Magnetic field lines · the magnetic effect of current · electromagnets · Fleming's left & right hand rules
A magnet never touches the iron it pulls. Around it lies a magnetic field — a region of space where a magnetic force acts — and that field has a real, mappable shape. A compass needle is the simplest instrument that reads it. Then in 1820 Oersted found the twist that wires the modern world: an electric current makes a field too.
This page covers magnetic fields and field lines, the magnetic effect of an electric current, solenoids and electromagnets, and Fleming's two hand rules. By the end you'll be able to:
Magnetism is unusual: you meet it at ten, again at fifteen, and again at seventeen — each time with the same picture and harder maths. Class 12's Moving Charges and Magnetism is one of the highest-scoring chapters in the boards and in JEE/NEET, and it rests entirely on the field lines you first met with a bar magnet.
Searched as: magnetic field lines, magnetic effect of electric current, right hand thumb rule, Fleming's left hand rule, electromagnet, solenoid.
Pick a source, then drag the compass anywhere on the stage. The needle turns to the real field direction at that exact spot — every line here is traced from the field maths, not drawn by hand. 🟢 real field equations
Drag the compass to read the field.
Three ideas, in plain terms: what a field is, why a current makes one, and how those two facts become a motor.
A magnetic field is the region around a magnet or a current where a magnetic force acts. It is not a story we tell about the magnet — it is a real, measurable thing at every point in that space, and it has both a size and a direction. We map it with field lines, drawn to run out of the north pole, round, and into the south. Their crowding is the code for strength: tight lines mean a strong field. And they can never cross — a crossing would mean the field pointed two ways at one point, and a compass needle sitting there cannot point in two directions at once.
In 1820 Hans Christian Oersted was lecturing when he noticed a compass needle twitch as he switched on a current. It was the first hint that electricity and magnetism are one subject: every current drags a magnetic field around with it. For a straight wire that field is a set of circles, and the right-hand grip rule gives its direction — point your thumb along the current and your fingers curl the way the field goes. Coil the wire into a solenoid and those circles add up inside to make a strong, near-uniform field: a bar magnet you can switch on and off. Put iron inside and you have the electromagnet lifting cars in a scrapyard. 🟡 Class 10 core
The lab above does not draw a picture of a field — it computes one. At any point it works out the real field vector: for the wire it evaluates B = μ₀I/2πr and takes the direction from the grip rule; for the magnet and the solenoid it adds the pull of a north and a south pole. The compass needle simply reports that vector, and every line you see is traced by taking small steps along it. That is why the picture can never disagree with the physics — and why, when you switch the solenoid's current to zero, the lines do not fade politely: they vanish, because there is no longer a field to draw.
Exams want the method, not just the idea. Here is one fully worked, the way a marker wants to see it.
A long straight wire carries a current of 4 A. Find the magnetic field at a point 5 cm from the wire. (μ₀ = 4π × 10⁻⁷ T·m/A)
Sixteen microtesla — about a third of Earth's own field, produced by a wire you could hold in your hand. Move twice as far away and it halves, because B ∝ 1/r.
Exams reward the method, not just the answer. Work each line out yourself, or tap to reveal it. Every numeric step is marked, so you can check your own arithmetic as you go.
The Field Lab shows the shape. These three make the numbers behind it move — the field round a wire, the field inside a coil, and the force that turns both into a motor.
Field lines and Fleming's two hands cause more confusion than any other part of school physics. Tap a card to bust the myth.
Earth's core makes a field of about 50 µT — feeble, but everywhere. For a thousand years a floating needle reading that field was the only way to hold a course out of sight of land.
Your phone buzzes because a current sits in a magnetic field and gets shoved — F = BIL. The same sentence explains the fan overhead and the train you catch.
An MRI wraps you in a field thousands of times Earth's, nudges your protons with radio, and listens. No blade, no X-rays — just magnetism, asked politely.
Oersted's twitching compass needle in 1820 was a curiosity. Within a lifetime it was driving factories — and it now moves, stores and images almost everything.
Field strength is measured in tesla (T). The word hides how violent the range is: your compass reads millionths of a tesla, and the universe goes up to a hundred million.
A planet moves your compass with fifty millionths of a tesla — and a fridge magnet, held close, beats it a hundred times over. Distance is everything.
Magnetism is unusually kind to home builders: a battery, a nail and some wire already make a real electromagnet. The measuring is what turns a demo into a project.
Build: wind insulated wire round an iron nail and touch the ends to a 1.5 V cell. The nail picks up pins — and drops them the instant you disconnect.
Measure: pins lifted against number of turns. Graph it: the line should climb with turns, because B ∝ nI.
Build: stroke a sewing needle 40 times one way with a magnet, push it through a cork, float it in water. It swings north.
Measure: time it takes to settle, with and without a magnet nearby — you are measuring interference with a planet.
Build: paper over a bar magnet, sprinkle iron filings, tap gently. The filings queue up along the field lines.
Measure: photograph one magnet, then two facing N–N and N–S. Compare with the Field Lab's traced lines above.
Build: a cell, a neodymium disc and a bent copper wire. The wire spins the moment it closes the circuit — Fleming's left hand made of scrap.
Measure: spin rate against number of cells. More current, more force, faster spin.
Build: a flat coil taped to a paper cup, a magnet beneath, wired to a phone's headphone output. Faint, but unmistakably music.
Measure: loudness against number of coil turns — the same B, I and L, doing a job you can hear.
Build: a copper-wire coil tube; a cell with magnets on both ends slides in and rockets through.
Measure: speed against coil tightness (turns per cm) — B = μ₀nI, running down a tube on your desk.
The questions students actually type — each answer reads on its own, lifted clean off the page.
Because a crossing would mean the field pointed in two directions at the same point — and it cannot. A field line's direction is simply the way a compass north pole turns there, and a single compass cannot point two ways at once. So the lines can crowd together, curve, or run side by side, but they never intersect.
A magnetic field is the region around a magnet or an electric current where a magnetic force acts. It is not a picture we invent — it has a real size and direction at every point, which is why a compass placed anywhere in it turns to a definite angle. We draw it as field lines running out of the north pole and into the south.
Left hand is for motors, right hand is for generators. The reliable way to pick is to ask what causes what. In a motor the current and the field already exist and they produce a force, so use the LEFT hand. In a generator the motion and the field already exist and they produce a current, so use the RIGHT hand. Causes first, then the hand follows.
Every moving charge drags a magnetic field around with it, so a current-carrying wire is always surrounded by one. Oersted found this in 1820 when a compass twitched beside a live wire. Around a straight wire the field forms circles, and the right-hand grip rule gives their direction: point your thumb along the current and your fingers curl the way the field goes.
A bar magnet is permanently magnetic; an electromagnet is a coil whose magnetism exists only while a current flows. That switch is the whole point: an electromagnet can be turned on, turned off, reversed, or made stronger by raising the current or adding turns. It is why a scrapyard crane can pick up a car and then drop it.
You get two smaller magnets, each with its own north and south pole — not a separate north piece and south piece. Keep cutting and it keeps happening, all the way down to individual atoms. Isolated single poles, called magnetic monopoles, have never been found.
Use B = μ₀I/2πr, where I is the current, r is the distance from the wire and μ₀ = 4π × 10⁻⁷ T·m/A. A 4 A current gives about 16 µT at 5 cm. Note that the field falls off as 1/r, not 1/r², so doubling the distance only halves the field.
Because the Earth itself is a giant, weak magnet — its molten core generates a field of roughly 50 microtesla, and a compass needle is a small magnet free to line up with it. Strictly, the needle's north end points toward Earth's magnetic north pole, which is near, but not exactly at, the geographic North Pole.
Spot these before the exam does.
Seven question formats, the way Beyond Dictionary serves them — multiple choice, multiple-correct, fill-in-the-blank, match, sequence, read-think-connect, and write-your-own. Every question has layered hints: a quick nudge, the reasoning, then a deeper connection — so a wrong answer opens a door, never a dead end. 🟢 received from a board-tagged question bank · seed toward 2,000
Pick your board — the set re-tunes to its wording and emphasis. Competitive draws the JEE / NEET / Olympiad lane.
A magnet never touches what it moves. It only changes the space around it — and everything else, from the compass that found the world to the scanner that reads your body, is a consequence of that one quiet fact.