Question 1Multiple choice · 1 markA* challenge
Point charges of +4 μC and +1 μC are 0.4 m apart.
What is the electric potential at the midpoint between them?
- A 1.1 × 105 V
- B 1.3 × 105 V
- C 2.2 × 105 V
- D 1.1 × 106 V
Show answer
Answer: C
10 free IB Physics D Fields questions written in the style of IB Physics papers: 4 multiple-choice and 6 structured, each with its answer and mark scheme. 4 of them are A* challenge questions, built to catch the mistakes strong students still make.
This topic covers D.1 Gravitational fields, D.2 Electric & magnetic fields, D.3 Motion in electromagnetic fields, D.4 Induction. Work through the questions below, then generate a full D Fields paper with a fresh set every time.
Generate a full D Fields paper →Point charges of +4 μC and +1 μC are 0.4 m apart.
What is the electric potential at the midpoint between them?
Answer: C
A coil of 500 turns and area 5 × 10−3 m2 is perpendicular to a magnetic field. The field changes uniformly from 0.1 T to 0.05 T in 0.02 s.
What is the magnitude of the average induced emf?
Answer: C
The gravitational field strength at the Earth’s surface is g. What is the field strength at a height of 2R above the surface, where R is the radius of the Earth?
Answer: B
An electron is accelerated from rest through a potential difference of 500 V.
What is its final speed?
Answer: C
Electrons are accelerated from rest through a potential difference of 1000 V and then enter the region between two parallel plates, 4 cm apart and 5 cm long, moving parallel to the plates. The potential difference between the plates is 600 V.
Show that the speed of the electrons entering the plates is about 1.9 × 107 m s−1.
Calculate the electric field strength between the plates and the acceleration of an electron.
Determine the vertical deflection of an electron as it leaves the plates.
Describe the shape of the path between the plates and after it leaves them.
A magnetic field is applied so that the electrons pass through undeflected. Calculate the magnetic flux density and state its direction relative to the electric field.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 1.87 × 107 m s−1 eV = ½mv2 ✓ v = 1.87 × 107 m s−1 ✓ | 2 |
| (b) | 15000 V m−1; 2.63 × 1015 m s−2 E = Vd = 15000 V m−1 ✓ a = eEm = 2.63 × 1015 m s−2 ✓ | 2 |
| (c) | 9.38 mm time between plates = Lv = 2.67 × 10−9 s ✓ y = ½at2 ✓ 9.38 mm ✓ | 3 |
| (d) | parabola then straight line parabolic between the plates (constant force perpendicular to initial velocity) ✓ straight line after leaving the field ✓ | 2 |
| (e) | 8.0 × 10−4 T eE = evB ✓ B = Ev = 8.0 × 10−4 T ✓ perpendicular to both the electric field and the velocity ✓ | 3 |
A coil of 150 turns and area 4.0 × 10−3 m2 is perpendicular to a magnetic field of 0.20 T. The field falls uniformly to zero in 0.050 s. The resistance of the coil is 3.0 Ω.
Calculate the initial magnetic flux linkage.
Calculate the average induced emf.
Calculate the charge that flows around the coil.
State Lenz’s law and use it to describe the direction of the induced current.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 0.12 Wb NBA = 0.12 Wb ✓ | 1 |
| (b) | 2.4 V ε = Δ(NΦ)/Δt ✓ = 2.4 V ✓ | 2 |
| (c) | 0.040 C I = 0.80 A ✓ Q = 0.80 × 0.050 = 0.040 C ✓ | 2 |
| (d) | opposes change induced emf opposes the change that produces it ✓ current produces a field in the same direction as the original field (trying to maintain the flux) ✓ | 2 |
A metal rod of length 0.30 m slides at a constant 2.0 m s−1 along two parallel conducting rails in a uniform vertical magnetic field of 0.60 T. The rails are connected by a resistor of 1 Ω.
Calculate the induced emf.
Calculate the current in the circuit.
Calculate the force needed to keep the rod moving at constant speed.
Show that the mechanical power supplied equals the electrical power dissipated.
Outline how Lenz’s law applies to this system.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 0.360 V ε = BLv = 0.360 V ✓ | 1 |
| (b) | 0.360 A 0.360 A ✓ | 1 |
| (c) | 0.0648 N F = BIL ✓ = 0.0648 N ✓ | 2 |
| (d) | 0.130 W P = Fv = 0.130 W ✓ I2R = 0.130 W, the same ✓ | 2 |
| (e) | induced current opposes motion induced current flows in a direction to oppose the change producing it ✓ the force on the current-carrying rod opposes its motion, so work must be done ✓ | 2 |
A satellite orbits the Earth at a height of 400 km. Earth mass = 5.97 × 1024 kg, radius = 6.37 × 106 m.
Calculate the gravitational field strength at this height.
Calculate the orbital speed.
Explain why astronauts in the satellite appear weightless.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 8.7 N kg−1 g = GM/r2 ✓ 8.7 N kg−1 ✓ | 2 |
| (b) | 7.7 km s−1 v = √(GM/r) ✓ 7.67 × 103 m s−1 ✓ | 2 |
| (c) | free fall together astronaut and satellite are both in free fall ✓ with the same acceleration, so no contact force ✓ | 2 |
Ions of charge +e and mass 3.8 × 10−26 kg are accelerated through 2.0 kV and enter a magnetic field of 0.25 T at right angles.
Calculate the speed of the ions.
Calculate the radius of their path.
State why the kinetic energy of the ions does not change in the field.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 1.3 × 105 m s−1 qV = ½mv2 ✓ 1.30 × 105 m s−1 ✓ | 2 |
| (b) | 0.12 m r = mv/qB ✓ 0.12 m ✓ | 2 |
| (c) | force perpendicular to velocity the magnetic force is perpendicular to the velocity, so no work is done ✓ | 1 |
An electron enters the region between two horizontal plates at 3.0 × 107 m s−1, moving parallel to the plates. The field between the plates is 1.0 × 104 V m−1 and the plates are 6.0 cm long.
Calculate the time the electron spends between the plates.
Calculate the vertical deflection of the electron.
State the shape of the path between the plates.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 2.0 × 10−9 s 0.060/3.0 × 107 = 2.0 × 10−9 s ✓ | 1 |
| (b) | 3.5 mm a = eE/m = 1.76 × 1015 m s−2 ✓ s = ½at2 ✓ = 3.5 × 10−3 m ✓ | 3 |
| (c) | parabola parabolic ✓ | 1 |
In the SL & HL syllabus, D Fields covers D.1 Gravitational fields, D.2 Electric & magnetic fields, D.3 Motion in electromagnetic fields, D.4 Induction.
This page has 10 D Fields questions with answers. For more, the free generator on mentorriyaz.in builds a new D Fields paper with a full mark scheme each time you click.
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Meet Riyaz →Original IB-style practice questions written for mentorriyaz.in. These are not IB past paper questions, and this site is not affiliated with or endorsed by the International Baccalaureate Organization. Updated 2026-11-04. Paper generator · mentorriyaz.in