A Level Physics 20.1 Concept of a Magnetic Field MCQs – 30 Practice Questions with Answers (Cambridge 9702)
This page covers Cambridge 9702 A Level Section 20.1, Concept of a Magnetic Field. Work through 30 original multiple-choice questions on field lines, magnetic flux density and field patterns, then open the worked answers to check your reasoning.
Formula recap (see the full A Level Physics Formula Sheet):
- A magnetic field is a region in which a magnetic pole, a moving charge or an electric current experiences a force.
- Field lines outside a magnet run from north to south; the direction of a field line is the direction of the force on a free north pole. Lines closer together mean a stronger field.
- Magnetic flux density B is measured in tesla (T): 1 T = 1 N A-1 m-1 (the full definition through F = BIL is developed in Section 20.2).
- Magnetic flux Φ = BA when the field is perpendicular to an area A; unit: weber (Wb), where 1 Wb = 1 T m2.
- Flux linkage for a coil of N turns = NΦ.
Magnetic Fields & Field Lines
Q1. A magnetic field is best described as a region of space in which
- (A) any object experiences a force towards the Earth
- (B) a magnetic pole experiences a force
- (C) an electric charge at rest experiences a force
- (D) a beam of light is always deflected
Show Answer
Answer: B – A magnetic field is the region around a magnet (or a current) in which a magnetic pole placed there experiences a force. Moving charges and currents also experience forces in a magnetic field. A charge at rest responds to an electric field, not a magnetic one.
Q2. Outside a bar magnet, the magnetic field lines are directed
- (A) from the north pole to the south pole
- (B) from the south pole to the north pole
- (C) radially outwards from both poles
- (D) radially inwards towards both poles
Show Answer
Answer: A – By convention, field lines outside a magnet leave the north pole and enter the south pole. Inside the magnet they continue from south to north, so every field line is a closed loop.
Q3. The direction of a magnetic field line at a point gives the direction of
- (A) the force on a south pole placed at that point
- (B) the force on an electric charge at rest at that point
- (C) the force on a free north pole placed at that point
- (D) the motion of any charge placed at that point
Show Answer
Answer: C – The field direction is defined as the direction of the force on a free (isolated) north pole. A south pole would be pulled in the opposite direction, and a stationary charge feels no magnetic force at all.
Q4. In a magnetic field diagram, the field is strongest where the field lines are
- (A) longest
- (B) perfectly straight
- (C) farthest apart
- (D) closest together
Show Answer
Answer: D – The spacing of field lines represents the flux density: the more crowded the lines, the larger B is at that point. Near the poles of a bar magnet the lines are most crowded, so the field is strongest there.
Q5. Magnetic field lines never cross one another because
- (A) the field is too weak away from the poles
- (B) at any point the field has only one direction
- (C) field lines carry electric charge
- (D) a compass cannot be placed at a crossing point
Show Answer
Answer: B – The resultant field at a point has a single direction (the tangent to the line there). If two lines crossed, the field would need two different directions at the same point, which is impossible.
Q6. When a magnetic field pattern is plotted with a small plotting compass, the compass needle
- (A) aligns itself along the magnetic field at that point
- (B) always points towards the south pole of the Earth
- (C) keeps spinning until the magnet is removed
- (D) settles at right angles to the field lines
Show Answer
Answer: A – The needle is a tiny magnet; it experiences a turning effect until it lines up with the local field. Moving the compass step by step traces out one field line, and the needle’s north pole points in the field direction.
Q7. Iron filings sprinkled on a card above a bar magnet reveal
- (A) the exact value of B in tesla at each point
- (B) the direction of the field, including which way the arrows point
- (C) the pattern of the field lines, but not their direction
- (D) only the field inside the magnet
Show Answer
Answer: C – Each filing becomes a small induced magnet and lines up with the field, so the filings trace the shape of the pattern. They give no arrow direction and no numerical value of B; a compass is needed to find the direction.
Q8. Inside a bar magnet, the magnetic field lines run
- (A) from the north pole to the south pole
- (B) only in circles around the centre of the magnet
- (C) in no definite direction, because there is no field inside a magnet
- (D) from the south pole to the north pole
Show Answer
Answer: D – Field lines are continuous closed loops: north to south outside the magnet, then south to north through the inside of the magnet back to the start. Unlike electric field lines, magnetic field lines have no beginning or end.
Q9. A plotting compass is placed at a point in the field of a bar magnet. The north pole of the compass needle points
- (A) always directly towards the magnet’s north pole
- (B) in the direction of the magnetic field at that point
- (C) always directly towards the magnet’s south pole only if it is near the centre
- (D) perpendicular to the field line at that point
Show Answer
Answer: B – The needle aligns with the local field, its north pole pointing along the field line in the field direction (towards a south pole region, away from a north pole region).
Q10. The magnetic field of a bar magnet is strongest
- (A) near its poles, where the field lines are most crowded
- (B) at the exact centre of the magnet
- (C) at a large distance from the magnet
- (D) equally at all points around the magnet
Show Answer
Answer: A – Field lines emerge and converge most densely at the poles, showing that B is greatest there. The field weakens rapidly with distance from the magnet.
Magnetic Flux Density
Q11. Magnetic flux density, symbol B, is a measure of
- (A) the number of lines drawn in a field diagram
- (B) the total magnetic flux through any closed surface
- (C) the strength of a magnetic field
- (D) the energy stored in a magnet
Show Answer
Answer: C – B, the magnetic flux density, quantifies how strong the field is at a point. It is a vector, directed along the field line, and is measured in tesla.
Q12. The unit of magnetic flux density is the
- (A) tesla
- (B) weber
- (C) henry
- (D) newton
Show Answer
Answer: A – B is measured in tesla (T). The weber is the unit of magnetic flux, not of flux density.
Q13. One tesla is equivalent to
- (A) 1 N C-1
- (B) 1 J kg-1
- (C) 1 Wb m-1
- (D) 1 N A-1 m-1
Show Answer
Answer: D – This follows from F = BIL: B = F/(IL), so the tesla is one newton per ampere per metre of conductor. (N C-1 is the unit of electric field strength.) The full definition is used in Section 20.2.
Q14. The magnetic flux density of the Earth’s magnetic field at the surface is approximately
- (A) 5 T
- (B) 5 x 10-5 T
- (C) 5 x 10-2 T
- (D) 5 x 103 T
Show Answer
Answer: B – The Earth’s field is very weak, of the order of 10-5 T (a few tens of microtesla depending on location). Notice how this compares with the field near a bar magnet (Q19).
Q15. A uniform magnetic field of flux density 0.40 T passes perpendicularly through a flat area of 0.025 m2. The magnetic flux through the area is
- (A) 16 Wb
- (B) 0.10 Wb
- (C) 0.010 Wb
- (D) 1.0 x 10-4 Wb
Show Answer
Answer: C – For a field perpendicular to the area, Φ = BA = 0.40 x 0.025 = 0.010 Wb.
Q16. The unit of magnetic flux is the
- (A) weber
- (B) tesla
- (C) volt
- (D) ohm
Show Answer
Answer: A – Flux Φ = BA is measured in webers (Wb). Since B is flux per unit area, 1 Wb = 1 T m2.
Q17. A flat coil lies in a uniform magnetic field so that the plane of the coil is parallel to the field lines. The magnetic flux through the coil is
- (A) equal to BA, the maximum possible
- (B) equal to half of BA
- (C) slightly less than BA
- (D) zero
Show Answer
Answer: D – Flux counts the field passing through the area. With the plane parallel to the field, no field lines pass through the coil; equivalently, the component of B perpendicular to the area is zero, so Φ = 0. Flux is maximum when the plane is perpendicular to the field.
Q18. A coil of 200 turns is threaded by a magnetic flux of 3.0 x 10-4 Wb through each turn. The flux linkage of the coil is
- (A) 6.0 Wb
- (B) 6.0 x 10-2 Wb
- (C) 1.5 x 10-6 Wb
- (D) 3.0 x 10-4 Wb
Show Answer
Answer: B – Flux linkage = NΦ = 200 x 3.0 x 10-4 = 6.0 x 10-2 Wb (weber-turns). Flux linkage, rather than flux alone, decides the e.m.f. induced in a coil (Section 20.5).
Q19. Which of the following is the largest magnetic flux density?
- (A) the Earth’s field at the surface, about 5 x 10-5 T
- (B) the field near the pole of a bar magnet, about 10-2 T
- (C) the field of a laboratory electromagnet, about 1 T
- (D) the field at the surface of a fridge magnet, about 5 x 10-3 T
Show Answer
Answer: C – A current-carrying electromagnet with an iron core can reach about 1 T, far stronger than permanent magnets of everyday size or the Earth’s field. These typical orders of magnitude are worth remembering for estimates.
Q20. The flux density through a flat area is doubled, while the area is halved. The magnetic flux through the area
- (A) is unchanged
- (B) is doubled
- (C) is halved
- (D) is quadrupled
Show Answer
Answer: A – Φ = BA. Doubling B and halving A gives (2B)(A/2) = BA, so the flux is unchanged.
Field Patterns & Interactions
Q21. A north pole of one magnet faces the south pole of another magnet, a short distance apart. The field in the gap between the poles is best described as
- (A) zero, because the two fields cancel
- (B) strong and fairly uniform, with field lines running from the north pole to the south pole
- (C) weak, with field lines running from the south pole to the north pole
- (D) circular, looping around both magnets
Show Answer
Answer: B – Lines leave N and enter S, so in the gap the fields of the two magnets act in the same direction and reinforce. The lines are almost straight and evenly spaced, giving a strong, nearly uniform field – the arrangement used between the poles of laboratory magnets.
Q22. Two north poles of equal strength face each other a short distance apart. In the region between the poles there is
- (A) a very strong uniform field
- (B) a field pointing in two directions at once
- (C) a set of field lines that cross each other
- (D) a neutral point at which the resultant field is zero
Show Answer
Answer: D – Along the line joining the poles, the two fields point in opposite directions. At the midpoint they are equal and opposite, giving a neutral point where the resultant flux density is zero.
Q23. A compass needle’s north pole points roughly towards geographic north because
- (A) the Earth behaves as if it has a magnetic south pole near geographic north, which attracts the needle’s north pole
- (B) a magnetic north pole at geographic north attracts the needle’s north pole
- (C) electric charges in the atmosphere pull the needle round
- (D) the needle is attracted by the gravity of the North Pole ice cap
Show Answer
Answer: A – Opposite poles attract. Since the needle’s north pole is drawn towards geographic north, the Earth’s field must be produced as if by a magnet whose south pole lies near geographic north (the equivalent bar magnet inside the Earth has its north pole pointing roughly south).
Q24. An electron is placed at rest in a strong magnetic field. The magnetic force on the electron is
- (A) along the field lines
- (B) opposite to the field lines
- (C) zero
- (D) towards the nearest pole
Show Answer
Answer: C – A magnetic field exerts a force only on a moving charge (or a current). A stationary charge experiences no magnetic force, although an electric field would still act on it.
Q25. A magnetic field can exert a force on
- (A) a stationary electric charge
- (B) a moving charge and a current-carrying conductor
- (C) every mass placed in the field
- (D) an uncharged plastic rod at rest
Show Answer
Answer: B – Magnetic forces act on magnetic poles, on charges in motion and on currents (which are moving charges). This is the key contrast with the gravitational field, which acts on every mass, and the electric field, which acts on every charge, moving or not.
Q26. A sheet of soft iron can be used to shield a sensitive instrument from a magnetic field because
- (A) soft iron destroys magnetic field lines
- (B) soft iron is completely non-magnetic
- (C) soft iron repels both poles of a magnet
- (D) field lines pass very easily through soft iron, so they are diverted through the shield instead of through the instrument
Show Answer
Answer: D – Soft iron has a high permeability: it provides an easy path for field lines, which crowd into the shield and follow it around the protected region. This is why transformer cores and magnetic screens are made of soft iron.
Q27. Which comparison of electric and magnetic fields is correct?
- (A) An electric field exerts a force on a charge whether it is stationary or moving; a magnetic field exerts a force only if the charge is moving
- (B) Both fields exert forces only on moving charges
- (C) A magnetic field exerts a force on every mass, but an electric field does not
- (D) Electric field lines can begin on a negative charge, but magnetic field lines can begin on a south pole
Show Answer
Answer: A – The electric force F = EQ acts on any charge in the field. The magnetic force requires motion (F = BQv for a charge moving across the field). Also, magnetic field lines are closed loops and never begin or end, unlike electric field lines, which begin and end on charges.
Q28. Two identical bar magnets have their north poles facing each other. The neutral point on the line joining the poles is located
- (A) inside one of the magnets
- (B) outside the pair, beyond the weaker pole
- (C) midway between the poles, where the two fields cancel
- (D) at no position, because like poles cannot produce a neutral point
Show Answer
Answer: C – For two equal poles, the fields along the joining line are equal in magnitude at the midpoint but point in opposite directions, so the resultant is zero there. For unequal poles the neutral point shifts closer to the weaker pole.
Q29. A bar magnet is suspended freely so that it can rotate horizontally. Away from other magnets, it comes to rest
- (A) pointing east-west
- (B) aligned roughly north-south, along the Earth’s magnetic field
- (C) pointing vertically downwards
- (D) still spinning slowly
Show Answer
Answer: B – Like a compass needle, the magnet aligns itself with the horizontal component of the Earth’s field, its north pole towards geographic north. This is the principle of the magnetic compass.
Q30. Which statement correctly compares the gravitational and magnetic fields?
- (A) A gravitational field exerts a force only on moving masses
- (B) A magnetic field exerts a force on all matter placed in it, at rest or moving
- (C) Field lines of both fields start and end on the objects causing them
- (D) A gravitational field exerts a force on every mass placed in it, whereas a magnetic field acts only on poles, moving charges and currents
Show Answer
Answer: D – Gravity acts on every mass (F = mg in a field of strength g), whatever its motion. A magnetic field is selective: it acts on magnetic poles, on charges only when they move, and on currents. Neither field needs contact with the object it acts on.
Keep practising: work through the other A Level Physics topics on the Physics MCQs hub, and keep the key equations at hand with the A Level Physics Formula Sheet.
These are original practice questions written for revision. They are not Cambridge past-paper questions, and this page is not affiliated with Cambridge Assessment International Education.