Resistance and resistivity
Cambridge A-Level Physics (9702) · Unit 9: Electricity · 9 flashcards
Resistance and resistivity is topic 9.3 in the Cambridge A-Level Physics (9702) syllabus , positioned in Unit 9 — Electricity , alongside Electric current and Potential difference and power. In one line: Resistance (R) is the opposition to the flow of electric current in a circuit. It is defined as the ratio of voltage (V) across a component to the current (I) flowing through it: R = V/I.
Marked as AS Level: examined at AS Level in Paper 1 (Multiple Choice), Paper 2 (AS Structured Questions) and Paper 3 (Advanced Practical Skills). The same content may also be assumed in Paper 4 (A Level Structured Questions).
The deck below contains 9 flashcards — 2 definitions, 6 key concepts and 1 calculation — covering the precise wording mark schemes reward. Use the 2 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Electrical resistance
Resistance (R) is the opposition to the flow of electric current in a circuit. It is defined as the ratio of voltage (V) across a component to the current (I) flowing through it: R = V/I.
What the Cambridge 9702 syllabus says
Official 2025-2027 spec · AS LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- define resistance
- recall and use V = IR
- sketch the I–V characteristics of a metallic conductor at constant temperature, a semiconductor diode and a filament lamp
- explain that the resistance of a filament lamp increases as current increases because its temperature increases
- state Ohm’s law
- recall and use R = ρL / A
- understand that the resistance of a light-dependent resistor (LDR) decreases as the light intensity increases
- understand that the resistance of a thermistor decreases as the temperature increases (it will be assumed that thermistors have a negative temperature coefficient)
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9702 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Resistance and resistivity
- › Calculate the resistance of one strand using the resistivity formula, then divide by the number of strands to find the total resistance in parallel.
- › Always calculate resistance R as V/I for a specific point on a graph rather than using the gradient for non-ohmic components.
- › Use standard symbols provided in the syllabus glossary, such as ρ for resistivity and λ for wavelength.
- › Always record raw data to the full resolution of the measuring instrument (e.g., 0.1 cm for a meter rule).
- › Use V = Al; if length increases, cross-sectional area must decrease proportionally to keep volume constant when calculating new resistance.
Define electrical resistance.
Resistance (R) is the opposition to the flow of electric current in a circuit. It is defined as the ratio of voltage (V) across a component to the current (I) flowing through it: R = V/I.
State Ohm's Law.
Ohm's Law states that the current through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature and other physical conditions remain constant. Mathematically: V = IR.
Sketch the I-V characteristic for a metallic conductor at constant temperature.
The I-V characteristic is a straight line passing through the origin. This indicates a linear relationship between voltage and current, obeying Ohm's Law.
Sketch the I-V characteristic for a semiconductor diode.
The I-V characteristic shows very little current flow for negative voltages (reverse bias). Current increases rapidly for positive voltages above a threshold voltage (forward bias).
Sketch the I-V characteristic for a filament lamp.
The I-V characteristic is a curve that starts linear, but the slope decreases as voltage/current increases. This indicates that resistance increases with temperature/current.
Explain why the resistance of a filament lamp increases as current increases.
As current increases, the temperature of the filament increases. This increased temperature causes greater vibrations in the metal lattice of the filament, hindering the flow of electrons and therefore increasing resistance.
How do you calculate the resistance of a wire given its resistivity, length and cross-sectional area?
The resistance (R) is calculated using the formula: R = ρL/A, where ρ is resistivity, L is the length of the wire, and A is its cross-sectional area.
Describe how the resistance of a Light Dependent Resistor (LDR) changes with increasing light intensity.
The resistance of an LDR decreases as light intensity increases. This is because more photons excite more electrons in the semiconductor material, leading to a greater number of charge carriers and therefore a lower resistance.
Describe how the resistance of a thermistor changes with increasing temperature.
The resistance of a thermistor (specifically an NTC thermistor as used at AS level) decreases as temperature increases. Higher temperatures provide more energy to charge carriers, allowing them to move more freely and thus lowering the resistance.
Review the material
Read full revision notes on Resistance and resistivity — definitions, equations, common mistakes, and exam tips.
Read NotesMore topics in Unit 9 — Electricity
Resistance and resistivity sits alongside these A-Level Physics decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Resistance and resistivity deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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