The shape of the lesson
By the end of the lesson, learners can
- Reading a field pattern: Switch between a single charge, a dipole, like charges and parallel plates.
- From field to force: The field strength E = F/q is defined using a positive test charge, so the force on a positive charge is along the field and on a negative charge is opposite to it.
- Uniform field: E = V / d; the force on a charge is F = qE, and a charge crossing the field follows a parabola.
- Point charges: Coulomb's law F = Q₁Q₂ / (4πε₀r²) and field E = Q / (4πε₀r²) are both inverse-square.
- Potential: V = Q / (4πε₀r) falls off as 1/r; the work to move charge q is W = qΔV, and E = −dV/dr.
One picture every idea returns to
An electric field is a region where a charge feels a force. The field strength is the force per unit positive charge, E = F / q, so the force on a charge is F = qE. Field lines run from positive to negative charges.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Mapping the field.. An electric field is invisible, but field lines make its shape and strength visible. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Reading a field pattern | Switch between a single charge, a dipole, like charges and parallel plates. Project tab 1 of the lab (Fields and field lines) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: From field to force | The field strength E = F/q is defined using a positive test charge, so the force on a positive charge is along the field and on a negative charge is opposite to it. | Jigsaw |
| 30 to 43 min | Check and plenary | The quick-check questions from the student lesson, answered on mini whiteboards one at a time; re-teach the one most get wrong, then a random call on the last. | Individual, then whole class |
| 43 to 45 min | Exit and homework | Set the worksheet below (2 questions, 22 marks) from the topic practice set. Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in From field to force, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Jigsaw
Split the content into expert pieces. Expert groups master one piece each, then regroup so every home team has one expert per piece, and each expert teaches their piece to the team.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
No examiner trap is recorded for this lesson. Use the quiz key below to spot the distractor most of the class picks.
Electric fields and field lines: topic worksheet
2 questions, 22 marks. Written in the style of the papers; no past paper question is reproduced. These questions cover the whole topic, so sibling lessons share them.
This question is about electric field strength and field lines.
(a) Define electric field strength. [2]
(b) State the equation relating force, charge and field strength. [1]
(c) A charge of +2.0 μC experiences a force of 0.30 N. Calculate the field strength. [2]
(d) State the unit of electric field strength in two equivalent forms. [1]
(e) Describe the field lines of (i) a uniform field and (ii) the radial field of a positive point charge. [2]
(f) State what the direction of a field line shows and what the spacing of the lines shows. [2]
Two point charges, A = +5.0 nC and B = −5.0 nC, are fixed 8.0 cm apart in a vacuum. P is the midpoint, as shown in Fig. 10.1. This question uses field strength, Coulomb’s law, the field of a point charge, and potential.
(a) Define electric field strength. [1]
(b) Calculate the force between A and B. [2]
(c) Calculate the field strength at P due to each charge, and the resultant field at P. [4]
(d) Calculate the electric potential at P. [3]
(e) State what your answer to (d) implies about the work done in bringing a charge to P from infinity. [2]
Answer key and marking notes
- (a)The force per unit positive charge at a point ✓✓
- (b)F = qE ✓
- (c)E = F/q = 0.30 / 2.0 × 10⁻⁶ ✓ = 1.5 × 10⁵ N C⁻¹ ✓
- (d)N C⁻¹ or V m⁻¹ ✓
- (e)Uniform: parallel, equally spaced lines ✓; radial: straight lines directed outward from the charge ✓
- (f)The direction shows the force on a positive charge ✓; the spacing shows the field strength (closer lines mean a stronger field) ✓
- (a)The force per unit positive charge at a point ✓
- (b)F = 8.99 × 10⁹ × (5.0 × 10⁻⁹)²/(0.080)² ✓ = 3.5 × 10⁻⁵ N (attractive) ✓
- (c)Each at r = 0.040 m: E = 8.99 × 10⁹ × 5.0 × 10⁻⁹/(0.040)² = 2.8 × 10⁴ V m⁻¹ ✓✓; at P both fields point from A toward B, so they add: resultant = 5.6 × 10⁴ V m⁻¹ ✓✓
- (d)V = ΣQ/(4πε₀r) = 8.99 × 10⁹ × (5.0 × 10⁻⁹/0.040 − 5.0 × 10⁻⁹/0.040) ✓✓ = 0 V ✓
- (e)No net work is needed, since the potential at P is zero ✓✓
The lesson's own checks as a quiz
The quick-check and exam-style questions from the student lesson, printable with the key.
The electric field strength at a point is defined as:
- Athe force per unit positive charge
- Bthe force per unit mass
- Cthe charge per unit force
- Dthe work per unit charge
The force on a charge q placed in a field of strength E is:
- AF = qE
- BF = E/q
- CF = q/E
- DF = qE²
Electric field lines:
- Astart on positive charges and end on negative charges
- Bstart on negative charges
- Cform closed loops
- Dalways point upward
Where field lines are drawn closer together, the field is:
- Astronger
- Bweaker
- Czero
- Dreversed
Quiz key
E = F/q: the force experienced per unit positive charge placed at the point.
Rearranging E = F/q gives F = qE.
By convention field lines leave positive charges and enter negative ones, in the direction of the force on a positive charge.
The density of field lines represents the field strength: closer lines mean a stronger field.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.