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Flip to reveal answersWhat is the emf of a cell?
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All 12 Flashcards — EMF and internal resistance
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Question
What is the emf of a cell?
Answer
The **energy given to each coulomb** of charge by the cell — its 'pushing voltage'. Unit: **volt (V)**.
Question
What is internal resistance?
Answer
The **resistance inside the cell itself** (symbol r). The current flows through it, so some energy is lost inside the cell.
Question
What is the terminal p.d.?
Answer
The voltage **actually delivered** across the cell's terminals to the circuit: $V = \varepsilon - Ir$.
Question
Formula linking emf, current and resistance?
Answer
$\varepsilon = I(R + r)$ — emf = current × (load + internal resistance). **Given** in the data booklet.
Question
Formula for terminal p.d.?
Answer
$V = \varepsilon - Ir$ — emf minus the lost volts (Ir).
Question
What are the 'lost volts'?
Answer
**Ir** — the volts used up inside the cell by its internal resistance. They grow as the current grows.
Question
Why is the terminal p.d. less than the emf?
Answer
Because some of the emf is used to push current through the **internal resistance r**, losing Ir volts inside the cell.
Question
How do you find r from emf and terminal p.d.?
Answer
Lost volts = ε − V = Ir, so $r = \dfrac{\varepsilon - V}{I}$.
Question
What happens to the terminal p.d. when more current is drawn?
Answer
It **drops** — bigger I means bigger lost volts Ir, so less voltage reaches the circuit.
Question
When is the terminal p.d. ≈ the emf?
Answer
When the internal resistance **r is negligible** (or the current is very small), so Ir ≈ 0.
Question
If r is negligible, what does ε = I(R + r) become?
Answer
The simple **ε = IR** — the emf is just current × external resistance.
Question
What does a voltmeter across a cell read?
Answer
The **terminal p.d.** V = ε − Ir (the same as IR, the voltage across the load).
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Current and circuits
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