B.5 Current and circuits
Practice exam-style IB Physics questions for Current and circuits, aligned with the syllabus and grouped by topic.
Question 1
The electromotive force (emf) of a cell is best described as the
A.
resistance inside the cell per unit current delivered.
B.
force exerted on each electron in the external circuit.
C.
energy supplied by the cell per unit charge passing through it.
D.
rate at which charge leaves the positive terminal of the cell.
Question 2
A steady current transfers 18 C of charge through a lamp in 45 s. What is the current in the lamp?
A.
0.40 A
B.
810 A
C.
27 A
D.
2.5 A
Question 3
The main difference between an electrical conductor and an electrical insulator is that a conductor has
A.
no resistance to the flow of charge at room temperature.
B.
mobile charge carriers that can drift when an electric field is applied.
C.
a smaller number of charged particles in its atoms.
D.
more protons than electrons in each atom.
Question 4
A metal wire obeys Ohm's law when
A.
the potential difference is proportional to current at constant temperature.
B.
the current is proportional to time for a fixed potential difference.
C.
the graph of current against potential difference has increasing gradient.
D.
its resistance is zero for all currents.
Question 5
A resistor of resistance 8.0 Ω is connected across a 12 V supply. What is the power dissipated in the resistor?
A.
18 W
B.
1150 W
C.
1.5 W
D.
96 W
Question 6
A sensor circuit has a steady current of 0.24 A.
Calculate the charge passing a point in the circuit in 35 s.
[1]
State the direction of electron drift in a metal wire relative to conventional current.
[1]
Question 7
A charge of 6.0 C passes through a small motor. The motor transfers 48 J of electrical energy to mechanical and thermal energy.
Calculate the potential difference across the motor.
[1]
State why potential difference and emf have the same unit.
[1]
Question 8
A 6.0 Ω resistor is connected in parallel with a 3.0 Ω resistor. This combination is connected in series with a 2.0 Ω resistor. What is the total resistance?
A.
9.0 Ω
B.
5.0 Ω
C.
4.0 Ω
D.
11 Ω
Question 9
Two wires are made from the same material and have the same length. Wire X has twice the diameter of wire Y. The resistance of X is
A.
four times the resistance of Y.
B.
one half of the resistance of Y.
C.
twice the resistance of Y.
D.
one quarter of the resistance of Y.
Question 10
A solar panel is made from identical cells. To obtain a larger emf while keeping the same maximum current as one cell, the cells should be connected
A.
in parallel.
B.
with an open switch between each cell.
C.
in series.
D.
with alternating polarity.
Question 11
A potentiometer used as a potential divider has its two outer terminals connected to a 9.0 V supply. The output is taken between the sliding contact and the negative terminal. Moving the sliding contact towards the positive terminal causes the output potential difference to
A.
increase towards 9.0 V.
B.
decrease towards 0 V.
C.
remain fixed at 4.5 V.
D.
become larger than 9.0 V.
Question 12
A uniform metal wire has length 1.50 m, diameter 0.40 mm and resistance 5.8 Ω.
Calculate the cross-sectional area of the wire.
[1]
Calculate the resistivity of the metal.
[1]
Question 13
A metal resistor becomes warm when a current passes through it.
Describe the microscopic origin of electrical resistance in the metal.
[1]
State the energy change in the resistor.
[1]
Question 14
Three resistors of 4.0 Ω, 6.0 Ω and 12 Ω are connected as follows: the 6.0 Ω and 12 Ω resistors are in parallel, and this combination is in series with the 4.0 Ω resistor. The supply has negligible internal resistance and emf 12 V.
Calculate the equivalent resistance of the circuit.
[1]
Calculate the current in the 4.0 Ω resistor.
[1]
Question 15
A student measures the resistance of a filament lamp at one operating point.
State how the ammeter and voltmeter should be connected to the lamp.
[1]
State one reason why the resistance measured at high current may differ from that at low current.
[1]
Question 16
A 24 Ω heating resistor is connected to a 12 V dc supply.
Calculate the power dissipated in the resistor.
[1]
Calculate the energy transferred in 5.0 min.
[1]
Question 17
A student measures the potential difference across a metal wire for different currents while attempting to keep the wire at constant temperature. The results are shown.
Determine the resistance of the wire from the graph.
[1]
State whether the data support Ohm's law for this wire.
[1]
Suggest one reason why the highest-current point may not lie exactly on the best-fit line.
[1]
Question 18
A network of resistors is connected to an ideal 12 V supply. Current readings in three branches are shown in the table.
| Branch | Resistance / Ω | Current / A |
|---|---|---|
| A | 5.0 | 0.90 |
| B | 25 | 0.30 |
| C | 12.5 | 0.60 |
Identify which two branches are in parallel across the same pair of nodes.
[1]
Determine the total current supplied by the cell.
[1]
Explain how the data are consistent with conservation of charge at a junction.
[1]
Question 19
A cell has emf 6.0 V and internal resistance 0.50 Ω. It supplies a current of 2.0 A to an external resistor. What is the terminal potential difference of the cell?
A.
5.0 V
B.
1.0 V
C.
6.0 V
D.
7.0 V
Question 20
A voltmeter of resistance 20 kΩ is connected across a 10 kΩ resistor in a circuit. What is the resistance of this parallel combination?
A.
6.7 kΩ
B.
10 kΩ
C.
30 kΩ
D.
15 kΩ
Question 21
A negative-temperature-coefficient thermistor is connected in series with a fixed resistor and a constant dc supply. The output potential difference is measured across the fixed resistor. The thermistor is warmed. The output potential difference
A.
increases because the thermistor resistance increases.
B.
decreases because the circuit current decreases.
C.
decreases because the fixed resistor resistance decreases.
D.
increases because the circuit current increases.
Question 22
A filament lamp has a curved graph of potential difference against current . The resistance of the lamp at one operating point is found from
A.
the ratio V/I at that point.
B.
the gradient of the tangent to the V–I graph at that point.
C.
the reciprocal of the potential difference at that point.
D.
the area under the V–I graph up to that point.
Question 23
A cell of emf and internal resistance is connected to an external resistor . The fraction of the cell emf appearing across the external resistor is
A.
$\dfrac{R+r}{R}$
B.
$\dfrac{r}{R+r}$
C.
$\dfrac{R-r}{R+r}$
D.
$\dfrac{R}{R+r}$
Question 24
An LDR and a fixed resistor are connected in series to a constant supply. The output potential difference is measured across the LDR. The illumination is increased. The output potential difference across the LDR
A.
decreases because the LDR resistance decreases.
B.
increases because the circuit current decreases.
C.
decreases because the LDR resistance increases.
D.
increases because the supply emf increases.
Question 25
Two resistors of resistance R and 3R are connected in parallel to a cell of negligible internal resistance. The power dissipated in the resistor R is P. What is the total power dissipated by both resistors?
A.
$\dfrac{4P}{3}$
B.
4P
C.
$\dfrac{3P}{4}$
D.
2P
Question 26
A cell of emf 9.0 V and internal resistance 1.5 Ω is connected to a 4.5 Ω resistor.
Calculate the current in the circuit.
[1]
Explain why the terminal potential difference is less than 9.0 V when current flows.
[1]
Question 27
A 6.0 V supply is connected to a potential divider consisting of a 2.0 kΩ fixed resistor in series with a thermistor. The output potential difference is taken across the fixed resistor. At one temperature the thermistor resistance is 4.0 kΩ.
Calculate the output potential difference.
[1]
The thermistor is warmed. State and explain the change in the output potential difference.
[1]
Question 28
A metal wire and a filament lamp are each connected separately to a variable dc supply. Measurements of potential difference and current are taken.
State the expected shape of a graph of against for the metal wire at constant temperature.
[1]
Explain why the graph for the filament lamp is not a straight line.
[1]
Question 29
A remote weather station may be powered either by primary chemical cells or by a solar cell with rechargeable storage.
State one advantage of using primary chemical cells.
[1]
State one limitation of using primary chemical cells.
[1]
Explain one advantage and one limitation of using the solar-cell system.
[1]
Question 30
A student investigates how the resistance of wires made from the same metal depends on diameter. All wires have the same length.
State a suitable quantity to plot on the horizontal axis so that a graph of resistance on the vertical axis should be linear.
[1]
Suggest two precautions to improve the reliability of the measured resistivity.
[1]
Question 31
A cell has emf 12.0 V and internal resistance 2.0 Ω. It is connected to a variable external resistor. At one setting the terminal potential difference is 8.0 V.
Calculate the current delivered by the cell.
[1]
Calculate the external resistance at this setting.
[1]
Question 32
The terminal potential difference of a cell is measured for different currents drawn from it.
Determine the emf of the cell.
[1]
Determine the internal resistance of the cell.
[1]
Explain why the terminal potential difference decreases as current increases.
[1]
Question 33
A student measures the resistance of wires of the same material and diameter but different lengths .
Describe the relationship between and .
[1]
Determine the resistivity of the wire material using the graph and the wire diameter stated on the figure.
[1]
Suggest one improvement to reduce uncertainty in the cross-sectional area.
[1]
Question 34
A thermistor is connected in series with a fixed resistor across a constant supply. The graph shows how the potential difference across the fixed resistor varies with thermistor temperature.
State the trend shown by the graph.
[1]
Explain the trend using the behaviour of the thermistor.
[1]
Suggest why the graph becomes less steep at high temperature.
[1]
Question 35
A resistor of resistance 12 kΩ is connected to a supply. A non-ideal voltmeter of resistance 36 kΩ is connected across the resistor.
Calculate the equivalent resistance of the resistor and voltmeter in parallel.
[1]
Explain why the voltmeter reading may be less than the potential difference across the resistor before the voltmeter was connected.
[1]
Question 36
An automatic night-light uses an LDR and a fixed resistor connected in series across a 5.0 V supply. A control circuit turns the lamp on when the potential difference across the fixed resistor is small. The LDR is connected between the positive terminal and the junction; the fixed resistor is connected between the junction and the negative terminal.
State what happens to the resistance of the LDR when the room becomes dark.
[1]
Explain why this arrangement can turn the lamp on in the dark.
[1]
Question 37
A student claims that crossing wires in a circuit diagram always indicate that the wires are electrically connected.
State the convention used in circuit diagrams to show that two crossing wires are connected.
[1]
real circuit does not behave as predicted by its diagram. Suggest three checks the student should make before concluding that a component is faulty.
[1]
Question 38
A filament lamp is tested using a variable dc supply. The graph shows potential difference against current for positive and negative currents.
Determine the resistance of the lamp at the labelled operating point.
[1]
Compare the resistance near the origin with the resistance at larger current.
[1]
Explain the shape of the graph in terms of the filament temperature.
[1]
Question 39
The table shows the output potential difference from a potential divider containing an LDR and a fixed resistor for different illumination levels.
| Illumination / a.u. | Output p.d. / V | Point |
|---|---|---|
| 0 | 0.24 | |
| 20 | 1.20 | |
| 40 | 2.40 | X |
| 60 | 3.15 | |
| 80 | 3.60 | |
| 100 | 3.90 | |
| 120 | 4.12 |
Identify the illumination range over which the output changes most rapidly.
[1]
Determine the resistance of the LDR at one labelled illumination level using the supply voltage and fixed resistor value shown.
[1]
Suggest one design change that would make the circuit more sensitive at low illumination.
[1]
Question 40
A small monitoring device is tested with three possible energy sources. The table gives operating data collected over one week.
| Source | Output time / h | Charge / A h | Mean voltage / V | Availability or service |
|---|---|---|---|---|
| Primary cell | 168 | 14.0 | 3.0 | continuous; replace after week |
| Rechargeable cell | 168 | 21.0 | 2.4 | continuous if recharged daily |
| Solar cell | 42 | 32.0 | 2.0 | output only in illumination |
Identify which source provides the largest total electrical energy during the week.
[1]
Determine the mean output power of the rechargeable cell during the week.
[1]
Evaluate one reason, other than total energy, why the solar-cell option may still be unsuitable.
[1]
Question 41
A resistor is immersed in water in a well-insulated container. The electrical measurements are shown for several time intervals.
| Interval label | Time interval / s | Current / A | p.d. / V | Water ΔT / °C |
|---|---|---|---|---|
| A | 0–60 | 2.10 | 11.8 | 1.7 |
| B | 60–120 | 2.05 | 12.0 | 1.8 |
| C | 120–180 | 2.20 | 11.6 | 1.9 |
| D | 180–240 | 2.15 | 11.9 | 1.8 |
Determine the electrical power dissipated in the resistor for the labelled interval.
[1]
Determine the electrical energy transferred during that interval.
[1]
Suggest two reasons why the increase in internal energy of the water may be less than the electrical energy transferred by the resistor.
[1]
Question 42
A cell of emf 10.0 V and internal resistance 1.0 Ω is connected to two external resistors, 4.0 Ω and 6.0 Ω, in series.
Calculate the current in the circuit and the terminal potential difference of the cell.
[1]
Explain, using energy-per-charge ideas, how the emf is shared between the internal resistance and the two external resistors. Include the potential difference across each external resistor.
[1]
Question 43
A student investigates whether a component is ohmic. The student varies the supply potential difference and records the current through the component.
Outline how the ammeter and voltmeter should be connected and state the expected graph for an ohmic conductor.
[1]
Discuss why a filament lamp does not obey Ohm's law over a wide range of currents, referring to both the graph and the particle model of resistance.
[1]
Question 44
A school plans to install emergency lighting. The lighting could be powered by primary chemical cells, secondary chemical cells, or solar cells with storage.
Define a primary cell and a secondary cell.
[1]
Evaluate the three energy-source options for this application, referring to reliability, environmental issues and practical use.
[1]
Question 45
A high-resistance component is connected in series with a resistor and a dc supply. A non-ideal voltmeter is used to measure the potential difference across the high-resistance component. Data for two voltmeters are shown.
| Voltmeter | Supply voltage / V | Series resistance / MΩ | Voltmeter resistance / MΩ | Reading across X / V |
|---|---|---|---|---|
| 1 | 12.0 | 2.00 | 5.00 | 8.00 |
| 2 | 12.0 | 2.00 | 50.0 | 10.5 |
State which voltmeter gives the smaller disturbance to the circuit.
[1]
Determine the effective resistance of the high-resistance component when measured with voltmeter 1.
[1]
Explain why voltmeter resistance affects the reading more strongly for a high-resistance component than for a low-resistance component.
[1]
Question 46
A student determines the resistivity of a metal wire using a dc supply, ammeter, voltmeter, metre rule and micrometer.
State the measurements needed to determine the resistivity of the wire and the equation used.
[1]
Evaluate the main sources of uncertainty or systematic error in the experiment and suggest how they may be reduced.
[1]
Question 47
A temperature sensor uses a negative-temperature-coefficient thermistor in a potential divider connected to a fixed dc supply. The output potential difference is sent to a data logger with very high input resistance.
Explain why the thermistor resistance decreases as temperature increases.
[1]
Evaluate how the choice of placing the thermistor in the upper or lower part of the potential divider affects whether the output increases or decreases with temperature. Include the role of the data logger input resistance.
[1]
Question 48
A student uses a variable resistor to investigate a real cell. For each setting, the student records the current delivered by the cell and the terminal potential difference.
Describe how the emf and internal resistance are obtained from a graph of terminal potential difference against current.
[1]
Discuss the limitations of the model for a real cell during the experiment and how the student can reduce their effect.
[1]
Question 49
Current can occur in metals, electrolytes and semiconductors, but the charge carriers and resistance mechanisms differ.
State the charge carriers responsible for conduction in a metal and in an electrolyte.
[1]
Compare and contrast conduction and resistance in metals and semiconductors, including the behaviour of a thermistor and an LDR.
[1]
Question 50
A 12 V supply of negligible internal resistance is connected to a network. A 3.0 Ω resistor is in series with a parallel combination of 6.0 Ω and 12 Ω resistors.
Determine the equivalent resistance of the network and the current supplied by the source.
[1]
Evaluate the distribution of current, potential difference and power in the three resistors, and explain why the total power from the source equals the sum of the powers dissipated in the resistors.
[1]
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