Give reason?
(a) We can feel the heat of a table lamp, under it
(b) The upper layers of water in a deep well are cool but those in a pond are warm.
(c) Sawdust is a bad conductor of heat.
MSCERT Class 7 General Science Ch 4 Transmission of Heat
Answers
saw dust is not very dense
electrons are less
Answer:
(a) Heat is a form of energy that can be transferred from a body having high temperature to a body having low temperature .since the atmosphere around us and oue body is having low temperature in comparison to the tablelamp therefore heat get transferred from tablelamp to our sorrounding and then to our body.So we feel heat near tablelamp
(b)It’s because water is a very unusual substance. It contracts and becomes denser as it gets colder, like most liquids, but only down to 4C. It then starts expanding, and as we all know ice floats on water.
(c) A pond in winter will actually be colder on top; ice, and water colder than 4C, will float on the underlying water at 4C. There won’t be any convection currents to mix the water, so cooling is a very slow process from the top down. Before the pond can freeze all the way down, Spring arrives.
Come Spring, and heat transfers from the air and sun above to the surface of the pond. The ice melts. The surface water warms from below 4C to above 4C, at no point becoming denser than the underlying water at 4C so convection does not mix warmer water on top with the underlying water. So the pond can only warm slowly from the top down …
And before it does, Winter is back.
If ice sank in liquid water like the solid phase of most materials does in the liquid phase, oceans would be thin skins of seasonally liquid water sitting on top of kilometers of ice, and life would in all likelyhood not ever have got started.
cMaybe the questioner is referring to Pykrete, which is a mixture of water and sawdust which was trialed for the production of a giant iceberg in WW2. This was intended for mooring in mid-Atlantic Ocean to serve as an Allied aircraft carrier as part of the battle against Nazi submarines.
The addition of 14% sawdust to the water before freezing had two major beneficial effects. Firstly it drastically reduced the thermal conductivity of the water. This slowed down the rate at which the iceberg thawed. Secondly it increased the toughmess of the iceberg, making it netter able to withstand the impact of shelling or bombing.
They started to make a Pykrete iceberg in Canada but it was a slow process, and the Battle of the Atlantic was already won by the Allies before it was big enough to be useful.
There is a good documentary on Youtube about this. Also an entry in Wikipedia on Pykrete.