EMF, Internal Resistance and Potential Dividers
Modelling sources with emf and internal resistance, using terminal potential difference and designing potential-divider and sensor circuits.
How to study A-level Physics
Define the system, represent the physics, select a justified model, calculate transparently, then test the result against units, limits and evidence.
Core concepts
Concept 1
Emf is energy supplied per unit charge; terminal potential difference is reduced by energy transfer in internal resistance when delivering current.
Exam cue: Distinguish source emf from terminal potential difference.
Concept 2
A potential divider produces an output determined by resistance ratios and loading conditions.
Exam cue: Identify which resistor the output is measured across before using a divider ratio.
Concept 3
Thermistors, light-dependent resistors and other transducers convert physical changes into electrical signals.
Exam cue: Explain a sensor response through the resistance change and circuit relationship.
Risk pitfalls and guardrails
Treating emf as a force.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Assuming terminal potential difference always equals emf.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Ignoring the effect of an output load on a potential divider.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Memory anchors
EMF
Emf is energy supplied by a source per unit charge.
Lost Volts
For a discharging source, lost volts equal current times internal resistance.
Terminal PD
Terminal potential difference equals emf minus lost volts while the source delivers current.
Potential Divider
The output fraction follows the resistance across which the output is measured.
Transducer
A transducer converts one form of physical input into another signal, often electrical.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What is the emf of a source?
What is terminal potential difference?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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