| English | Internal resistance |
|---|---|
| Bahasa Melayu | Rintangan dalam |
| 中文 | 内阻 |
Definition
The resistance within a source of electromotive force, such as a cell, which causes some energy to be lost inside the source.
In exam terms
Internal resistance, symbol r, is the resistance within a source of emf such as a cell, measured in ohms (Ω). Because the source itself opposes current, some energy is lost inside it as heat.
The full circuit equation is ε = IR + Ir, or equivalently V = ε − Ir, where V is the terminal potential difference.
Example of correct use: a cell of emf 3.0 V and internal resistance 0.5 Ω driving a current of 2 A loses a potential difference of Ir = 2 A × 0.5 Ω = 1.0 V inside itself, giving a terminal p.d. of 3.0 V − 1.0 V = 2.0 V.
Do not confuse it with…
Do not confuse internal resistance with external (load) resistance. Internal resistance r is inside the source; load resistance R is the resistance of the external components.
As the current increases, the lost voltage Ir grows, so the terminal p.d. falls below the emf.
In Paper 2, the standard experiment plots terminal p.d. V against current I: the gradient of the line equals −r and the vertical intercept equals the emf ε.
Be ready to state what the gradient and intercept represent.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English) (Bahagian Pembangunan Kurikulum (BPK), KPM)