Magnetism¶
Syllabus mapping
PY-11-03 — Electricity and Magnetism. Covers: magnetic field lines around a bar magnet, a current-carrying wire, and a current-carrying loop; field strength around a current-carrying wire; solenoid structure and field; field strength inside a solenoid; investigating factors affecting electromagnet strength.
Wording above is paraphrased for teaching use — cross-check against the official syllabus PDF (see Resources) before treating any line as verbatim NESA text. Note: this page covers the static magnetic fields produced by steady currents — motors, generators, transformers and electromagnetic induction are Year 12 content.
You need to know: how to sketch magnetic field lines for a bar magnet, a straight current-carrying wire, and a solenoid, how to apply the right-hand grip rule, and how to calculate field strength around a wire and inside a solenoid.
Assumed knowledge check¶
Before this page makes sense, you should be comfortable with:
- Electric Circuits — current
Core¶
Magnetic field lines¶
🖼️ Diagram needed: field lines for a bar magnet, a straight current-carrying wire (concentric circles), and a current-carrying loop, all with direction correctly shown. TODO — placeholder until sourced/drawn.
A bar magnet's field lines emerge from the north pole and curve around into the south pole (outside the magnet). A current-carrying wire produces a magnetic field in concentric circles around itself. A current-carrying loop produces a field that looks like a bar magnet's — strong and roughly uniform through the centre of the loop, curving back around outside it.
Right-hand grip rule (straight wire): point your thumb in the direction of conventional current flow — your curled fingers show the direction of the magnetic field circling the wire.
Field strength around a current-carrying wire¶
where \(\mu_0\) is the magnetic permeability constant (\(4\pi\times10^{-7}\ \text{N A}^{-2}\), from your data sheet) and \(r\) is the distance from the wire.
Worked example
Find the magnetic field strength \(0.02\ \text{m}\) from a wire carrying \(5\ \text{A}\).
Solenoids¶
A solenoid is a coil of wire, wound with \(N\) turns over a length \(L\). Current through the coil produces a field that's strong and largely uniform inside the solenoid (much like a bar magnet's), and comparatively weak outside it.
Worked example
A solenoid has \(400\) turns over a length of \(0.25\ \text{m}\), carrying a current of \(2\ \text{A}\). Find the field strength inside it.
Electromagnets¶
An electromagnet is essentially a solenoid built to exploit this effect deliberately — and its strength depends on more than just \(N\), \(I\) and \(L\) from the formula above. Adding a ferromagnetic core (commonly soft iron) inside the coil dramatically increases the field strength, because the core material itself becomes magnetised and adds its own field on top of the coil's. This is a genuine research point (secondary-source investigation) rather than something derivable from the solenoid formula alone: different core materials produce meaningfully different field strengths for the same coil and current, and that's an empirical, materials-science question.
Practical¶
Suggested prac: factors affecting electromagnet strength — build a simple electromagnet (coil around a nail or core rod) and investigate how its strength (measured e.g. by how many paperclips it can lift, or with a field sensor) changes with number of turns, current, and core material. TODO — link this school's actual prac instructions once written.
Advanced
Superposition of magnetic fields — when a point is influenced by more than one current-carrying wire, calculate each wire's field contribution separately using \(B=\dfrac{\mu_0I}{2\pi r}\), then combine them as vectors (magnitude and direction — use the right-hand rule for each wire individually) using the same component method used throughout this course.
Combining wire and solenoid problems — e.g. finding the number of turns needed for a solenoid to match a target field strength, or comparing the field just outside a solenoid's end to the field of an equivalent straight wire.
Extension
Beyond the Physics 11–12 syllabus — won't appear in the HSC, included for interest / depth study inspiration.
📎 Depth study idea
Everything on this page is a static picture — steady currents producing steady magnetic fields. The genuinely interesting physics happens when that picture starts changing: a moving magnet, or a changing current, induces a voltage in a nearby conductor (electromagnetic induction) — the principle behind generators, transformers, and (run in reverse) motors. That's Year 12 content, not covered here, but a depth study now could research the real-world electromagnet applications this static picture already supports and does explain properly — MRI machines (extremely strong, precisely controlled electromagnets), maglev trains, industrial scrap-metal lifting magnets, or particle accelerators — as a bridge between "here's the formula" and "here's why anyone builds one of these."
Video/visual resources¶
- 🖥️ PhET Simulation — Magnet and Compass / Magnets and Electromagnets — TODO: confirm current PhET link
- 🎥 Khan Academy — TODO: source a magnetic fields explainer
- 🎥 Physics High — TODO: check for a NSW-syllabus-aligned magnetism video
Check yourself¶
-
Find the magnetic field strength \(0.05\ \text{m}\) from a wire carrying \(10\ \text{A}\).
Answer
\(B = \dfrac{\mu_0I}{2\pi r} = \dfrac{(4\pi\times10^{-7})(10)}{2\pi(0.05)} = 4\times10^{-5}\ \text{T}\)
-
A solenoid with \(250\) turns over \(0.2\ \text{m}\) needs a field strength of \(3\times10^{-3}\ \text{T}\) inside it. Find the required current.
Answer
\(B = \dfrac{\mu_0NI}{L} \implies I = \dfrac{BL}{\mu_0N} = \dfrac{(3\times10^{-3})(0.2)}{(4\pi\times10^{-7})(250)} \approx 1.91\ \text{A}\)
-
State the right-hand grip rule, and use it to describe the magnetic field direction around a vertical wire carrying current straight upward.
Answer
Point your right thumb in the direction of conventional current (upward, in this case); your curled fingers show the field direction. For a vertical wire with current flowing upward, the field circles the wire horizontally, anticlockwise when viewed from above.