In simple terms
A friendly intro before the formal notes — no formulas yet.
Electric fields and field lines
This lesson covers the fundamental concepts of electric fields, including field strength, Coulomb's Law, and field line patterns. You will learn to calculate forces and field strengths for point charges and in uniform fields, and understand the motion of charged particles within these fields.
- 1
Electric field strength is defined as the electrostatic force per unit positive charge acting on a stationary point charge at that point.
- 2
The SI unit for electric field strength is Newtons per Coulomb (N C⁻¹) or Volts per metre (V m⁻¹).
- 3
Electric field is a vector quantity, possessing both magnitude and direction.
- 4
The direction of the field is the direction of the force on a positive charge.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 18.1.1
Understand that an electric field is an example of a field of force and define electric field as force per unit positive charge
- 18.1.2
Recall and use for the force on a charge in an electric field
- 18.1.3
Represent an electric field by means of field lines
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
Electric field strength is defined as the electrostatic force per unit positive charge acting on a stationary point charge at that point.
11 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.
11 simulations · 4 to start with
Start herein this order — each one shows a different piece of the topic
- 3JCN PhysicsStart here · 19702 18.1 · 9702 18.4 · IB D.2
Electric Field of Charged Particles
Place positive and negative charges; see field lines and field vectors
Why this one: Place a positive and a negative charge and see the field lines run from + to −.
Try this
- Place one positive charge and look at the field lines.
- Add a negative charge and watch the lines link the two.
- Place two like charges and look for the region where the field vectors cancel.
Look for Field lines start on positive charge and end on negative charge, and are densest where the field is strongest.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- The Physics ClassroomStart here · 29702 18.1 · 9702 18.4 · IB D.2
3D Electric Field Mapping
Drag source charges onto the workspace, see the lines of force, and tap any point for the field vector; 3D view
Why this one: Tap any point near two charges and read the resultant field vector — superposition in action.
Try this
- Place a charge and tap a point to read the field.
- Double the distance and read it again.
- Add a second charge.
Look for Field strength falls as 1 divided by r² from a point charge.
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
- 3JCN PhysicsStart here · 39702 18.1 · 9702 18.4 · IB D.2
Electric Field of a Spherical Shell
Electric field inside and outside a charged spherical shell
Why this one: Probe inside and outside a charged sphere: zero inside, a point-charge field outside.
Try this
- Read E inside the shell.
- Read E just outside the shell and then at twice the radius.
Look for E is zero inside the shell and falls as 1/r² outside, as if the charge were at the centre.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- The Physics ClassroomStart here · 49702 18.1 · IB D.2
Electric Field Lines
Drag positive or negative charges onto the space and observe the electric field lines
Why this one: Drag like and unlike charges together and compare the two classic field-line patterns.
Try this
- Place one positive charge.
- Add a negative charge and watch the lines.
- Add two charges of the same sign.
Look for Field lines run from positive to negative charge and crowd together where the field is strong.
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
More simulations7 more on this topic — core ones first
- oPhysicsCore9702 18.1 · 9702 18.5 · 9702 18.4
Electric Field & Potential
Drag +1 nC / -1 nC point charges; see field lines and the electric potential map update
Try this
- Drag a +1 nC and a −1 nC charge near each other.
- Replace the negative charge with a second positive one.
- Move the charges apart and watch the potential map.
Look for Field lines run from positive to negative and cross equipotentials at right angles.
Simulation by Tom Walsh, oPhysics.com — made with GeoGebra · Licensed to MarkScheme (site: free for non-profit educational use; applets made with GeoGebra)
- SimuPhysicsCore9702 18.1 · IB D.2
Electric Field Lines
Field lines traced from the computed field; drag the test charge around and flip its sign to see it pushed along the lines
Try this
- Drag the test charge along a line.
- Flip its sign and compare.
- Drag it to where the lines crowd.
Look for Field lines leave positive charges, end on negative ones, never cross and crowd where the field is strong.
Open on SimuPhysicsRuns on their siteSimuPhysics by Mohamed Abdelsalam · Licensed to MarkScheme (site publishes no licence; served with frame-ancestors self)
- 3JCN PhysicsCore9702 18.5 · 9702 18.1 · 9702 18.4
Electric Field & Equipotential Mapping
Probe a 3D field and plot equipotential lines around configured charges
Try this
- Probe the field around the configured charges and plot a few equipotential lines.
- Compare the direction of the field with the equipotentials where they cross.
- Note where the equipotentials are closest together.
Look for Field lines cross equipotentials at right angles, and closely spaced equipotentials mark a strong field.
3JCN Physics Simulation by Thomas Nguyen · CC BY 4.0
- PhETJava · best on a laptop9702 18.1–18.2 · IB D.2
Electric Field of Dreams
Add point charges and an external field; watch each charge accelerate along the field.
Try this
- Add one charge and switch on an external field — it accelerates in a straight line.
- Add a second, opposite charge — the two curve toward each other.
- Raise the external field strength — the acceleration grows.
Look for F = qE: a charge accelerates along the field, with a = qE/m.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- PhETJava · best on a laptop9702 18.1 · 18.3 · IB D.2
Electric Field Hockey
Place fixed positive and negative charges to steer a moving puck into the goal using electrostatic force.
Try this
- Place one positive charge behind the puck and start — it is pushed away.
- Add a negative charge near the goal to pull the puck in.
- Turn on the field lines to see where the puck will be pushed.
Look for Force follows the field lines; like charges push, unlike charges pull, and closer charges act more strongly.
Simulation by PhET Interactive Simulations, University of Colorado Boulder · Licensed to MarkScheme (public licence CC BY-NC 4.0 since 2026-03-30)
- The Physics Classroom9702 18.1 · 9702 18.2 · IB D.2
Put the Charge in the Goal (Electric Field Hockey)
Place charges to steer a puck around obstacles and into the goal
Try this
- Place one charge behind the puck.
- Add charges to curve the puck's path.
- Try a level with obstacles.
Look for The puck accelerates along the direction of the net electric field.
Physics Interactives by The Physics Classroom · Licensed to MarkScheme (site terms otherwise permit linking only)
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
Tap a symbol — great for exam definitions
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 is an Electric Field?
An electric field is a specific region in space around any charged object where another charged object would experience an electrostatic force. Think of it as the 'sphere of influence' of a charge. This force acts without direct contact, meaning it's a non-contact force, much like gravity or magnetism.
Electric Field Strength ($E$)
To quantify how strong an electric field is at any point, we use electric field strength, denoted by the symbol . It's defined as the force experienced per unit positive test charge placed at that point. This 'test charge' is theoretical; it's considered so small that it doesn't significantly alter the field it's testing.
Electric field strength is defined as the electrostatic force per unit positive charge acting on a stationary point charge at that point.
The SI unit for electric field strength is Newtons per Coulomb (N C⁻¹) or Volts per metre (V m⁻¹).
Electric field is a vector quantity, possessing both magnitude and direction.
The direction of the field is the direction of the force on a positive charge.
Coulomb's Law: Force Between Point Charges
When two point charges interact, the electrostatic force between them is described by Coulomb's Law. This fundamental law tells us that the force is directly proportional to the product of the charges and inversely proportional to the square of the distance separating them.
Force is proportional to the product of the magnitudes of the charges ().
Force is inversely proportional to the square of the separation distance ().
is the permittivity of free space (), a constant indicating how an electric field permeates a vacuum.
Like charges (e.g., positive-positive) will repel, while opposite charges (e.g., positive-negative) will attract.
Electric Field Strength from a Point Charge
By combining the definition of electric field strength () with Coulomb's Law, we can determine the electric field strength generated by a single point charge at a distance . This formula is crucial for understanding fields around individual charges.
Visualising Electric Fields: Field Lines
Electric field lines are a powerful visual tool to represent the invisible electric field. These imaginary lines show the direction and strength of the field at every point, making complex field patterns easier to understand.
Lines originate from positive charges and terminate on negative charges, or extend to infinity.
Arrows on the lines indicate the direction of the force on a positive test charge.
The density of the lines (how close they are) indicates the field strength; denser lines mean a stronger field.
Field lines never cross each other, as this would imply two directions of force at one point.
Field lines are always perpendicular to the surface of conductors and equipotential lines.
Common Electric Field Patterns
Understanding the patterns for common charge arrangements is key. These are often tested in exams.
Isolated Point Charge: Lines radiate outwards from a positive charge and inwards towards a negative charge, becoming less dense with distance.
Electric Dipole (+ and -): Field lines originate from the positive charge and curve to terminate on the negative charge.
Two Like Charges (+ and +): Field lines from each charge repel each other, creating a 'null point' (zero field) exactly between them if the charges are equal.
Parallel Plates: A uniform field is created between two oppositely charged parallel plates, with straight, parallel, equally spaced lines pointing from the positive to the negative plate.
The Principle of Superposition
When multiple charges are present, the total electric field at any point is the vector sum of the electric fields that each charge would create individually at that point. This is known as the principle of superposition. For two fields and at a point, the resultant field is . You must use vector addition (e.g., resolving components or using the parallelogram/triangle law) to find the resultant.
Uniform Electric Fields
A uniform electric field is one where the electric field strength is constant in both magnitude and direction throughout a region. This type of field is typically found between two large, parallel, oppositely charged plates. The force on a charge within this field is constant (), leading to constant acceleration. This creates a situation analogous to projectile motion under gravity.
Represented by parallel, equally spaced straight electric field lines.
The field strength between parallel plates can be found using , where V is the potential difference and d is the plate separation.
A charged particle moving perpendicular to a uniform electric field will follow a parabolic trajectory.
Positive charges accelerate in the direction of the field; negative charges accelerate opposite to it.
Motion of a Charged Particle in a Uniform Field
When a charged particle enters a uniform electric field, it experiences a constant electrostatic force. If the particle's initial velocity is perpendicular to the field, its path will be a parabola. The motion can be analysed by considering two components: a constant velocity component parallel to the plates (no force in this direction) and a constantly accelerated component perpendicular to the plates (due to the constant electric force, and ).
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A point charge of +5.0 nC is placed in a vacuum. Calculate the electric field strength at a distance of 10 cm from the charge. ()
- 1
Identify given values: Charge C, distance , permittivity of free space .
Two parallel metal plates are separated by 2.0 cm in a vacuum. A potential difference of 500 V is applied across them, creating a uniform electric field. An electron is released from rest at the surface of the negative plate. Calculate: (a) the electric field strength between the plates, (b) the force on the electron, and (c) the acceleration of the electron. (Use C, kg)
- 1
Identify given values: Potential difference V, distance .
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.
- electric field strength
To quantify how strong an electric field is at any point, we use electric field strength, denoted by the symbol . It's defined as the force experienced per unit positive test charge placed at that point.
- Coulomb's Law
When two point charges interact, the electrostatic force between them is described by Coulomb's Law. This fundamental law tells us that the force is directly proportional to the product of the charges and inversely proportional to the square of the distance separating them.
- vector sum
When multiple charges are present, the total electric field at any point is the vector sum of the electric fields that each charge would create individually at that point. This is known as the principle of superposition.
- uniform electric field
A uniform electric field is one where the electric field strength is constant in both magnitude and direction throughout a region.
Quick check
Write your answer first, then compare it with the model one — the gap is what you would have lost.
Teach it back
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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.
Electric field strength is defined as the electrostatic force per unit positive charge acting on a stationary point charge at that point.
The SI unit for electric field strength is Newtons per Coulomb (N C⁻¹) or Volts per metre (V m⁻¹).
Electric field is a vector quantity, possessing both magnitude and direction.
The direction of the field is the direction of the force on a positive charge.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
On Fig. 5.1, draw four field lines to represent the electric field between the plates.
Define electric field.
Extra simulations & links
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Frequently asked
Checkpoint
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