E.m.f. & Internal Resistance
E.m.f. & Internal Resistance
- A source transfers energy to charge. Components outside the source transfer that electrical energy into other forms.
- This energy account separates from p.d.: both are measured in volts, but they describe different transfers.
Energy gained and energy transferred
- E.m.f. is energy supplied by the source per coulomb around the complete circuit.
- P.d. across a component is energy transferred from each coulomb in that component.
- In a complete loop, energy supplied per coulomb equals the total energy transferred per coulomb.
Definition
2 marksWhat is meant by electromotive force (e.m.f.)?
Model answer: The energy transferred by a source per unit charge in driving charge around a complete circuit.
A real source loses some energy inside
- A real source behaves like an ideal source of e.m.f. in series with an internal resistance .
- When current flows, the source transfers joules per coulomb inside itself. This is the lost volts.
- The terminal p.d. is what remains for the external circuit.
Definition
2 marksWhat is meant by internal resistance?
Model answer: The resistance within a source that causes energy to be transferred inside the source when current flows.
Symbols
- = terminal potential difference (V)
- = electromotive force (V)
- = current supplied by the source (A)
- = internal resistance (\Omega)
useful: V = 4.80 Vlost inside: I r = 1.20 V
I = E / (R + r) = 1.20 A·terminal p.d. = 4.80 V of the 6.00 V label
Common mistake
E.m.f. and terminal p.d. are equal only when no current is drawn, or when internal resistance is negligible. A larger current makes the lost volts larger.
Your turn9702/22/O/N/25 Q5(b)(i)
A cell has e.m.f. 5.0 V and internal resistance 4.7 Ω. It supplies 38 mA. Calculate its terminal p.d. [2] (9702/22/O/N/25 Q5(b)(i))
Show worked answer
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