Physics • Year 12 • Module 6: Electromagnetism • Lesson 6
Electric vs Magnetic Fields — Integrated Comparison
Lock in the key vocabulary, the velocity selector formula v = E/B, and the core rules for how each field type acts on a charged particle before tackling harder problems.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: velocity selector, crossed fields, mass spectrometer, electric force, magnetic force, work done by a field, parabolic path, circular path, velocity selector condition, mass-to-charge ratio. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | A region where perpendicular electric and magnetic fields act simultaneously on a moving charged particle, allowing only those at a specific speed to pass through undeflected. | |
| 1.2 | Electric and magnetic fields oriented perpendicular to each other and to the initial particle velocity. | |
| 1.3 | A device that uses a velocity selector followed by a uniform magnetic field to separate ions by their mass-to-charge ratio. | |
| 1.4 | The force on a charged particle due to an electric field: F = qE; it acts on stationary or moving charges. | |
| 1.5 | The force on a charged particle due to a magnetic field: F = qvB sinθ; it acts only on moving charges. | |
| 1.6 | The energy transferred to or from a particle by a force acting along its direction of motion; the magnetic force never does this to a charged particle. | |
| 1.7 | The curved trajectory followed by a charged particle moving perpendicular to a uniform electric field (analogous to projectile motion). | |
| 1.8 | The closed curved trajectory followed by a charged particle moving perpendicular to a uniform magnetic field at constant speed. | |
| 1.9 | The equation v = E/B, stating the unique particle speed for which the electric and magnetic forces balance exactly in a crossed-fields device. | |
| 1.10 | The quantity m/q, measured in kg C−1, which determines the radius of a particle’s circular path in a uniform magnetic field after velocity selection. |
2. True or false — with correction
Circle T or F for each statement. If the statement is false, write the corrected version on the line below it. 12 marks (1 T/F + 1 correction each)
2.1 A stationary charged particle in a magnetic field experiences a magnetic force. T / F
2.2 A magnetic force on a moving charged particle can increase the particle’s kinetic energy. T / F
2.3 In a velocity selector, the condition v = E/B applies only to positively charged particles, not to electrons. T / F
2.4 A charged particle moving parallel to a magnetic field (angle = 0°) experiences zero magnetic force. T / F
2.5 When a proton enters perpendicular to a uniform electric field, its path is a circle. T / F
2.6 In a mass spectrometer, heavier ions follow a larger circular radius than lighter ions of the same charge after passing through the velocity selector. T / F
3. Fill-in-the-blank paragraph
Use the word bank to complete the passage. Each word or phrase is used once. 8 marks (1 per blank)
Word bank:
perpendicular · parabola · circle · E/B · qE · qvB · mass · speed
Electric fields exert a force of F = ___________ on all charges regardless of their motion. Magnetic fields exert a force of F = ___________ only on charges that are moving. The magnetic force is always ___________ to the velocity, so it changes the particle’s direction but never its ___________. When a charge moves perpendicular to a uniform electric field, it follows a ___________; when it moves perpendicular to a uniform magnetic field, it follows a ___________. In a velocity selector, the electric and magnetic forces cancel when v = ___________. After the velocity selector, ions enter a pure magnetic field where heavier ions (larger ___________) curve through a larger radius.
4. Function recall
Answer each question in 1–2 sentences using precise terms from the lesson. 8 marks (2 each)
4.1 Why does a magnetic field do no work on a moving charged particle?
4.2 What is the function of the velocity selector stage in a mass spectrometer?
4.3 Why is the velocity selector condition v = E/B independent of the sign and magnitude of the particle’s charge?
4.4 What physical quantity determines whether a particle’s path in a combined E and B field will curve upward, downward, or remain straight?
5. Build a concept map
Draw labelled arrows between the six terms below to show how they are connected. Each arrow must carry a linking phrase (e.g. “acts on”, “can change”, “is derived from”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: electric field (E) · magnetic field (B) · moving charge · velocity selector · kinetic energy · mass spectrometer.
6. Complete the field comparison table
Fill in each empty cell. Use the lesson’s Side-by-Side Comparison card as a reference. 6 marks (1 each)
| Property | Electric field | Magnetic field |
|---|---|---|
| Acts on stationary charges? | ||
| Can change particle’s speed? | ||
| Shape of path (field perpendicular to v) |
Q1 — Term–definition match
1.1 velocity selector • 1.2 crossed fields • 1.3 mass spectrometer • 1.4 electric force • 1.5 magnetic force • 1.6 work done by a field • 1.7 parabolic path • 1.8 circular path • 1.9 velocity selector condition • 1.10 mass-to-charge ratio.
Q2 — True / false with correction
2.1 False. A stationary charge in a magnetic field experiences zero magnetic force. The magnetic force formula is F = qvB sinθ; with v = 0, F = 0.
2.2 False. The magnetic force is always perpendicular to the particle’s velocity, so it does no work and cannot change kinetic energy. It can only change the direction of motion, not the speed.
2.3 False. The velocity selector condition v = E/B is independent of charge sign and mass. When charge sign is reversed, both the electric force and the magnetic force reverse direction simultaneously, so they still cancel at the same speed.
2.4 True. F = qvB sinθ; at θ = 0°, sin 0 = 0, so F = 0 regardless of the particle’s speed or charge.
2.5 False. A proton moving perpendicular to a uniform electric field follows a parabolic path, analogous to a projectile in a gravitational field. A circular path occurs in a uniform magnetic field.
2.6 True. After velocity selection, all ions have the same speed v. In a uniform magnetic field the radius is r = mv/(qB); with v, q, and B constant, r ∝ m, so heavier ions have larger radii.
Q3 — Cloze paragraph
In order: qE / qvB / perpendicular / speed / parabola / circle / E/B / mass.
Q4.1 — Why B-field does no work
Work is defined as W = F cosθ × d; the magnetic force is always perpendicular (θ = 90°) to the particle’s velocity, so the component of force along the direction of displacement is zero. Therefore the magnetic force does no work and cannot change the particle’s kinetic energy or speed.
Q4.2 — Function of velocity selector in mass spectrometer
The velocity selector ensures that only ions with exactly the speed v = E/B pass through the exit slit undeflected. This means all ions entering the mass-analysis stage have the same known speed, so differences in radius of curvature in the magnetic field reflect differences in mass-to-charge ratio only, not differences in speed.
Q4.3 — Why v = E/B is charge-independent
When q is cancelled from both sides of qE = qvB, the result is v = E/B, which contains no q term. This means charge sign and magnitude do not affect the balance condition. If charge sign is reversed, both forces reverse simultaneously, so they still cancel at v = E/B.
Q4.4 — What determines direction of deflection
The particle’s speed relative to the selector speed vsel = E/B determines the direction. If v > E/B the magnetic force dominates, deflecting the particle in the direction of the magnetic force. If v < E/B the electric force dominates, deflecting the particle in the direction of the electric force. At v = E/B the particle travels straight.
Q5 — Sample concept map
Correct maps should include arrows such as:
- electric field (E) — acts on → moving charge (and stationary charges)
- magnetic field (B) — acts only on → moving charge
- electric field (E) — can change → kinetic energy
- magnetic field (B) — cannot change → kinetic energy
- electric field (E) and magnetic field (B) — combined create → velocity selector
- velocity selector — is the second stage of → mass spectrometer
Award 1 mark per valid labelled arrow with a linking phrase (minimum 6).
Q6 — Field comparison table
Acts on stationary charges? Electric field: Yes • Magnetic field: No (only on moving charges).
Can change particle’s speed? Electric field: Yes (does work, changes KE) • Magnetic field: No (force perpendicular to v, no work done).
Shape of path (field perpendicular to v): Electric field: Parabola • Magnetic field: Circle.