CAIE A-Level Physics 10.3 Potential Dividers

Practise calculating potential-divider outputs, using potentiometers and null methods, and predicting sensor-controlled voltages from thermistor or LDR resistance changes.

Exam points

calculate and interpret potential-divider output p.d. and resistor ratios
use potentiometers and galvanometer null methods to compare potential differences
apply thermistor and LDR resistance changes in potential-divider circuits to predict sensor-dependent output

Covered syllabus points

10.3.1—The principle of a potential divider circuit 10.3.2—The principle of the potentiometer as a means of comparing potential 10.3.3—The use of a galvanometer in null methods 10.3.4—The use of thermistors and light-dependent resistors in potential dividers

[Maximum number: 2]

The slider of the potentiometer in (c) is moved to end A .

State and explain the effect on the brightness of lamps P and Q .
lamp P :
lamp Q:

[Maximum number: 4]

(a)

A battery of electromotive force (e.m.f.) 9.0 V and negligible internal resistance is connected to a light-dependent resistor (LDR) and a fixed resistor, as shown in Fig. 7.1.

Fig. 7.1

The LDR and fixed resistor have resistances of $1800 \Omega$ and $1200 \Omega$ respectively.
Calculate the potential difference across the LDR.
potential difference = V

[ 2 ]

V / 9.0=1800 /(1800+1200)

C1

$V=5.4 \mathrm{~V}$

A1

or

$I=9.0 /(1800+1200)=3.0 \times 10^{-3}(\mathrm{~A})$ $V=3.0 \times 10^{-3} \times 1800$

(C1)

$=5.4 \mathrm{~V}$

(A1)

(b)

The circuit in (a) is now modified by adding a uniform resistance wire XY and a galvanometer, as shown in Fig. 7.2.

Fig. 7.2 (not to scale)

The length of the wire XY is 1.2 m . The movable connection Z is positioned on the wire XY so that the galvanometer reading is zero.

[ 2 ]

(i)

Calculate the length XZ along the resistance wire.

length XZ =m [2]

[ 2 ]

L / 1.2=5.4 / 9.0 or XZ / 1.2=5.4 / 9.0

C1

$L=0.72 \mathrm{~m}$

A1

or

L / 1.2=1800 /(1800+1200) or X Z / 1.2=1.8 /(1.8+1.2)

(C1)

$L=0.72 \mathrm{~m}$

(A1)

[Maximum number: 1]

In the circuit shown, a battery of negligible internal resistance is connected in series with a pair of fixed resistors $\mathrm{R}_{1}$ and $\mathrm{R}_{2}$ .

Figure for Question 10.3 Potential dividers question 3 — CAIE A-Level Physics AS

The circuit is to be used to test whether the electromotive force (e.m.f.) of a particular cell is 1.5 V . The cell is connected between terminals X and Y in parallel with $R_{2}$ and in series with a galvanometer.

Which statement about the test is correct?

Any non-zero reading on the galvanometer means the cell has an e.m.f. of 1.5 V .

The battery does not need to have an e.m.f. of 6.0 V .

The cell may be connected either way round between X and Y .

The galvanometer does not need a scale calibrated in amperes.

All question bank results loaded

Covered syllabus points

10.3 Potential dividers question 1

10.3 Potential dividers question 2

Question (a)

Question (b)

Question (i)

10.3 Potential dividers question 3