Heat and Temperature
Heat is a form of energy which passes from a body at high temperature to a body at a lower temperature.
When a body receives heat energy its temperature increases whereas the temperature of a body that gives away energy decreases.
Condition when if two bodies at the same temperature are in contact, there is no net flow from one body to the other.
The SI unit of heat is joules.
Heat cannot be measured directly by an instrument as temperature is measured by a thermometer.
Modes of heat transfer
Heat can travel through a medium and also in a vacuum.
There are three (3) modes of heat transfer namely;
- Conduction – takes place in solids.
- Convection – takes place in fluids (liquids and gases).
- Radiation – takes place in gases (vacuum)
In stirring hot tea the handle of a spoon becomes warm. The mechanism to this is explained below,
- Heat energy entering the spoon from the hot end increases vibrations of the atoms at this ends. These atoms in turn collide with neighbouring atoms, increasing their vibrations and hence passing the heat energy along.
- Metals have free electrons which travel throughout the body of the metal. Heat energy injected at the hot end of the metal spoon increases the vibration of the particles at the end. The free electrons in that region gain more kinetic energy and because they are free to move, they spread heat energy to the other parts of the spoon.
Thermal conductivities of various conductors
Different materials have different thermal conductivities. Metals are generally good conductors of heat. Non-metals are poor conductors of heat (insulator).
Solids that are good conductors of heat use both atom vibration and free electrons to conduct heat.
Solids that are poor conductors of heat like glass, wood, rubber make use of atom vibration as a mechanism to conduct heat because they have no free or mobile electrons.
The table below shows some of the good and poor conductors in decreasing order of thermal conductivity.
|Good Conductors||Poor Conductors|
NOTE: During thermal condition, heat flows through the materials with the material shifting or flowing. Conduction is therefore transfer of heat as a result of vibration of particles.
Conductivity of wood and iron rods
Set the apparatus as shown
Observation and explanation
The paper gets charred (blackened) on the region covering the wooden rod. This is because the wood does not conduct heat from the paper.
Wood is said to be a bad conductor of heat while iron is a good conductor.
Factors affecting thermal conductivity
Thermal conductivity in materials depends on the following factors;
- Temperature difference ( Ñ²) between the ends of the conductor.
- The length of the conductor.
- The cross-sectional area (A) of the conductor.
- The nature of the material (K)
To demonstrate how temperature difference ( Ñ²) affects thermal conductivity
The rod placed in the flame becomes too hot faster than the one placed in the boiling water.
The rate of heat flow (thermal conduction) increases with increase in temperature.
Thermal conduction in metals is by two mechanisms i.e. vibration of atoms and by free electrons.
A high temperature difference between the ends of the conductors sets the atoms into vibrations more vigorously and the vibrations are passed more quickly to the cooler end. The electrons on the other hand gain a lot of kinetic energy causing them to spread the heat energy to cooler parts of the metal within a short time.
Length of the conductor
Consider the set up below.
The end of metal B held in hand becomes too hot earlier than metal A. Thermal conductivity increases with decrease in length.
Heat travels within a conductor along imaginary lines called lines of heat flow.
These lines diverge from the hot end as shown.
The graph of temperature (Ñ²) against length (l) is as shown.
When the heat energy gets to the surface of the metal it is easily lost to the surroundings.
The lines of heat are more divergent near the hot end than they are far away. (position A and B).
The slope of the graph in the above figure is steeper at A (near the hot end) than at B further away. This indicates that the shorter the length of the material, the higher the rate of heat flow.
The cross-sectional area of the conductor
Consider the set up below,
The end of metal A held in the hand becomes too hot earlier than metal B.
Thermal conductivity increases with increase in area of cross-section of the conducting material.
The number of free electrons per unit length of the thicker length A is more than those in the thin metal rod B.
The nature of the material K
Different materials have different strength of force bonding the atoms within the material. The number of free electrons also differs from one material to another material.
Materials with many free electrons are better conductors of heat e.g. copper has more free electrons than iron.
This is the covering of good conductors of heat with insulators to reduce heat loss through surface effects. For example, iron pipes carrying hot water from boilers are covered with thick asbestos material.
The figure below shows lines of heat flow in a lagged metal bar.
A graph of temperature (Ñ³) against the position along the lagged conductor is as shown below.
THERMAL CONDUCTIVITY IN LIQUIDS
To demonstrate that water is a poor conductor, consider the set up below,
Observation and explanation
Water at the top of the boiling tube boils while ice remains unmelted. This shows that water is a poor conductor.
NOTE: The boiling tube is made of glass (poor conductor of heat) which limits possible conduction of heat down the tube.
The ice is wrapped in wire gauze to ensure it does not float. The fact that the wire gauze is a good conductor of heat and yet ice remained unmelted shows that there is very little heat conduction in water, unable to melt the ice.
Water is heated at the top to eliminate possibility of heat transfer to the ice by convection.
Although liquids are in generally poor conductors of heat, some liquids are better heat conductors than others e.g. mercury is a better conductor of heat than water.
Why liquids are poor conductors of Heat
Pure liquids have molecules further apart from each other. Although molecules move about within the liquid, they are slow to pass heat to other regions compared to the free electrons in metals. This is because there are large intermolecular distances between liquid molecules. There are also fewer and rare collisions between the molecules.
Electrolytes e.g. salt solution, are better conductors of heat than pure liquids because of increased compactness of the particles.
Mercury is a metal existing as a liquid at room temperature. Bromine, the only non-metal existing as a liquid at room temperature, is a poor conductor.
Thermal conductivity in gases
Since thermal conductivity is by means of vibration of atoms and presence of free electrons, gases are worse conductors of heat because of large intermolecular distance.
A match stick held within the unburnt gas region of a flame cannot ignited by the heat from the hot part of the flame. This is because gas is a poor conductor of heat.
Applications of good and poor conductors
- Cooking utensils, soldering irons and boilers are made of metals which conduct heat rapidly. For cooking utensils, the handles are made of insulators such as wood or plastic. Metal pipes carrying hot water from boilers are lagged with cloth soaked in a plaster of paris to prevent heat losses.
- Overheating of integrated circuits (ICs) and transistors in electronic devices can drastically affect their performance such components are fixed to a heat sink (a metal plate with fins) to conduct away undesired heat. The fins increase the surface area of heat sink and conduct more heat away to the surrounding.
- Fire fighters put on suits made of asbestos material to keep them safe while putting out fire.
- Birds flap their wings after getting wet as a means of introducing air pockets in their feathers. Air being a poor conductor reduces heat loss from their bodies.
- In modern buildings where desired inside temperatures is to be stabilised, double walls are constructed. Materials that are good insulators of heat and can trap air put between the walls. Examples of such materials are glass, wool (fibre glass) and foam plastic. Air on its own may not effectively give the desired insulation because it undergoes convection. Double glazed windows used for the same purpose have air trapped between two glass sheets.
- In experiment involving heating water or liquid, the beaker is placed on the wire gauze. The gauze is heated and spreads the heat to a large area of the beaker. If the gauze is not used, heat from the Bunsen burner may concentrate on a small area and may make the beaker crack.
Convection is the process by which heat is transferred through fluids (liquids and gases). The heat transfer is by actual movement of the fluid called convection currents, which arise out of the following;
Natural convection – It involves change in density of the fluid with temperature.
Forced convection – Mixing of hot and cold parts of the fluid through some external stirring like a fan or pump.
Convection in liquids
To demonstrate convection in liquids the set up below is used
A purple colourisation rises up from the potassium permanganate, forming a loop.
The colourisation arising from the potassium permanganate flow in clockwise direction.
From the experiments, it is clear that when a liquid is heated, it rises while cold liquid replaces it.
When a liquid is heated, it expands and this lowers its density. The less dense liquid rises and its place is taken by more dense colder liquid. This movement of liquid forms convection currents
CONVECTION IN GASES
To demonstrate convection currents in gases, consider the set up below
Smoke is sucked into the box through chimney A and exists through chimney B. When the candle is put off, the smoke is not drawn into the box.
This shows convection currents are set up when air or gas is heated.
The candle heats up the air above it, which expands and rises up because of lower density. Cold heavier air particles is drawn into chimney A, carrying along the smoke which replaces the air that is escaping through chimney B.
Molecular explanation of convection in fluids
Molecules in fluids are further apart and have negligible cohesive force. Heating a fluid increases the kinetic energy of the vibrating molecules and their random movement.
As the fluid rises, these molecules pass energy to the molecules in the colder regions which have less energy. Because the molecules are further away from the heating source, their temperature is reduced.
Pressure near the heating source decreases because of the depletion of molecules as they rise. Colder molecules move into the low pressure zone to fill up the void being created.
This movement of molecules constitutes convection currents. Convection currents are set up much faster in gases than in liquids because of relatively low cohesive force in gases.
APPLICATION OF CONVECTION IN FLUIDS
1. Domestic hot water system
Initially, the two beakers A and B have cold water. Water in beaker A is coloured to distinguish it from that in beaker B. When the water in beaker A is heated, it is observed to rise up through tube X and emerges on top of cold water in beaker B. The cold water flows down from beaker B to beaker A.
As long as heating continues, there will be movement of hot water into beaker B and cold water will flow down into beaker A. Thermometer will show increase in temperature for water in beaker B.
The commercial domestic hot water system utilises the same principle of operation. The hot water rises up because of the effective lowering of density.
The force of gravity helps the cold water to flow down from the cold tank.
The hot water tap and expansion pipe are connected to the upper region of the cylinder. The expansion pipe is an outlet for excess water that could have resulted from overheating.
Once the cold water flows down the cylinder, the main pipe allows more cold water to flow into the tank. When filled to capacity, the ball cork floating on water closes a valve in the main pipe, stopping further in flow of cold water.
An overflow pipe lets out water from the cold tank when the valve is not sufficiently functional.
Lagging is done on the pipe that conveys hot water to minimise heat losses.
This is the supply of fresh air into the room. Air expelled by the room occupants is warm and less dense. It rises up and escapes through the ventilation holes.
Cold fresh air flows into the room to replace the rising warm air. The room gets continuous flow of fresh air.
NOTE: Some devices are fitted with air conditioning devices which cause forced convection of air, giving out cold dry air and absorbing warm moist air.
3. CAR ENGINE COOLING SYSTEM
Heat conduction and convection play a very crucial role of taking away heat from a car engine that would reduce its efficiency.
The engine is surrounded by a metal water jacket that is connected to the radiator. The metal surface conducts heat away from the engine. This heats up the water, setting up convection currents. The hot water is pumped into the radiator which has thin copper fins that conduct away heat from water.
Fast flowing air past the fins speeds up the cooling process.
4. LAND AND SEA BREEZES
This is a natural convection of air, and occurs at sea shores because of temperature difference between the mass of water and the land.
The mass of water takes longer time than land nearby land by the same temperature from the sun. Water also takes a longer time to cool than the land after being raised at the same temperature.
During the day, the land heats up much faster than the sea. The air just above the land gets heated up and rises because of reduced density. Cold air above the sea blows towards the land to replace the void created by warm air rising. This is called sea breeze.
In the evening, temperature of the sea water is higher than that of the land. The air above the sea gets heated up and rises. Cold air from the land blows to the sea. This is called land breeze.
Heat from the sun to the earth reaches us by radiation.
Thermal radiation is heat transfer through a vacuum.
All bodies absorb and emit radiation. The higher the temperature of the object, the greater the amount of radiation
A body emitting thermal radiation can also emit visible light when it is hot enough.
An electric bulb in a room produces both light and radiant heat. The radiant heat is absorbed by the materials in the room, which in turn give out radiant heat of lower energy.
NATURE OF RADIANT HEAT
To demonstrate the radiant heat
Consider light rays travelling from sun light to hand lens as shown,
When light rays are focussed onto the paper, it burns out.
Radiant heat, like light can be concentrated to a point using a lens. Thermal radiation is a wave like light and can be reflected. Because of the nature of production, radiant heat is an electromagnet wave which causes heating effect in objects that absorb it.
Radiation can also be described as the flow of heat from one place to another by means of electromagnetic waves.
EMISSION AND ABSORPTION OF RADIATION
To compare radiation from different surfaces (shiny and black surfaces),
Consider the set up below,
The two surfaces are heated to a certain temperature say 800C. The temperatures of the two tins taken after sometime.
After sometime, it is noted that the temperature recorded by TB is lower than that recorded by TS.
The experiment shows that black surfaces are better emitters than shiny surfaces.
A graph of temperature against time for temperatures recorded by each thermometer.
The graph shows water in a shiny tin lost heat less rapidly than the blackened tin (good emitter).
To compare absorption of radiant heat by different surfaces
Set up the apparatus as shown
The cork fixed on the dull/black surface falls off after the wax, melts, while the cork polished/shiny plate remains fixed for a longer time.
Consider also the set up below,
The thermometer TB immersed in water in the blackened tin records higher reading than that of thermometer TS, when the heater is placed mid-way between tin A and tin B.
A graph of temperature (oC) against time (minutes) is as shown,
The graph shows that temperature of water in the polished tin does not increase as fast as temperature of water in blackened tin.
Black surfaces are good absorbers of radiant heat than polished surfaces.
NOTE: Good absorbers of radiant heat also good emitters while poor absorbers of heat are also poor emitters.
Poor emitters of heat are also good reflectors.
APPLICATIONS OF THERMAL RADIATION
- Kettles, cooking pan and iron boxes have polished surfaces to reduce heat lose through radiation.
- Petrol tanks are painted silvery bright to reflect away as much heat as possible.
- Houses in hot areas have their walls and roofs painted with bright colours to reflect away heat, while those in cold areas have walls and roofs painted with dull colours.
- In solar concentrators, the electromagnetic waves in form of radiant heat are reflected to a common point (focus) by a concave reflector. The temperature at this point can be sufficiently high to boil water as shown
- The green house effect
A green house has a glass roof through which radiant heat energy from the sun passes. This heat is absorbed by objects in the house, which then emit radiation of lower energy that cannot penetrate through glass.
The cumulative effect is that temperature of the houses increases substantially. Greenhouses are used in providing appropriate conditions for plants in cold regions.
NOTE: Carbon dioxide (CO2) and other air pollutants in the lower layers of the atmosphere show the same properties of glass, raising the temperature on earth to dangerous levels.
- Solar heater
The solar heater uses solar energy to heat water. The figure below shows the solar heater.
The solar heater consists of a coiled blackened copper pipe on an insulating surface. Radiant heat from the sun passes through glass and is absorbed by black copper pipes that contain water, which is heated up.
Copper pipes are used because they are good conductors and they are painted black to increase their absorbing power. Lower energy emitted after absorption of radiant energy does not escape because it cannot penetrate the glass. The temperature of the air above the pipe thus increases boosting the heating of water. A good insulating material is used at the base.
- Thermos flask( vacuum flask)
A thermos flask is designed such that heat transfer by conduction, convection and radiation between the contents of the flask and its surrounding is reduced to a minimum.
The vacuum is a double walled glass vessel with a vacuum in the space between the walls. This minimises the transfer of heat by conduction and convection. The inside of glass walls, in the vacuum side, is silvered to reduce heat losses by radiation (Poor emitter and absorber). The felt pads on the sides and at the bottom support the vessel vertically. The heat loss by evaporation from the liquid surface is prevented by a well fitting cork.