IB Physics SL B 5 14 Emf and Internal Resistance Questions

Practise IB Physics SL B.5.14 by relating emf, internal resistance, terminal potential difference and current in real-cell circuits.

Exam points

Calculate emf, terminal potential difference or internal resistance using ε = I(R + r) and V = ε − Ir for a cell supplying an external circuit.
Interpret V–I data: use the y-intercept for emf and the negative gradient for internal resistance, including readings from a variable-resistance investigation.
Explain how changing external resistance changes current, terminal potential difference, lost volts and power dissipated inside the cell.
Determine internal resistance from power or energy dissipated within the cell, using the I²r model and relevant circuit data.
Apply emf and internal-resistance calculations to cells connected in series or parallel, finding current or terminal potential difference.

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Study B.5.14—Emf and internal resistance conceptsOpen the corresponding Concept lesson for explanations, examples and exam connections.

[Maximum number: 5]

(a)

A cell is connected to an ideal voltmeter, a switch S and a resistor R. The resistance of R is $4.0 \Omega$ .

When S is open the reading on the voltmeter is 12 V . When S is closed the voltmeter reads 8.0 V .

[ 2 ]

(i)

Deduce the internal resistance of the cell.

[ 2 ]

I=2.0 A OR 12=I(r+4) OR 4=I r OR 8=4 I

«Correct working to get » r=2.0 « $\Omega$ »

Marking guidance:

Allow ECF from (b)(i)

(b)

The voltmeter is used in another circuit that contains two secondary cells.

Cell A has an emf of 10 V and an internal resistance of $1.0 \Omega$ . Cell B has an emf of 4.0 V and an internal resistance of $2.0 \Omega$ .

Calculate the reading on the voltmeter.

[ 3 ]

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IB Physics SL B 5 14 Emf and Internal Resistance Questions question 1

Question (a)

Question (i)

Question (b)