A-Level Physics May/June 2024 Q5(b)(iii): The resistance of X is increased. The temperature of Y remains at 190°C. By reference t…

1 answer

• ▲0
  accepted ✓

  The resistors X and Y are in series, so the total resistance of the circuit is $R_{total} = R_X + R_Y$. When the resistance of X increases, the total resistance of the circuit also increases.

  According to Ohm's law for the entire circuit, the current is given by $I = \frac{V}{R_{total}}$, where $V$ is the constant e.m.f. of the supply. Since the total resistance has increased, the current $I$ in the circuit must decrease.

  The potential difference across Y is given by $V_Y = I \times R_Y$. The resistance of Y, $R_Y$, is stated to be constant. As the current $I$ has decreased, the potential difference across Y must therefore decrease.

  ---

  How the marks are awarded

  • B1 — Stating that the total resistance of the circuit increases as a result of the resistance of X increasing.
  • B1 — Correctly deducing that the current in the circuit decreases because the total resistance has increased while the supply e.m.f. is constant.
  • B1 — Concluding that the potential difference across Y decreases, by correctly applying Ohm's law ($V_Y = I R_Y$) with a decreased current and constant resistance.

  Common mistakes

  • Failing to refer to the current as explicitly requested, instead only using a potential divider argument (e.g., 'X takes a bigger share of the voltage, so Y gets less').
  • Stating that the total resistance increases and the current decreases, but then incorrectly concluding that the p.d. across Y stays the same because its own resistance is constant.
  • Only stating the final effect (p.d. decreases) without any explanation of the intermediate steps involving total resistance and current.
  • Incorrectly assuming that because the resistance of X increases, the current also increases, which is a fundamental misunderstanding of Ohm's law in a series circuit.

  Examiner tip: For 'state and explain' questions, always build a logical chain of cause and effect, ensuring each step directly follows from the previous one and addresses all parts of the prompt.

  ---

  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

  @markscheme_answers

Your answer

Sign in to answer this question.