D.2 Electric and magnetic fields
Practice exam-style IB Physics questions for Electric and magnetic fields, aligned with the syllabus and grouped by topic.
Question 1
A plastic rod becomes negatively charged when rubbed with a cloth.
A.
It has lost electrons to the cloth.
B.
It has gained protons from the cloth.
C.
It has gained electrons from the cloth.
D.
It has lost neutrons to the cloth.
Question 2
Two identical small conducting spheres carry charges of +6.0 nC and −2.0 nC. They touch and are then separated.
A.
+2.0 nC
B.
+8.0 nC
C.
+4.0 nC
D.
−2.0 nC
Question 3
A positive point charge is placed near a neutral conducting sphere.
A.
Electrons in the sphere move nearer to the point charge.
B.
Protons in the sphere move nearer to the point charge.
C.
The sphere gains a net negative charge from the point charge.
D.
The point charge becomes neutral by induction.
Question 4
A potential difference of 120 V is applied across two large parallel plates separated by 4.0 mm.
A.
3.0 × 10¹ V m⁻¹
B.
3.0 × 10⁴ V m⁻¹
C.
4.8 × 10² V m⁻¹
D.
4.8 × 10⁻¹ V m⁻¹
Question 5
The magnetic field around a long straight wire is viewed from above. The conventional current is out of the page.
A.
Straight parallel lines across the page
B.
Straight radial lines away from the wire
C.
Anticlockwise circles centred on the wire
D.
Clockwise circles centred on the wire
Question 6
A charge q is moved along an equipotential surface in an electric field.
A.
kqQ/r
B.
qV, where V is the potential of the surface
C.
Zero
D.
qE/r
Question 7
Electric potential at a point in the field of a point charge is defined using a reference value.
A.
At the point where electric field strength is largest
B.
At the surface of the charge
C.
At Earth potential in all calculations
D.
At infinite distance from the charge
Question 8
A negatively charged rod is brought near a neutral metal sphere on an insulating stand.
State the sign of charge induced on the side of the sphere nearest the rod.
[1]
Outline why the sphere is attracted to the rod even though it remains neutral overall.
[1]
Question 9
Two point charges separated by distance r exert a force F on each other. The separation is increased to 3r and the charges are unchanged.
A.
F/9
B.
F/3
C.
3F
D.
9F
Question 10
A charge of +3.0 μC experiences an electric force of 0.18 N to the east at a point in a field.
A.
1.7 × 10⁻⁵ N C⁻¹ west
B.
6.0 × 10⁴ N C⁻¹ west
C.
6.0 × 10⁴ N C⁻¹ east
D.
5.4 × 10⁻⁷ N C⁻¹ east
Question 11
A conducting hollow sphere has a net positive charge and contains no charge in its cavity.
A.
Uniform from the upper surface to the lower surface
B.
Radially inward towards the centre
C.
Zero everywhere in the cavity
D.
Radially outward from the centre
Question 12
Two point charges +Q and −Q are at equal distances from point P.
A.
2kQ/r
B.
kQ/r²
C.
Zero
D.
It depends on the directions of the fields at P.
Question 13
The electric potential decreases uniformly from 90 V to 30 V over a distance of 0.20 m in the positive x-direction.
A.
+300 V m⁻¹
B.
−300 V m⁻¹
C.
+600 V m⁻¹
D.
−600 V m⁻¹
Question 14
Equipotential lines are drawn closer together in region X than in region Y.
A.
It is zero in region X.
B.
It has the same magnitude in both regions.
C.
It is greater in region X.
D.
It is greater in region Y.
Question 15
Two point charges of +4.0 μC and −2.0 μC are separated by 0.30 m in air.
Calculate the magnitude of the electrostatic force between the charges.
[1]
State the nature of the force between the charges.
[1]
Question 16
A neutral conducting sphere is charged positively by induction using a negatively charged rod.
Describe what happens to electrons in the sphere when the rod is first brought near it.
[1]
State the role of grounding while the rod is still nearby.
[1]
State the order in which the ground connection and rod must be removed to leave the sphere positively charged.
[1]
Question 17
In a simplified Millikan oil-drop experiment, an oil drop is held stationary between two horizontal plates.
State the two vertical forces acting on the charged oil drop when it is stationary.
[1]
Explain how repeating the experiment for many drops provides evidence for quantization of charge.
[1]
Question 18
A single isolated positive point charge is shown at the centre of a square region.
Sketch the electric field lines around the charge.
[1]
State what the spacing of the field lines indicates about field strength.
[1]
Question 19
A long air-core solenoid carries a steady direct current.
Describe the magnetic field pattern inside the solenoid.
[1]
State two changes that would increase the magnetic field strength inside an air-core solenoid.
[1]
Question 20
A point charge Q = +3.0 μC is in a vacuum.
Calculate the electric potential at a point 0.50 m from the charge.
[1]
State the reference point for zero electric potential used in this calculation.
[1]
Question 21
A student investigates the force between two small charged spheres. The same charges are used throughout and the separation r is varied. A force sensor gives readings proportional to the electrostatic force.
State the relationship being tested by plotting force against 1/r².
[1]
Use the graph to describe whether the data support Coulomb's law.
[1]
Suggest one experimental reason why the data may deviate from the expected relationship at small separations.
[1]
Question 22
A uniform electric field is produced between two parallel plates. The plate separation is changed while the potential difference remains constant.
Identify the dependent variable in the investigation.
[1]
Use the graph to describe how electric field strength depends on plate separation.
[1]
Explain the dependence using an equation from the data booklet.
[1]
Question 23
A plotting compass is placed at different positions around a straight vertical wire carrying a steady conventional current upward. The compass directions are shown.
Describe the shape of the magnetic field lines around the wire.
[1]
Use the compass directions to determine the direction of current in the wire.
[1]
Explain how the right-hand grip rule applies to the pattern.
[1]
Question 24
Two charges +2q and −q are separated by a distance r.
A.
+kq²/r
B.
−2kq²/r²
C.
−2kq²/r
D.
+2kq²/r
Question 25
A proton moves from a point at electric potential 20 V to a point at electric potential 75 V.
A.
+55 eV
B.
+95 eV
C.
−95 eV
D.
−55 eV
Question 26
The surface of a charged conducting sphere is an equipotential.
A.
The electric potential is zero everywhere on the sphere.
B.
The electric field outside the sphere is zero.
C.
The charge is uniformly distributed throughout the volume.
D.
Mobile charges would move if a potential difference existed on the surface.
Question 27
Two parallel metal plates are separated by 8.0 mm and connected to a 240 V supply.
Calculate the electric field strength between the plates, away from the edges.
[1]
State one way in which the field near the edges differs from the field near the centre.
[1]
Question 28
A small charge q = −5.0 nC is placed at a point where the electric field strength is 2.4 × 10⁵ N C⁻¹ to the right.
Calculate the magnitude of the force on the charge.
[1]
State the direction of the force on the charge.
[1]
State the direction of the force on a positive test charge placed at the same point.
[1]
Question 29
Two point charges +4.0 nC and −6.0 nC are separated by 0.20 m.
Calculate the electric potential energy of this two-charge system.
[1]
Explain the significance of the sign of your answer.
[1]
Question 30
The electric potential varies with position x as shown by a straight-line graph.
Determine the electric field strength from the gradient of the graph.
[1]
State the direction of the electric field relative to increasing x.
[1]
Question 31
Equipotential lines are drawn around a positive point charge.
State the shape of the equipotential surfaces around the charge.
[1]
Explain why the equipotential lines in a two-dimensional diagram are farther apart at larger distances for equal potential intervals.
[1]
Question 32
An electron is moved through a potential difference of +12 V, so that ΔV_e = +12 V.
Calculate the change in electric potential energy of the electron in joules.
[1]
Express the change in electric potential energy in electronvolts.
[1]
Question 33
A solid conducting sphere carries a net positive charge and is in electrostatic equilibrium.
State how the electric potential varies inside the conducting material.
[1]
State how the electric field strength varies inside the conducting material.
[1]
Explain the relationship between your answers to
[1]
and (b).
[1]
Question 34
Students map an electric field between two shaped electrodes using small particles suspended in oil. A sketch of the observed particle alignment is shown.
Identify the region where the electric field is strongest.
[1]
Explain how the sketch indicates the region of strongest field.
[1]
State one safety precaution for this experiment.
[1]
Suggest one limitation of using the particle pattern to determine electric field strength.
[1]
Question 35
A table shows results from a Millikan-type experiment. The charges calculated for several oil drops are listed.
| Oil drop | Charge / 10⁻¹⁹ C |
|---|---|
| A | 3.18 |
| B | 4.83 |
| C | 6.39 |
| D | 7.98 |
| E | 9.61 |
| F | 11.18 |
| G | 12.82 |
| H | 14.38 |
Use the table to identify the common smallest charge interval.
[1]
Explain how the data provide evidence for quantization of charge.
[1]
Suggest why an individual measured charge might not be exactly an integer multiple of the elementary charge.
[1]
Question 36
The electric potential around a point charge is investigated using a simulation. A graph of electric potential V_e against 1/r is obtained.
State the relationship between V_e and r for a point charge.
[1]
Use the graph to determine the sign of the source charge.
[1]
Explain how the magnitude of the source charge could be found from the graph.
[1]
State why electric potential values from more than one point charge can be added directly.
[1]
Question 37
A graph shows electric potential V_e as a function of position x between two large parallel plates.
Determine the electric field strength between the plates from the graph.
[1]
State the direction of the electric field.
[1]
Explain why the graph is a straight line in the central region between the plates.
[1]
Question 38
Three point charges are fixed at the corners of an equilateral triangle. The charges are +Q, +Q and −Q. The side length is r.
Write an expression for the total electric potential energy of the three-charge system.
[1]
State why potential energy is easier to combine than electric field strength in this situation.
[1]
Question 39
Two large oppositely charged parallel plates produce an approximately uniform electric field.
Describe the equipotential surfaces between the central regions of the plates.
[1]
Explain why electric field lines must meet these equipotential surfaces at 90°.
[1]
Question 40
A conducting-paper experiment maps equipotential lines between two small electrodes. The measured equipotential lines are shown.
Identify where the electric field strength is greatest.
[1]
Draw or describe the direction of the electric field at point P.
[1]
Explain how the equipotential map can be used to infer electric field lines.
[1]
Suggest one reason for uncertainty in the map.
[1]
Question 41
A charged conducting sphere is investigated. The graph shows electric potential V_e against distance r from the centre of the sphere.
Identify the region of the graph corresponding to points inside the conductor.
[1]
Use the graph to explain why the electric field inside the conductor is zero.
[1]
State how the graph would differ outside the sphere if the charge on the sphere were doubled.
[1]
Question 42
A positively charged metal sphere is brought into contact with an identical neutral metal sphere. The spheres are then separated.
State and apply the principle of conservation of charge to this situation.
[1]
Explain, using ideas about mobile charge carriers and conductors, how charge is redistributed during contact and why both spheres have the same final charge.
[1]
Question 43
Two small charged spheres are used in a school experiment to test Coulomb's law.
Outline how the variables should be processed to test the inverse-square dependence.
[1]
Discuss three experimental factors that could affect the reliability of the conclusion.
[1]
Question 44
Electric field lines and magnetic field lines are both used to represent fields.
State two rules for drawing electric field lines.
[1]
Compare and contrast electric field-line patterns with magnetic field-line patterns for the cases of a point charge, a bar magnet and a current-carrying straight wire.
[1]
Question 45
A simulation displays equipotential lines for four fixed point charges. The values of potential on selected lines are labelled.
Identify one point or region where the electric field is approximately zero or very small.
[1]
Explain your choice using the equipotential pattern.
[1]
Evaluate one advantage and one limitation of representing this field using equipotentials rather than field lines.
[1]
Question 46
A hollow conducting sphere with no charge inside its cavity is given a net negative charge.
State where the excess charge is found and the electric field in the conducting material.
[1]
Explain the electric field pattern inside the cavity, inside the conducting material and outside the sphere.
[1]
Question 47
A student represents the field around a system of point charges using both electric field lines and equipotential lines.
State the relationship between electric field lines and equipotential lines.
[1]
Evaluate the usefulness of equipotential diagrams compared with field-line diagrams for analysing energy and force in electric fields.
[1]
Question 48
Two fixed point charges +Q and −Q are separated by a distance d. A small positive test charge is moved slowly from infinity to different points near the charges.
Define electric potential at a point and state why it is a scalar.
[1]
Explain how the sign and magnitude of the electric potential and electric potential energy depend on the position of the test charge.
[1]
Question 49
A point charge and a pair of oppositely charged parallel plates both produce electric fields.
State the expressions for electric potential in the point-charge field and electric field strength between uniform parallel plates.
[1]
Compare and contrast the potential, electric field and equipotential surfaces for these two situations.
[1]
Question 50
A charged conducting spherical shell has no charge inside its hollow cavity.
Describe the electric potential and electric field inside the cavity and inside the conducting material.
[1]
Discuss how the potential and field vary outside the shell and how these ideas explain electrostatic shielding.
[1]
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