18.1.2Electric fields and capacitance (A Level)
Electric Field Lines
5 min read 9 parts Lesson 4 of 19 1 quiz
Lesson summary
18.1.2 Electric Field Lines — AS PHYSICS 9702 notes: Draw and interpret electric field lines for point charges, pairs of charges and parallel plates.Arrow — gives the direction of the force on a positive charge, which is the direction of the fieldStart and end — lines start on po…
- Lines never cross
- At conductors
- Positive point charge or sphere
- Negative charge
- Falling strength
- Charged sphere
Section 1 of 9Rules for field lines
Arrow
- Arrow gives the direction of the force on a positive charge, which is the direction of the field
Start and end
- Start and end lines start on positive charges (or at infinity) and end on negative charges (or at infinity)
Tangent
- Tangent the tangent to a line at a point gives the direction of the field there
Strength
- Strength closer lines mean a stronger field and wider spacing means a weaker field
Lines never cross
- Lines never cross the field at a point has only one direction
At conductors
- At conductors lines meet charged conducting surfaces at right angles
Section 2 of 9Point charges and spheres
Point charges and spheres
Positive point charge or sphere
- Positive point charge or sphere radial lines pointing outwards, like spokes of a wheel
Negative charge
- Negative charge radial lines pointing inwards
Falling strength
- Falling strength lines spread out with distance and the field falls as 1/r²
Charged sphere
- Charged sphere outside it the pattern equals that of a point charge of the same total charge at its centre
Comparison with gravity
- Comparison with gravity gravitational lines look like those of a negative charge because gravity is only attractive
Section 3 of 9Pairs of charges
Opposite equal charges
- Opposite equal charges lines leave the positive charge and curve round to enter the negative charge
- Between them the lines are close and nearly parallel, so the field is strong
Like charges
- Like charges lines bend away from each other
Neutral point
- Neutral point halfway between equal like charges the fields cancel and E = 0, with no lines passing through
Unequal charges
- Unequal charges the larger charge has more lines, and the neutral point lies closer to the smaller charge
Section 4 of 9Parallel plates and uniform fields
Parallel plates and uniform fields
Between the plates
- Between the plates straight, parallel, equally spaced lines from the positive to the negative plate, perpendicular to both
Uniform field
- Uniform field the same magnitude and direction at every point between the plates
Edges
- Edges lines bulge outwards where the field is not uniform
Outside the plates
- Outside the plates the field is very weak
Point charge near a plate
- Point charge near a plate the lines resemble those of two opposite charges and become parallel at the plate
Section 5 of 9Using field lines
Using field lines
Force on a charge
- Force on a charge find the line direction at that point, then push a positive charge along the line and a negative one against it
Comparing strengths
- Comparing strengths compare the spacing of the lines
Comparing charges
- Comparing charges the body with twice as many lines has twice the charge
Three dimensions
- Three dimensions lines represent a 3D field, where density falls with distance faster than the picture shows
Comparison with magnetic fields
- Comparison with magnetic fields both use line density for strength and conventions for direction
Section 6 of 9Sharp points and drawing rules
Irregular conductors
- Irregular conductors the field is strongest where the surface is most sharply curved
- Lightning conductors have sharp tips and sparks jump from pointed objects first
Drawing
- Drawing include at least five lines, put an arrow on each, touch the surface of the source charge or plate, and never let lines touch or cross
Section 7 of 9More from the specification
1Vector field lines and Coulomb forces
- Vector field lines and Coulomb forces tangential tangents along electric field lines define the directional electrostatic forces experienced by positive test charges.
2Stationary
- Not moving.
Direction
- Direction a field line shows the direction of the force on a small positive test charge.
Strength
- Strength where the lines are closer together the field is stronger.
Ends
- Ends lines start on positive charges and end on negative charges.
No crossing
- No crossing field lines never cross each other.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Field line | the path a small positive test charge would follow in a field |
| Radial field | a field with lines pointing towards or away from a point |
| Uniform field | a field with parallel equally spaced lines |
| Neutral point | a point where the resultant field is zero |
| Dipole | two equal and opposite charges close together |
| Fringing | the bulging of field lines at the edges of parallel plates |
Section 9 of 9Exam technique
1
Letting field lines cross — the field has only one direction at a point.
2
Leaving out the arrows — always label the direction.
3
Drawing lines that do not meet conducting surfaces at right angles.
4
Drawing lines starting or stopping in empty space — they start on positive charges and end on negative charges.