What the Simulation Shows – Electric fields
The electric fields simulation has five interactive modes, each demonstrating a different configuration of electric field lines using physically accurate Coulomb’s law calculations.
＋ Single Positive Charge — a single orange charge sits at the centre of the canvas. Field lines radiate symmetrically outward in all directions, with arrows confirming the direction of the field away from the positive charge. Moving the mouse over the canvas shows a gold vector arrow at the cursor position indicating the exact local field direction, and the strength readout updates from “very weak” far from the charge to “very strong” close to it.
－ Single Negative Charge — field lines converge inward toward the blue charge from all directions. The lines are seeded from a ring around the charge and traced inward, with arrows pointing toward the charge, showing that a negative charge acts as a sink for the electric field.
⊕⊖ Dipole — a positive and a negative charge sit on either side of the canvas. Field lines leave the positive charge and curve around to terminate on the negative charge, producing the classic dipole pattern. No field lines pass through or beyond the negative charge — they all terminate there, demonstrating that unlike charges attract field lines between them.
⫿ Parallel Plates — a positive plate on the left and a negative plate on the right produce animated field lines that travel horizontally from left to right — from the positive plate to the negative plate — parallel and evenly spaced in the region between the plates. Arrowheads animate along the lines to show direction. Fringe field lines bow outward at the top and bottom edges, showing the non-uniform field at the plate edges. The label “UNIFORM FIELD →” confirms the key property of this configuration.
✦ Place Charges — an open canvas where students tap or click to place positive charges and right-click or long-press to place negative charges anywhere they choose. The combined electric field of all placed charges is calculated and drawn in real time using superposition, letting students experiment with any configuration.
Across all modes, a gold vector arrow follows the mouse and shows the instantaneous field direction at the cursor, giving a point-by-point feel for how the field varies across space.
Suggested Classroom Activity
Topic: Electric fields — field lines, direction, strength and superposition Level: A-Level Physics (Year 12/13) or advanced GCSE Duration: 25–30 minutes Group size: Pairs
Starter (5 min)
Draw a positive charge on the board and ask: “If I placed a small positive test charge at different points around this charge — which way would it be pushed each time?” Take four or five answers for different positions. Establish that the direction changes depending on where the test charge is, and that field lines are a way of mapping all those directions at once. Tell students the simulation calculates this mathematically for every point on the screen.
Guided Exploration (15 min)
Part A — Single charges (5 min) Switch between Single + and Single −. For each one, observe the field line pattern and write one sentence describing the direction, then hover the mouse close to the charge and far away and record what happens to the strength readout.
Answer: Why does the field get weaker further away? What law does this follow? (Inverse square law — field strength ∝ 1/r²)
Part B — Dipole (4 min) Switch to Dipole mode. Hover the mouse along the horizontal line between the two charges and observe the gold arrow. Then hover above and below the midpoint.
Answer these questions:
- Between the charges, which way does the field point?
- At the midpoint between the charges, what direction does the arrow show and why?
- Where on the canvas does the field appear strongest?
Part C — Parallel Plates (4 min) Switch to Parallel Plates. Observe the field lines between the plates and compare them to the dipole pattern.
Answer: What is different about the spacing and direction of lines between the plates compared to the dipole? What does this tell you about the strength of the field across the gap?
Key point to draw out: the lines are parallel and equally spaced between the plates — this means the field is uniform in strength and direction, unlike the radial fields around point charges.
Part D — Place Charges (2 min) Place two positive charges side by side, then a positive and a negative charge. Observe how the field pattern changes. Then try placing three or more charges in different configurations.
Results Table
| Configuration | Field direction | Field uniform? | Lines terminate on a charge? |
|---|---|---|---|
| Single + | |||
| Single − | |||
| Dipole | |||
| Parallel Plates (between) | |||
| Two + charges |
Key Discussion Questions
- Why do field lines never cross each other? What would it mean physically if they did?
- The field between parallel plates is uniform — why is this useful in technology such as cathode ray tubes or capacitors?
- How does the mouse vector arrow in the dipole mode change as you move from near the + charge to near the − charge?
- In the Place Charges mode, what happens to the field pattern when you place equal and opposite charges very close together versus far apart?
Plenary (5 min)
Ask students to use the Place Charges mode to recreate one of the four standard configurations from memory — single positive, single negative, dipole, or parallel plate equivalent — and describe the resulting field pattern to their partner using the words: radiate, converge, uniform, terminate, strength, inverse square.
Real-World Links to Mention
- Parallel plate capacitors in circuits store energy in the uniform electric field between the plates — this is the basis of RAM memory, camera flash circuits, and defibrillators
- The electron gun in old CRT televisions accelerated electrons using a strong uniform electric field between plates — exactly the parallel plate configuration in the simulation
- Lightning rods work because electric field lines concentrate at sharp points — high field strength at the tip triggers discharge safely to the ground
- Electrocardiogram (ECG) machines measure the tiny electric field produced by the heart’s electrical activity, mapping it across the body in real time