Story by Ms. Rani Subramaniam
Ram and Sarala visited their paternal grandparents one weekend. They called their grandfather, thatha, and grandmother, paati.
Thatha and paati lived close to the beach. Before Ram and Sarala went to bed, paati told them she would take them to the beach
the next morning provided they got up early and finished their morning chores on time.
Paati got up next morning and went about her usual tasks. She tried her best to wake up Ram and Sarala. All her efforts
were useless, as they refused to get up.
Paati gave up trying to wake them. She decided that she would tell them about the proverb “early to bed and early to rise makes
a man healthy, wealthy and wise”.
The sun was up in the sky when the duo got up. It went further up by the time they finished brushing, prayers and breakfast.
“Let us go to the beach paati”, they said.
“You are very late. It is hot and we cannot go to the beach now”, said paati.
“Please, please take us.”, they pleaded.
Paati agreed but added, “do not say I did not warn you.”
Paati told them that they would go walking to the beach.
They would save petrol and also prevent pollution by not using the car.
“Why do we need to save petrol?”, Ram asked.
Paati said that petrol took thousands of years to be made but we were using it
up so fast that soon we would not have petrol anymore.
Paati said that she would tell them all about it later, as they had reached the beach.
Ram and Sarala flung their slippers and ran on the sand. Soon they were hopping on their legs and yelling “ouch!! ouch!! ooii!! ooii”.
“Oh Paati! we are not dancing. We are jumping. The sand is so hot.
The soles of our feet are burnt”.
“Come let us go to the water”, paati said. Ram and Sarala hopped, skipped and jumped to the water. What a relief the cool water was.
“Paati, why is the water so cool and the sand so hot?”
“It is like this – if I give each one of you two dosais to eat, one of you may be full and the other may still be a little hungry. Similarly, the same amount of heat from the sun will make the sand’s temperature rise a lot, but not that of the water”, paati explained.
“If I take one gram of sand, it takes a small amount of heat for a temperature rise of one degree centigrade. If I take one gram of water, it takes a much larger amount of heat for its temperature to rise by one degree centigrade. In fact, out of all liquids and many of the common substances.
we deal with, water needs the maximum amount of heat per gram for a one degree rise in temperature.”
Paati said that when she heated water on the gas stove for drinking and bathing, her gas cylinder ran out very fast. She also told them that when she visits temples in the late morning or afternoon, she finds the stone in the open areas so hot that her feet get burnt. In fact, many temples scatter wet gunny bags all over the place so that it is comfortable for people to walk on.
Paati started off her favourite statement, “Nature is great… this hot sand and cool sea causes breeze!”.
“How is that? Paati, please tell us”.
“During the night, the sand gives up its heat and so does the water. But the temperature of the sand falls much faster. The air above the water is warm. Air above the sand is cooler. Air pressure above the water is less and so the
breeze blows from land to the sea”.
“We’ve learnt so much today!”, said Ram and Sarala.
Paati told them that she was going to ask them some questions. They should think about it and get back with their answers.
She also told them that just as we express length in meters, mass in kilograms and time in seconds, heat given to a body is expressed in Joules.
Paati asked them to see this table that she got from the internet. She told the children that from this table one could make out that a substance like water needed 4186 Joules of heat for one kilogram of water to rise in temperature by one degree Kelvin, or one degree Centigrade.
Q1. Which is the substance that needs the least amount of heat per Kilogram per degree rise in temperature.
Q2. How much heat does iron need per kilogram for a one degree rise in temperature.
Q3. To save heat energy would you use Aluminum vessels or iron vessels in the kitchen?
Q4. When working near a gas stove would gold bangles feel hotter than similar copper bangles, on the wrist?
Q5. There are two exactly similar vessels and one contains one kilogram of cooking oil. The other vessel contains one kilogram of water. Both are placed on exactly similar burners. Which liquid will reach a temperature of 50 degree centigrade first and why?