IB Physics SL B.5 Current & circuits Question Bank

Practise extracting circuit quantities from diagrams and graphs, applying charge, resistance and power relations and checking cell or meter assumptions.

Exam points

define emf as energy per charge and calculate current from charge flow or conventional-current direction
calculate resistance or resistivity and distinguish ohmic from non-ohmic V-I behaviour
reduce series-parallel networks and apply Kirchhoff junction or loop rules with ideal meters
calculate terminal potential difference using emf and internal resistance as load changes
calculate electrical power, energy or rms current and compare useful transfer or dissipation

Covered syllabus points

B.5.1—Cells provide a source of emf B.5.2—Electrical energy sources B.5.3—Resistance B.5.4—Ohmic behaviour B.5.5—Electrical power B.5.6—Electrical energy transfer B.5.7—Electric cells B.5.8—Conventional current B.5.9—Current and charge flow B.5.10—DC and AC B.5.11—Kirchhoff’s laws B.5.12—Resistance and resistivity B.5.13—Series and parallel circuits B.5.14—Emf and internal resistance B.5.15—Variable resistance

[Maximum number: 8]

This question is in two parts. Part 1 is about a thermistor circuit. Part 2 is about vibrations and waves.
Part 1 Thermistor circuit
The circuit shows a negative temperature coefficient (NTC) thermistor X and a $100 \mathrm{k} \Omega$ fixed resistor R connected across a battery.

Figure for Question B.5 Current and circuits question 1 — IB Physics SL

The battery has an electromotive force (emf) of 12.0 V and negligible internal resistance.

(a)

Define electromotive force (emf).

[ 1 ]

(b)

The graph below shows the variation with temperature T of the resistance $R_{\mathrm{x}}$ of the thermistor.

[ 7 ]

(i)

Determine the temperature of X when the potential difference across R is 4.5 V .

[ 4 ]

$V_X = 7.5\ ext{V}$ .
$I = \frac{4.5}{100 imes 10^3} = 4.5 imes 10^{-5}\ ext{A}$ OR $\frac{V_X}{V_R}=\frac{R_X}{R_R}$ .
$R_X = \frac{7.5}{4.5 imes 10^{-5}} = 1.67 imes 10^5\ \Omega$ OR $R_X=\frac{7.5}{4.5} imes100 imes10^3=1.67 imes10^5\ \Omega$ .
T=-37 or $-38\ ^\circ ext{C}$ .

(ii)

State the range of temperatures for which the change in the resistance of the thermistor is most sensitive to changes in temperature.

[ 1 ]

50 to (up to) $-30^{\circ} \mathrm{C} /$ at low temperatures;

(iii)

State and explain the effect of a decrease in temperature on the ratio

\frac{\text { voltage across } \mathrm{X}}{\text { voltage across } \mathrm{R}}

Part 2 Vibrations and waves

The cone and dust cap D of a loudspeaker L vibrates with a frequency of 1.25 kHz with simple harmonic motion (SHM).

Figure for Question (iii) — IB Physics SL

[ 2 ]

as the temperature decreases $R_{X}$ increases; same current through R and X so the ratio increases or $V_{X}$ increases and $V_{R}$ decreases so the ratio increases;

Part 2 Vibrations and waves

[Maximum number: 9]

(a)

Identify the laws of conservation that are represented by Kirchhoff's circuit laws.

[ 2 ]

(b)

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 .

[ 3 ]

(i)

State the emf of the cell.

[ 1 ]

$12 \mathrm{~V}$

(ii)

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)

(c)

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 ]

(d)

Outline why research into solar cell technology is important to society.

[ 1 ]

[Maximum number: 10]

(a)

Part 2 Electric current and resistance
The graph below shows how the current I in a tungsten filament lamp varies with potential difference V across the lamp.

[ 10 ]

(i)

Define the electrical resistance of a component.

[ 1 ]

(ii)

Explain whether or not the filament obeys Ohm's law.

[ 2 ]

(iii)

Calculate the resistance of the filament lamp when the potential difference across it is 2.8 V .

[ 2 ]

(iv)

The length of the filament in a lamp is 0.40 m . The resistivity of tungsten when the potential difference across it is 2.8 V is $5.8 \times 10^{-7} \Omega \mathrm{~m}$ . Calculate the radius of the filament.

[ 3 ]

(v)

Two identical filament lamps are connected in series with a cell of emf 6.0 V and negligible internal resistance. Using the graph on page 26, calculate the total power dissipated in the circuit.

[ 2 ]

All question bank results loaded

Covered syllabus points

B.5 Current and circuits question 1

Question (a)

Question (b)

Question (i)

Question (ii)

Question (iii)

B.5 Current and circuits question 2

Question (a)

Question (b)

Question (i)

Question (ii)

Question (c)

Question (d)

B.5 Current and circuits question 3

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (v)