Temperature is a property of a body which decides which way heat will flow when it is placed in contact with another body i.e. Temperature is the degree of hotness or coldness.
Temperature must not be confused with heat itself. Heat is a transfer of energy due to temperature energy. The S.I unit of temperature is Kelvin known as the “absolute” or “thermodynamic scale” on this scale temperature is measured on Kelvin. Its symbol is K
Commonly thermometers encounter the unit “Celsius degree” with symbol 0C. Another scale is called “Fahrenheit scale
A reliable measurement of temperature is done by using thermometer.
Thermometers use measurable physical properties that change linearly with temperature to give temperature readings
Physical properties that change with temperature are called “thermometric properties of a thermometer” which include;
i)    •Expansion of liquid when heated (e.g. alcohol and mercury).
ii)   •Expansion of compound strip of two metals.
iii)  •Thermometric property change in which when junction of two different metals is heated an electric current is generate.
iv)  •Change of resistance of a wire e.g. platinum , resistance thermometer.
•We can investigate some of thermometric properties by studying expansion of mercury in an evacuated tube in the experiment shown below.
To investigate thermometric properties of mercury.
Water, heater and mercury evacuated narrow tub.
1.      Put crushed ice in a filter funnel and insert the ice shown below.
2.      Put the beaker underneath the funnel to collect the water.
3.      Wait until the mercury in the thermometer stops falling.
4.      Mark the position of mercury meniscus.
The marked position of the meniscus is the upper fixed point of the thermometer, in Celsius scale, the upper fixed point is 1000C or 373K.
When you want to construct a thermometer, you must establish two constant temperatures called “fixed points”.
The two temperatures are called upper fixed points and lower fixed points. The fundamental interval of thermometer is the difference between the upper fixed point and the lower fixed point of the thermometer.
The upper fixed point is the temperature of pure steam from water boiling at standard pressure of 760mmHg. Carefully here we use “pure steam” and not boiling water because boiling water has inclusion of impurities and local overheating of the vessel which may alter the boiling point. The lower fixed point is the temperature of pure melting ice.
Note: Impurities lower the melting point of the ice.
The marked position of the meniscus is the upper fixed point of the thermometer. In Celsius temperature scale, the upper fixed point is 1000C or 373K.
The working of this thermometer is based on the fact that liquid expands when heated and contracts when cooled e.g. mercury and alcohol thermometers. These two thermometers are called;
1 1. Mercury – in - glass thermometer
2  2. Alcohol - in – glass thermometer
These thermometers have bulbs which are reservoirs of liquids and stems with fine bores through which liquid rises and falls during the variation of temperature.
The liquids used in the thermometers are called thermometric liquids.

It is a good conductor of heat
It is fairly good conductor
It expands linearly
It expands rapidly (not linearly)
It is clearly seen
It is colorless
It boils at 3600C
It boils at 780C
It freezes at -390C
It freezes at -1120C
Does not wet the glass
It wet the glass
The working of this thermometer is based on the expansion of the liquid in bulb. When the bulb touches a hot body the liquid contained warms up and expand there by rising through the bore and into the stem proportionally to the amount of temperature felt. When the bore touches something cold the liquid in the stem contracts proportionally to the amount of temperature felt and it falls into the bulb.
   By this rise and fall of the liquid level in the steam the different temperature reading can be obtained. The figure below shows mercury – in – glass thermometer that can be found in the Hospitals, Laboratories, and even Homes.

Sustainable energy sources are the natural resources that are used in production of electricity without polluting the environment.
Examples of these resources are;-
The sun, water, wind, sea waves and geothermal fields. These sources occur naturally and are readily available.
When we use oil and natural gas as the sources of energy there are lot of disadvantages. They pollute the environment and their supply is limited.
  1. Water energy
Hydroelectric power is the production of electricity by using water energy. In Tanzania we use water from Kidatu and Mtera hydroelectric power plants.
Production of electricity by using water
A dam is built to trap water. Water is allowed to flow using tunnels in the dam.
In the tunnels they cause turbines to generator and generator produce electricity.
  1. Solar energy
Is the energy from the sun. It is considered renewable energy because sunlight is unlimited, it is estimated that, the sun will shine for at least another five billions of years
We can convert sunlight  to electricity by using special devices called solar cells.
Solar cells
Solar cells are devices that use sunlight to produce electricity; The central part of solar cells is made of two thin layers of substances called semiconductors. A semiconductor is an element that conduct better than non-metal though not as well as a metal.
These layers are placed between metal contacts that connect the solar cells with an electricity circuit.
Glass or any transparent materials are used to cover the solar cells.
When sunlight strikes the solar cell, some electron gains enough energy to break free from atoms in the semiconductor. This electron flow creates an electric current through the electric circuit.
                                     A Solar cell


           Wind energy
When the sun heats the  atmosphere some area which are close to the sun become warmer than the other. The warmer air rise and cold air moves to replace them. That is the formation of wind.
Wind mill
Wind mill is a device which is used to convert wind energy into electric energy.
Wind mill is a tall structure with a larger propeller on top.
When the wind blows it rotates the propeller which causes the attached generators to produce electricity. For more electricity to produce more propellers are needed. The best places for construction of wind mill are the coastal area and the top of rounded hills.
Sea waves
This is the production of electricity by using sea waves, water pushed by the sea waves cause turbines to turn as the result turbines turn the generator. The production of electricity by this method is similar to the production of electricity by HEP (Hydro electric power).
Geothermal energy
Geothermal energy is the energy generated by the flow of heat from the surface of the earth it is associated with area of frequent earthquakes and high volcanic activities
The heat from the inside of the earth is used to heat water into steam. The steam is used to turn turbines and as the result this cause turbines to turn generator and produce electricity.

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