IB Diploma Physics chapter 4: Fields MCQs with answers
By Team Megalecture · Updated 4 Nov 2026
36 exam-style questions on Fields, each with the answer and a short explanation. Ten of them are below to try. The chapter test draws ten fresh questions each time and is marked the moment you finish, with your score by topic.
Try each one, then open the answer. These ten stay the same; the test draws new ones every time.
1. In a velocity selector with perpendicular electric and magnetic fields, why does the speed of undeflected particles not depend on their charge?
A
the electric force depends on the mass as well as the charge
B
the magnetic force does not depend on charge
C
only neutral particles pass through undeflected
D
both the electric force and the magnetic force are proportional to the charge, so it cancels
Show answer
Answer: D. qE = qvB gives v = E/B: q cancels. Particles of any charge or mass with this speed pass straight through.
2. How are the gravitational field lines drawn over a small region near the Earth's surface?
A
parallel, equally spaced and pointing vertically downwards
B
circles running round the Earth
C
radial lines pointing away from the Earth
D
parallel lines pointing vertically upwards
Show answer
Answer: A. Over a small region the field is uniform, so the lines are parallel and evenly spaced. They point in the direction of the force on a mass: downwards.
3. Two identical small metal spheres carry charges of +6.0 nC and −2.0 nC. They touch and are then separated. What is the charge on each?
A
+8.0 nC
B
+2.0 nC
C
+4.0 nC
D
−2.0 nC
Show answer
Answer: B. Charge is conserved: total = +6.0 − 2.0 = +4.0 nC, shared equally between identical spheres: +2.0 nC each.
4. What is the pattern of magnetic field lines around a long straight wire carrying a current?
A
straight lines parallel to the wire
B
concentric circles centred on the wire, in planes perpendicular to it
C
a uniform field running along the wire
D
straight lines radiating outwards from the wire
Show answer
Answer: B. The field circles the wire; its direction follows the right-hand grip rule, and the circles get further apart with distance as the field weakens.
5. A planet moves in an elliptical orbit around the Sun. According to Kepler's laws, where is it moving fastest?
A
midway between the closest and furthest points
B
it moves at the same speed everywhere
C
at the point closest to the Sun
D
at the point furthest from the Sun
Show answer
Answer: C. Equal areas in equal times: near the Sun the radius line is short, so the planet must cover a longer arc in the same time and therefore moves fastest.
6. What is the definition of gravitational field strength at a point?
A
the work done per unit mass in bringing a mass there
B
the gravitational force per unit mass on a small test mass placed there
C
the gravitational force on any object placed there
D
the acceleration of any object moving through that point
Show answer
Answer: B. g = F/m, measured in N kg−1 (equivalent to m s−2). Work done per unit mass defines gravitational potential, not field strength.
7. A positively charged rod is held near an uncharged metal sphere on an insulating stand. The sphere is earthed briefly, the earth is removed, and then the rod is taken away. What is the final charge on the sphere?
A
positive
B
zero
C
negative
D
positive on one side and negative on the other
Show answer
Answer: C. The rod attracts electrons up from earth to the sphere. With the earth removed first, those extra electrons are trapped, so the sphere ends up negative: charging by induction gives the opposite sign to the rod.
8. An electron enters the uniform electric field between two parallel plates, moving at right angles to the field lines. What path does it follow between the plates?
A
a circular arc, curving towards the positive plate
B
a parabola, curving towards the positive plate
C
a parabola, curving towards the negative plate
D
a straight line, slowing down
Show answer
Answer: B. The electric force is constant and perpendicular to the initial velocity, just like gravity on a projectile, so the path is a parabola. The negative electron is pulled towards the positive plate.
9. A horizontal wire carries a current due north in a uniform magnetic field that points vertically downwards. In which direction is the force on the wire?
A
north
B
vertically upwards
C
west
D
east
Show answer
Answer: C. The force is perpendicular to both the current and the field, so it must be horizontal east or west. By Fleming's left-hand rule (field down, current north) the thrust points west.
10. Which statement is Kepler's second law?
A
a line joining a planet to the Sun sweeps out equal areas in equal times
B
planets move in circles with the Sun at the centre
C
planets move around the Sun at constant speed
D
the square of the orbital period is proportional to the cube of the mean orbital radius
Show answer
Answer: A. Kepler's first law: orbits are ellipses with the Sun at one focus. Second: equal areas in equal times. Third: T² ∝ r³.
36, each with the answer and a short explanation, across 3 topics: D.1 Gravitational fields, D.2 Electric and magnetic fields, D.3 Motion in electromagnetic fields. Every chapter test draws ten of them, so you can take it many times.
Is the IB Diploma Physics chapter 4 test free?
Yes. The test, the answers and the explanations are free and need no sign-up. Sign in only if you want your scores kept across devices.
How is the chapter test marked?
The moment you press Finish: your score, the right answer and an explanation for every question, and the ones you got wrong saved in My mistakes to retry.
