometer can be used to measure temperatures at a point
3. It is very responsive to rapidly changing temperature due to its small mass and because metals are good conductors of heat.
4. As the output is an electrical signal, it can be connected to suitable electrical equipment for checking rapid or sudden temperature changes.

Other thermometers

Experiments can be carried out to show that liquids tend to expand at different rates over different temperature ranges. There are exceptinos like merury which expands quite uniformly for a good range of temperatures.

Mercury thermometers.

The mercury thermometer is a common type of thermometer in everyday use. The mercury in the thermometer expands when heated. The expansion pushes a thread of the liquid out of the bulb and up the capillary tube.
The narrow bore of te capillary tube makes the thermometer more sensitive. This is because a small expansion of the mercury in the bulb will cause a big change in the length of the mercury thread. The bulb is made of thin glass so that heat can be conducted quickly to the liquid. The round glass stem acts as a mgnifying lens enabling the temperature to be read easily.

Alcohol thermometers.

This is a cheaper liquid in glass thermometer. The fact that alcohol is safer than mercury is often an important consideration. It can also meeasure a lower temperature than a mercury thermometer wheich is an advantage in cold countries.
However, alcohol has severe disadvantages including its irregular expansion over different ranges of temperature, it slow reactino to temperature changes and its transparncy and tendency to stick to glass which make it difficult to see its meniscus. A dye is often added to the alcohol to make it more visible. Fortunately, in science laboratories, alcohol expands rather linearly over the temperature range encountered.

Clinical Thermometers

A clinical thermometer is a type of maximum thermometer specially designed for measuring the temperature of the human body. It is an ordinary mercury thermometer with two modifcaitions:

1. It shows a short range in temperature from about 35- 42 degrees celsius, centred around the normal body temperature which is about 37 degrees celius
2. It ihas a constriction in the capillary tube just above the bulb of the thermometer.


The bulb of the thermometer is held gently under the patient's tongue. When the temperature rises, the mercury expands, forcing its way through the constriction and into the capillary tube. On cooling, the mercury in the blub contracts but the constriction prevents the mercury from falling back. This enables the patient's temperature to be read at leisure. The mercury thread is shaken into the bulb before the thermometer is used again.

Transfer of Thermal Energy

Thermal Energy is transferred only when there is a difference in temperature. Thermal energy always flows from a region of higher temperature to a region of lower temperature.

How is thermal energy transferred?

Most objects near the barbecue pit are not in direct contact with the burning charcoal. So how is the thermal energy from the red hot charcoal transferred to these objects?
Thermal energy is transferred by ant of these three processes: conduction, convetion and radiation.

Conduction

What is conduction?
If one end of a metal rod is heated with a flame, the other end will also get heated up after a while. Fromt this, we can see that thermal energy is transferred from one end of the metal rod to the other. The transfer of thermal energy through a medium, without the medium moving, is called conduction.

Conduction is the process of thermal energy transfer without any flow of the material medium.

Experiment 1:
Objective: To investigate the transfer of thermal energy through solids
Apparatus: bath, rods of the same dimensions but of different materials, stopwartch
Procedure:
1. coat the parts of the rods that are on the outside of the tank evenly with melted wax
2. pour boiling water into the bath, so that the ends of the rods are submerged
3. record the length of wax that melts in a given interval of time for each of the four rods.
Observation: the wax melts furthest along the copper rod, followed by iron, glass and wood.

In this experiment, thermal energy is transferred from a region of higher temperature to a region of lower temperature. The wax on the rods melts as thermal energy is transferred from the boiling water towards the colder end of the rods. But the length of melted wax on each of the four rods is different. In other words, the lenth of unmelted wax is shortest for the copper rod and longest for the wooden rod.

2 important conclusons:
1. thermal energy flows through the material of the rods without any flow of the material itself. such a transfer of thermal energy without any movement of the material medium is called conduction
2. different materials conduct heat at different rates. SInce the length of unmelted wax is shortest for copper and longest for wood, it can be concluded that copper is a good conductor of heat and wood is a poor conductor of heat. Another term for poor conductors of heat is insulator.

How does conduction work?

All solids (metals and non metals) are made up of tiny particles called atoms and molecules. The difference between metals and non metals is thst metals contain many free electrons which move randomly between the atoms or molecules, while non metals do not have such free electrons. WHen a metal and non metal is heated from one end, with thermal energy supplied to one end of the rod, the particles will collide with neighbouring particles, making them vibrate as well. Thus, the kinetic energy of the vibrating particles at the hot end is transferred to the neighbouring particles. The process of transferring thermal energy from the hot end to the cold end by atomic or molecular vibration take place in both metal and non metal. In this process, there is no transfer of particles. it is quite slow to transfer thermal enrergy this way. however, in metals, another much faster mechanism of thermal energy transfer takes place at the same time: free electron diffucsion. When the copper rod is heated, the free electrons in the copper gain kinetic energy and move faster as a result. These fast moving elecrtrons then diffuse or spread into the cooler parts of the metal and transfer their kinetic energies to them. this explains why good conductors like metals are cpable of transferring thermal energy much faster than insulators.

Convection and Radiation

Convection is another process of thermal energy transfer. It is the transfer of thermal energy by means of currents in a fluid (liquids or gases).
How does Convectino work?
When water at the bottom of a falsk is heated, it expands. This expanded water is less dense than the surrounding water and therefore starts to rise. In doing so, the cooler regions of the water in the upper part of the flask, being denser, sink. This movement of the liquid due to a difference in its density sets up a convection current.
Convection currents occur only in fluids such as liquids and gases but not solids. This is because convection involves the bulk movements of the fluids which carry thermal energy with them. For solids, the thermal energy is transferred from one particle to another through vibrations, without any bulk movement of the particles themselves.


Radiation
Radiation is the continual emission of infrared waves from the surface of all bodies, transimitted without the aid of a medium.
Unlike conduction and convection, radiation does nto require a medium for energy transfer. This means that radiation can take place in a vacuum. For example, thermal eernegy from the sun reaches the Earth by the processof radiation. Conduction or convection is not possilbe because of the vacuum between the Sun and the Earth.
The sun emits electromagnetic waves. A part of this family of electromagnettic waves emitted by the sun makes us feel warm. This group of electromagnetic waves is called infrared waves. Thermal energy from infrared waves is called radiant heat. In fact, all objects emit some radiant heat. The hotter the object, the greater the amount of radiant heat absorbed.

Factors affecting rate of infrared radiation:
1. Colour and texture of the surface:
Black absorbers of infrared radiation than shiny, white surfaces.
2. Surface temperature:
The rate of infrared radiation also depends on the surface temperature. The higher the temperatureof the sufrface of the object relative to the surrounding temperature, the higher the rate of infrared radiation.
3. Surface Area
The third factor whci haffects the rate of radiation emission and absorption is the surface area of the object. If we can compare two objects of the same mass and material, but with different surface areas, the object with the larger surface area will emit infrared radiation at a higher rate.

Applications of thermal energy tranfer

Use of good conductors of heat:
If thermal energy has to be transferred quickly througha substance, good conductors of heat such as metals are used. Some examples of the use of metals are :
1. Cooking utensils like kettles, saucepans and boilers are usually made of aluiminium or stainless steel where direct heating is involved.
2. Soldering iron rods are made of iron with the tip made of copper, as copper is much better conductor heat than iron.
3. Heat exchangers, such as those used in a large laundry facility, help save energy.

Use of bad conductors of heat:
1. Handles of appliances and utensils such as saucepens, kettles, teapots etc. In this way, the hot utensil or iron can be picked up with scalding of our hands.
2. Table mats are usually made out of cork so that hot kitchenware can be planced on them without damaging the table top.
3. Sawdust is used to cover ice blocks because of its good insulating property
4. Wooden ladles are very useful for stirring or scooping hot stuffs
5. Woollen clothes are used to keep people warm on cold days
6. Fibre glass, felt and expanded polysterene foam which trap large amounts of air are emplyed as insulators in the walls of hosues, ice boxes and fridges.

Common applicatinos of convection
1. Electrical kettles
The heating coil of an electric kettle is always placed at the bottom of the kettle to aid transfer of thermal energy by convection. when the power is swiched on, the water near the heating coil is heated up, expands and becomes less dense. The heated water therefore rises while the cooler regions in the upper part of the body of water descend to replace the heated water.
2. Household hot water systems
3. Air conditioners
An air conditoner is always installed near to the ceiling of a room to faciliate setting up of convection currents. The rotary fan inside an air condiitioner releases cool dry air onto the room. As cool air is denser, it sinks.
4. Refrigerators

Common applications of radiation
1. teapot
shiny surfaces are bad emitters of a radiation, shiny teapots acan keep tea warm for a longer time than black ones. SInce shiny surfaces are bad abosrbers of radiationn, shiny containers can keep cold liquids cool for a longer time
2. Greenhouse
3. Vacuum Flask
The vacuum flask is designed to keep liquid hot by minimising heat loss in a few ways, namely conduction, convection, radiatino and evaporation.