June 2002 Paper 1:Q18A bar made of half wood and half copper has a piece of paper wrapped tightly round it. The bar is heated Strongly at the centre for a short time, and the paper goes brown on one side only. Which side goes brown and what does this show about wood and copper?A-Brown side-copper (Wood-conductor Copper-insulator)B-Brown side-copper (Wood-insulator Copper-conductor)C-Brown side-wood (Wood-conductor Copper-insulator)D-Brown side-wood (Wood-insulator Copper-conductor)I chose B, but I was told the answer was D, how is this possible?
The wood side goes brown, because wood is a poor thermal conductor, so the answer is indeed D. The metal part of the bar has a good thermal conductivity, and draws the heat away from the paper before it has a chance to catch or brown. However, the heat is not conducted as well by the wood, meaning that it stays near the paper longing causing it to singe.
Look up for the definition of insulator Look up for the definition of conductor Ask yourself what is the difference between a conductor and an insulator? Here the comparison is between i)paper and copper ii)paper and wood Wood is a worse insulator than wood,so no heat flows through wood.Most of it flows through the paper. Copper is a a good conductor and paper is an insulator.So all the heat flows through copper.No heat flows through the paper.So only paper on the wood side gets burnt. Check the coefficients of thermal conductivity.Paper has a higher coefficient than wood.Which shows that paper is a relatively poorer insulator as compared to wood.
The heat will affect the wood because it's a poor conductor (insulator) of heat and cannot take the heat away from the paper. The copper is conducting the heat away. The answer therefore is 'D' (This is similar to boiling water in a paper cup. The water boils at 100°C .this keeps the cup at the same temp it won't burn).
