Electromotive Force & Internal Resistance Flashcards | A-Level Physics AQA

What does e.m.f. stand for?

Electromotive force

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What is meant by the term 'electromotive force'?

Electromotive force (e.m.f.) is the energy supplied by a power supply per coulomb of charge.

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What is a power supply's internal resistance?

The resistance of the power supply itself.

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What is meant by 'terminal potential'?

Terminal potential is the voltage measured across the terminals of a source.

The voltage measured will be equal to the e.m.f. when there is no current flowing.

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How can the e.m.f of a cell be measured?

Connect a high resistance voltmeter across the terminals of a cell with no current flowing.

When measuring the e.m.f. of a cell disconnect the cell from the rest of the circuit before connecting the voltmeter. This will ensure no current is flowing.

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The graph below shows how the terminal p.d. varies with current across a cell.

Which feature of the graph represents the e.m.f of the cell?

The y-intercept is the e.m.f.

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The graph below shows how the terminal p.d. varies with current across a cell.

What does the magnitude of the gradient of this graph represent?

Internal resistance of the power supply.

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What is the formula linking e.m.f. ( $\epsilon$ ), current (I), load resistance (R) and internal resistance (r)?

ϵ = I(R + r)

$\epsilon$ = e.m.f of the supply (V)

I = current (A)

R = load resistance ( $\Omega$ )

r = internal resistance ( $\Omega$ )

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What is the formula linking e.m.f ( $\epsilon$ ), terminal potential (V), current (I) and internal resistance (r)?

ϵ = V + Ir

$\epsilon$ = e.m.f. of supply (V)

V = terminal potential (V)

I = current (A)

r = internal resistance ( $\Omega$ )

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What is meant by the term 'lost volts'?

Lost volts is the difference between the electromotive force of a power supply and the terminal potential.

Lost volts is due to the potential difference across the internal resistance.

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