Electromotive force and terminal voltage
An emf is an energy-per-charge quantity, not a force.
Key knowledge
A source emf measures supplied work per unit charge and has units of volts. An ideal emf source with a series internal resistance r models a discharging battery’s terminal voltage as V_terminal=ε−Ir, where I leaves its positive terminal. At zero current this model gives V_terminal=ε; under load internal dissipation lowers terminal voltage.
Values and equations
ε=dW_source/dq; V_terminal=ε−Ir
- ε,V_terminal
- Source emf and terminal voltage V
- W_source
- Work supplied by source mechanisms J
- q
- Transported charge in the stated source orientation C
- I
- Current leaving the positive terminal during discharge A
- r
- Series internal resistance Ω
Conditions and scope
- Quasistatic two-terminal source model with a stated emf and linear internal resistance r≥0 over the operating range.
- Charging reverses the current sign and can make terminal voltage exceed emf. Real electrochemical sources have state, temperature, history and rate dependence; the simple model does not make r a universal constant.
Sources · 1
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Read §10.1 emf work-per-charge definition and Internal Resistance and Terminal Voltage10.2. Exclude the source’s blanket “terminal voltage is always less” claim; the discharge sign is explicit.
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Entry details
Permanent link to this entry ↗- Record identity
- foundation-physics-emf-internal-resistance
- Revision
- foundations-refresh.1
- Structured reference record
- Foundational reference additions