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
2025-2027 syllabus, in our words · AS LevelWhat the syllabus expects you to be able to do in this topic, in plain English. For the exact wording, see the official syllabus (PDF).
- Define resistance.
- Recall V = IR and use it.
- Sketch I-V characteristic graphs for a semiconductor diode, a filament lamp, and a metallic conductor held at constant temperature.
- Explain that a filament lamp's resistance rises as the current rises, because the filament gets hotter.
- State Ohm's law.
- Recall R = ρL / A and use it.
- Understand that a light-dependent resistor (LDR) has a lower resistance when the light falling on it is brighter.
- Understand that a thermistor's resistance falls as its temperature rises; thermistors are assumed to 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.
- › Identify if a cable contains a return path; if so, double the length used in the resistance formula R = ρL/A.
- › Use V = Al; if length increases, cross-sectional area must decrease proportionally to keep volume constant when calculating new resistance.
- › Calculate resistance as R = V/I at the point in question; do not read R from a gradient (on an I-V graph the gradient is 1/R).
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 an NTC thermistor (the type used at AS level) decreases as temperature increases. In the semiconductor, a higher temperature releases more charge carriers, so the number density of charge carriers rises and the resistance falls.
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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