HEAT

    Heat:-

Heat is a form of energy (heat energy/thermal energy), that is transferred from one body to another, due to the existence of a temperature difference between them.

Transfer of heat energy/thermal energy always takes place from the body at a higher temperature to that at a lower temperature.

•  HEAT AND TEMPERATURE :-

We often observe that when we hear a body, it's temperature rises. This indicates that heat and temperature are related to each other. Heat is the energy of a body that is due to the motion of its constitution particles (molecules). Temperature, on the other hand, is just ab indicator of this energy. Even a cold object possesses heat energy due to the motion of its particles (molecules). When we add, (or remove) heat (from) a body, motion of its molecules becomes faster (or slower).

• EFFECT OF HEAT :-

When a body is heater, various types of physical and chemical changes are observed to take place. We list below some of the main effect of year.

    ● PHYSICAL CHANGES :-

       (a) Change in temperature: We all know that addition, or removal, of heat to, or from, a body brings about a change in its temperature. Addition of heart to a body (usually) raises its temperature, whereas, removal of heat , from a body (usually), lowers is temperature.

    (b) Change of state: Under appropriate conditions, addition, or removal, of heat to, or from, a body, can also bring about a change in its state. For example, on putting a tray of water (liquid form) in the freezer compartment of a refrigerator, the water freezes to form ice. Ice, as we know, is the solid form of water. Similarly, when water is heated over a flame, it starts boiling and gets converted into steam. Steam, as we know, is the gaseous or vapour form of water.

     (c) Thermal expansion: Most of solids, liquid and gases, are known to expand on hearing. This phenomenon is called thermal expansion. For example, the metal rim, to be put on a cart wheel, is designed to have a(slightly) smaller diameter than that of the wheel. When this rim is heated, it becomes 'red-hot', expands, and slips on to the wheel easily. When it is cooled, it contacts and grips the wheel firmly.

   ◆ CHEMICAL CHANGES:-

  Many chemical changes take place only when the reactants are heated up. For example, we can prepare oxygen, in the laboratory, by hearing potassium chlorate (along with manganese oxide as a catalyst) over a flame.

• Thermometer:-

A more reliable method to know the relative hotness or coldness, I.e. the temperature of a given body. We use special instruments fir this purpose. All such instruments, or devices, are known as thermometers.

It is based on the thermal expansion of mercury.

It consist of a narrow capillary tube of glass that is closed at its upper end and has a bulb at its lower end. The bulb is filled with mercury. Inside the capillary tube we can see a small shining thread of mercury. The mercury, present in the bulb of the thermometer, expands when heated. The extent of its expansion, and, therefore, the length of murcury thread in the capillary tube, depends on the extent of hearing of the thermometer. This, in turn, depends on the temperature of the object with which the thermometer bulb has been put in contact. The length of the mercury thread (in the thermometer) can, this, give us a measure of the temperature of the object.

◆Two scales, that have been in common use, are the Fahrenheit and the Celsius.

【the temperature 'T' in degree Celsius (°C) is equal to the temperature 'T' in degree Fahrenheit (°F) minus 32, times 5/9. 】

           T (°C) = [T (°F) - 32] × 5/9

◆The range of a laboratory thermometer can be from -10°C to 110°C and 0°C to 100°C are found more often in school laboratories.

Transfer of heat:-

● Conduction 

In this method, heat is transferred from one particle (molecule), of the object, to the next, and then to the next, and so on, without the particles leaving their places. The particles just vibrate about their mean position, and keep on passing heat energy, to their 'next-in-line' neighbours.

● Convection

The molecule in liquids (and gases), are quite free to move. This is unlike solids whose molecule are quite rigidly fixed. It is this ability of liquid molecules (to move around) that causes a liquid to get heated up. We can this method of heart transfer, in liquid (and gases), as convection.

◆ Land and Sea Breezes:-

Water gets heated up slowly than karne during day time. Hence, the air above the land gets heater up more quickly and rises up. Cooler air, from above the sea, then rushes in to take it's place. We call this movimiento of air as a Sea Breeze.

During night, water cooled more slowly than land. The air, above the water, therefore, because warmer and rises up. It's place is taken up by the cooler air above the land. This movement of air is called as Land Breeze.

◆Radiation

It may be defined as a method of heat transfer in which no material medium is required. The heat energy, received by an object through radiation, is called radiant energy. We say that all hot objects give out, or radiate, heat energy.