Where can I learn Force on a moving charge for Cambridge 9702?

This free lesson on MarkScheme covers syllabus point 20.3 (Force on a moving charge) for Cambridge A-Level Physics. It includes visual revision, key points, and links to real past-paper questions with mark-scheme marking at markscheme.app/courses/9702/20-3-force-on-a-moving-charge.

Is Force on a moving charge (20.3) on 9702 free to study?

Yes. The Physics 9702 course on MarkScheme is 100% free — including premium visual lessons, exam tips, and past-paper marking. No paywall for Force on a moving charge.

How do I revise Force on a moving charge for 9702 exams?

Start with the visual steps on this page, note the key exam points, then attempt a 9702 past-paper question on Force on a moving charge using MarkScheme's /mark tool. Mark strictly against the official scheme and re-read any marks you lost.

MarkScheme 9702

Opening…

20.3Physics25 min readPREMIUM

Force on a moving charge

Imagine tiny charged particles speeding through space, only to be suddenly pushed sideways by an invisible force! This is the magnetic force on a moving charge, a fundamental concept in electromagnetism. We'll explore how this force arises, how to predict its direction using Fleming's Left-Hand Rule, and its fascinating applications in velocity selectors and the Hall effect. This pilot aligns with Senpai Corner A Level notes (2025–2027) for topic 20.3.

BY THE END, YOU CAN…

exam checklist — tick these off

1Describe the magnetic force on a moving charge and apply F = BQvsinθ.
2Explain the function of a velocity selector and how it utilises electric and magnetic fields.
3Outline the principles of the Hall effect and its application in Hall probes.

STUDY MODE

READING

TYPEFACE

Literata — a reading serif made for e-books; set like a textbook, sits well beside maths.

SIZE

AIRY SPACING

Wider letter and line spacing. The one change with solid evidence for readers who lose their place — worth a try whoever you are.

Pick whatever reads fastest for you — it differs from person to person, by up to a third.

EXPLAIN SIMPLER

The same notes in plain English — no jargon. Key points and exam tips stay.

DIAGRAM NOTES

A small sketch beside each block of notes.

In simple terms

A friendly intro before the formal notes — no formulas yet.

Force on a moving charge

Cambridge 9702 Paper 4 — Force on a moving charge (20.3). Senpai Corner diagram-backed pilot with premium structure and live visuals.

1
The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).
2
The force is zero when the charge moves parallel to the field ( $\theta=0^\circ$ ).
3
Fleming's Left-Hand Rule determines the force direction for a positive charge. For a negative charge, the force is in the opposite direction.
4
The magnetic force is always perpendicular to both the velocity of the charge and the magnetic field.

What this topic covers

The official Cambridge syllabus points this lesson works through.

20.3.1
Determine the direction of the force on a charge moving in a magnetic field
20.3.2
Recall and use $F = BQv \sin \theta$
20.3.3
Understand the origin of the Hall voltage and derive and use the expression $V_H = BI/(ntq)$, where t = thickness
20.3.4
Understand the use of a Hall probe to measure magnetic flux density
20.3.5
Describe the motion of a charged particle moving in a uniform magnetic field perpendicular to the direction of motion of the particle
20.3.6
Explain how electric and magnetic fields can be used in velocity selection

Explore the concept

Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.

Figure Live interactive1 / 4

Loading simulation…

Fetching from 3JCN Physics Simulation by Thomas Nguyen…

Charge in Magnetic Field

r = mv / (qB): faster charges make bigger circles and stronger fields make smaller ones.

If the embed does not load, use Open simulation.

3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0

Step-synced diagram — highlights what to look for in the simulation above.

Step 1

The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).

10 more simulations for this topic — run them in the Simulations section below

Simulations

Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.

10 simulations · 5 to start with

Start herein this order — each one shows a different piece of the topic

3JCN PhysicsStart here · 19702 20.3 · IB D.3
Hall Effect
Pass current through a slab in a B field and read the Hall voltage
Why this one: Pass current through the slab in a field and watch charge pile up on one side: the Hall voltage.
Try this
1. Pass current through the slab and read the Hall voltage.
2. Increase B and compare.
3. Reverse the current and read the sign.
Look for The Hall voltage is proportional to B and to the current, and its sign reveals the charge carriers.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
3JCN PhysicsStart here · 29702 20.3 · 9702 18.2 · IB D.3
J.J. Thomson's Experiment
Balance electric and magnetic deflection of a cathode ray, as J.J. Thomson did
Why this one: Balance electric and magnetic deflection so the beam goes straight: qE = Bqv gives the speed.
Try this
1. Deflect the cathode ray with the electric field alone.
2. Add the magnetic field and adjust until the beam is straight again.
3. Use the balance condition to find the beam speed.
Look for When eE = evB the beam is undeflected, so v = E / B.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
The Physics ClassroomStart here · 39702 20.3 · IB D.3
The Mass Spectrometer
Fire ions down the chamber through the charge accelerator, velocity selector and detector; analyse each section
Why this one: Send ions through the velocity selector then the field and see heavier ions curve on wider circles.
Try this
1. Fire an ion and find where it lands in the detector.
2. Double the mass and fire again.
3. Change the charge and compare.
Look for The radius in the detector is proportional to mass divided by charge.
Open on The Physics Classroom
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
3JCN PhysicsStart here · 49702 20.3 · IB D.3
Helical Electron
Launch an electron at an angle to a B field and see its helical path
Why this one: Launch the electron at an angle to B and get a helix: only v sin θ feels the force.
Try this
1. Launch the electron at an angle to the field and watch the helix.
2. Reduce the angle toward zero and compare the path.
Look for The velocity component along B is unchanged while the perpendicular component circles, giving a helix.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
oPhysicsStart here · 59702 20.3 · IB D.3 · IB E.1
Electron Charge to Mass Ratio Lab
Thomson e/m experiment: balance electric and magnetic deflection of an electron beam and compute e/m
Why this one: Bend an electron beam with a known B and known speed and compute e/m from the radius.
Try this
1. Apply the electric field and note the beam deflection.
2. Add the magnetic field and adjust until the beam is straight.
3. Compute e/m from the balanced fields.
Look for When eE = evB the beam is undeflected, and e/m follows from the field values and beam speed.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)

More simulations5 more on this topic — core ones first

oPhysicsCore9702 20.3 · IB D.3
Charged particle in a Magnetic Field
Shoot a charged particle into a uniform magnetic field; vary q, m, v and B; see the circular path and radius
Try this
1. Double v and read the radius.
2. Double B instead.
3. Reverse the sign of q.
Look for Radius r = mv/qB grows with speed and shrinks with field; reversing the charge reverses the sense of rotation.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
oPhysicsCore9702 20.3 · IB D.3
Charged Particle in a Magnetic Field 3D
3D charged particle in a magnetic field with velocity components along and across B; see helical motion
Try this
1. Set the velocity purely across B and watch the circle.
2. Add a component along B.
3. Increase the along-B component.
Look for The across-B component makes the circle and the along-B component stretches it into a helix.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
SimuPhysicsCore9702 18.2 · 9702 20.3 · IB D.3
Cathode Ray Tube
An electron gun, accelerating anode and deflecting plates; drive the glowing spot around the screen
Try this
1. Raise the anode voltage and watch the spot.
2. Change the plate voltage and move the spot.
3. Reverse the plate voltage.
Look for The spot deflects towards the positive plate by an amount proportional to the plate voltage.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
SimuPhysicsCore9702 20.3 · 9702 18.2 · IB D.3
Electron and Photon Field Effect
Fire an electron and a photon into the same electric and magnetic fields and see which one bends
Try this
1. Fire both through the electric field.
2. Fire both through the magnetic field.
3. Reverse the field and compare.
Look for The electron bends in both fields while the photon, having no charge, goes straight.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
3JCN PhysicsCore9702 20.3 · IB D.3
Charge to Mass Ratio Electron
Bend an electron beam in a known B field to measure e/m
Try this
1. Bend the electron beam in the known B field and read the radius.
2. Increase B and compare the radius.
3. Compute e/m from the beam speed, B and radius.
Look for r = mv / (eB), so a stronger field gives a tighter circle and e/m follows from the measured radius.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0

Key formulas

Tap any symbol to reveal exactly what it means and its units.

KEY RELATION

$F = BQv\sin\theta$

Tap a symbol — great for exam definitions

KEY RELATION

$F_B = F_E$

Tap a symbol — great for exam definitions

KEY RELATION

$V_H = \frac{BI}{nQt}$

Tap a symbol — great for exam definitions

Full topic notes

Formal explanation with the rigour you need for the exam.

Magnetic Force on Individual Charges

When a charged particle travels through a magnetic field, it can experience a force that's completely different from an electric force. Crucially, this magnetic force only appears if the particle's velocity has a component perpendicular to the magnetic field lines. If it moves perfectly parallel or anti-parallel to the field, no magnetic force acts on it.

$F = BQv\sin\theta$ Where:

$F$ = magnetic force (N)
$B$ = magnetic flux density (T)
$Q$ = magnitude of the charge (C)
$v$ = speed of the particle (m s⁻¹)
$\theta$ = angle between the velocity vector ( $\vec{v}$ ) and the magnetic field vector ( $\vec{B}$ )

The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).
The force is zero when the charge moves parallel to the field ( $\theta=0^\circ$ ).
Fleming's Left-Hand Rule determines the force direction for a positive charge. For a negative charge, the force is in the opposite direction.
The magnetic force is always perpendicular to both the velocity of the charge and the magnetic field.
Since the force is perpendicular to the direction of motion, the magnetic force does no work on the charge, and its kinetic energy does not change.

Not clear?

Circular Motion and Velocity Selectors

If a charged particle enters a uniform magnetic field perpendicularly, the magnetic force always acts at right angles to its velocity. This constant perpendicular force acts as a centripetal force, compelling the particle to move in a circular path. An important consequence is that the magnetic force does no work on the particle, so its speed and kinetic energy remain constant. This principle is key to devices like mass spectrometers.

A velocity selector is a clever arrangement of perpendicular electric and magnetic fields. Only particles moving at a specific velocity will pass through undeflected, because the magnetic force ( $F_B$ ) will exactly cancel out the electric force ( $F_E$ ). Particles too fast or too slow will be deflected.

For undeflected particles: $F_B = F_E$ $BQv = EQ$ $v = \frac{E}{B}$ Where:

$v$ = selected velocity (m s⁻¹)
$E$ = electric field strength (V m⁻¹ or N C⁻¹)
$B$ = magnetic flux density (T)

A uniform magnetic force acting perpendicular to velocity provides the centripetal force ( $F = \frac{mv^2}{r}$ ), leading to circular motion.
Velocity selectors filter particles based on speed, useful in experimental physics.
The balance $BQv = EQ$ means the charge $Q$ cancels out, so the selection is independent of the particle's charge magnitude.

Not clear?

The Hall Effect: Measuring Magnetic Fields

The Hall effect is a fascinating phenomenon observed when a current-carrying conductor is placed in a magnetic field perpendicular to the current. The magnetic force pushes the charge carriers (e.g., electrons) to one side of the conductor, creating a build-up of charge. This charge separation generates an electric field and, consequently, a measurable potential difference across the conductor, known as the Hall voltage ( $V_H$ ).

$V_H = \frac{BI}{nQt}$ Where:

$V_H$ = Hall voltage (V)
$B$ = magnetic flux density (T)
$I$ = current (A)
$n$ = charge carrier number density (m⁻³)
$Q$ = charge of one carrier (C)
$t$ = thickness of the conductor perpendicular to B and I (m)

The Hall voltage is directly proportional to the magnetic flux density ( $B$ ).
Hall probes are compact devices that use the Hall effect to precisely measure magnetic field strengths.
The sign of the Hall voltage can reveal the sign of the charge carriers (e.g., electrons or holes).
The thickness 't' is perpendicular to both current and magnetic field, affecting the path for charge accumulation.

Not clear?

Worked examples

See the formulas applied — reveal one step at a time, like the exam.

reveal slowly — mark each step

EXAMPLE 1

A proton enters a velocity selector with an electric field strength of $2.0 \times 10^4 \, \text{V m}^{-1}$ and a magnetic flux density of $0.50 \, \text{T}$ . Calculate the speed at which the proton will pass through undeflected.

1
Identify the given values: $E = 2.0 \times 10^4 \, \text{V m}^{-1}$ , $B = 0.50 \, \text{T}$ .

EXAMPLE 2

An electron travels at a speed of 5.0 x 10^6 m/s and enters a region of uniform magnetic field of flux density 0.020 T. The electron's path is perpendicular to the magnetic field. Calculate the radius of the circular path it follows. (Charge of an electron = 1.60 x 10^-19 C, mass of an electron = 9.11 x 10^-31 kg)

1
Identify the forces acting on the electron. The magnetic force provides the centripetal force for the circular motion.

How it all connects

The big idea sits in the middle — tap a linked idea to explore the link.

MAIN IDEAForce on a moving charge

Tap a linked idea to see how it connects back to the main topic — that connection is what examiners reward.

Glossary

Key terms for this topic — skim now; the Check step will test them.

velocity selector: A velocity selector is a clever arrangement of perpendicular electric and magnetic fields.
Hall effect: The Hall effect is a fascinating phenomenon observed when a current-carrying conductor is placed in a magnetic field perpendicular to the current.
Hall voltage: This charge separation generates an electric field and, consequently, a measurable potential difference across the conductor, known as the Hall voltage ( $V_H$ ).
Fleming's Left-Hand Rule: The predicted force direction is opposite to that for a positive charge, or the middle finger points opposite to the actual velocity.
purpose of a velocity: To allow only charged particles moving at a specific velocity to pass through undeflected by balancing electric and magnetic forces.

10Apply

Quick check

Write your answer first, then compare it with the model one — the gap is what you would have lost.

0 of 6 answered

11Understand

Teach it back

If you can explain it simply, you own it — gaps here are marks you’d lose.

Teach it back

Explain this topic as if teaching a friend. We name the gaps an examiner would still dock.

Your explanation — no notes

12Remember

Revision flashcards

Guess first, then flip — retrieval beats re-reading.

Your guess — write it before you look

1 / 15 · ← → space

Key takeaways

Review these before you close the topic — retrieval beats re-reading.

close the tab only after these stick

The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).
The force is zero when the charge moves parallel to the field ( $\theta=0^\circ$ ).
Fleming's Left-Hand Rule determines the force direction for a positive charge. For a negative charge, the force is in the opposite direction.
The magnetic force is always perpendicular to both the velocity of the charge and the magnetic field.
Since the force is perpendicular to the direction of motion, the magnetic force does no work on the charge, and its kinetic energy does not change.

Practice — then mark it

The whole point: a real Cambridge question, marked mark-by-mark.

Real past paper9702/41 · Q5(c)(ii)

PhysicsMay/June 2024

Explain, with reference to the forces exerted by the two fields on the electron, why the path of the electron is undeviated.

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

9702/41/M/J/24

Extra simulations & links

PhET, GeoGebra and other curated tools — open in a new tab.

Frequently asked

How does the magnetic force differ from the electric force on a charged particle?

Why does a charged particle move in a circle in a uniform magnetic field?

Can the Hall effect be used to determine the type of charge carriers in a material?

Checkpoint

One marked question is worth ten re-reads — close the loop before you move on.

Checkpoint

Reading it isn’t knowing it — prove it.

Before you move on: do 9702/41 · Q5(c)(ii) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.

Attempt & get marked →Takes about a minute · you’ll land back on this lesson

← PREVIOUS20.2 Force on a current-carrying conductor NEXT →20.4 Magnetic fields due to currents

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Revise Force on a moving charge for Cambridge 9702 (20.3)

Looking for free Physics 9702 notes on Force on a moving charge? This A-Level lesson covers official syllabus point 20.3 on Paper 4: A Level Structured Questions. Use the visual steps, key terms, and quick checks below — then practise a real 9702 past-paper question with mark-scheme marking. Built for students searching "Force on a moving charge 9702 revision", "9702 20.3 notes", or a free alternative to paid note sites.

Full Physics 9702 course Mark a past paper →9702 subject hub

20.3Physics25 min readPREMIUM

Force on a moving charge

BY THE END, YOU CAN…

exam checklist — tick these off

1Describe the magnetic force on a moving charge and apply F = BQvsinθ.
2Explain the function of a velocity selector and how it utilises electric and magnetic fields.
3Outline the principles of the Hall effect and its application in Hall probes.

STUDY MODE

READING

TYPEFACE

Literata — a reading serif made for e-books; set like a textbook, sits well beside maths.

SIZE

AIRY SPACING

Wider letter and line spacing. The one change with solid evidence for readers who lose their place — worth a try whoever you are.

Pick whatever reads fastest for you — it differs from person to person, by up to a third.

EXPLAIN SIMPLER

The same notes in plain English — no jargon. Key points and exam tips stay.

DIAGRAM NOTES

A small sketch beside each block of notes.

In simple terms

A friendly intro before the formal notes — no formulas yet.

Force on a moving charge

Cambridge 9702 Paper 4 — Force on a moving charge (20.3). Senpai Corner diagram-backed pilot with premium structure and live visuals.

1
The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).
2
The force is zero when the charge moves parallel to the field ( $\theta=0^\circ$ ).
3
Fleming's Left-Hand Rule determines the force direction for a positive charge. For a negative charge, the force is in the opposite direction.
4
The magnetic force is always perpendicular to both the velocity of the charge and the magnetic field.

What this topic covers

The official Cambridge syllabus points this lesson works through.

20.3.1
Determine the direction of the force on a charge moving in a magnetic field
20.3.2
Recall and use $F = BQv \sin \theta$
20.3.3
Understand the origin of the Hall voltage and derive and use the expression $V_H = BI/(ntq)$, where t = thickness
20.3.4
Understand the use of a Hall probe to measure magnetic flux density
20.3.5
Describe the motion of a charged particle moving in a uniform magnetic field perpendicular to the direction of motion of the particle
20.3.6
Explain how electric and magnetic fields can be used in velocity selection

Explore the concept

Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.

Figure Live interactive1 / 4

Loading simulation…

Fetching from 3JCN Physics Simulation by Thomas Nguyen…

Charge in Magnetic Field

r = mv / (qB): faster charges make bigger circles and stronger fields make smaller ones.

If the embed does not load, use Open simulation.

3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0

Step-synced diagram — highlights what to look for in the simulation above.

Step 1

The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).

10 more simulations for this topic — run them in the Simulations section below

Simulations

Every simulation here runs the real model — try the steps on a card, then check what you see against the notes.

10 simulations · 5 to start with

Start herein this order — each one shows a different piece of the topic

3JCN PhysicsStart here · 19702 20.3 · IB D.3
Hall Effect
Pass current through a slab in a B field and read the Hall voltage
Why this one: Pass current through the slab in a field and watch charge pile up on one side: the Hall voltage.
Try this
1. Pass current through the slab and read the Hall voltage.
2. Increase B and compare.
3. Reverse the current and read the sign.
Look for The Hall voltage is proportional to B and to the current, and its sign reveals the charge carriers.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
3JCN PhysicsStart here · 29702 20.3 · 9702 18.2 · IB D.3
J.J. Thomson's Experiment
Balance electric and magnetic deflection of a cathode ray, as J.J. Thomson did
Why this one: Balance electric and magnetic deflection so the beam goes straight: qE = Bqv gives the speed.
Try this
1. Deflect the cathode ray with the electric field alone.
2. Add the magnetic field and adjust until the beam is straight again.
3. Use the balance condition to find the beam speed.
Look for When eE = evB the beam is undeflected, so v = E / B.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
The Physics ClassroomStart here · 39702 20.3 · IB D.3
The Mass Spectrometer
Fire ions down the chamber through the charge accelerator, velocity selector and detector; analyse each section
Why this one: Send ions through the velocity selector then the field and see heavier ions curve on wider circles.
Try this
1. Fire an ion and find where it lands in the detector.
2. Double the mass and fire again.
3. Change the charge and compare.
Look for The radius in the detector is proportional to mass divided by charge.
Open on The Physics Classroom
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
3JCN PhysicsStart here · 49702 20.3 · IB D.3
Helical Electron
Launch an electron at an angle to a B field and see its helical path
Why this one: Launch the electron at an angle to B and get a helix: only v sin θ feels the force.
Try this
1. Launch the electron at an angle to the field and watch the helix.
2. Reduce the angle toward zero and compare the path.
Look for The velocity component along B is unchanged while the perpendicular component circles, giving a helix.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
oPhysicsStart here · 59702 20.3 · IB D.3 · IB E.1
Electron Charge to Mass Ratio Lab
Thomson e/m experiment: balance electric and magnetic deflection of an electron beam and compute e/m
Why this one: Bend an electron beam with a known B and known speed and compute e/m from the radius.
Try this
1. Apply the electric field and note the beam deflection.
2. Add the magnetic field and adjust until the beam is straight.
3. Compute e/m from the balanced fields.
Look for When eE = evB the beam is undeflected, and e/m follows from the field values and beam speed.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)

More simulations5 more on this topic — core ones first

oPhysicsCore9702 20.3 · IB D.3
Charged particle in a Magnetic Field
Shoot a charged particle into a uniform magnetic field; vary q, m, v and B; see the circular path and radius
Try this
1. Double v and read the radius.
2. Double B instead.
3. Reverse the sign of q.
Look for Radius r = mv/qB grows with speed and shrinks with field; reversing the charge reverses the sense of rotation.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
oPhysicsCore9702 20.3 · IB D.3
Charged Particle in a Magnetic Field 3D
3D charged particle in a magnetic field with velocity components along and across B; see helical motion
Try this
1. Set the velocity purely across B and watch the circle.
2. Add a component along B.
3. Increase the along-B component.
Look for The across-B component makes the circle and the along-B component stretches it into a helix.
Open on oPhysics
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
SimuPhysicsCore9702 18.2 · 9702 20.3 · IB D.3
Cathode Ray Tube
An electron gun, accelerating anode and deflecting plates; drive the glowing spot around the screen
Try this
1. Raise the anode voltage and watch the spot.
2. Change the plate voltage and move the spot.
3. Reverse the plate voltage.
Look for The spot deflects towards the positive plate by an amount proportional to the plate voltage.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
SimuPhysicsCore9702 20.3 · 9702 18.2 · IB D.3
Electron and Photon Field Effect
Fire an electron and a photon into the same electric and magnetic fields and see which one bends
Try this
1. Fire both through the electric field.
2. Fire both through the magnetic field.
3. Reverse the field and compare.
Look for The electron bends in both fields while the photon, having no charge, goes straight.
Open on SimuPhysicsRuns on their site
SimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
3JCN PhysicsCore9702 20.3 · IB D.3
Charge to Mass Ratio Electron
Bend an electron beam in a known B field to measure e/m
Try this
1. Bend the electron beam in the known B field and read the radius.
2. Increase B and compare the radius.
3. Compute e/m from the beam speed, B and radius.
Look for r = mv / (eB), so a stronger field gives a tighter circle and e/m follows from the measured radius.
Open on 3JCN Physics
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0

Key formulas

Tap any symbol to reveal exactly what it means and its units.

KEY RELATION

$F = BQv\sin\theta$

Tap a symbol — great for exam definitions

KEY RELATION

$F_B = F_E$

Tap a symbol — great for exam definitions

KEY RELATION

$V_H = \frac{BI}{nQt}$

Tap a symbol — great for exam definitions

Full topic notes

Formal explanation with the rigour you need for the exam.

Magnetic Force on Individual Charges

$F = BQv\sin\theta$ Where:

$F$ = magnetic force (N)
$B$ = magnetic flux density (T)
$Q$ = magnitude of the charge (C)
$v$ = speed of the particle (m s⁻¹)
$\theta$ = angle between the velocity vector ( $\vec{v}$ ) and the magnetic field vector ( $\vec{B}$ )

The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).
The force is zero when the charge moves parallel to the field ( $\theta=0^\circ$ ).
Fleming's Left-Hand Rule determines the force direction for a positive charge. For a negative charge, the force is in the opposite direction.
The magnetic force is always perpendicular to both the velocity of the charge and the magnetic field.
Since the force is perpendicular to the direction of motion, the magnetic force does no work on the charge, and its kinetic energy does not change.

Not clear?

Circular Motion and Velocity Selectors

For undeflected particles: $F_B = F_E$ $BQv = EQ$ $v = \frac{E}{B}$ Where:

$v$ = selected velocity (m s⁻¹)
$E$ = electric field strength (V m⁻¹ or N C⁻¹)
$B$ = magnetic flux density (T)

A uniform magnetic force acting perpendicular to velocity provides the centripetal force ( $F = \frac{mv^2}{r}$ ), leading to circular motion.
Velocity selectors filter particles based on speed, useful in experimental physics.
The balance $BQv = EQ$ means the charge $Q$ cancels out, so the selection is independent of the particle's charge magnitude.

Not clear?

The Hall Effect: Measuring Magnetic Fields

$V_H = \frac{BI}{nQt}$ Where:

$V_H$ = Hall voltage (V)
$B$ = magnetic flux density (T)
$I$ = current (A)
$n$ = charge carrier number density (m⁻³)
$Q$ = charge of one carrier (C)
$t$ = thickness of the conductor perpendicular to B and I (m)

The Hall voltage is directly proportional to the magnetic flux density ( $B$ ).
Hall probes are compact devices that use the Hall effect to precisely measure magnetic field strengths.
The sign of the Hall voltage can reveal the sign of the charge carriers (e.g., electrons or holes).
The thickness 't' is perpendicular to both current and magnetic field, affecting the path for charge accumulation.

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Worked examples

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reveal slowly — mark each step

EXAMPLE 1

1
Identify the given values: $E = 2.0 \times 10^4 \, \text{V m}^{-1}$ , $B = 0.50 \, \text{T}$ .

EXAMPLE 2

1
Identify the forces acting on the electron. The magnetic force provides the centripetal force for the circular motion.

How it all connects

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MAIN IDEAForce on a moving charge

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Glossary

Key terms for this topic — skim now; the Check step will test them.

velocity selector: A velocity selector is a clever arrangement of perpendicular electric and magnetic fields.
Hall effect: The Hall effect is a fascinating phenomenon observed when a current-carrying conductor is placed in a magnetic field perpendicular to the current.
Hall voltage: This charge separation generates an electric field and, consequently, a measurable potential difference across the conductor, known as the Hall voltage ( $V_H$ ).
Fleming's Left-Hand Rule: The predicted force direction is opposite to that for a positive charge, or the middle finger points opposite to the actual velocity.
purpose of a velocity: To allow only charged particles moving at a specific velocity to pass through undeflected by balancing electric and magnetic forces.

10Apply

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11Understand

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12Remember

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1 / 15 · ← → space

Key takeaways

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close the tab only after these stick

The force $F = BQv\sin\theta$ is maximum when the charge moves perpendicular to the field ( $\theta=90^\circ$ ).
The force is zero when the charge moves parallel to the field ( $\theta=0^\circ$ ).
Fleming's Left-Hand Rule determines the force direction for a positive charge. For a negative charge, the force is in the opposite direction.
The magnetic force is always perpendicular to both the velocity of the charge and the magnetic field.
Since the force is perpendicular to the direction of motion, the magnetic force does no work on the charge, and its kinetic energy does not change.

Practice — then mark it

The whole point: a real Cambridge question, marked mark-by-mark.

Real past paper9702/41 · Q5(c)(ii)

PhysicsMay/June 2024

Explain, with reference to the forces exerted by the two fields on the electron, why the path of the electron is undeviated.

Do it on paper → mark itMARKED MARK-BY-MARK · B1 / M1 / A1 · OFFICIAL SCHEME

9702/41/M/J/24

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Frequently asked

How does the magnetic force differ from the electric force on a charged particle?

Why does a charged particle move in a circle in a uniform magnetic field?

Can the Hall effect be used to determine the type of charge carriers in a material?

Checkpoint

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Checkpoint

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Before you move on: do 9702/41 · Q5(c)(ii) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.

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Revise Force on a moving charge for Cambridge 9702 (20.3)

In simple terms

What this topic covers

Explore the concept

Simulations

Hall Effect

J.J. Thomson's Experiment

The Mass Spectrometer

Helical Electron

Electron Charge to Mass Ratio Lab

Charged particle in a Magnetic Field

Charged Particle in a Magnetic Field 3D

Cathode Ray Tube

Electron and Photon Field Effect

Charge to Mass Ratio Electron

Key formulas

Full topic notes

Magnetic Force on Individual Charges

Circular Motion and Velocity Selectors

The Hall Effect: Measuring Magnetic Fields

Worked examples

How it all connects

Glossary

Quick check

Teach it back

Revision flashcards

Key takeaways

Practice — then mark it

Extra simulations & links

Frequently asked

Checkpoint

Reading it isn’t knowing it — prove it.

Discuss Force on a moving charge

Revise Force on a moving charge for Cambridge 9702 (20.3)

In simple terms

What this topic covers

Explore the concept

Simulations

Hall Effect

J.J. Thomson's Experiment

The Mass Spectrometer

Helical Electron

Electron Charge to Mass Ratio Lab

Charged particle in a Magnetic Field

Charged Particle in a Magnetic Field 3D

Cathode Ray Tube

Electron and Photon Field Effect

Charge to Mass Ratio Electron

Key formulas

Full topic notes

Magnetic Force on Individual Charges

Circular Motion and Velocity Selectors

The Hall Effect: Measuring Magnetic Fields

Worked examples

How it all connects

Glossary

Quick check

Teach it back

Revision flashcards

Key takeaways

Practice — then mark it

Extra simulations & links

Frequently asked

Checkpoint

Reading it isn’t knowing it — prove it.

Discuss Force on a moving charge