Scatter iron filings around a magnet and they snap into graceful curves, you are seeing a magnetic field made visible. Field lines are the language physicists use to describe where a field points and how strong it is, and the same simple rules let you sketch the field of a bar magnet, a straight wire, or a loop of current.
You sprinkle iron filings on paper over a bar magnet and they line up into curved patterns.
Before reading: where are the filings packed most closely together, and what does that tell you about the field there? Write your prediction and a one-sentence reason.
Direction, strength and the bar magnet pattern
A field line is a map: which way it points tells you the field's direction, and how tightly the lines are packed tells you how strong the field is.
Magnetic field lines are continuous, they form complete closed loops and never cross. The arrow on a line shows the direction of the field (the way a free north pole would be pushed); outside a magnet the lines run from the north pole to the south pole, and they continue inside the magnet to close the loop. Where the lines are close together the field is strong; where they spread out, it is weak. That is why iron filings cluster most densely at the poles.
Magnetic field lines are continuous closed loops that never cross. Their arrows show the field direction (N to S outside a magnet) and their spacing shows the strength, closely packed lines mean a stronger field.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: magnetic field lines are strongest where they are spaced furthest apart.
Circular field lines and the right-hand grip rule
We just saw the field of a permanent magnet. That raises a question: can a current make a magnetic field too? This card answers it → yes, a current-carrying wire is wrapped in circular field lines, with their direction set by the right-hand grip rule.
A current produces a magnetic field that circles around the wire, and the right hand gives you both the shape and the direction at once.
A current-carrying wire creates a magnetic field made of concentric circles centred on the wire, lying in planes perpendicular to it. To find the direction, use the right-hand grip rule: point your right thumb in the direction of the conventional current, and your curled fingers show the way the field circles the wire. The circles are packed most tightly close to the wire, so the field is strongest there and weakens with distance.
A vertical wire carries conventional current straight upward. Using the right-hand grip rule, in which direction does the magnetic field point at a place directly in front of the wire (between you and it)?
A straight current-carrying wire is surrounded by circular magnetic field lines in planes perpendicular to it. The right-hand grip rule (thumb along the conventional current, fingers curl with the field) gives their direction; the field is strongest close to the wire.
Add the highlighted rule to your notes before the check below.
Quick check: The magnetic field around a long straight current-carrying wire is best described as:
Pattern B, Construct then justify
In your book, draw the magnetic field lines for: (1) a bar magnet, with arrows from N to S and lines closest at the poles; (2) a straight wire carrying current out of the page (use the right-hand grip rule to show the circle direction); (3) a single current-carrying loop, showing the field lines threading through the centre. For each, write one sentence saying how you decided the direction and where the field is strongest.
Odd one out: three of these statements about magnetic field lines are correct. Which one is not?
Dot point: Identify that magnetic field lines are continuous and used to represent the strength and direction of magnetic fields
Bending a wire into a loop concentrates the field
We just learned the field around a straight wire. That raises a question: what happens if we bend the wire into a loop? This card answers it → the circular fields add together through the centre, making a loop behave like a tiny magnet.
Bending the wire into a loop forces all its circular fields to pass the same way through the middle, so they reinforce and the loop gains a north and a south face.
When a straight wire is bent into a loop, the circular field lines around every part of the wire all pass through the centre of the loop in the same direction, so they add together. The result is a strong, fairly uniform field through the middle of the loop and a weaker field spreading out around the edges, the same shape as a short bar magnet. One face of the loop acts as a north pole and the other as a south pole; the right-hand grip rule (fingers following the current around the loop, thumb pointing through the centre) tells you which face is which. Stacking many loops into a coil makes this effect much stronger, the idea behind the solenoid you meet soon.
Bending a current-carrying wire into a loop makes its circular fields add through the centre, giving a concentrated field and a north and south face like a small bar magnet. The right-hand grip rule (fingers follow the current, thumb gives the field through the loop) sets the direction.
Pause, copy the highlighted loop result into your book before the check below.
Last check: Decide if the statement is true or false.
In a single current-carrying loop, the circular fields from each part of the wire add together through the centre, so one face acts as a north pole and the other as a south pole.
Match the pairs: Match each current arrangement to the shape of its magnetic field.
Dot point: use field line diagrams to analyse the magnetic field around a bar magnet, current-carrying wire and current-carrying loop
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 4(4 marks) 1. Explain what the direction and spacing of magnetic field lines represent, and describe the field pattern of a bar magnet and of a straight current-carrying wire.
1 mark: the arrow direction shows the field direction (N to S outside a magnet) · 1 mark: line spacing shows strength, closer lines mean a stronger field · 1 mark: bar magnet field lines loop continuously from N to S, closest at the poles · 1 mark: a straight wire has concentric circular field lines around it (direction from the right-hand grip rule)
ExplainBand 4(3 marks) 2. Explain what is meant by saying magnetic field lines are continuous, and describe how field-line spacing and direction convey information about the field.
1 mark: magnetic field lines form closed loops with no start or end, passing through the magnet · 1 mark: the direction of the line gives the field direction at that point (north-seeking pole direction) · 1 mark: closer spacing means a stronger field
Dot point: Identify that magnetic field lines are continuous and used to represent the strength and direction of magnetic fields
ApplyBand 5(3 marks) 3. A vertical wire carries a conventional current directly upwards. Use the right-hand grip rule to determine the direction of the magnetic field at a point due north of the wire, and describe the overall shape of the field around the wire.
1 mark: applies the grip rule with the right thumb pointing up along the current · 1 mark: fingers curl anticlockwise viewed from above · 1 mark: at a point due north the field points due west, and the field forms concentric horizontal circles around the wire
Dot point: use field line diagrams to analyse the magnetic field around a bar magnet, current-carrying wire and current-carrying loop
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): The direction of a magnetic field line (its arrow) shows the direction of the magnetic field at that point, the way a free north pole would be pushed, which outside a magnet runs from the north pole to the south pole. The spacing of the lines represents the strength of the field: where the lines are close together the field is strong, and where they spread apart it is weak. A bar magnet's field lines are continuous closed loops that emerge from the north pole, curve around to the south pole, and are most closely packed at the poles where the field is strongest. A straight current-carrying wire is surrounded by concentric circular field lines lying in planes perpendicular to the wire; their direction is given by the right-hand grip rule (thumb along the conventional current, fingers curling in the direction of the field), and the field is strongest close to the wire.
Q2 (3 marks): Magnetic field lines are continuous in the sense that they form complete closed loops. Unlike electric field lines, which begin on positive charge and end on negative charge, a magnetic field line has no beginning and no end: outside a bar magnet the lines run from the north pole round to the south pole, and they continue through the interior of the magnet from south back to north, closing the loop. This reflects the fact that isolated magnetic poles are not observed. The direction of a line at any point gives the direction of the magnetic field there, which is the direction a small compass needle's north-seeking pole would point. The spacing carries the strength: where the lines are crowded close together the field is strong, and where they spread apart the field is weak, which is why the field is strongest near the poles of a bar magnet.
Q3 (3 marks): Point the right thumb upwards, along the direction of the conventional current. The fingers then curl around the wire, and viewed from above they curl anticlockwise. At a point due north of the wire, the anticlockwise circulation is heading towards the west, so the magnetic field there points due west. The field overall consists of concentric circles lying in horizontal planes, centred on the wire and perpendicular to it. The circles are closer together near the wire and spread out further away, showing that the field weakens with distance from the wire. On the opposite side of the wire, due south, the same circulation means the field points due east, which is consistent with a single set of closed circular loops.
You can now read and draw field lines for a bar magnet, a straight wire and a loop, and use the right-hand grip rule. Next lesson makes the wire's field quantitative with $B = \mu_0 I / 2\pi r$.
↩ Back to focus-area hubAt the start you predicted where the iron filings cluster. The answer: most densely at the poles, where the field lines are closest together and the field is strongest. If you thought the field was uniform everywhere, revisit Card 1, line spacing is the visual code for field strength.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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