Heat and energy
Heat is a degree of temperature
In simple words, heat is a degree of temperature, but in scientific terms, heat is a type of energy found in an object that has a higher temperature than its surroundings. When an object is heated, its molecules gain energy and start moving faster. As they vibrate, they bump into each other at a faster rate and push the particles in front of them. As this movement continues, heat travels from one end of the object to the other.
Conductors and insulators
Materials like iron, steel, and copper that allow heat to travel through them are called good conductors of heat. Materials, like rubber, wood, and plastic that do not allow heat to travel through them are called bad conductors of heat. Materials, like wool, fur, and feathers that do not conduct heat or electricity easily are called insulators. The handles of pots and pans are made of wood or plastic, so that heat is not conducted from the pan to the handle and we do not burn our hands. Firefighters wear clothes made of an insulating material called fibreglass. Air is a good insulator. We wear woollen clothes in winter to keep warm because wool traps air between its fibres.
methods of heat transfer
Conduction
Heat can travel through solids by conduction. If one end of a metal spoon is dipped in hot water, the other end heats up after a short while. This happens because the molecules at the dipped end gain energy from the hot water, start vibrating, and push the molecules in front of them. On gaining energy, these molecules also start vibrating, and in this way a chain reaction is set up by which the other end of the spoon heats up. Conductivity is the ability of a material to conduct heat.
Copper wires
To compare the conductivity of different materials
Take some rods which are of the same length and thickness but gre made of different materials, such as wood, iron, copper, aluminium, etc. Dip them in wax and allow them to cool, then pass them through a specially-made metallic box which has holes in its sides. Pour hot water in the box. After a few minutes, remove the rods. You will observe that the wax melts to a different distance on eqach rod.
To prove that water is a bad conductor of heat
Wrap a cube of ice in a piece of gauze and drop it into a test tube full of water. Hold the top of the tube close to a Bunsen flame. You will observe that although the water at the top boils, the ice at the bottom of the tube does not melt.
Convection
Convection occurs when heat is transferred from one part of a fluid to another by the movement of the fluid itself. The ventilation of a room and heating water in a pan are examples of convection.
To demonstrate convection in liquids
Fill a conical flask with tap water and drop a large crystal of potassium permanganāte into it. Heat the flask. An upward current of coloured water will rise and spread outwards. It will then move down the sides of the flask, showing that a convection current has been set up.
To demonstrate convection in air
Take a cardboard box and make two holes in it at the top. Place a lit candle underneath one hole and hold a smouldering piece of paper over the other. The direction of the smoke will indicate the flow of the convection current.
Wind
Hot air from the Earth rises and cold air rushes to take its place. This moving air is called wind.
Monsoon winds
Monsoon winds are convection current winds which blow due to the difference in temperature between the land and the sea.
Thermals
The upward currents of warm air that rise by convection are called thermals. The air in a thermal can rise at very great speeds. This can be of great use to birds and humans who fly by gliding because they can use the thermal to gain height without using any of their own energy.
Pilots of light aircraft try to avoid flying through convection clouds because the air inside the cloud is turbulent. This means that it is swirling around at great speeds. This can make the aircraft lose control.
Land and sea breezes
During the daytime, the land heats up quickly by the heat of the Sun. As a result, the air above it becomes hot. As this air expands and rises, the cooler air above the sea blows in to take its place. This is called a sea breeze.
At night, the sea water, which has absorbed heat during the day, remains hot for a longer time than the land and so, the air above the sea warms up. It expands and rises and in this way the convection current is reversed. The breeze now blows from the cooler land to the sea and is called a land breeze.
Radiation
Radiation is a particular way in which energu travels. This energy need not be carried by a medium such as a solid, liquid, or gas. It can travel in space because it is not carried by moving particles which need a medium to travel in. The Earth is heated by the heat radiation of the Sun. This heat travels through space and reaches the Earth.
Some of it is absorbed and some of it is reflected back. Clouds cut down the amount of heat radiation reaching the Earth.
White reflects more heat and black absorbs more heat.
Some surfaces are better at absorbing radiation than others.
Standing in the sunshine, a black car warms up more quickly than any other colour. Some surfaces are better at sending out or emitting radiation than others. A black saucepan cools down more quickly than any other. Could you design an experiment to test it for yourself ?
Good absorbers of heat radiation are also good emitters. Dull black surfaces are the best absorbers of radiation. They hardly reflect any radiation at all. Shiny silvery surfaces are the worst absorbers of radiation. They reflect nearly all the radiation that strikes them. Dull black surfaces are the best emitters of radiation. Silvery surfaces are the worst emitters of radiation.
Shinu aluminium foil helps keep food dishes warm when they are served on the table.
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