An electric field acts on any electric charge, including one at rest, whereas a magnetic field exerts a force on a charge only when the charge is moving relative to the field and its velocity has a component perpendicular to the field. Electric forces act along or opposite the electric field; magnetic forces act perpendicular to both the particle's velocity and the magnetic field.
This distinction controls how charged particles accelerate, gain energy, and follow curved paths. It is central to IB Physics fields, especially D.2 Electric and magnetic fields and D.3 Motion in electromagnetic fields in the course first assessed in 2025. This article focuses specifically on the electric field vs magnetic field comparison; for broader coverage, use the IB Physics Fields explained exam guide.
Electric field vs magnetic field at a glance
Both electric and magnetic fields are vector fields. At every point in space, they have a magnitude and direction, and they describe how an object would respond if placed there. The critical differences concern their sources, the conditions under which they exert force, and the direction of that force.
| Feature | Electric field, E | Magnetic field, B |
|---|---|---|
| Principal static source | Electric charge | Moving charge, electric current, or magnetic dipole |
| Force on a point charge | F = qE | F = qv × B |
| Condition for force | Charge may be stationary or moving | Charge must move with a component of velocity perpendicular to B |
| Force magnitude | F = |q|E | F = |q|vB sin θ |
| Force direction for positive charge | Parallel to E | Perpendicular to both v and B |
| Effect on particle speed | Can change speed and direction | By itself, changes direction but not speed |
| Work on a point charge | Can do work and change kinetic energy | Does no work because force is perpendicular to motion |
| SI unit | N C⁻¹, equivalent to V m⁻¹ | tesla, T, equivalent to N A⁻¹ m⁻¹ |
| Field-line pattern | Begins on positive charge and ends on negative charge, or extends to infinity | Forms continuous closed loops |
The compact expression combining both forces is the Lorentz force equation:
F = q(E + v × B)
This equation shows why the fields cannot be treated as interchangeable. The electric contribution depends on charge and field strength, while the magnetic contribution also depends on the particle's velocity.
What is an electric field?
An electric field is the electric force per unit positive test charge at a point:
E = F/q
Its direction is defined as the direction of the force on a positive test charge. A positive source charge therefore produces a field directed radially outward, while a negative source charge produces a field directed inward.
For a point charge Q, the magnitude at distance r is:
E = k|Q|/r²
This is an inverse-square relationship. Doubling the distance reduces the field strength to one quarter of its original value, provided the point-charge model remains appropriate.
What an electric field acts on
An electric field acts on every charged particle within it, regardless of whether that particle is initially moving. The force is:
F = qE
For a positive charge, the force points in the same direction as E. For a negative charge, such as an electron, it points opposite to E because q is negative.
In a uniform electric field, a freely moving particle of constant mass experiences constant acceleration:
a = qE/m
An electric field can therefore start a stationary charge moving, speed it up, slow it down, or change its direction. Because the force can have a component parallel to displacement, the field can transfer energy to or from the particle.
Electric potential and energy
Electric fields are closely connected to electric potential, which is energy per unit charge. Across a uniform field, the magnitude of the potential gradient is related to field strength by:
E = -ΔV/Δx
The negative sign indicates that the electric field points toward decreasing electric potential. When a charge moves through a potential difference, its potential energy changes according to ΔEp = qΔV.
This energy description is a major difference from the usual treatment of magnetic forces on individual particles. An electrostatic field is conservative, so work between two points depends on the endpoints rather than the path taken.
What is a magnetic field?
A magnetic field is a vector field produced by electric currents, moving charges, and magnetic dipoles such as permanent magnets. Magnetic effects also arise from changing electric fields, an important part of Maxwell's unified description of electromagnetism.
The direction of B at a point is conventionally the direction in which the north-seeking pole of a small compass would point. Around a straight current-carrying wire, the field forms concentric circles, with direction found using a right-hand grip rule.
The SI unit is the tesla. Unlike electric field strength, magnetic flux density cannot be defined simply as force per unit charge because the force also depends on velocity and orientation.
What a magnetic field acts on
For a point charge moving through a magnetic field:
F = qv × B
Its magnitude is:
F = |q|vB sin θ
Here, θ is the angle between v and B. This gives three important cases:
- If v is parallel or antiparallel to B, then θ = 0° or 180°, so the magnetic force is zero.
- If v is perpendicular to B, then θ = 90°, so the force has its maximum magnitude, |q|vB.
- At any intermediate angle, only the component of velocity perpendicular to the field contributes to the force.
A magnetic field also exerts forces on current-carrying conductors because a current consists of moving charge. For a straight conductor of length L in a uniform magnetic field:
F = BIL sin θ
Permanent magnets and magnetic dipoles can also experience forces or torques in magnetic fields. It is therefore incomplete to say that magnetic fields act only on moving point charges, although that is the central model used for charged-particle motion.
Why a magnetic field does no work on a point charge
The magnetic force is perpendicular to the instantaneous velocity. Consequently, it has no component in the direction of displacement and does no work on an isolated point charge:
W = Fs cos 90° = 0
The particle's kinetic energy and speed remain constant, but its momentum changes because momentum is a vector. The magnetic field can bend a trajectory without speeding up the particle.
If v is perpendicular to a uniform B, the magnetic force supplies the centripetal force:
qvB = mv²/r
so that:
r = mv/(|q|B)
A larger momentum produces a larger radius, while a larger charge magnitude or magnetic field produces tighter curvature. If the velocity has both parallel and perpendicular components, the result is a helical path.
The statement that magnetic forces do no work applies specifically to the magnetic force on a point charge. Energy can still be transferred in electromagnetic systems through induced electric fields, power supplies, resistance, or mechanical forces on current-carrying conductors.
How the fields are generated
A useful introductory summary is that charges produce electric fields and moving charges produce magnetic fields. This is correct for common static situations, but the complete relationship is more closely connected.
- Electric charges are sources of electric fields.
- Electric currents and moving charges produce magnetic fields.
- A changing magnetic field produces an electric field, as described by Faraday's law.
- A changing electric field contributes to the production of a magnetic field, as described by the Maxwell-Ampère law.
Electricity and magnetism are therefore not independent interactions. They are two aspects of the electromagnetic field, and their separation can depend on the observer's frame of reference. For example, a charge at rest in one reference frame may be moving in another, so observers can assign different electric and magnetic components while agreeing on the physical predictions.
This deeper connection explains electromagnetic waves. A changing electric field and a changing magnetic field propagate together through space, carrying energy and momentum.
Comparing field-line diagrams
Field lines are visual models, not physical threads. At any point, the tangent to a field line gives the field's direction, while closer line spacing represents a stronger field.
Electric field lines
Electric field lines point away from positive charges and toward negative charges. They may begin or end at infinity when the diagram does not contain an opposite charge.
Between large oppositely charged parallel plates, straight, parallel, equally spaced lines represent an approximately uniform electric field. Edge effects are normally ignored in basic IB calculations unless the question makes them relevant.
Magnetic field lines
Magnetic field lines always form closed loops. Outside a bar magnet, they are drawn from north to south; inside the magnet, they continue from south to north.
This difference reflects the fact that isolated electric charges exist, whereas isolated magnetic poles have not been observed in classical electromagnetism. Cutting a bar magnet in half produces two smaller dipoles, not a separate north pole and south pole.
For both types of field, lines must never cross. Crossing would assign two field directions to the same point, which is impossible for a well-defined vector field.
Worked comparison: a proton in uniform fields
Consider a proton moving to the right.
If it enters a uniform electric field directed upward, the electric force is upward because the proton is positively charged. Its velocity gradually gains an upward component, its speed changes, and its path is curved rather than circular in general.
Now suppose the proton instead enters a magnetic field directed into the page. Using the right-hand rule for v × B, the magnetic force is upward. As the proton turns, the force direction also turns so that it remains perpendicular to the velocity, producing circular motion if the field is uniform and no other forces act.
The initial force may point upward in both situations, but the later motion differs fundamentally. The electric force retains the direction of E, whereas the magnetic force continually changes direction because it depends on the particle's changing velocity.
If the particle were an electron, both force directions would reverse. A common exam error is to apply the right-hand rule and stop; for a negative charge, the direction obtained for a positive charge must be reversed.
What happens when both fields are present?
When electric and magnetic fields act together, add their forces as vectors:
Ftotal = qE + qv × B
In crossed fields, the forces can oppose each other. If E, v, and B are mutually perpendicular and the particle passes undeflected, their magnitudes are equal:
qE = qvB
Therefore:
v = E/B
This is the operating principle of a velocity selector. Only particles with the selected speed travel straight through, independent of their charge magnitude and mass, provided they have the correct direction of motion. The cancellation condition is about velocity, not kinetic energy.
For more applications involving curved paths and crossed fields, consult how electromagnetic fields guide charged particles and the focused D.2 Electric and Magnetic Fields notes.
Common IB exam mistakes
Treating field direction as force direction in every case
The electric field direction is the force direction for a positive charge only. A negative charge experiences the opposite force. In a magnetic field, B is not the force direction at all; the force is perpendicular to v and B.
Forgetting the angle term
The expression F = qvB applies only when motion is perpendicular to the magnetic field. In the general case, use F = |q|vB sin θ and identify the angle between v and B, not between the force and field.
Claiming that every moving charge feels a magnetic force
A charge moving parallel to a magnetic field experiences no magnetic force. Motion alone is insufficient; there must be a velocity component perpendicular to B.
Saying that magnetic fields increase particle speed
A static magnetic field alone cannot change the speed or kinetic energy of a point charge. If a particle speeds up in an electromagnetic situation, look for an electric field or another force doing work.
Confusing a field with a force
The field exists at a location independently of the test particle used to detect it. Force depends on both the field and the particle placed there. Always distinguish E from F = qE, and B from F = qvB sin θ.
A reliable exam method
For any electric or magnetic field question:
- Identify the source of the field and the object experiencing the force.
- Draw separate arrows for E or B, particle velocity v, and force F.
- Decide whether the charge is positive or negative.
- Select F = qE or F = qvB sin θ, checking whether both fields are present.
- Determine direction before substituting numerical values.
- For magnetic motion, ask whether the force supplies centripetal force.
- Check whether speed, direction, or both should change.
Practise this routine using the Topic D Fields Questionbank. The broader IB Physics resource hub can then support recall with notes and flashcards, while Jojo AI can help identify whether an error came from vector direction, equation selection, or algebra.
Conclusion
The core difference between electric and magnetic fields is what determines their force. An electric field acts on stationary and moving charges through F = qE, and it can change a particle's kinetic energy. A magnetic field acts on moving charges through F = qv × B, with a force perpendicular to the motion that changes direction but not speed.
Electric field lines can begin and end on charge, while magnetic field lines form closed loops. Although these fields behave differently in calculations, both belong to the unified electromagnetic interaction. After learning the distinctions, use RevisionDojo's Topic D notes, Questionbank, and Jojo AI feedback to practise applying them under IB exam conditions.
Sources and referenced URLs
- Official IB Physics subject brief for first assessment in 2025
- Official IB Physics specimen papers for first examinations in 2025
- OpenStax explanation of electric fields
- OpenStax University Physics: magnetic fields and field lines
- OpenStax University Physics: Maxwell's equations and the Lorentz force
- RevisionDojo IB Physics Fields explained exam guide
- RevisionDojo D.2 Electric and Magnetic Fields notes
- RevisionDojo guide to how electromagnetic fields steer charged particles
- RevisionDojo Topic D Fields Questionbank
- RevisionDojo IB Physics resource hub