The questions are on the ohm's law test. Take it under real conditions before reading the mark scheme.
Quick answer: Ohm's law says that the current through an ohmic conductor is proportional to the potential difference across it, so V = I × R. This mark scheme gives the model answers and the marks for the 8 questions of the grade 8 science test on ohm's law, from meters to wire resistance, with the equivalent answers to accept.
Marks are shown in each model answer. Accept equivalent wording and give error-carried-forward credit if a correct method follows an earlier slip.
Level: Grade 8 science (Year 9 · Class 8) · Curriculum: MS-PS2-3
Question 1 Meters (2 marks)
- In series.
- In parallel across the component.
Marking: 1 mark each.
Question 2 Find the current (3 marks)
- I = V ÷ R = 9.0 ÷ 45 = 0.20 A.
- R = 90 Ω, so I = 9.0 ÷ 90 = 0.10 A.
Marking: 1 mark for the formula and substitution, 1 for 0.20 A; 1 for 0.10 A.
Question 3 Unit conversion (2 marks)
- 0.33 A.
- 2200 Ω.
Marking: 1 mark each.
Question 4 Reading a graph (3 marks)
- R = V ÷ I = 10.0 ÷ 0.40 = 25 Ω.
- I = 6.0 ÷ 25 = 0.24 A.
Marking: 1 mark for substitution, 1 for 25 Ω; 1 mark for 0.24 A (accept 0.23 to 0.25 A from graph).
Question 5 Filament lamp (3 marks)
- As the current increases the filament gets hotter. The metal atoms vibrate more, so the moving charges collide with them more often. The resistance rises with temperature, so the current grows more slowly than the voltage and the graph bends.
Marking: 1 mark filament heats up; 1 mark atoms vibrate/more collisions; 1 mark resistance increases so line curves.
Question 6 An LED resistor (3 marks)
- 6.0 − 2.0 = 4.0 V.
- R = V ÷ I = 4.0 ÷ 0.020 = 200 Ω.
Marking: 1 mark for 4.0 V; 1 mark for the substitution, 1 for 200 Ω.
Question 7 Wet hands (2 marks)
- Wet skin has a much lower resistance, so for the same voltage the current is much larger (I = V ÷ R), and a large current through the body can be fatal.
Marking: 1 mark lower resistance; 1 mark larger current for the same voltage.
Question 8 Wire resistance (2 marks)
- Use a shorter wire and a thicker wire. (Also accept a metal such as copper, or a lower temperature.)
Marking: 1 mark each.
