The clinical thermometer
The clinical thermometer. This thermometer is used to measure the human body temperature.
The thermometer has a very fine bore (narrow capillary tube) which makes it sensitive.
-Expansion of mercury makes it shoot along the tube.
-The glass from which the tube is made is very thin which enables heat to reach the mercury quickly to read body’s temperature.
-The bulb is the fluid reservoir. Thus it should be large enough to hold all the fluid. It is thin walled for quick response to heat.
-The glass stem is thick to act as a magnifying glass for the temperature readings.
-When thermometer bulb is placed into the mouth or armpit, the mercury expands and it is forced past the constriction along the tube.
-When removed, the bulb cools and the mercury in it contracts quickly.
-The mercury column breaks at the constriction leaving mercury in the tube. The constriction prevents flow back of mercury to the bulb when the thermometer is temporary removed from the patients mouth or armpits.
The thermometer is reset by shaking the mercury back in the bulb.
Properties/qualities of a thermometer.
This refers to the ability of a thermometer to measure temperature in the shortest time possible. This is attained by using;
-A thin walled bulb and using a liquid which is a good conductor of heat e.g. mercury.
This is the ability of a thermometer to detect very small changes in temperature. It is attained by:
-Using a thermometer with a big bulb
-Using a liquid which has a high linear expansivity.
-Using a narrow bore or reducing the diameter of the bore hole.
Effect of heat on matter:
-When a solid is heated, the cohesive forces between its molecules are weakened and the molecules begin to vibrate vigorously causing the solid to (expand or) change into a liquid state.
-The temperature at which a solid changes into liquid is called the melting point. At melting point the temperature remains constant until the solid has melted.
-When the entire solid has melted and more heat is applied, the temperature rises. The heat gained weakens the cohesive forces between the liquid molecules considerably causing the molecules to move faster until the liquid changes into gaseous state.
-The temperature at which a liquid changes into gaseous state is called the boiling point. At boiling point temperature of the liquid remains constant since heat supplied weakens the cohesive forces of attraction in liquid molecules.
-If the heated substance is water its temperature rises with time as shown below.
Internal combustion engine
A Heat engine is a machine which changes heat energy obtained by burning fuel to kinetic energy (Mechanical energy).
Engines are always less than 100% efficient because.
(i) Some of the energy is lost in overcoming friction between walls of the cylinder and pistons.
(ii) Some heat energy is lost to the surrounding due to conduction.
(iii) Some of the energy is also wasted in lifting useless loads like pistons.
A Petrol engine gets its energy from an exploded mixture of air and petrol vapour.
Petrol engine are also called four stroke cycle engines because four piston stroke or monuments inside the cylinder repeat themselves continuously. The piston strike is in the order intake, compression, power and exhaust.
-As the piston moves down the cylinder due to the starter motor in a car (or kick start in a motor cycle) it reduces the pressure inside the cylinder.
-The inlet value opens and the petrol air mixture from the carburettor is forced into the cylinder by atmospheric pressure.
-In short intake involves the piston moving down the cylinder, inlet valve opening and allowing petrol – air mixture into the cylinder.
-Both valves close and the piston moves up compressing the mixture to about a sixth of its original volume.
-Near the end of the stroke, the fuel is ignited by a spark from the spark plug.
c) Power stroke
A spark jumps across the points of the sparking plug and explodes the mixture, forcing the piston to move down.
d) Exhaust stroke
The outlet value opens and the piston rises, pushing the exhaust gasses out of the cylinder
The operation of a diesel engine is similar to that of a petrol engine.
However, there are some differences.
In a diesel engine air is drawn into the cylinder on down stroke of the piston. On upstroke of the piston, it compresses reducing the volume of the cylinder. The very high compression increase the stroke, oil is pumped into the cylinder by a fuel injector, it ignites automatically. The resulting explosion drives the piston down on its power stroke.
Note: Diesel engine is also called compression ignition (CI). It is heavier than a petrol engine. Diesel engine is reliable and economical. The efficiency is about 40% higher than any other heat engine.
(b) Heat transfer
Heat flows from a region of high temperature to a region of low temperature. There are three ways by which heat can be transferred, namely;
Conduction is the flow of heat through matter from a region of higher temperature to one of lower temperature without movement of matter as a whole.
Conduction in solids
Heat transfer in solids can occur as a result of;;
(i) Excess energy of vibrations being passed from one atom to another.
(ii) There excess Kinetic energy given to the free electrons near the source of heat being carried by these electrons as they move to colder region.
For heat to be transferred by conduction there should be a material medium. Metal are good conductors of heat because metals are made up of atom having free electrons that are loosely held.
Examples of metals which are good conductors are;
New utensils or kettles, saucepans boilers, radiators are made of metals because metals are good conductors of heat as their atoms have free electrons that are loosely held. Non-metals, according to Kinetic theory do not have free electrons that are loosely held so that heat does not pass through them easily. This is why non-metals are called bad conductors or insulators e.g plastics, cork, wood.
Rate of heat transfer
The rate of heat transfer along a metal bar depends on the following factors;
(i) The temperature difference between the ends.
(i) The length and area of cross in short time when the cross section of the metal bar. Much heat is passed across in a short time when the cross sectional area of the bar is large and when the bar is short.
(ii) Material from which the solid is made of.
Comparing the conductivities of different metals.
-The rate of conduction is compared by dipping the ends of four rods coated with wax in hot water.
-The rods are identical but made of different materials.
-After a short while, the wax begins to melt along the rods. It melts fastest along the copper rod and slowest along wood.
-This shows that copper is the best conductor and wood is the poorest of them.
Bad and good conductors and their applications.
(i) Good conductors like aluminium are used in cooking utensils because they allow heat to pass through them easily. Copper is one of the best conductor but aluminium is usually used in making cooking utensils because it is much cheaper.
(ii) Bad conductor also called insulators are used in making of handles of cooking utensils because they do not allow heat to pass through easily.
A metal always feels cold when touched on a cold day because it loses heat from the body and transfer it to the surrounding very fast.
Explain why metals feel colder when touched than bad conductors
This is because metals carry heat away from the hands due to high degree of conduction while bad conductors do not conduct heat.
This also explains why a cemented floor feels colder than a carpeted floor.
Liquids conduct heat very slowly. This is because their molecules are apart.
Experiment to show that water is a poor conductor of heat
Convection is the flow of heat through fluid from a region of higher temperature to one of lower temperature by the movement of the fluid itself.
It is the heat transfer which involves bulk movement of molecules of the medium.
Convection cannot occur in vacuum because it requires a material medium. It occurs in fluids (liquid and gases) because they flow easily.
When a liquid is heated it expands and becomes less dense than the surrounding cold liquid.
Convection current is the cyclic motion of rising hot fluid and falling itself. The hot fluid rises because when heated it becomes less dense.
Explanation of convection current
-When the fluid is heated it expands and becomes less dense.
-The heated fluid is forced upward by the surrounding cooler fluid which moves under it.
-As the warm fluid rises, it gives heat to the surrounding cooler fluid.
Experiment to demonstrate convection current.
When the flask is heated from the bottom as shown, the coloured solution of water rises upwards from the crystals and on reaching the top; it spreads as shown in the diagram.
-The solution rises because on heating, it expands and becomes less dense, so it is forced upwards by nearby cooler (denser) water.
Difference between convection and conduction
Application of convection current to hot water domestic supply system.
How it works
-Cold water is supplied to the boiler along the cold water supply pipe.
-In the boiler the cold water warms up, expands and becomes less dense, so it rises up.
-As more cold water is applied to the boiler, hot water is displaced upwards and supplied to the hot water taps along hot water pipes A and D.
-The ventilation pipe, D is used to release steam.
The expansion pipe A allows pipe D allows escape of:
(i) Dissolved air which comes out of the water when it is heated.
(ii) Steam, if the water is boiled.
If the expansion pipe is not there;
(i) The dissolved air which comes out when water is heated causes air lock in the pipe.
(ii) The steam if the water is boiled causes explosion.
When working convection current of less dense hot water from boiler raises up through pipe A to the hot water tank. At the same time the more
A circulation is set up in the hot water involving filling hot water from top down wards.
When a volume of hot water flows to the hot water tank through pipe A, an equal volume of cold water flows to the boiler through pipe B.
-At the same time an equal volume flows through pipe C to the hot water tank.
-Pipe A leaves the boiler at the top and enters the hot water tank at the top because it carries more dense hot water.
-Pipe B is connected to the bottom of the hot water tank and to the bottom of the because it carries more dense cold water.
Convection in gases
Experiment to demonstrate convection in gases
The hot air above the candle rises up and gets out through B.
-A lighted piece of paper will produce smoke at point A.
-Cold air enters at point A and sweeps all the smoke to go and replace the hot air.
-The movements of smoke from A across the box and out through B shows convection of gases.
Explanation of how smoke moves:
Smoke moves by convection because;
-The air above the candle warms up, becoming less dense and then rises up through C.
-The dense cold air from the paper (smoke) enters X through chimney A to replace the risen air (smoke) causing convection currents.
Application of convection in gases:
-Chimneys in kitchens and factories
-Ventilation pipes in VIP latrines
-Ventilators in houses
-Land and sea breezes
Land and sea breezes
a) Cooling of land
At night land cools faster than sea because land is a better emitter of heat and has lower specific heat capacity than sea water.
b) Movement of air
The air above the sea is heated more than air above land, so the warmer air above sea rises and the cooler air from land occupy the space left. This results in a land breeze blowing towards the sea.
a) Heating land.
During day land is heated by the sun more than the sea because land is a better absorber of heat and has lower specific heat capacity than the sea.
b) Movement air
The much more heating of land causes the air above it to expand and rise as it becomes less dense. The space left is occupied by more dense air from the relatively cooler sea. So a cool sea breeze blows from the sea towards the land.
-Air inside a room, air gets heated up on hot days.
-Roofs are usually provided with small openings called ventilators above the building so that the warm air which is less dense rises up and flow out through them.
-At the same time cool fresh air enters the building through the doors and windows. In this way circulation of air convection is set up
Radiation is the flow of heat from one place to another by means of electromagnetic waves.
Heat energy is transfer from the sun to the earth by means of radiation. Radiation is the means by which heat can travel through the vacuum.
The energy from the hot body is called radiant energy.
Radiation is emitted by the bodies above absolute zero.
Radiant heat is mainly comprised of infrared which makes the skin feel warm. It travels as fast as light and it is the fastest means of heat transfer. It can travel through a vacuum.
Factors affecting the rate of radiation of heat energy
Good and bad absorbers
Shiny surface are bad absorbers of radiation while dull black surface are good absorbers. This implies that shiny surface
reflects most of the heat radiations instead of absorbing. The dull black surface absorb most of the heat radiations and reflect very few.
Experiment to show absorbing of radiation in surface
Some surfaces absorb heat radiation better than others as illustrated below;
-Stick two pieces of cork using molten wax onto two vertical metal plates.
- The heat source is placed midway between the vertical plates so that the same amount of radiations are received by the two surfaces.
-It will be observed that after a few minutes the wax on the dull black plate melts and the cork falls off before that on the shiny polished plate.
-This indicates that dull black surfaces are better absorbers.
Requirements: – A Leslie cube
-Thermopile (instrument that converts heat to electrical energy). A galvanometer.
One side of the cube is dull black, the other is dull white and the last one is made shiny polished.
-The cube is filled with hot water and radiation from each surface is detected by a thermopile.
-When the radiant heat falling on the thermopile is much, it registers a large deflection of the point.
With different surfaces of the tube made to face the thermopile one at a time.
-The greatest deflection of the pointer is obtained when dull dark surface faces the thermopile.
-The least deflection is obtained when a highly polished shiny surface faces the thermopile.
-The dull and black surface is a good radiator or emitter of heat radiation while a polished shiny surface is a poor emitter of heat radiation.
Application of absorbers.
(i) Building in hot countries are painted white and roof surfaces are shinny because white and shinny surface are bad absorbers of heat radiation.
(ii) Reflection on electric devices are made up of polished metals because they have good reflecting properties.
Good and Bad emitter.
If the backs of the hands are held on either sides of the sheet, one first feels much heat from the black surface.
This shows that a black surface is a better emitter of heat than a shiny one.
-In short, black surface are good absorbers as well as good emitters of heat radiations.
-Shinny surfaces or polished surfaces are bad absorbers as well as bad emitters of radiations.
(i) Cooling fins on the heat exchanger of refrigerator are painted black so that they emit heat more quickly.
(ii) Tea pots and kettles are polished so that they keep heat longer as polished surface are poor emitters of heat radiation.
Laws of radiation:
Heat radiation travels in a straight line.
Good absorbers of heat radiation are also good emitters.
Temperature of the body remains constant when the rate at which it absorbs heat radiation is equal to the rate at which it radiates heat energy.
Bodies only radiate heat when their temperatures are higher than those of the surroundings and absorb heat from the surroundings if their temperatures are low.
Application of radiation:
Black and dull surfaces
(i) Car radiators are painted black to easily emit heat
(ii) Cooling fins of a refrigerator are black to easily emit heat.
(iii) Solar plates or panels are black to easily emit heat.
Polished and white surfaces
(i) White washed buildings keep cool in summer.
(ii) Roots and petro tanks are aluminium painted to reflect radiant heat.
(iii) White coloured clothes are won in summer to keep us cool.
(iv) Silver tea pots, kettles and saucepan retain heat for a long time.