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Natural sciences

Energy transfers and forces

Educator section

Memorandum

a)

  • Iron
  • Copper
  • Aluminium
  • Lead

All four are good conductors, but copper, for example, is a better conductor than iron. Determine positions by doing an experiment. Try to use laboratory rods that are of the same size.

(b) An eiderdown (or down duvet) keeps one warm because body heat is poorly conducted by it. The air between the down is also a poor heat conductor. Bricks also have tiny holes that are filled with air. That makes a brick a poorer conductor of heat than for example copper, which is more solid/compact.

(c)

(i) Clay.

(ii) It conducts heat poorly from outside; therefore it will remain cooler inside than outside.

(iii) Wool is a poor conductor and therefore it keeps the body heat inside.

(iv) The learner should choose a good conductor for the bottom of the pot especially, in order to get the heat to the contents of the pot rapidly to speed up the preparation time of the contents. He/she should choose a poor conductor for the handles of the pot, so that one can handle the pot without having to use potholders or a cloth.

(v) Water is a poor conductor.

(vi) Tins are made of aluminium, which is a better conductor than glass.

Leaner section

Content

Activity 6: to compare the conductivity of various substances [lo 2.2]

(a) Test the following metals by warming them in a flame or in boiling water. Let each member of the group hold a different metal rod. Report it as soon as the heat reaches your hand. Arrange the metals in such a manner that the metal that is the fastest conductor – the one that gets hot first – is placed in the first position.

Metal Position
Iron _______________________
Copper _______________________
Aluminium _______________________
Lead _______________________

(b) Why is it that some substances are better conductors than other substances?

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(c) Use your knowledge of conduction to answer the following questions.

(i) Which metal would you use to build a hut that needs to be cool in summer – clay or corrugated iron?

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(ii) Why have you chosen this as the building material?

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(iii) Have you ever thought about the reason for wearing woollen clothing in winter? Try to explain why winter garments contain wool.

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(iv)You have an opportunity to design a pot for cooking food. Which materials would you use, and why? (Remember that a pot also needs handles!)

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(v) Why do we put a bottle of cool drink in cold water when we want to keep it cool?

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(vi) Try to explain why cool drink in a bottle would remain cold longer than cool drink in a tin.

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Assessment

Learning Outcome 2: The learner will know and be able to interpret and apply scientific, technological and environmental knowledge.

Assessment Standard 2.2: We know this when the learner categorises information: compares features of different categories of objects, organisms and events.

Questions & Answers

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At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
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Source:  OpenStax, Natural sciences grade 7. OpenStax CNX. Sep 16, 2009 Download for free at http://cnx.org/content/col11077/1.1
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