In simple terms
A friendly intro before the formal notes — no formulas yet.
Uniform electric fields
Cambridge 9702 Paper 4 — Uniform electric fields (18.2). Senpai Corner diagram-backed pilot with premium structure and live visuals.
- 1
18.2 Uniform electric fields.
- 2
E is now also defined by the units Vm -1.
- 3
The equation above can only be used for two charged parallel plates.
- 4
A charged particle will move through an electric field due to a force on it that is caused by said electric field.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 18.2.1
Recall and use to calculate the field strength of the uniform field between charged parallel plates
- 18.2.2
Describe the effect of a uniform electric field on the motion of charged particles
Explore the concept
Use the live diagram, PhET or GeoGebra sim, and synced steps — play it, drag controls, or tap a step.
Step-synced diagram — highlights what to look for in the simulation above.
Step 1
18.2 Uniform electric fields.
15 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.
15 simulations · 4 to start with
Start herein this order — each one shows a different piece of the topic
- 3JCN PhysicsStart here · 19702 18.2 · IB D.2 · IB D.3
Charge in Electric Field
Launch a charge into a uniform electric field and watch the parabolic deflection
Why this one: Launch a charge across the plates and watch it curve in a parabola — constant force, like a projectile.
Try this
- Launch the charge into the field and watch its path.
- Increase the field and compare the deflection.
- Launch faster and compare.
Look for Constant force gives a parabola, just like a projectile under gravity.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsStart here · 29702 18.2 · IB D.2 · IB D.3
E Field of 2 Parallel Charged Metal Plates
Electric field between two parallel charged metal plates
Why this one: See the evenly spaced parallel field lines between the plates and the fringing at the edges.
Try this
- Read E between the plates.
- Read E outside the plates and compare.
Look for The fields of the two plates add between them and cancel outside, giving a uniform field inside.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- oPhysicsStart here · 39702 18.2 · 9702 11.1 · IB D.3
The Millikan Oil-Drop Experiment
Simplified Millikan oil-drop: adjust plate voltage to balance a charged droplet and deduce its charge
Why this one: Adjust the plate voltage until the droplet hangs still: qE balances mg.
Try this
- Raise the plate voltage until the droplet hangs still.
- Deduce the charge from the balancing voltage.
- Repeat with a new droplet and compare the charges.
Look for Balancing voltages give charges that are whole-number multiples of e.
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
- 3JCN PhysicsStart here · 49702 18.2 · 9702 4.2 · 9702 4.1
Charge on a String in Electric Field
Hang a charged ball on a string in a uniform field; find the deflection angle
Why this one: Hang a charged ball in the field and resolve tension, weight and qE to find the angle.
Try this
- Hang the charged ball in the field and read the deflection angle.
- Increase the field and compare the angle.
- Resolve tension, weight and electric force to check the angle.
Look for In equilibrium tan θ = qE / mg.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
More simulations11 more on this topic — core ones first
- oPhysicsCore9702 18.2 · IB D.3
Charged Particle in an Electric Field
Shoot a charged particle into a uniform electric field; adjust charge, mass, speed and field; see parabolic path
Try this
- Fire the particle in and watch the parabolic path.
- Double the field strength.
- Reverse the sign of the charge.
Look for The path is a parabola because the force is constant and perpendicular to the entry velocity; a negative charge curves the other way.
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
- Raise the anode voltage and watch the spot.
- Change the plate voltage and move the spot.
- 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 siteSimuPhysics 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
- Fire both through the electric field.
- Fire both through the magnetic field.
- 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 siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- 3JCN PhysicsCore9702 18.2 · 9702 18.4 · IB D.2
E Field of an Infinite Charged Sheet
Electric field of an infinite charged sheet
Try this
- Read E near the sheet.
- Move far from the sheet and compare.
Look for The field of an infinite sheet is uniform and independent of distance.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsCore9702 18.2 · 9702 11.1 · IB D.2
Milikan's Oil Drop Exp
Suspend oil drops between charged plates to find the electron charge
Try this
- Adjust the plate voltage until an oil drop hangs still.
- Compute the drop's charge from the balanced forces.
- Repeat for several drops and compare the charges.
Look for Every drop's charge is a whole-number multiple of the electron charge e.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- 3JCN PhysicsCore9702 18.5 · 9702 18.2 · IB D.2
Equipotential Surfaces Capacitor
Equipotential surfaces between capacitor plates
Try this
- Look at the spacing of the equipotential surfaces between the plates.
- Compare the equipotentials near the plate edges with those in the middle.
Look for Between the plates the equipotentials are evenly spaced planes, showing a uniform field.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
Key formulas
Tap any symbol to reveal exactly what it means and its units.
Tap a symbol — great for exam definitions
Tap a symbol — great for exam definitions
Full topic notes
Formal explanation with the rigour you need for the exam.
What Makes a Field Uniform?
A uniform electric field is a special region where the electric force on a charged particle is the same in both strength and direction, no matter where the particle is located within that region. This consistency is represented visually by straight, parallel, and equally spaced electric field lines.
18.2 Uniform electric fields.
E is now also defined by the units Vm -1.
The equation above can only be used for two charged parallel plates.
A charged particle will move through an electric field due to a force on it that is caused by said electric field.
The trajectory, as shown in the diagram above is parabolic .
The direction of parabola depends on the charged of the particle.
Defining Electric Field Strength
At its core, electric field strength (E) quantifies the force exerted per unit of positive charge. This fundamental definition applies to all electric fields, uniform or not. When a charge 'q' is placed in a field 'E', it experiences a force 'F'.
or
Uniform Fields Between Parallel Plates
The most common way to create a uniform electric field is by applying a potential difference () across two large, parallel metal plates separated by a distance (). The field lines emerge perpendicularly from the positive plate and terminate perpendicularly on the negative plate.
This formula highlights that the electric field strength is also the magnitude of the potential gradient. It means for every metre you move across the field, the electric potential changes by 'E' Volts. The units for electric field strength can therefore be N C⁻¹ or V m⁻¹.
Equipotential Surfaces
Imagine contour lines on a map showing points of equal height. Equipotential surfaces (or lines in 2D) are similar, connecting all points within the field that have the same electric potential. In a uniform field, these are simple.
Equipotential lines are parallel to the charged plates.
They are always perpendicular to the electric field lines.
No work is done by the electric field when a charge moves along an equipotential line.
Work is done when a charge (Q) moves between different equipotentials ().
Motion of Charged Particles
When a charged particle enters a uniform electric field, it experiences a constant force (), much like the constant gravitational force on a mass near the Earth's surface. According to Newton's second law, this constant force produces a constant acceleration (). This allows us to use the standard kinematic equations (suvat) to predict the particle's motion. The trajectory depends on the initial velocity relative to the field.
Positive charges accelerate in the same direction as the field lines.
Negative charges accelerate in the opposite direction to the field lines.
If fired parallel to the field, motion is linear.
If fired perpendicular to the field, the path is parabolic, as the particle has constant horizontal velocity and constant vertical acceleration.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Two parallel plates are separated by 2.0 cm and have a potential difference of 500 V across them. a) Calculate the electric field strength between the plates. b) What force does an electron (charge = C) experience in this field?
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Convert plate separation to metres: .
An electron (mass kg, charge C) is fired horizontally with a speed of m s⁻¹ into a uniform electric field. The field is created by two parallel plates, 5.0 cm long and 1.5 cm apart, with a potential difference of 300 V. Calculate the vertical deflection of the electron as it exits the plates.
- 1
Calculate Electric Field Strength (E): The field is uniform between the plates.
How it all connects
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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.
- fundamental definition
The force per unit positive test charge ().
- In which direction does
Opposite to the direction of the electric field lines.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
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.
Revision flashcards
Guess first, then flip — retrieval beats re-reading.
Key takeaways
Review these before you close the topic — retrieval beats re-reading.
18.2 Uniform electric fields.
E is now also defined by the units Vm -1.
The equation above can only be used for two charged parallel plates.
A charged particle will move through an electric field due to a force on it that is caused by said electric field.
The trajectory, as shown in the diagram above is parabolic .
The direction of parabola depends on the charged of the particle.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
An electron travels at a speed of 2.6 × 10⁷ m s⁻¹ towards the region between the plates, as shown in Fig. 5.1. On Fig. 5.1, draw the path of the electron as it moves between and beyond the plates.
Determine the strength E of the electric field between the plates.
Extra simulations & links
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Frequently asked
Checkpoint
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Reading it isn’t knowing it — prove it.
Before you move on: do 9702/41 · Q5(b)(iii) on paper, snap a photo, and get examiner-style feedback on exactly where you win and lose marks.
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