Physics • Year 12 • Module 6 • Lesson 12
Magnetic Flux and Changing Flux
Lock in the definition of magnetic flux, the formula Φ = BA cos θ, the weber unit, and the key principle that only changing flux induces an emf.
1. Term–definition match
The definitions below are shuffled. In the right-hand column write the matching term from this list: magnetic flux, weber, tesla, normal to the area, rate of change of flux, induced emf, magnetic field strength, three ways to change flux, stationary loop, changing flux. 10 marks (1 each)
| # | Definition | Matching term |
|---|---|---|
| 1.1 | The quantity Φ = BA cos θ that measures how much magnetic field passes through a given area. Measured in webers (Wb). | |
| 1.2 | The SI unit of magnetic flux, equal to 1 T m². Abbreviated Wb. | |
| 1.3 | The SI unit of magnetic field strength, B. 1 T = 1 kg s⁻² A⁻¹. | |
| 1.4 | The direction perpendicular to the plane of the loop, used as the reference direction when measuring the angle θ in the flux formula. | |
| 1.5 | The quantity ΔΦ/Δt — how quickly magnetic flux changes with time. Units: Wb s⁻¹ (or V). | |
| 1.6 | A voltage generated in a conductor as a result of changing magnetic flux through it. | |
| 1.7 | The symbol B — a measure of the force a magnetic field exerts per unit charge per unit velocity. | |
| 1.8 | The three methods by which the flux through a loop can change: varying the magnetic field strength B, altering the area A of the loop, or rotating the loop to change the angle θ. | |
| 1.9 | A loop held still in a constant, uniform magnetic field. No emf is induced in this configuration. | |
| 1.10 | The necessary condition for electromagnetic induction — the only scenario in which an emf can appear in a conductor. |
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 Magnetic flux through a loop is greatest when the magnetic field is parallel to the plane of the loop. T / F
2.2 When θ = 90° (field parallel to plane of loop), the flux is zero because cos 90° = 0. T / F
2.3 A strong stationary magnetic field threading a stationary loop will continuously induce a large emf. T / F
2.4 Doubling the area of a loop (keeping B and θ constant) will double the magnetic flux. T / F
2.5 Flux can be changed by rotating a loop, by strengthening the magnetic field, or by altering the loop’s area. T / F
2.6 The angle θ in Φ = BA cos θ is the angle between the magnetic field B and the plane of the loop. 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:
area · changing · cos θ · magnitude · normal · rate · stationary · webers
Magnetic flux is defined as Φ = BA ___________, where B is the magnetic field strength, A is the ___________ of the loop, and θ is the angle between B and the ___________ to the area. Flux is measured in ___________ (Wb). A ___________ loop in a constant field has constant flux and produces no induced emf. An emf is only induced when the flux is ___________. The ___________ of the induced emf depends on the ___________ of change of flux.
4. Function recall
Answer each question in 1–2 sentences using precise physics terms from the lesson. 8 marks (2 each)
4.1 What does it mean, physically, when the magnetic flux through a loop is zero?
4.2 State the single condition that must be met for an emf to be induced in a conducting loop.
4.3 In a generator, which variable in Φ = BA cos θ is continuously changed to produce an alternating emf?
4.4 State the relationship between 1 Wb and tesla and metres squared (T m²).
5. Build a concept map
Draw labelled arrows between the six terms below to show how they connect. Each arrow must carry a linking phrase (e.g. “is measured in”, “can be changed by”, “requires”). Aim for at least 6 labelled arrows. 6 marks (1 per valid labelled arrow)
Supplied terms: magnetic flux · B, A, θ · weber · changing flux · induced emf · rate of change.
Q1 — Term–definition match
1.1 magnetic flux • 1.2 weber • 1.3 tesla • 1.4 normal to the area • 1.5 rate of change of flux • 1.6 induced emf • 1.7 magnetic field strength • 1.8 three ways to change flux • 1.9 stationary loop • 1.10 changing flux.
Q2 — True / false with correction
2.1 False. Flux is greatest when the field is perpendicular to the plane of the loop (i.e., parallel to the normal), giving θ = 0° and cos 0° = 1 so Φ = BA.
2.2 True. When the field is parallel to the plane, it makes a 90° angle with the normal, so cos 90° = 0 and Φ = 0. No field lines thread the loop.
2.3 False. A stationary loop in a constant field has constant (unchanging) flux. Since the flux does not change, no emf is induced, regardless of how strong the field is.
2.4 True. Φ = BA cos θ. Doubling A while keeping B and θ constant doubles Φ.
2.5 True. These are the three ways to change Φ = BA cos θ: change B, change A, or change θ (rotate the loop).
2.6 False. The angle θ is measured between the magnetic field B and the normal to the plane of the loop, not the plane itself. When B is perpendicular to the plane, θ = 0° and flux is maximum.
Q3 — Cloze paragraph
In order: cos θ / area / normal / webers / stationary / changing / magnitude / rate.
Q4.1 — Zero flux
Zero flux means no magnetic field lines are passing through the loop. This occurs when the plane of the loop is parallel to the field (field lines skim past the loop without threading through it), i.e. θ = 90°.
Q4.2 — Condition for induced emf
The magnetic flux through the loop must be changing. A constant flux, no matter how large, produces no induced emf. It is the rate of change of flux, ΔΦ/Δt, that drives the emf.
Q4.3 — Generator variable
In a generator, the angle θ is continuously varied by rotating the coil in the magnetic field. This makes cos θ oscillate between +1 and −1, causing Φ — and hence the induced emf — to vary sinusoidally.
Q4.4 — Weber definition
1 Wb = 1 T m². The weber is derived from the definition of flux: Φ = BA (when θ = 0°), so Wb = T × m².
Q5 — Sample concept map
Correct maps should include arrows such as:
- B, A, θ — determine → magnetic flux
- magnetic flux — is measured in → weber
- changing flux — produces → induced emf
- rate of change — determines magnitude of → induced emf
- magnetic flux — when changing gives → changing flux
- rate of change — is the rate of change of → magnetic flux
Award 1 mark per valid labelled arrow (minimum 6, maximum 6 marked).