CAIE A-Level Physics 10.2.7 Kirchhoff Circuit Problems

Practise solving multi-branch circuits with Kirchhoff current and energy equations, checking simultaneous results, signs and conservation conditions.

Exam points

use Kirchhoff’s first law to write current equations at junctions
use Kirchhoff’s second law to write energy equations around loops
solve simultaneous circuit equations for currents, p.d.s, resistances or source values
check current directions, signs and conservation conditions in a complete circuit

[Maximum number: 4]

(a)

Fig. 5.2 shows a circuit with a battery of electromotive force (e.m.f.) 12.0 V connected to a linear potentiometer AB and two identical filament lamps P and Q .

Fig. 5.2

The battery has negligible internal resistance and the lamps each have the same I-V characteristic shown in Fig. 5.1.

When the slider of the potentiometer is at its midpoint, as shown in Fig. 5.2, the current I in the battery is 1.78 A .

Determine:

[ 4 ]

(i)

the current in lamp P
current = A

[ 1 ]

$I=1.55 \mathrm{~A}$

A1

(ii)

the resistance of the potentiometer between its ends A and B.
resistance = $\Omega$

[ 3 ]

$\begin{gathered} I=1.78-1.55 (=0.23 \mathrm{~A}) \end{gathered}$

C1

$\begin{aligned} R =12.0 / 0.23 =52 \Omega \end{aligned}$

A1

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10.2.7—Kirchhoff’s laws to solve simple circuit problems question 1

Question (a)

Question (i)

Question (ii)