IB Physics SL B.5.14 Emf & internal resistance

Connect emf, terminal potential difference and internal resistance by combining circuit equations with straight-line graph readings and valid model choices.

Exam points

read the y-intercept of a terminal p.d.-current graph or use ε = I(R + r) to determine emf
use the gradient of a terminal p.d.-current graph to calculate the internal resistance r in ohms
explain with V = ε - Ir that changing a variable resistor changes current, so terminal p.d. changes too

[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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B.5.14—Emf and internal resistance question 1

Question (a)

Question (i)

Question (b)