Electrical Sources and Internal Resistance
Model real electrical sources using emf, internal resistance, terminal voltage and lost volts.
How to study Higher Physics
Move between physical models, relationships, units, graphs and justified conclusions, while using current official materials for exact paper and assignment conditions.
Core concepts
Concept 1
electromotive force
Exam cue: Treat emf as energy supplied per unit charge.
Concept 2
internal resistance
Exam cue: Separate internal and external potential drops.
Concept 3
terminal potential difference
Exam cue: On V against I, identify intercept and gradient with signs.
Concept 4
experimental determination from V–I data
Risk pitfalls and guardrails
Calling emf a force.
Guardrail: Check assumptions, prefixes, units, significant figures and whether the statement is supported by evidence.
Assuming terminal voltage always equals emf.
Guardrail: Check assumptions, prefixes, units, significant figures and whether the statement is supported by evidence.
Reporting internal resistance as a negative physical value from a negative graph gradient.
Guardrail: Do not infer proportionality from shape alone; test the predicted transformed graph and intercept.
Memory anchors
Emf
Energy supplied by the source per unit charge.
Lost volts
The internal potential drop Ir.
Terminal voltage
V = ε − Ir while the source supplies current.
Open circuit
With I = 0, terminal voltage equals emf.
V–I intercept
The vertical intercept is ε.
V–I gradient
The gradient is −r, so internal resistance is its magnitude.
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 an electrical source?
What is terminal potential difference when a cell supplies current?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
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