Magnetic fields and materials - TCE Physics (Tasmania)

Syllabus dot point

“Describe magnetic fields from magnets and currents and represent them with field lines.”

TCEPhysicsUnit 3: Gravity and Electromagnetism9 min readUpdated 2 Jun 2026

Quick answer

Magnetic fields come from moving charges, including the electrons in magnetic materials. Field lines run from north to south outside a magnet and never cross. A current in a wire makes a circular field (right-hand grip rule), a solenoid makes a field like a bar magnet, and Earth itself has a magnetic field with a measurable angle of dip.

What this dot point is asking

This dot point describes where magnetic fields come from and how to represent them, the groundwork for the motor effect and induction.

Poles and field lines

Every magnet has two poles, north and south. Like poles repel and unlike poles attract. A magnetic field is represented by field lines that, by convention, point from the north pole to the south pole outside the magnet and continue from south to north inside it, forming closed loops. The lines never cross, and where they bunch together the field is stronger.

A compass needle is a small magnet that aligns with the field, so the direction a compass north points defines the field direction at that spot.

Magnetic materials

At a deeper level, magnetism comes from the motion of charges, specifically the spin and orbital motion of electrons. In most materials these effects cancel, but in ferromagnetic materials like iron, regions called domains can align to give a net field.

A permanent magnet keeps its alignment, producing a steady field.
A temporary magnet, such as soft iron, becomes magnetised only while in another field and loses it afterward.
An electromagnet is a coil of wire whose magnetism can be switched on and off with the current, and strengthened with an iron core.

Magnetic field

A magnetic field is a region in which a moving charge or a magnet experiences a magnetic force. Its direction at a point is the direction in which the north pole of a compass needle points, and its strength is measured in tesla.

Fields from currents

A straight current-carrying wire produces a circular magnetic field around it, with strength $B = \dfrac{\mu_0 I}{2\pi r}$ that falls off with distance $r$ from the wire. The right-hand grip rule gives the direction: point your right thumb along the conventional current and your curled fingers show the field looping around the wire.

A coil or solenoid carrying current produces a field very like a bar magnet, with a north and south pole at its ends, of strength $B = \mu_0 n I$ along the axis, where $n$ is the number of turns per metre. A second right-hand rule applies: curl your fingers in the direction of the current around the loops and your thumb points to the solenoid's north pole. Adding an iron core concentrates the field, making a strong electromagnet used in motors, relays and lifting magnets.

Moving charge is the single source of magnetism

There are no isolated magnetic poles. Every magnetic field, whether from a current or a permanent magnet, ultimately arises from moving electric charge. In a bar magnet that motion is the aligned electrons inside the iron.

Adding fields from several wires

Because the field at a point is a vector, the total field from two or more wires is the vector sum of each wire's contribution. Find each magnitude with $B = \dfrac{\mu_0 I}{2\pi r}$ using its own distance, work out each direction with the grip rule, then add them as vectors. Between two parallel wires carrying current in the same direction the fields partly cancel, which is linked to the attractive force the wires exert on each other.

Earth's magnetic field

Earth behaves like a giant bar magnet tilted slightly from its spin axis. A compass aligns horizontally with this field, but a needle free to rotate vertically also tilts downward; the angle below the horizontal is the angle of dip. The dip is near zero at the equator and close to vertical near the magnetic poles, which tells us the field has a vertical component that varies with latitude.

Worked example: field direction around a wire

Apply the grip rule

A vertical wire carries conventional current upward. Point the right thumb up along the current; the curled fingers show the field looping around the wire.

Read off the direction at the point

North of the wire the curled fingers point toward the west, so the field at a point due north is directed toward the west. Its size at $r = 0.10\ \text{m}$ for $I = 4.0\ \text{A}$ would be $B = \dfrac{(4\pi\times10^{-7})(4.0)}{2\pi(0.10)} = 8.0 \times 10^{-6}\ \text{T}$ .

Treating magnetic field lines as starting and stopping

Unlike electric field lines, magnetic field lines have no start or end; they always form closed loops, continuing through the inside of the magnet. Drawing them stopping at the poles is a common error. Show them as complete loops to reflect that there are no magnetic monopoles.

In the exam, use the right-hand grip rule for the field around a wire and the coil version for a solenoid. Add fields from several wires as vectors, and when describing Earth's field mention both the horizontal compass direction and the angle of dip, which shows the field is genuinely three dimensional.

Exam-style questions

Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.

Practice4 marks

Two parallel wires

5.00\ \text{cm}

apart carry currents of

5.00\ \text{A}

and

10.00\ \text{A}

. At point P, located

4.00\ \text{cm}

from the

5.00\ \text{A}

wire and

3.00\ \text{cm}

from the

10.00\ \text{A}

wire, calculate the magnetic flux density

B_1

due to the

5.00\ \text{A}

wire and

B_2

due to the

10.00\ \text{A}

wire.

Show worked answer →

The field around a long straight wire is $B = \dfrac{\mu_0 I}{2\pi r}$ , with $\mu_0 = 4\pi \times 10^{-7}\ \text{T m A}^{-1}$ .

From the $5.00\ \text{A}$ wire at $r = 0.04\ \text{m}$ :

B_1 = \frac{(4\pi\times10^{-7})(5.00)}{2\pi(0.04)} = \frac{(2\times10^{-7})(5.00)}{0.04} = 2.50 \times 10^{-5}\ \text{T}.

From the $10.00\ \text{A}$ wire at $r = 0.03\ \text{m}$ :

B_2 = \frac{(4\pi\times10^{-7})(10.00)}{2\pi(0.03)} = \frac{(2\times10^{-7})(10.00)}{0.03} = 6.67 \times 10^{-5}\ \text{T}.

The directions are tangent to circles around each wire (right-hand grip rule) and must be added as vectors for the total field. Markers want $B = \dfrac{\mu_0 I}{2\pi r}$ applied with the correct distance for each wire.

Practice2 marks

A vertical wire carrying a current of

5.00\ \text{A}

upwards is placed at the edge of a table. Calculate the magnetic flux density due to the current at a point P located

20\ \text{cm}

north of the wire, and state its direction.

Show worked answer →

Use the field of a long straight wire: $B = \dfrac{\mu_0 I}{2\pi r}$ .

B = \frac{(4\pi\times10^{-7})(5.00)}{2\pi(0.20)} = \frac{(2\times10^{-7})(5.00)}{0.20} = 5.0 \times 10^{-6}\ \text{T}.

The flux density is $5.0 \times 10^{-6}\ \text{T}$ (that is $5.0\ \mu\text{T}$ ). By the right-hand grip rule, with the current upward and P to the north, the field at P points west. Markers want the straight-wire formula and the field direction from the grip rule.

Practice2 marks

In a science museum an aluminium plate is levitated over a large coil carrying

800\ \text{A}

of alternating current at

800\ \text{Hz}

. Describe and explain what would happen if direct current (rather than alternating current) was used.

Show worked answer →

With direct current the coil produces a steady, unchanging magnetic field. A steady field does not change the magnetic flux through the aluminium plate, so no EMF is induced and no eddy currents flow in the plate (Faraday's law requires a changing flux).

Without induced eddy currents there is no opposing magnetic field in the plate, so there is no repulsive force to support it. The plate would not levitate; it would fall or rest on the coil.

Markers want: DC gives a constant field, no changing flux, no induced eddy currents, therefore no levitating force.

Practise this

Exam-style questions 3 on this page
Flashcards Physics deck

Sources & how we know this

Physics Level 4 (PHY415115) course document (2015 onwards) — TASC