It is a matter of common experience that hot liquids like milk, water, etc., when left in a room, start cooling. The cooling of hot liquids is fast if their temperature is higher than the temperature of their surroundings. But the cooling of hot liquids is slow if their temperature is not much higher than the temperature of their surroundings. The cooling of the hot liquids continues till they attain the temperature of their surroundings. The temperature of the hot liquid left in the room decreases with the passage of time.

Newton’s Law of Cooling

Newton studied the cooling of many hot liquids and stated a law known as Newton’s law of cooling. According to this law, the rate at which a body (liquid or solid) loses heat is directly proportional to the temperature difference between the body and its surroundings. The heat loss rate also depends upon (i) the area of the exposed surface of the body and (ii) the nature of the surface of the body.

Consider a body of mass m, and specific heat c, at temperature θ. Let θ₀ be the temperature of the surroundings of the body. According to Newton’s law of cooling, the heat loss rate is proportional to the temperature difference between the body and its surroundings.

(-dQ/dt) ∝ (Ө –  Ө₀)

Newton’s Law of Cooling Limitations

• The temperature difference between the body and the environment must be minimal.
• Only radiation should be used to remove heat from the body.
• The biggest drawback of Newton’s law of cooling is that the temperature of the environment must not change while the body is cooling.

Capillary Action

A glass tube with a very thin bore throughout the length of the tube is called a capillary tube. If the capillary tube is dipped in water, it wets the inner side of the tube and rises in the tube. If the same capillary tube is dipped in the mercury, then the mercury in the tube is depressed. Capillary action is the term used to describe the phenomenon of liquids rising or falling in a capillary tube. A sheet of paper absorbs water in the same way when it is placed on a puddle of water. This occurs due to water absorption into the tiny openings in the paper’s fibres.

Practical Applications of Capillarity

• The oil in a lamp rises in the wick to its top by capillary action.
• The tip of a pen’s nib is split up to make a narrow capillary so that the ink rises to the tip of the nib continuously. This enables us to write with a pen.
• Water rises to the top of the leaves of the tree by capillary action.
• If one end of the towel dips into a bucket of water and the other end hangs over the bucket, the towel soon becomes wet throughout due to capillary action.
• Ink is absorbed by the blotting paper due to capillary action.
• The moisture rises in the capillaries of soil to the surface, where it evaporates. To preserve the moisture in the soil, capillaries must be broken up. This is done by ploughing and levelling the fields.