Q: What does A-Level Physics: 17) Electromagnetic Forces Guide cover? A: From field-line sketches to velocity selectors, this post unpacks Section V Topic 17 of the 2026 H2 Physics syllabus for IP students and parents.
TL;DR Magnetic fields are everywhere current flows. Mastering the three B-formulae, Fleming's rule and the Lorentz force lifts marks in Paper 2 calculation tricks and Paper 4 practicals. This guide turns the SEAB bullet-points into parent-approved check-lists, mini-drills and WA timing hacks.
Concrete example: how to use this page
If a moving charge enters a magnetic field, first check whether its velocity is perpendicular to the field. Then use F=BQv and connect that force to circular motion if the path curves.
Need the rest of the electromagnetism chain (induction, EM waves, Modern Physics)? Stay in sync via the H2 Physics notes hub, which links this topic to the remaining Section V and VI refreshers.
If you searched "sensitive balance" with Lenz's law
SEAB 9478 splits the idea across two neighbouring topics. Topic 17 Electromagnetic Forces includes force on a current-carrying conductor and measuring magnetic flux density with a current balance. Topic 18 Electromagnetic Induction includes Faraday's law and Lenz's law.
Reviewed by
Chee Wei Jie·Academic Advisor (Physics)
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sensitive balance
The balance reading changes because a current-carrying conductor feels F=BIlsinθ.
The question asks why an induced current opposes a magnet's motion.
current balance
You need Δmg, Fleming's left-hand rule, or B=F/Il.
The current is induced by changing flux rather than supplied by a power source.
Lenz's law sensitive balance
Use this page to interpret the force that changes the balance reading.
Use this map before choosing a formula. The same magnetic field can produce different equations depending on whether the object is a wire, a single charge, or a beam balanced by an electric field.
What the question shows
First move
Main route
Misconception check
Wire, circular coil, or solenoid producing a field
Identify the geometry before substituting numbers.
Use the matching field expression for wire, coil centre, or solenoid interior.
The distance symbol is not interchangeable: wire questions use radial distance; coil questions use coil radius.
Current-carrying conductor in an external field
Mark current direction, field direction, and the angle between them.
Use F=BIlsinθ, then Fleming's left-hand rule for direction.
A conductor parallel to the field has zero magnetic force even if the current is large.
Single charged particle entering a magnetic field
Check charge sign, velocity direction, and whether the path bends.
Use F=BQvsinθ; if the path is circular, set magnetic force equal to centripetal force.
Magnetic force changes direction, not speed, because it is perpendicular to velocity.
Crossed electric and magnetic fields
Decide whether the beam is undeflected.
Balance QE with QvB, giving v=E/B.
The charge cancels in the speed condition, but charge sign still matters for the separate force directions.
1 What counts as a magnetic field?
A magnetic field is a region of space where a moving charge or a current-carrying conductor experiences a force. In the H2 syllabus it is treated as a vector field of force produced by currents or permanent magnets.
2 Field patterns produced by currents
2.1 Long straight wire
Field lines form concentric circles centred on the wire. The magnitude falls off with radial distance d according to
B=2πdμ0I.(2.1)
Equation (2.1) can be derived via Ampère's law or the Biot-Savart law.
2.2 Flat circular coil (N turns, radius r)
Near the centre the field is approximately uniform and given by
B=2rμ0NI.(2.2)
A quick way to remember: halve the solenoid result, replace length with diameter.
2.3 Long solenoid (n=N/L turns per metre)
Inside a tightly wound solenoid the field is nearly uniform:
B=μ0nI.(2.3)
Inserting a ferrous core (iron) multiplies B by the material's relative permeability μr-the working principle of electromagnets used in MRI machines.
Mini-drill Predict the shape of the field outside a solenoid. (Answer: similar to a bar magnet-closed loops emerging from one end and re-entering the other.)
3 Force on a current-carrying conductor
3.1 Core equation and direction
When a conductor of length l carrying current I sits in an external field B, the magnetic (Lorentz) force is
F=BIlsinθ.(3.1)
with direction given by Fleming's left-hand rule.
3.2 Defining magnetic flux density
Re-arranging (3.1) for perpendicular orientation (θ=90∘) gives
B=IlF.(3.2)
Hence magnetic flux density is “force per unit current per unit length.”
3.3 Measuring B with a current balance
Suspend the test conductor on a sensitive balance, run a known I and record the mass difference Δm. From F=Δmg and (3.2) deduce B.
This is a Topic 17 current-balance use of a sensitive balance: a supplied current in a known conductor experiences a magnetic force. It is different from a Topic 18 Lenz's-law demonstration, where changing flux induces a current and the induced current's magnetic effect opposes the motion that caused it.
Current-balance reading checkpoint
A current balance question is not just a direct substitution into F=BIl. First decide whether the magnetic force makes the balance reading increase or decrease, then convert the mass change into a force.
Observation in the setup
Force on conductor
Balance reading change
Calculation move
Magnet assembly is on the balance and conductor pushes it down
Downward force on magnet assembly
Reading increases
Use F=Δmg.
Magnet assembly is on the balance and conductor lifts it
Upward force on magnet assembly
Reading decreases
Use the magnitude of the mass change, then keep the force direction separate.
Wire is on the balance instead of the magnet
Force is on the wire itself
Reading follows the wire's force direction
Apply Fleming's left-hand rule to the wire before interpreting the sign.
Worked check: if the balance reading rises by 1.8g, the magnetic force magnitude is F=(1.8×10−3)g=1.77⋅10−2N. For a 5.0cm wire carrying 3.0A perpendicular to the field,
B=IlF=(3.0)(0.050)1.77×10−2≈0.12T.
Misconception check: the balance does not measure magnetic flux density directly. It measures a change in weight reading, which you convert to force before using B=F/Il.
3.4 Parallel-wire interactions
Two long, parallel wires carrying currents I1 and I2 a distance r apart exert equal and opposite forces
lF=2πrμ0I1I2.(3.3)
Currents in the same direction attract; opposite directions repel-an idea embedded in the SI definition of the ampere.
Parallel-wire direction checkpoint
For parallel-wire questions, separate the field made by one wire from the force felt by the other wire. This prevents sign errors when the currents are shown into or out of the page.
Current arrangement
Field from wire 1 at wire 2
Force on wire 2
What to conclude
Both currents in the same direction
Use the right-hand grip rule around wire 1.
Apply Fleming's left-hand rule to wire 2.
The wires attract.
Currents in opposite directions
Use the same field direction from wire 1 at wire 2.
Reverse the current direction in wire 2 before applying the force rule.
The wires repel.
Question asks for force per unit length
Use lF=2πrμ0I1I2.
Direction still comes from the rule sequence, not from the formula.
Quote both magnitude and attraction or repulsion.
Worked check: if two vertical wires carry current upward, the magnetic field from the left wire at the right wire points into the page. Fleming's left-hand rule then gives a force on the right wire toward the left wire, so the wires attract.
Misconception check: the formula gives a positive magnitude. Attraction or repulsion comes from the current directions, not from putting a negative sign into I1I2.
4 Force on an isolated moving charge
4.1 Lorentz force
A charge Q entering a uniform field with speed v feels
F=BQvsinθ.(4.1)
The force is always perpendicular to both v and B, producing circular or helical motion.
4.2 Uniform circular motion
For θ=90∘ the radius is
r=QBmv.(4.2)
This relationship underpins the mass spectrometer.
Moving-charge direction checkpoint
Before deciding the path, separate the force direction from the radius calculation.
Situation
Direction move
Radius move
What to say
Positive charge
Use Fleming's left-hand rule with conventional current in the same direction as velocity.
Use r=QBmv if velocity is perpendicular to the field.
Force is perpendicular to velocity, so the particle curves without speeding up.
Negative charge
First find the positive-charge force direction, then reverse it.
Use the magnitude of charge in the radius calculation.
The path bends the opposite way from a positive charge with the same speed.
Velocity partly parallel to field
Split velocity into perpendicular and parallel components.
The perpendicular component sets circular motion; the parallel component continues along the field.
The path becomes helical rather than a flat circle.
Misconception check: the magnetic field does not pull the particle along the field line. Only the component of velocity perpendicular to the magnetic field produces magnetic force.
5 Crossed fields and velocity selection
Set up perpendicular electric E- and magnetic B-fields such that QE=QvB. Only particles with
v=BE.(5.1)
exit undeflected-perfect for ion-implantation or cathode-ray oscilloscopes.
Velocity-selector checkpoint
A crossed-field question is a force-balance question before it is a substitution question. Draw the electric force and magnetic force separately, then check whether the beam is meant to pass through undeflected.
Question clue
First force to mark
Balance condition
Common trap
Positive ion beam enters perpendicular E and B fields
Electric force is in the direction of E.
Undeflected path needs QE=QvB, so v=E/B.
Forgetting to reverse the magnetic-force direction if the velocity direction changes.
Electron beam enters the same selector
Electric force is opposite to E.
Magnetic force must oppose that electric force for no deflection.
Using the positive-charge force direction without reversing it.
Question asks which particles pass through
Compare each particle speed with E/B.
Only particles with the selected speed are undeflected.
Thinking the selector filters charge or mass directly.
Worked check: if E=2.4⋅104V⋅m−1 and B=0.080T, the selected speed is
v=BE=0.0802.4×104=3.0⋅105m⋅s−1.
Misconception check: charge cancels from the speed condition, but the charge sign still controls the direction of each separate force. A selector chooses speed; downstream magnetic bending can then separate particles by m/Q.
6 IP-style study hacks
Weak spot
Quick fix
Forgetting which rule (left vs right hand)
Write "FBI" on your lab glove: Force, B-field, I current direction.
Pre-draw axis arrows and annotate charge sign before any algebra.
6.1 Why parents should care
IP schools allot fewer contact hours for classical electromagnetism, banking on student independence. Targeted tuition bridges the gap with timed drilling and data-logger labs, preventing the common Term 3 “dip” before promos.
6.2 Tuition tip
Look for centres that demo the current-balance experiment live-students internalise B-calculations better when they see the metre reading. Group formats are cost-effective and harness peer reinforcement.
7 Three WA timing rules (reprise)
Use syllabus pacing as a guide: Paper 2/3 average ~1.6 min/mark; Paper 4 ~3 min/mark.
Copy units before numbers to avoid mis-scaling Tesla ↔ mT.
Always quote answers to the least precise s.f. among inputs.
Need structured practice on Electromagnetic Forces? Our H2 Physics tuition programme covers this topic with weekly problem sets and Paper 4 practical drills.
Comprehensive revision pack
9478 Section V, Topic 17 Syllabus outcomes
Candidates should be able to:
(a) show an understanding that a magnetic field is an example of a field of force produced either by current-carrying conductors or by permanent magnets.
(b) sketch magnetic field lines due to currents in a long straight wire, a flat circular coil and a long solenoid.
(c) use B=2πdμ0I, B=2rμ0NI and B=μ0nI for the magnetic flux densities of the fields due to currents in a long straight wire, a flat circular coil and a long solenoid respectively.
(d) show an understanding that the magnetic field due to a solenoid may be influenced by the presence of a ferrous core.
(e) show an understanding that a current-carrying conductor placed in a magnetic field might experience a force.
(f) recall and solve problems using the equation F=BIlsinθ, with directions as interpreted by Fleming's left-hand rule.
(g) define magnetic flux density as the force acting per unit current per unit length on a conductor placed perpendicular to the magnetic field.
(h) show an understanding of how the force on a current-carrying conductor can be used to measure the magnetic flux density of a magnetic field using a current balance.
(i) explain the forces between current-carrying conductors and predict the direction of the forces.
(j) predict the direction of the force on a charge moving in a uniform magnetic field.
(k) recall and solve problems using the equation F=BQvsinθ.
(l) describe and analyse deflections of beams of charged particles by uniform electric fields and uniform magnetic fields.
(m) explain how perpendicular electric and magnetic fields can be used in velocity selection for charged particles.
Concept map (in words)
Generate fields with currents (wire, coil, solenoid). Use left-hand rule to determine force on conductors and charges. Combine with circular motion ideas for particle trajectories. When electric and magnetic fields coexist, tune one to balance the other (velocity selector).
Boundary check: this page covers magnetic force once a current or moving charge is present. If the question first asks how the current was induced, move to Topic 18 Electromagnetic Induction for magnetic flux, Faraday's law, and Lenz's law.
Key relations
Situation
Expression / comment
Long straight wire field
B=2πdμ0I
Circular coil centre
B=2rμ0NI
Solenoid interior
B=μ0nI
Conductor force
F=BIlsinθ
Flux density definition
B=IlF (perpendicular)
Moving charge (Lorentz)
F=BQvsinθ
Circular motion radius
r=QBmv
Velocity selector
v=BE
Hall effect (extension)
VH=nqtBI
Derivations & reasoning to master
Biot-Savart/Ampere: understand proportionalities leading to wire/coil/solenoid formulas.
Lorentz force to circular motion: show how F=BQvsinθ supplies the centripetal requirement, giving r=QBmv.
Force between parallel wires: derive attraction/repulsion relationship and relate to the ampere definition.
Velocity selector: equate electric and magnetic forces; discuss filter bandwidth.
Worked example 1 - mass spectrometer bend
An ion with charge +2e and kinetic energy 3.2keV enters a 0.25T magnetic field perpendicular to velocity. Determine its speed and radius of curvature if mass equals 4.0⋅10−26kg.
Solution: convert energy to joules, compute v=m2qV, then r=QBmv.
A 5cm conductor carrying 3.0A experiences an upward force equivalent to 1.8g when placed in a uniform field. Calculate B and deduce the direction of current relative to field lines.
Steps: F=Δmg⇒B=IlF. Apply the left-hand rule to relate directions.
