Theme D — Fields

D.2.6 Electric Field Lines — IB Physics HL Revision Notes & Topical Questions

5 min read 11 parts Lesson 21 of 39 1 quiz

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D.2.6 Electric Field Lines — IB PHYSICS SL notes: Draw and interpret electric field lines for point charges, conducting spheres, pairs of charges and parallel plates.Electric field lines — lines whose tangent shows the direction of the force on a small positive test chargeStart a…

Start and end
No crossing
Conductors
Line density
Uniform field
Radial field

Part 1 of 9Rules for drawing

Electric field lines

Electric field lines lines whose tangent shows the direction of the force on a small positive test charge

Start and end

Start and end lines begin on positive charges (or at infinity) and end on negative charges (or at infinity)

Arrows

Arrows they show the direction of the force on a positive charge, so label them on every line

No crossing

No crossing the field has only one direction at a point

Conductors

Conductors lines meet the surface of a conductor at right angles

Line density

Line density the closer the lines, the stronger the field
The number of lines leaving a charge is proportional to its size.

Model

Model the field exists in the gaps between lines too

Part 2 of 9Uniform and radial fields

Uniform field

Uniform field equally spaced lines mean constant field strength and a force of constant size and direction on a test charge

Radial field

Radial field lines are equally spaced as they leave the surface of the charge but spread out with distance
The field strength and the force on a test charge decrease with distance.

Non-uniform

Non-uniform radial fields are non-uniform

Part 3 of 9Point charges

Isolated positive charge

Isolated positive charge radial lines pointing outwards, equally spaced and getting further apart with distance, matching the 1/r² fall in E

Isolated negative charge

Isolated negative charge radial lines pointing inwards

Comparison with gravity

Comparison with gravity the pattern for a negative charge resembles gravitational lines around a mass, but electric lines can point either way

Part 4 of 9Charged conducting sphere

Charge distribution

Charge distribution repulsion spreads the charges evenly over the surface

Outside field

Outside field it is the same as if all the charge were at the centre, so the sphere can be treated as a point charge

Perpendicular lines

Perpendicular lines field lines meet the surface at 90°
A parallel component would move surface charges until it fell to zero.

Inside

Inside the field is zero at all points, because forces on a test charge cancel

Part 5 of 9Pairs of charges

Pairs of charges

Two opposite charges (dipole)

Two opposite charges (dipole) curved lines run from the positive to the negative charge, densest between them where the field is strong
The lines connect the surfaces of the charges, representing attraction.
Bringing the charges closer strengthens the attraction.

Two like charges

Two like charges lines curve away from each other and do not connect the surfaces, representing repulsion
Midway between equal like charges is a neutral point where E = 0.

Point charge and plate

Point charge and plate lines resemble those between opposite charges and become parallel where they meet the plate

Part 6 of 9Parallel plates

Pattern

Pattern straight, parallel, equally spaced lines run from the positive plate to the negative plate

Edges

Edges the lines bulge outwards ("fringing"), and the field beyond the edges is non-uniform

Dependence on p.d.

Dependence on p.d. a larger potential difference makes the lines denser, the field stronger and the force on a test charge greater

Spacing

Spacing in a uniform field the lines stay equally spaced, with the spacing set by the field strength

Part 7 of 9Field lines and the force on charges

Direction

Direction a positive charge feels a force along the tangent in the direction of the arrow and a negative charge feels the opposite

Not trajectories

Not trajectories in a curved field a charge released from rest starts along the tangent but is not constrained to follow the line

Uniform field

Uniform field a charge released from rest follows a straight line
A charge entering with sideways velocity follows a parabola.

Magnitude comparison

Magnitude comparison where the lines are twice as dense, the field is twice as strong

Part 8 of 9Interpreting sketches

Sign of a charge

Sign of a charge lines leaving a body mean positive and lines arriving mean negative

Size of a charge

Size of a charge the larger charge has more lines starting or ending on it

Strongest field

Strongest field it is where lines are closest
Near a sharp point the lines crowd together, which explains discharge at points.

Hollow charged conductor

Hollow charged conductor the interior has no lines, so the field is zero
A car or aircraft protects occupants from lightning for this reason.

Gravitational lines

Gravitational lines the same rules apply, but gravity has only inward lines

Magnetic field lines

Magnetic field lines they form closed loops

Part 9 of 9More from the specification

1Coulomb's Law and Electric Permittivity

Coulomb's law states that the electrostatic force between two point charges Q₁ and Q₂ separated by distance r is given by F = (1 / 4πε)·(Q₁·Q₂ / r²),
where ε is the electric permittivity of the surrounding material.
- (ε₀ for free space or air)

2Electric Field Lines and Force Vectors

Electric field lines originate on positive charges and terminate on negative charges, indicating the direction of force on a positive test charge;
- the density of lines represents field strength,
which means closer line separation corresponds to stronger electrostatic field intensity.

3Comparing Electric and Magnetic Field Patterns

While electric field lines begin on positive and end on negative charges, magnetic field lines from a current or permanent magnet form closed continuous loops directed from north to south externally.
- (or dissipate to Earth upon grounding contact)

4W = mg

What the symbols mean: W = weight (N) · m = mass (kg) · g = gravitational field strength (9.81 N kg⁻¹ on Earth)

5Permittivity

A property of a medium affecting the electric force within it.

6Coulomb

Charge passing a point when a current of 1 A flows for 1 s.

7Current

Flow of electrons in a circuit.

8Relative permittivity

Ratio of the permittivity of a material to that of free space.

Key terms

Term	Meaning
Field line	Line whose tangent shows the direction of the force on a positive charge.
Dipole	Pair of equal and opposite charges.
Neutral point	A point where the resultant field is zero.
Fringing	Bulging of field lines at the edges of parallel plates.
Radial field	Field lines pointing away from, or towards, a point.

Avoid these mistakes

Drawing lines that cross or that start in empty space.

Showing arrows pointing away from a negative charge.

Believing that charges always move along field lines.

Thinking that no lines in a region means no field there.

Drawing lines that meet a conductor's surface at an angle other than 90°.

Last updated: 15 October 2026