Q: What does A-Level Physics: 17) Electromagnetic Forces Guide cover? A: From field-line sketches to velocity selectors, this post unpacks Section I 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.
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.
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.
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.
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.
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)
1 mark ≈ 1.5 min - same as SEAB design.
Copy units before numbers to avoid mis-scaling Tesla ↔ mT.
Always quote answers to the least precise s.f. among inputs.
Comprehensive revision pack
9478 Section I, Topic 17 Syllabus outcomes at a glance
Outcome (a) - describe magnetic fields due to currents and magnets.
Outcome (b) - calculate forces on current-carrying conductors and moving charges.
Outcome (c) - analyse motion of particles in uniform magnetic fields.
Outcome (d) - explain applications such as velocity selectors and mass spectrometers.
Outcome (e) - perform experiments to measure magnetic flux density (current balance, Hall probes).
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).
Key relations
Situation
Expression / comment
Long straight wire field
\( B = \dfrac{\mu_0 I}{2 \pi d} \)
Circular coil centre
\( B = \dfrac{\mu_0 N I}{2 r} \)
Solenoid interior
\( B = \mu_0 n I \)
Conductor force
\( F = B I l \sin \theta \)
Flux density definition
\( B = \dfrac{F}{I l} \) (perpendicular)
Moving charge (Lorentz)
\( F = B Q v \sin \theta \)
Circular motion radius
\( r = \dfrac{m v}{Q B} \)
Velocity selector
\( v = \dfrac{E}{B} \)
Hall effect (extension)
\( V_H = \dfrac{B I}{n q t} \)
Derivations & reasoning to master
Biot–Savart/Ampère: 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.
Worked example 2 - current balance measurement
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.
Practical & data tasks
Use a search coil and CRO to map B along a solenoid; integrate induced emf to estimate flux density.
Perform a current-balance experiment and compare measured B with theoretical μ0nI.
Simulate circular motion of charged particles using PhET and vary mass/charge to observe radius changes.
Common misconceptions & exam traps
Confusing left-hand (motor effect) with right-hand (generator) rules.
Forgetting that magnetic force does no work (speed unchanged, only direction).
Ignoring sign of charge when determining curvature direction.
Mixing units (mT vs T, cm vs m).
Quick self-check quiz
What is the direction of force on a positive charge moving east in a magnetic field pointing up? - South (using left-hand rule).
How does doubling current in a wire affect magnetic flux density at fixed distance? - It doubles B.
State the condition for zero deflection in crossed E and B fields. - QE=QvB; thus v=BE.
Why do heavier ions curve less in a mass spectrometer? - Larger mv leads to larger radius for same QB.
Does magnetic force change kinetic energy of a charge? - No, force perpendicular to velocity ⇒ speed constant.
Revision workflow
Redraw wire/coil/solenoid field diagrams weekly, noting right-hand rule orientation.
Practise Lorentz-force calculations that bridge to circular motion and energy conversions.
Complete at least two velocity-selector or mass-spectrometer questions from past papers.
Summarise experimental methods for measuring B (current balance, Hall probe) with strengths/limitations.
