Cambridge IGCSE Physics Notes 4.2: Electrical Quantities
Cambridge IGCSE Physics notes on charge, current, electromotive force, potential difference, resistance, electrical energy, and power.
Q: What does Cambridge IGCSE Physics Notes 4.2: Electrical Quantities cover?
A: It follows official Cambridge Physics section 4.2 for syllabuses 0625 and 0972, with Core ideas separated from Supplement depth.
Electrical quantities describe how charge moves and how energy is transferred: current is charge flow rate, voltage is energy per charge, and resistance links voltage to current.
The ideas that organise this section
- Conventional current direction is opposite to electron flow in metals.
- Electromotive force is energy supplied per charge; potential difference is energy transferred per charge in a component.
- Resistance depends on the component and conditions, so not every current-voltage graph is linear.
Core route
Core candidates should be ready to describe, calculate, interpret, and apply the following:
- Calculate charge, current, voltage, resistance, electrical energy, and power.
- Describe current in metals and the effect of changing resistance or voltage.
- Interpret simple current-voltage data and household energy use.
Supplement route
Extended candidates study all Core content and add the following depth:
- Distinguish electromotive force from potential difference through energy transfer.
- Explain non-ohmic behaviour through temperature or component response.
- Apply electrical relationships across unfamiliar devices and data sets.
Formula route
- Relationship: current = charge / time.
- Relationship: voltage = energy transferred / charge.
- Relationship: resistance = voltage / current.
- Relationship: electrical power = current x voltage.
- Relationship: electrical energy = power x time.
Write the relationship before substituting. Convert units first, keep extra figures during working, and round only the final answer to sensible precision.
Build a connected model
In Electrical Quantities, a strong answer connects the named quantity or model to observable evidence. Begin by defining the physical quantity in words or with its relationship. Identify which values are scalars and which require a direction. Represent the situation with a labelled diagram, graph, field pattern, ray or circuit when that makes the relationship visible. The representation is part of the reasoning: its labels, arrows and scale should agree with the written explanation.
For a calculation, write the governing relationship before substitution, convert prefixes and time units, keep unrounded values during working and attach the correct final unit. For a qualitative question, use a cause-link-consequence chain. Name what changes, state the physical mechanism that links the change to the system, and then give the measurable result. If two cases are compared, hold unrelated variables constant and use the same physical principle for both.
Check the answer against limiting cases. A zero input, a doubled input or a reversed direction often exposes an incorrect proportionality or sign. Also check whether the value is plausible for the apparatus or context. This final check is especially important when a calculator gives a precise-looking result from an incorrect unit conversion.
Worked reasoning example
A filament lamp's resistance rises as it heats, so current no longer increases in direct proportion to voltage.
For explanation questions, state the physical principle, apply it to the named system, then give the consequence. A formula or keyword by itself is not a complete explanation.
Extend the example by identifying the evidence that would distinguish the correct model from a tempting alternative. State what would be measured or observed, which variable must remain controlled, and how the conclusion follows from the result. This turns a numerical or descriptive answer into a testable physical argument. When the question asks for an explanation rather than a calculation, preserve the same chain but express it as principle, application and consequence.
A reliable answer method
- Identify the quantity, law, graph feature, or physical model being tested.
- Mark a positive direction or label the diagram when direction matters.
- Write the equation or principle before inserting data.
- Keep units consistent and show the main substitution.
- Check whether the magnitude, direction, trend, and unit are physically reasonable.
Common mistakes
- Calling current the amount of charge rather than its rate of flow.
- Assuming resistance is always constant.
- Confusing power with energy consumed.
Assessment guidance
Questions on Electrical Quantities can mix recall, calculation, graph or diagram interpretation and unfamiliar application. Read the command word first. A definition needs the precise physical meaning; a description reports a pattern; an explanation supplies the mechanism; and an evaluation weighs evidence or method quality. Show equations and substitutions so method marks remain visible. Label every diagram and state directions where relevant. In multi-step problems, carry forward unrounded values and make the final answer's unit and significant figures consistent with the data. Never rely on a memorised keyword when the question asks how or why a result occurs.
Retrieval practice
Without notes, rebuild the Core and Supplement lists for Electrical Quantities. Define every named quantity, reproduce the principal relationship or representation and explain one everyday application. Then solve one direct calculation and one unfamiliar context, checking units and limiting cases. Finish by writing two misconception corrections and one practical measurement that could test the topic's main relationship.
Theory and practical stay separate
Use the resistance and current-voltage practical post for meter placement, variable resistors, heating control, and graph interpretation.
Official source
Cambridge International, Physics 0625 syllabus for examinations in 2026, 2027 and 2028. Cambridge states that Physics 0972 is graded from 9 to 1 but otherwise follows the same subject content as 0625.
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