Year 8 Science · Unit 2 · Lesson 18
Challenge Worksheet
Learning Goals
1. What are allotropes? Name two allotropes of carbon other than carbon fibre.
Find the errors
A student’s poster text
“Diamond is so hard because its carbon atoms are a heavier kind of carbon than the atoms in graphite. Graphite is soft because its atoms are loosely packed in every direction. Diamond conducts electricity very well, which is why it is used for the electrodes in batteries. Carbon fibre is chosen for bike frames because it is heavy and strong.”
2. The poster has four errors, one in each sentence. For each, quote the error and write the correct science.
Copper or aluminium for an electric aircraft?
| Metal | Density (g/cm³) | Relative conductivity (silver = 100) |
|---|---|---|
| Copper | 8.96 | 95 |
| Aluminium | 2.70 | 59 |
Source: standard data. Because aluminium conducts less well, an aluminium cable needs a cross-section 1.6 times larger than a copper cable to carry the same current.
3. A copper cable 1 m (100 cm) long has a cross-section of 1 cm². (a) Find its volume and its mass. (b) Find the mass of the aluminium cable, also 100 cm long, that carries the same current. (c) What fraction of the copper cable’s mass is this? Use mass = density × volume.
Answer (c) of the mass
4. Every extra kilogram an aircraft carries uses more energy in flight. Use your answer to recommend a cable metal for the aircraft, and give one drawback of your choice.
Plan a short investigation
Does pencil grade (2H, HB, 2B, 6B) affect how well the lead conducts?
Hypothesis (softer leads hold more graphite): If ... then ... because ...
Variables
Method (a battery, wires, a small globe or ammeter, and pencil leads)
5. “Always choose the best conductor for an electrical job.” Evaluate this, using three metals as evidence.
Warm Up
1. Different forms of one element, with the atoms arranged differently. Diamond and graphite (buckminsterfullerene accepted). [2 marks]
2. (1) “heavier kind”: same carbon atoms; diamond is hard because each atom joins four neighbours in a rigid 3D framework. (2) “loosely packed”: atoms join three neighbours in flat sheets that are weakly held together and slide. (3) “Diamond conducts”: diamond is an insulator; graphite conducts and is used for electrodes. (4) “heavy”: carbon fibre is light and stiff. [4 marks: 1 per error found and corrected]
Your Turn
3. (a) Volume = 100 cm × 1 cm² = 100 cm³; mass = 8.96 × 100 = 896 g. (b) Aluminium cross-section = 1.6 cm², volume = 160 cm³; mass = 2.70 × 160 = 432 g. (c) 432 ÷ 896 = 0.48, so about half the mass. [4 marks: 1 copper volume and mass, 1 aluminium volume, 1 aluminium mass, 1 fraction]
4. Aluminium: for the same current its cable weighs about half as much as copper (432 g vs 896 g per metre), so the aircraft carries less mass and uses less energy. Drawback: the cable must be 1.6 times thicker in cross-section, so it takes up more space (or: it conducts less well, so more cable material is needed). [3 marks: 1 aluminium, 1 uses the calculation and weight, 1 drawback]
Go Deeper
Investigation: If the pencil is softer, more current flows, because it holds more graphite, which conducts. IV: pencil grade (2H, HB, 2B, 6B). DV: current (or globe brightness). Controlled (any one): length of lead between clips, lead thickness, battery voltage. Method: clip wires a fixed distance apart on a lead, close the circuit, record the current; repeat for each grade, three trials each.
Show What You Know
5. Marking guide (points expected): 1 position: disagree, conductivity is only one property; 1 silver is best (100) but copper (95) is nearly as good and far cheaper, so houses use copper; 1 aluminium conducts about 60 per cent as well as copper but is about a third as dense, so it wins where weight matters (aircraft; accept overhead power lines); 1 iron is cheap but conducts poorly and rusts, or gold (72) is used on circuit boards because it does not corrode; 1 uses numbers correctly; 1 conclusion: the choice balances conductivity with cost, density and corrosion. [6 marks]
Common error to watch for: in question 3, forgetting the 1.6 times larger cross-section (giving 270 g, a third). Equal current needs a thicker cable.