A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Electrical Characteristics and Ohm's Law — mark scheme explained
The short answer
In this section, we will explore the electrical characteristics of different components: ohmic conductors, semiconductor diodes, and filament lamps. We will also delve into Ohm’s law and how it applies to these components under constant physical conditions.
The question
A resistor has a resistance of 10 Ω. If a potential difference of 5 V is applied across it, calculate the current flowing through the resistor. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
Step 1: Identify the given values: R = 10 Ω, V = 5 V.
- S2
Step 2: Use Ohm's law to find the current: I = V / R = 5 V / 10 Ω = 0.5 A.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
Step 1: Identify the given values: R = 10 Ω, V = 5 V.
- S2
Step 2: Use Ohm's law to find the current: I = V / R = 5 V / 10 Ω = 0.5 A.
Final answer: 0.5 A
Common mistakes
- Assuming all components follow Ohm's law — Understand the specific characteristics of each component: ohmic conductors have a linear I-V relationship, while diodes and filament lamps do not.
- Confusing the roles of ammeters and voltmeters — Remember that an ideal ammeter has zero resistance and is connected in series, while an ideal voltmeter has infinite resistance and is connected in parallel.
- Forgetting the effect of temperature on filament lamps — Understand that the resistance of a filament lamp increases with temperature, causing the I-V curve to become less steep (it flattens and bends towards the voltage axis) as the voltage increases.
- Misinterpreting the forward and reverse bias regions of diodes — Recognize that in forward bias, the current increases exponentially with a small increase in voltage, while in reverse bias, the current remains very low until the breakdown voltage is reached.
- Incorrectly plotting I-V characteristics — Ensure that the I-V characteristic graph is plotted correctly with the appropriate axis (I on the horizontal or vertical) and that the shape of the curve reflects the component's behavior.
- Failing to consider constant physical conditions in Ohm's law — Understand that Ohm's law (V = IR) is valid only when the physical conditions (e.g., temperature) remain constant. For components like filament lamps, the resistance changes with temperature.
Where the marks go
- Full worked solution (all marking points)3 marks