Convection
Heat Convection is form of heatConvection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less dense, and rises (see Ideal Gas Law). Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy.
Radiation is electromagnetic radiation emitted from a material which is due to the heat of the material, the characteristics of which depend on its temperature. An example of thermal radiation is the infrared radiation emitted by a common household radiator or electric heater.
Conduction is heat transfer by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperature, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. For heat transfer between two plane surfaces, such as heat loss through the wall of a house, the rate of conduction heat transfer is:
What is Evaporation?
Evaporation takes place at all temperatures. Rate of evaporation increases with rise in temperature and becomes maximum at the boiling point of the liquid. The process of evaporation also increases with increase in surface area of the liquid.
Evaporation is an essential part of the water cycle. Solar energy drives evaporation of water from oceans, lakes, moisture in the soil, and other sources of water. Evaporation is caused when water is exposed to air and the liquid molecules turn into water vapor which rises up and forms clouds.
Evaporation requires energy. A liquid draws heat energy from the surrounding thereby cooling the surrounding.
Evaporation is the process by which water is converted from its liquid form to its vapor form and thus transferred from land and water masses to the atmosphere. Evaporation from the oceans accounts for 80% of the water delivered as precipitation, with the balance occurring on land, inland waters and plant surfaces.
As shown above, the rate of evaporation depends upon:
Wind speed: the higher the wind speed, the more evaporation
Temperature: the higher the temperature, the more evaporation
Water cycle in nature is initiated by the evaporation of water from lakes, ponds, rivers, sea, etc. Water evaporates due to sun's heat. Water vapor rises to the sky to from clouds. Clouds condense to form raindrops, which fall on the earth. And the water cycle continues.
Thus, The boiling point of a liquid substance is the temperature at which the vapor pressure of the liquid equals the external pressure on the liquid. Vapor bubbles form in the liquid, rise to the surface and burst, causing the liquid to boil.
a) Liquid particles are always in motion, Liquid particles are free to slide past one another.
b) liquids can both FLOW
c) However, liquid particles are attracted to each other, whereas gases are not
d) Particles of a liquid spin and vibrate while they move, thus contributing to their average kinetic energy e) But, most of the particles do not have enough energy to escape into the gaseous state; they would have to overcome their intermolecular attractions with other particles
f) The intermolecular attractions also reduce the amount of space between particles of a liquid Thus, liquids are more dense than gases
g) Increasing pressure on liquid has hardly an effect on it’s volume
h) Water in an open vessel or puddle eventually goes into the air
i) Increasing the pressure also has little effect on the volume of a solid For that reason, liquids and solids are known as the condensed states of matter
j) The conversion of a liquid to a gas or vapor is called vaporization.When this occurs at the surface of a liquid that is not boiling, the process is called evaporation k) Some of the particles break away and enter the gas or vapor state; but only those with the
minimum kinetic energy
l) A liquid will also evaporate faster when heated Because the added heat increases the average
kinetic energy needed to overcome the attractive forces
m) Cooling occurs because those with the highest energy escape first
n) The vapor pressure of a liquid can be determined by a device called a manometer
o) The boiling point (bp) is the temperature at which the vapor pressure of the liquid is just equal to the external pressure.
We can investigate pressure in a liquid by using the apparatus shown in figure below
It is a simple pressure gauge and it measures differences in pressure exerted at the two ends of the apparatus. It is called manometer. The mouth of a thistle funnel is tightly covered with a thin plastic sheet. The thistle funnel is connected to a U-tube manometer containing water, by a rubber tubing. If you press the plastic sheet lightly with a finger, the air inside the manometer gets compressed and it exerts greater pressure at that end than the atmospheric pressure exerted at the open end of the U-tube. As a result, the liquid level alters and goes up in the other open arm of the U-tube. When the pressure exerted on the plastic sheet is greater, the difference in the levels (h) is also greater. This difference is the measure of the difference in the pressures at the two ends of the manometer.
Now, lower the mouth of the funnel into a glass vessel containing water. You will notice that the deeper it goes, greater is the difference in the levels of the water in the manometer. This indicates that the pressure in a liquid increases with depth.
Repeat the experiment by turning the thistle funnel in different directions keeping the depth constant. You will observe that as long as the depth remains the same there is no change in the level of the water in the manometer. Thus, the pressure exerted by a liquid at a given depth is the same in all directions
Now, lower the thistle funnel to the same depth in a number of liquids having different densities. You will notice that in liquids having greater density the pressure at the same depth is greater. This indicates that greater the density of the liquid, greater is the pressure at the same depth.
Matter normally exists in three states: solid, liquid, and gas. A transition from one of these states to another is called a change of state. Two common changes of states are solid to liquid and liquid to gas (and vice versa). These changes can occur when the exchange of heat takes place between the substance and its surroundings.
The change of state from solid to liquid is called melting and from liquid to solid is called fusion. It is observed that the temperature remains constant until the entire amount of the solid substance melts. That is, both the solid and liquid states of the substance coexist in thermal equilibrium during the change of states from solid to liquid. The temperature at which the solid and the liquid states of the substance in thermal equilibrium with each other is called its melting point. It is characteristic of the substance. It also depends on pressure. The melting point of a substance at standard atomspheric pressure is called its normal melting point. Let us do the following activity to understand the process of melting of ice.
Take a slab of ice. Take a metallic wire and fix two blocks, say 5 kg each, at its ends. Put
the wire over the slab as shown in Fig. 11.10. You will observe that the wire passes through
the ice slab. This happens due to the fact that just below the wire, ice melts at lower
temperature due to increase in pressure. When the wire has passed, water above the wire freezes again. Thus the wire passes through the slab and the slab does not split. This phenomenon of refreezing is called regelation
Theboiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid. A liquid in a vacuum environment has a lower boiling point than when the liquid is at atmospheric pressure. A liquid in a high pressure environment has a higher boiling point than when the liquid is at atmospheric pressure. In other words, the boiling point of liquids varies with and depends upon the surrounding environmental pressure and elevation. Different liquids boil at different temperatures.
The normal boiling point of a liquid is the special case in which the vapor pressure of the liquid equals the defined atmospheric pressure at sea level, 1 atmosphere. At that temperature, the vapor pressure of the liquid becomes sufficient to overcome atmospheric pressure and lift the liquid to form bubbles inside the bulk of the liquid
Solid is the state in which matter maintains a fixed volume and shape; liquid is the state in which matter maintains a fixed volume but adapts to the shape of its container; and gas is the state in which matter expands to occupy whatever volume is available.
Let us learn about solid and its properties.
A solid has definite shape and size. In order to change the shape or size of a body, a force is required. If you stretch a helical spring by gently pulling its ends, the length of the spring increases slightly. When you leave the ends of the spring, it regains its original size and shape.The property of a body, by virtue of which it tends to regain its original size and shape when the applied force is removed, is known as elasticity and the deformation caused is known as elastic deformation.
We know that in a solid, each atom or molecule is surrounded by neighbouring atoms or molecules. These are bonded together by interatomic or intermolecular forces and stay in a stable equilibrium position. When a solid is deformed, the atoms or molecules are displaced from their equilibrium positions causing a change in the interatomic distances.When the deforming force is removed, the interatomic forces tend to drive them back to their original positions. Thus the body regains its original shape and size. The restoring mechanism can be visualised by taking a model of spring-ball system shown below.Here the balls represent atoms and springs represent interatomic forces.
If you try to displace any ball from its equilibrium position, the spring system tries to restore the ball back to its original position. Thus elastic behaviour of solids can be explained in terms of microscopic nature of the solid.
Convection
Radiation is electromagnetic radiation emitted from a material which is due to the heat of the material, the characteristics of which depend on its temperature. An example of thermal radiation is the infrared radiation emitted by a common household radiator or electric heater.
