IB Physics SL B.5 Current and Circuits Question Bank

Practise IB Physics SL B.5 by reading circuit diagrams, conserving charge and energy, and calculating resistance, power and terminal voltage.

Exam points

Define current and emf, then calculate charge flow, resistance, voltage and power with units.
Reduce series–parallel networks and apply junction, loop and meter rules to circuit evidence.
Use resistivity, internal resistance and V–I data to explain energy transfer and non-ohmic behaviour.

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 ]

$V_X=7.5\,\mathrm{V}$ $I=\frac{4.5\,\mathrm{V}}{100\times10^3\,\Omega}=4.5\times10^{-5}\,\mathrm{A}$ $R_X=\frac{7.5\,\mathrm{V}}{4.5\times10^{-5}\,\mathrm{A}}=1.67\times10^5\,\Omega$ $T=-37\text{ to }-38\,^{\circ}\mathrm{C}$

Alternatively, use the potential-divider relation V_X/V_R=R_X/R_R or the equivalent resistance calculation.

(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: 12]

(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)

A fully charged cell of emf 6.0 V delivers a constant current of 5.0 A for a time of 0.25 hour until it is completely discharged.

The cell is then re-charged by a rectangular solar panel of dimensions $0.40 \mathrm{~m} \times 0.15 \mathrm{~m}$ at a place where the maximum intensity of sunlight is $380 \mathrm{Wm}^{-2}$ .

The overall efficiency of the re-charging process is 18 %.
Calculate the minimum time required to re-charge the cell fully.

[ 3 ]

ALTERNATIVE 1

«energy output of the panel = » VIt OR $6 \times 5 \times 0.25 \times 3600$ OR 27000 «J»

«available power => $380 \times 0.4 \times 0.15 \times 0.18$ OR 4.1 «W»

$t=\ll \frac{27000}{4.1}=>6600<\mathrm{S} \gg$

ALTERNATIVE 2

«energy needed from Sun = » $\frac{V I t}{\text { eff }}$ OR $\frac{6 \times 5 \times 0.25 \times 3600}{0.18}$ OR 150000 «J»

« incident power=» $380 \times 0.4 \times 0.15$ OR 22.8 «W»

$t=\ll \frac{150000}{22.8}=>6600<\mathrm{S} \gg$

Marking guidance:

Allow ECF for MP3

Accept final answer in minutes (110) or hours (1.8).

(e)

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)

Question (e)

B.5 Current and circuits question 3

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

Question (v)