Chapter 4 · Magnetic Effects of Electric Current

4.2 Magnetic Field due to a Current: Wire, Loop and Solenoid

Every electric current is surrounded by a magnetic field: circles round a straight wire, a concentrated field through a loop, and inside a solenoid a uniform field that makes the coil behave like a bar magnet — an electromagnet you can switch on and off.

Press the button of an electric bell and a hammer hammers; switch on the crane in a scrapyard and a tonne of steel leaps up to a plate; at a station in Shanghai a train floats above its track. None of these machines contains a permanent magnet doing the work. Each uses a coil of copper wire, and the magnetism appears only when a current flows through it — and vanishes the moment the current stops.

This is the discovery Ørsted made in 1820: an electric current produces a magnetic field. In this section we map that field for the three shapes of wire that matter most — a straight wire, a circular loop and a long coil called a solenoid — learn the right-hand rule that gives the direction of the field in each case, and see how a solenoid wound on soft iron becomes an electromagnet.

The Magnetic Field of a Straight Wire

Stretch a straight wire north–south directly above a compass needle and close the switch: the needle swings to one side. Reverse the current and it swings the other way; open the switch and it returns to north. The current has made a magnetic field, and the direction of that field depends on the direction of the current.

To see the shape of the field, pass a thick straight wire vertically through a hole in a horizontal sheet of card, sprinkle iron filings on the card and switch on a large current. Tapped gently, the filings arrange themselves in concentric circles centred on the wire. A small compass placed on any circle sets along it, and when the current is reversed every compass turns round.

📐 Result 4.3 — The magnetic field of a straight current-carrying wire

The field lines are concentric circles centred on the wire, lying in planes perpendicular to it.
Reversing the current reverses the direction of the field lines.
The field at a given point is directly proportional to the current $I$: double the current, double the field.
The field becomes weaker farther from the wire: it is inversely proportional to the distance $r$ from the wire, so the circles are drawn farther apart as $r$ increases.

Together, $B\propto\dfrac{I}{r}$, so that $\dfrac{B_2}{B_1}=\dfrac{I_2}{I_1}\times\dfrac{r_1}{r_2}$.

📐 Law 4.4 — The right-hand thumb rule

Imagine holding a current-carrying straight wire in your right hand with the thumb pointing in the direction of the current. Then your fingers curl round the wire in the direction of the magnetic field lines. (The same rule is sometimes stated as Maxwell's corkscrew rule: a right-handed screw driven forward in the direction of the current turns in the direction of the field.)

Two pictures make the rule easy to use. Looking at the end of a wire with the current coming towards you (drawn ⊙, the point of an arrow), the field lines go anticlockwise. With the current going away from you (drawn ⊗, the tail feathers of an arrow), they go clockwise.

Left: looking down on a card with a vertical wire through it (⊙ current up, out of the card; ⊗ current down). The blue circles are field lines — drawn closer together where the field is stronger — and the arrows run along them faster where the field is strong. Eight compasses sit on a circle round the wire; the red one is the probe at distance $r$. Right: the right-hand thumb rule for the same current. Change the current (negative = downwards) and the probe distance; switch on the Earth's field to see what a real compass does far from the wire. Play runs a short tour.

current $I$:3.0 Aprobe $r$:2.0 cm Earth's field

current—

field lines (from above)—

field at probe—

probe needle points—

✏️ Example 1 — A vertical wire seen from above

A vertical wire carries a current upwards. Looking down from above, in which sense do the field lines circle the wire? Which way does a compass needle placed just east of the wire point? (Ignore the Earth's field.)

💡 Show solution▶

Rule Right thumb pointing up (along the current): seen from above, the fingers curl anticlockwise. Equivalently, the current comes towards the viewer (⊙), so the lines are anticlockwise.

Compass On an anticlockwise circle, at the point due east of the centre the circle is heading north. The needle's north end points north.

Check Set $I=+3\ \text{A}$ in Interactive 1 and look at the compass on the right of the wire. ✓

✏️ Example 2 — A wire above a compass

A horizontal wire runs north–south a short distance above a compass. A current flows through it from south to north. Which way does the north end of the needle turn? What happens if the wire is placed below the compass instead?

💡 Show solution▶

Rule Right thumb pointing north along the wire. The fingers curl over the top of the wire and come back under it. Under the wire they point west.

Needle The compass is below the wire, where the wire's field points west, so the north end turns towards the west. (It does not turn all the way: the Earth's field still pulls it north, so it settles between north and west.)

Below Above the wire the curling fingers point east, so with the wire under the compass the needle turns east instead.

Tip Reversing the current or moving the wire to the other side of the compass reverses the deflection; doing both leaves it unchanged.

✏️ Example 3 — How the field depends on $I$ and $r$

The magnetic field at $4.0\ \text{cm}$ from a long straight wire carrying $3.0\ \text{A}$ is $B_0$. Find the field (a) at $2.0\ \text{cm}$ from the wire when the current is $6.0\ \text{A}$; (b) at $12\ \text{cm}$ from the wire when the current is $1.5\ \text{A}$.

💡 Show solution▶

Rule $B\propto\dfrac Ir$, so $\dfrac{B_2}{B_0}=\dfrac{I_2}{I_1}\times\dfrac{r_1}{r_2}$.

(a) $\dfrac{B_2}{B_0}=\dfrac{6.0}{3.0}\times\dfrac{4.0}{2.0}=2\times2=4$, so $B_2=4B_0$.

(b) $\dfrac{B_2}{B_0}=\dfrac{1.5}{3.0}\times\dfrac{4.0}{12}=\dfrac12\times\dfrac13=\dfrac16$, so $B_2=\dfrac{B_0}{6}$.

Check More current and closer in both make the field stronger, so (a) must exceed $B_0$ and (b) must be smaller. ✓

The Field of a Circular Loop

Now bend the wire into a circle and pass a current round it. Every short piece of the loop is a little straight wire with its own circles of field round it. Near the wire the circles are small and clearly circles; farther in, they grow larger, and at the centre of the loop the arcs from all round the loop have become almost straight lines. The important point is that inside the loop every piece of the wire sends its field the same way — use the thumb rule on the top and the bottom of the loop and you will find the fingers point in the same direction through the middle. So the fields add up, and the field at the centre is strong.

📐 Result 4.5 — The magnetic field of a circular coil

Inside the loop the field lines all pass through in the same direction; near the centre they are nearly straight and parallel to the axis of the loop.
The field at the centre is directly proportional to the current $I$ and, for a coil of $n$ turns all carrying the current the same way round, directly proportional to $n$: each turn adds the same field. It is larger for a smaller radius. In short, at the centre $B\propto\dfrac{nI}{r}$.
Direction (right-hand rule for a loop): curl the fingers of your right hand round the loop in the direction of the current; your thumb points in the direction of the field through the centre.
Which face is which pole (clock rule): looking at one face of the coil, if the current flows anticlockwise that face behaves as a north pole; if it flows clockwise the face is a south pole.

✏️ Example 4 — A loop lying on a table

A circular loop of wire lies flat on a table. Seen from above, the current flows clockwise. (a) In which direction is the field at the centre of the loop? (b) Which face of the loop — the upper or the lower — behaves as a north pole?

💡 Show solution▶

(a) Curl the fingers of the right hand clockwise (as seen from above): the thumb points downwards, into the table. The field at the centre points vertically down.

(b) From above the current is clockwise, so the upper face is a south pole. From below, the same current looks anticlockwise, so the lower face is the north pole.

Check Field lines leave a north pole: they come out of the lower face, pointing down — the same as (a). ✓

✏️ Example 5 — Turns and current

Coil P has $50$ turns and carries $2.0\ \text{A}$. Coil Q has the same radius, $100$ turns and carries $0.50\ \text{A}$. Which has the stronger field at its centre, and by what factor?

💡 Show solution▶

Rule Same radius, so $B\propto nI$.

Working P: $nI=50\times2.0=100$; Q: $nI=100\times0.50=50$. $\dfrac{B_P}{B_Q}=\dfrac{100}{50}=2$.

Answer Coil P's field is $2$ times as strong. Doubling the turns does not make up for quartering the current.

The Solenoid and the Electromagnet

Wind many turns of insulated copper wire closely, side by side, on a cylinder — a pencil, a drinking straw, a cardboard tube. Each turn is a circular loop, and their fields add. Iron filings sprinkled round such a coil show a pattern you have seen before: exactly the pattern of a bar magnet. One end of the coil behaves as a north pole and the other as a south pole, and inside the coil the field lines run straight and parallel.

📘 Definition 4.4 — Solenoid

A solenoid is a coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder.

📐 Result 4.6 — The magnetic field of a current-carrying solenoid

Outside, the field is like that of a bar magnet: one end of the solenoid is a north pole and the other a south pole (use the clock rule or the right-hand rule for a loop on either end).
Inside, the field lines are straight, parallel and equally spaced: the field is uniform — the same strength and direction at all points inside.
The field is stronger when the current is larger and when there are more turns on each unit of length; placing a soft-iron core inside makes it very much stronger.
Reversing the current reverses the field and swaps the poles.

📘 Definition 4.5 — Electromagnet

An electromagnet is a solenoid wound on a core of soft iron. When a current flows, the field of the solenoid magnetises the core strongly; when the current is switched off, soft iron loses almost all its magnetism. (A steel core would stay magnetised — that is how permanent magnets are made.)

	Electromagnet	Permanent magnet
Magnetism	only while current flows — can be switched on and off	always present
Strength	can be changed (current, turns); can be made very strong	fixed; usually weaker
Poles	reversed by reversing the current	fixed
Core	soft iron	steel or special alloys

Electromagnets are everywhere: lifting magnets in scrapyards and steel works, electric bells and door locks, relays that let a small current switch a large one, loudspeakers, the huge solenoids of hospital MRI scanners, and the guide magnets of maglev trains.

✏️ Example 6 — Poles of a solenoid

A solenoid lies along a table with ends A (left) and B (right). Looking at end A, the current is seen flowing clockwise round the turns. (a) Which end is the north pole? (b) A compass is placed beyond end B on the axis of the solenoid. Which way does its needle point? (c) Give two ways to make the field stronger without changing the battery.

💡 Show solution▶

(a) Clockwise seen from A, so A is a south pole. Seen from B the same current looks anticlockwise, so B is the north pole.

(b) Field lines leave the N pole B and point away from it along the axis: the needle points away from the solenoid, to the right.

(c) Wind more turns on the same length; put a soft-iron core inside. (With a fixed battery, a shorter or thicker wire of lower resistance would also raise the current.)

Check Right hand: fingers curl with the current, thumb points through the coil from A to B — out of the N end. ✓

A slice through the middle of a coil: each turn cuts the page twice, as ⊙ (current coming out) at the top and ⊗ (going in) at the bottom. The blue field lines are computed from the currents and spaced so that crowded lines mean a strong field. Change the shape, the number of turns and the current; add a soft-iron core. The compass slides along the axis — watch it inside and outside the coil. Play moves the compass and tours the settings.

shape:turns:8current:1.5 A soft-iron corecompass:

current, seen from the right—

north pole—

field at the centre—

compass needle points—

⚠️ Common mistakes

1. Using the left hand. The thumb rule is a right-hand rule. (Fleming's left-hand rule, in Section 4.3, is for the force on a wire — a different question.)

2. Thumb and fingers swapped. For a straight wire the thumb is the current and the fingers the field. For a loop or solenoid it is the other way round: fingers follow the current round the turns, thumb gives the field through the middle (the N end).

3. "The field lines of a straight wire point away from it." They are circles round the wire; they never start or end on it.

4. Distance and field. Doubling the distance from a straight wire halves the field (not a quarter): $B\propto\frac1r$.

5. Steel instead of soft iron. An electromagnet needs a core that loses its magnetism when the current stops. Steel keeps it — good for permanent magnets, bad for a crane that must drop its load.

🚀 Beyond the board syllabus (JEE Main · NEET · JEE Advanced) — optional: the formulas behind the proportions

In JEE Main, NEET and JEE Advanced 2026; not in the CBSE Class 10 syllabus.

With $\mu_0=4\pi\times10^{-7}\ \text{T m/A}$ (the permeability of free space), the fields in tesla are:

$$B_{\text{wire}}=\frac{\mu_0 I}{2\pi r}$$ $$B_{\text{centre of coil}}=\frac{\mu_0 nI}{2r}$$ $$B_{\text{inside solenoid}}=\mu_0\,n'I$$

where $n'$ is the number of turns per metre of the solenoid. For example, $10\ \text{A}$ in a long wire gives $B=\frac{4\pi\times10^{-7}\times10}{2\pi\times0.050}=4.0\times10^{-5}\ \text{T}$ at $5.0\ \text{cm}$ — about the same as the Earth's field, which is why the compass under a wire swings so clearly. A solenoid with $1000$ turns per metre carrying $2.0\ \text{A}$ has $B=4\pi\times10^{-7}\times1000\times2.0\approx2.5\times10^{-3}\ \text{T}$ inside. You will derive these in Class 12 Physics, Moving Charges and Magnetism.

Exercises

The field of a straight wire.

1 Describe the pattern of the magnetic field lines round a long straight wire carrying a current.
2 State the right-hand thumb rule.
3 A vertical wire carries a current downwards. Seen from above, do its field lines go clockwise or anticlockwise?
4 A horizontal wire runs east–west with the current flowing from west to east. What is the direction of its field at a point directly below it? Directly above it?
5 The field at $3.0\ \text{cm}$ from a wire is $B$. What is it at $6.0\ \text{cm}$, for the same current?
6 The field at $1.0\ \text{cm}$ from a wire carrying $2.0\ \text{A}$ is $B$. What is the field at $2.5\ \text{cm}$ when the current is $5.0\ \text{A}$?
7 What happens to the field round a wire when the current is reversed?
8 Why are the circles that represent the field round a wire drawn farther apart as you go outwards?
9 A wire runs north–south directly above a compass. When the current is switched on, the north end of the needle turns east. In which direction does the current flow?

Check: field ratio for a straight wire, $\dfrac{B_2}{B_1}=\dfrac{I_2}{I_1}\times\dfrac{r_1}{r_2}$

$I_1$ (A):$r_1$ (cm):$I_2$ (A):$r_2$ (cm):

current factor—

distance factor—

$B_2/B_1$—

💡 Show solutions▶

1 Concentric circles centred on the wire, in planes perpendicular to it, closer together near the wire; their direction is given by the right-hand thumb rule.

2 Hold the wire in the right hand with the thumb along the current: the curled fingers show the direction of the field lines.

3 Thumb pointing down; seen from above the fingers curl clockwise (the current goes away from the viewer, ⊗).

4 Thumb pointing east: the fingers go over the top heading south and come back underneath heading north. Below the wire: north; above it: south.

5 $B\propto\frac1r$: twice the distance, $\tfrac B2$.

6 $\tfrac{B_2}{B}=\tfrac{5.0}{2.0}\times\tfrac{1.0}{2.5}=2.5\times0.4=1$: the field is again $B$.

7 The circles stay the same but every field line reverses its direction; compasses round the wire turn through $180^\circ$.

8 The field weakens with distance ($B\propto\frac1r$), and the spacing of lines shows the strength: weaker field, wider spacing.

9 The needle is below the wire. Below the wire the field must point east. Trying thumb north gives west underneath, so the thumb points south: the current flows from north to south.

Loops and coils.

1 Sketch the field lines of a current-carrying circular loop, seen in a plane through its axis.
2 Why is the field at the centre of a loop stronger than near a single short piece of the same wire?
3 How does the field at the centre of a coil change if the number of turns is doubled, the current and radius staying the same?
4 A coil of $20$ turns carrying $1.5\ \text{A}$ has a field $B$ at its centre. What is the field at the centre of a coil of the same radius with $60$ turns carrying $0.50\ \text{A}$?
5 The radius of a coil is halved, the turns and current staying the same. How does the field at the centre change?
6 Looking at one face of a coil, you see the current flowing clockwise. Is that face a north or a south pole?
7 A loop lies flat on a table, and seen from above the current is anticlockwise. Which way does the field at the centre point?
8 A coil has a field of $12$ units at its centre when the current is $3.0\ \text{A}$. What current gives $20$ units?

💡 Show solutions▶

1 Two wire ends (⊙ at the top, ⊗ at the bottom), each with small circles close to it; between them the lines pass through the loop together, nearly straight at the centre, and curve round outside to close on themselves. (See Interactive 2, "circular coil".)

2 Every part of the loop produces a field at the centre in the same direction, so all the contributions add up.

3 It doubles: $B\propto n$.

4 $nI$: $20\times1.5=30$ and $60\times0.50=30$. Equal, so the field is again $B$.

5 It doubles: $B\propto\frac1r$ at the centre.

6 A south pole (clock rule: clockwise = S).

7 Fingers curl anticlockwise (as seen from above): the thumb points upwards, out of the table.

8 $B\propto I$: $I=3.0\times\tfrac{20}{12}=5.0\ \text{A}$.

III

Solenoids and electromagnets.

1 What is a solenoid?
2 Draw the field lines of a current-carrying solenoid. What does the pattern show?
3 What kind of field exists inside a long solenoid, and how is it drawn?
4 State three ways of increasing the strength of the field of a solenoid.
5 What is an electromagnet? Why is its core made of soft iron and not steel?
6 Give two differences between an electromagnet and a permanent magnet.
7 Looking at end P of a solenoid, the current is clockwise. Name the poles at P and at the other end, Q.
8 A solenoid of $400$ turns carrying $0.50\ \text{A}$ has a field $B$ inside. It is rewound with $200$ turns on the same length. What current gives the same field?

💡 Show solutions▶

1 A coil of many circular turns of insulated copper wire wound closely in the shape of a cylinder.

2 Straight parallel lines inside running from one end to the other; outside they curve round from the end where they emerge back to the other end, as round a bar magnet. The solenoid behaves like a bar magnet, one end N and the other S.

3 A uniform field — drawn as straight, parallel, equally spaced lines.

4 Increase the current; increase the number of turns on each unit of length; put a soft-iron core inside.

5 A solenoid with a soft-iron core. Soft iron is magnetised strongly while the current flows and loses its magnetism when the current stops; steel would stay magnetised.

6 Any two: an electromagnet can be switched on and off, a permanent magnet cannot; its strength can be changed; its poles can be reversed by reversing the current; it can be made much stronger.

7 P is a south pole (clockwise); Q is a north pole.

8 Same length, so $B\propto(\text{turns})\times I$: $400\times0.50=200=200\times I$, so $I=1.0\ \text{A}$.

Applications.

1 Why does a scrapyard crane use an electromagnet rather than a permanent magnet?
2 In an electric bell, a soft-iron armature is pulled towards an electromagnet and then springs back. Why must the core be soft iron?
3 The two wires in a mains cable carry equal currents in opposite directions, very close together. Why does such a cable hardly affect a compass placed near it?
4 A compass $2\ \text{cm}$ below a wire is deflected only slightly by a current of $1\ \text{A}$. Suggest three ways of making the deflection larger.
5 The main magnet of an MRI scanner is a large solenoid that the patient lies inside. Why is a solenoid a good shape for this?
6 Challenge. Two long parallel wires are $10\ \text{cm}$ apart. Each, on its own, gives a field $B$ at the point midway between them. What is the field at the midpoint when (a) the currents are in opposite directions; (b) the currents are in the same direction?

💡 Show solutions▶

1 It can be switched off to drop the load; its strength can be made very large and adjusted.

2 The core must lose its magnetism as soon as the circuit breaks, so that the armature is released and springs back; steel would stay magnetised and hold it.

3 The two equal and opposite currents produce nearly equal and opposite fields at any point not very close to the cable, so the fields almost cancel.

4 Increase the current; bring the wire closer to the compass; loop the wire so that several turns pass above the compass in the same direction (each adds its field).

5 The field inside a long solenoid is strong and uniform — the same everywhere in the region where the patient lies.

6 (a) Opposite currents: by the thumb rule both fields at the midpoint point the same way, so they add — $2B$. (b) Same direction: the two fields at the midpoint are equal and opposite — zero.

🎯 Exam corner — board and entrance-exam practice BoardNEETJEE Main

E1. Board · 2 marks State the right-hand thumb rule. A long straight wire is vertical and the current in it flows upwards. What is the direction of its magnetic field at a point due east of the wire?

💡 Show solution▶

Rule Hold the wire in the right hand with the thumb along the current; the curled fingers give the direction of the field lines. [1 mark]

Answer Current up: the lines are anticlockwise seen from above, so due east of the wire the field points north. [1 mark] Exam tip: draw a tiny top view with ⊙ and an anticlockwise circle — the direction can then be read off without error.

E2. Board · 3 marks Draw the pattern of magnetic field lines of a current-carrying solenoid. What does the pattern indicate? How can the solenoid be used to make an electromagnet?

💡 Show solution▶

Diagram Coil with straight, parallel, evenly spaced lines inside; outside, lines curving round from one end to the other; arrows inside running from the S end to the N end and outside from N to S; current direction marked on the turns. [1½ marks]

Meaning The solenoid behaves like a bar magnet (one end N, one end S), and the field inside is uniform. [½ mark]

Electromagnet Place a soft-iron rod inside the solenoid and pass a current: the rod is strongly magnetised while the current flows. [1 mark] Exam tip: mark N and S on your diagram and check them with the clock rule.

E3. Board · Assertion–Reason Assertion (A): The magnetic field inside a long current-carrying solenoid is uniform. Reason (R): The field lines inside a long solenoid are straight, parallel and equally spaced. (A) Both A and R are true and R explains A. (B) Both are true but R does not explain A. (C) A is true but R is false. (D) A is false but R is true.

💡 Show solution▶

Idea Both are true. Parallel lines mean the same direction, equal spacing means the same strength — which is exactly what "uniform" means, so R explains A.

Answer (A). Exam tip: when R is the field-line description of the property in A, R explains A.

E4. Board · Case study In a laboratory, a straight wire is stretched north–south $3.0\ \text{cm}$ above a compass needle, which points north when there is no current. (i) A current flows from south to north. Which way does the north end of the needle turn? Explain with a rule. [2 marks] (ii) The wire is moved to $6.0\ \text{cm}$ above the needle with the same current. How does the wire's field at the needle change? [1 mark] (iii) State one way of reversing the deflection without touching the wire. [1 mark]

💡 Show solution▶

(i) Right-hand thumb rule with the thumb pointing north: under the wire the fingers point west. The north end of the needle turns towards the west.

(ii) $B\propto\frac1r$: doubling the distance halves the field, so the deflection is smaller.

(iii) Reverse the current (swap the connections at the battery). Exam tip: moving the compass to above the wire would also reverse it — but the question says the wire stays put; read the conditions.

E5. NEET The magnetic field at a distance $r$ from a long straight wire is $B$. If the current is doubled and the distance is made $3r$, the field becomes (A) $6B$ (B) $\dfrac{3B}{2}$ (C) $\dfrac{2B}{3}$ (D) $\dfrac{2B}{9}$

💡 Show solution▶

Working $B\propto\dfrac Ir$: $\dfrac{B'}{B}=\dfrac{2I}{I}\times\dfrac{r}{3r}=\dfrac23$.

Answer (C). Exam tip: (D) comes from wrongly using $\frac{1}{r^2}$ — that is a point charge or a bar magnet, not a long wire.

E6. JEE Main A wire bent into a single circular turn has a field $B$ at its centre. The same wire, carrying the same current, is rewound into a coil of $2$ turns. The field at the centre is now (A) $B$ (B) $2B$ (C) $4B$ (D) $\dfrac B2$

💡 Show solution▶

Idea The length of wire is fixed: $2\pi r=2\times2\pi r'$, so $r'=\dfrac r2$. Also $n$ goes from $1$ to $2$.

Working $B\propto\dfrac{nI}{r}$: $\dfrac{B'}{B}=\dfrac{2}{1}\times\dfrac{r}{r/2}=4$.

Answer (C). Exam tip: when the same wire is rewound, the radius changes too — with $n$ turns, $B$ grows as $n^2$.

🔬 Research–Teaching NexusResearch-tutored · optional, about 60 minutes

Research question. How does the lifting strength of a home-made electromagnet depend on the number of turns and on the current — and is there a point where adding more stops helping?

Why it matters. Electromagnets lift scrap steel in recycling plants, sort cans from rubbish, hold fire doors open and guide maglev trains. Engineers must choose the turns and current that give enough force without wasting energy as heat in the coil. Every extra ampere costs electricity, and an iron core can only be magnetised so far before it "saturates" — beyond that, more current gives little extra strength.

What to do.

Wind $20$ turns of insulated copper wire round a large iron nail (about $10\ \text{cm}$). Connect it through a switch to one $1.5\ \text{V}$ cell; switch on only while testing (the wire warms up).
Count how many small steel paper clips the nail can lift and hold. Repeat three times and take the average.
Increase the turns to $40$, $60$ and $80$ on the same length of nail, recounting each time. Then, with $40$ turns, use one, two and three cells in series (more current).
Plot the number of clips against the number of turns, and against the number of cells. Does the graph stay a straight line?

Tools: iron nail, enamelled or insulated copper wire, $1.5\ \text{V}$ cells and holder, switch, paper clips; optionally a phone magnetometer held at a fixed distance from the end of the nail.

Hand in: A table and two graphs, a sentence on whether strength is proportional to turns and to current in your data, and one reason why the results might level off.

Going further. Repeat with an aluminium rod, a steel bolt and no core at all. Which core gives the strongest magnet, and which keeps some magnetism after the switch is opened? Link your answer to the choice of soft iron for electromagnets.

Instructor note: with a single cell, the current through a thin coil can be several amperes — keep tests short and use a cell holder rather than holding wires. More turns of the same wire also add resistance, so with a fixed battery the current falls slightly as turns are added; students who measure the current with an ammeter can separate the two effects using the product (turns × current). Clip counts rise roughly linearly at first and then flatten as the nail approaches saturation and as the clip chain itself becomes the limit — a good discussion of experimental limits. A steel bolt will retain some magnetism; aluminium behaves like no core.

🧪 Self-check quiz

0/5 correct

Q1. The magnetic field lines round a long straight current-carrying wire are

Q2. In the right-hand thumb rule for a straight wire, the thumb shows

Q3. The field at $2\ \text{cm}$ from a long straight wire is $B$. With the same current, the field at $4\ \text{cm}$ is

Q4. The magnetic field inside a long current-carrying solenoid is

Q5. Looking at one end of a solenoid, you see the current flowing anticlockwise. That end is

← 4.1 Magnets, Magnetic Fields and Field Lines 4.3 Force on a Current-Carrying Conductor and Domestic Circuits →