Does Ohm’s Law apply to every component in IB Physics?

No, and the IB expects you to know that. Ohm’s Law applies to ohmic conductors, where resistance stays constant as voltage and current change. Many real components are non-ohmic, meaning their resistance changes due to temperature effects or material properties. Filament lamps are the classic example because heating changes resistance. Diodes are another because they have a threshold behavior rather than a linear one. In exams, you often get marks for stating why the law does not apply, not just that it doesn’t.

Why does the “constant temperature” condition matter so much?

Because temperature is a hidden variable that changes resistance. In a metal, higher temperature generally means more lattice vibrations, which increases collisions and reduces charge carrier drift speed. That shows up as higher resistance, which breaks the direct proportionality between V and I. In practical terms, it means your I–V graph stops being a straight line. In IB Physics, examiners love when you connect the qualitative idea (heating) to the quantitative consequence (changing gradient). If you can say “resistance increases so the slope changes,” you’re thinking like the markscheme.

How do I avoid messing up V, I, and R in longer circuit questions?

Start by labeling what each value refers to: the whole circuit or one component. Many errors happen when students mix total voltage with a component current, or treat a parallel branch like it shares the same voltage drop as a series element. Next, write Ohm’s Law once, then decide which form you need (V, I, or R). After that, apply series/parallel rules before calculating anything new. Finally, do a quick sanity check: if resistance increases and voltage is fixed, current should drop. For structured practice, RevisionDojo’s IB Physics Revision Notes plus its Questionbank and AI Chat are a strong combination.

IB Physics: What Does Ohm’s Law State?

IB Physics

RevisionDojo Team

•11/15/2025•6 min read

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IB Physics: What Does Ohm’s Law State?

IB Physics

RevisionDojo Team

•11/15/2025•6 min read

Ohm’s Law shows up in IB Physics the way a chorus shows up in a song: you might not notice it at first, but once you do, you hear it everywhere. It’s in the tidy one-mark calculations and the long, messy explanations where your diagram is correct but your reasoning drifts. And it’s especially present in that anxious moment when the question says “explain” and your brain replies with silence.

At its core, Ohm’s Law is a promise about patterns. Not a promise that everything in circuits behaves nicely (it doesn’t), but a promise that some components behave predictably if the conditions are right. That “if” is where most exam marks are hiding.

A stressed IB student sees a spaghetti circuit and clings to V=IR

Ohm’s Law in IB Physics (the statement you must know)

In IB Physics, Ohm’s Law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.

Written as an equation:

V = IR

Where:

V is potential difference (volts, V)
I is current (amperes, A)
R is resistance (ohms, Ω)

This relationship is the backbone of circuit calculations, but it’s also the logic behind common “explain the trend” questions. If you want the syllabus context around circuits, start at IB Physics B.5 Current and Circuits.

A 20-second Ohm’s Law checklist (before you calculate)

Before you rearrange anything in IB Physics, check:

Is the component ohmic (resistance constant)?
Is temperature roughly constant?
Are you using values for one component or the whole circuit?
Do series/parallel rules change what “V” and “I” refer to?
Does an I–V graph suggest a straight line or a curve?

For quick refreshers on the quantities themselves, RevisionDojo’s notes on Electrical Current and Voltage and Resistance and Resistivity are the cleanest place to reset your definitions.

The condition students forget: temperature must stay constant

Ohm’s Law is not saying “V and I always behave linearly.” It’s saying they behave linearly when resistance stays constant. And for many conductors, resistance changes when temperature changes.

That’s why the syllabus loves components that break the rule:

Filament lamps: as current increases, the filament heats up, resistance rises, and the graph curves.
Diodes: current stays tiny until a threshold is reached.
Thermistors: resistance changes strongly with temperature.

The exam skill is to spot when Ohm’s Law is valid, and when you should switch from calculating to explaining. If you need more practice on the typical traps, pair this with How Resistance Affects Current in a Circuit.

A resistor sweats as temperature changes and R changes too

I–V graphs: the fastest way to prove Ohm’s Law (or disprove it)

In IB Physics, an I–V graph is basically a lie detector for “ohmic vs non-ohmic.”

Ohmic conductor: straight line through the origin (constant resistance).
Non-ohmic conductor: curve (resistance changes).
Diode: near-zero current, then a sharp rise after a threshold.

A useful exam sentence: “Since the I–V relationship is linear and passes through the origin, the resistance is constant, so the component is ohmic and obeys Ohm’s Law.”

To build speed, use the B.5 Current and Circuits Questionbank on RevisionDojo and force yourself to justify the graph shape in words, not just numbers.

A cafe menu of I–V graphs with funny descriptions

How Ohm’s Law actually shows up in IB Physics exam questions

Most IB Physics questions use Ohm’s Law in one of three ways:

Single-step calculation

You’re given two of V, I, R and asked for the third. This is pure V = IR.

Multi-step circuit reasoning

You use series/parallel rules first, then apply Ohm’s Law to a part of the circuit. Power questions often do this too, so keep Power and Resistor Configurations Notes nearby.

Explanation/graph interpretation

You’re asked why current changes, why resistance isn’t constant, or what the graph implies. This is where that “temperature constant” clause earns marks.

If you like learning from tightly defined language (helpful for Paper 2 explanations), bookmark the IB Physics Glossary.

Close: make Ohm’s Law automatic in IB Physics

Ohm’s Law is small, but it’s not shallow. In IB Physics, it’s the bridge between neat formulas and messy reality: when conditions hold, V = IR is reliable; when they don’t, you’re expected to explain why.

If you want to turn this from “I get it” into “I can do it under time pressure,” use RevisionDojo as your circuit workbench: drill exam-style questions in the Questionbank, tighten your understanding with Study Notes and Flashcards, test yourself with Mock Exams and Predicted Papers, and use AI Chat plus Grading tools to improve explanations. If your goal includes an investigation, the Coursework Library and Tutors can help you shape a clean, high-scoring IA plan.

When you can look at a circuit, a graph, or a paragraph question and calmly think “this is just IB Physics plus Ohm’s Law,” you’re already most of the way to a top grade.