Magnet, magnetic field and compass
A magnet has two ends called poles: the north-seeking pole (N) and the south-seeking pole (S). Like poles repel and unlike poles attract. The space around a magnet where its pull or push can be felt is its magnetic field. A field has both strength and direction, so it is a vector quantity.
A compass is simply a tiny magnet (the needle) that can turn freely. Its N end points in the direction of the field at that place. With no magnet or current nearby, it points towards Earth's geographic north, because Earth itself behaves like a huge magnet.
We cannot see the field, so we draw magnetic field lines: paths along which a tiny free north pole would move. Sprinkle iron filings around a magnet and tap the card: the filings line up along these paths.
Properties of magnetic field lines
- Direction: outside a magnet they go from N to S; inside the magnet they go from S back to N.
- Closed loops: so every line is a closed curve with no start and no end.
- Never cross: if two lines crossed, a compass at that point would have to point in two directions at once, which is impossible.
- Density shows strength: where lines are close together the field is strong (near the poles); where they spread out the field is weak.
- The tangent to a field line at any point gives the field direction there.
Field around a straight current-carrying wire
In 1820 Hans Christian Oersted noticed that a compass needle turned whenever current flowed in a nearby wire. This showed that electric current produces a magnetic field: electricity and magnetism are linked.
Around a long straight wire the field lines are concentric circles centred on the wire, lying in planes at right angles to it.
Right-hand thumb rule
Hold the wire in your right hand with the thumb pointing along the current. Your curled fingers show the direction of the field lines. Reverse the current and the field reverses too, so the compass swings the other way.
What the strength depends on
The field is stronger when the current is larger and weaker as you move away from the wire. Close to the wire the circles are crowded; far away they spread out. (The strength is directly proportional to current and inversely proportional to distance.)
Field due to a circular loop (coil)
Bend the straight wire into a circle. Every small piece of the loop still makes circles of field around itself. Near the wire they look like small circles; towards the centre they get bigger and, at the centre, the lines from all parts of the loop point the same way and look almost like straight lines.
The field at the centre becomes stronger if you increase the current or the number of turns (n turns act like n times one loop). A smaller loop also gives a stronger field at its centre. Seen from one face, anticlockwise current makes that face a north pole and clockwise current makes it a south pole.
Solenoid: a coil that acts like a bar magnet
A solenoid is a long coil of many closely wound turns of insulated copper wire, shaped like a cylinder. When current flows, its field pattern is just like that of a bar magnet: one end behaves as N and the other as S.
Inside a long solenoid the field lines are straight, parallel and equally spaced. This means the field is uniform: the same strength and direction at every point inside. To find the N end, curl the fingers of your right hand along the current in the turns; the thumb points to the N end.
The field becomes stronger with (1) more turns, (2) more current and (3) a soft iron core placed inside.
Electromagnet vs permanent magnet
Put a soft iron rod inside a solenoid and pass current: the rod becomes a strong magnet called an electromagnet. Switch off the current and it loses almost all its magnetism.
| Electromagnet | Permanent magnet |
|---|---|
| Magnet only while current flows | Stays magnetised all the time |
| Strength can be changed (current, turns) | Strength is fixed |
| Poles can be reversed by reversing current | Poles are fixed |
| Core: soft iron | Made of steel, alnico, ferrite, neodymium |
| Can be made very strong | Usually weaker |
Uses of electromagnets: cranes for scrap iron, electric bells, relays, loudspeakers, motors, MRI machines and magnetic-levitation trains. Steel is used for permanent magnets because it keeps its magnetism; soft iron for electromagnets because it loses it quickly.
What comes in the board exam
Expect diagrams of field lines (bar magnet, straight wire, loop, solenoid), the right-hand thumb rule, reasons why field lines never cross, factors affecting field strength, and differences between an electromagnet and a permanent magnet. These are usually 1 to 3 mark questions; diagram questions carry marks for arrows showing direction, so always draw them.
Key formulas and definitions
- Straight wire: field strength B ∝ I and B ∝ 1/r (r = distance from wire)
- Circular loop (centre): B ∝ n I / r (n turns, radius r)
- Solenoid (inside): B ∝ n I per unit length; uniform field
- SI unit of magnetic field: tesla (T)
- Right-hand thumb rule: thumb = current, curled fingers = field
- Coil face: anticlockwise current = N face, clockwise current = S face
Worked examples
1. A compass is placed just east of a vertical wire. Current flows upward in the wire. Which way does the compass N end point? (Look from above.)
Use the right-hand thumb rule: thumb up, fingers curl anticlockwise when seen from above. On the east side, anticlockwise motion is towards the north. So the needle's N end points north (the wire's field and Earth's field agree here).
2. At 5 cm from a long straight wire the field has some value B. What is the field at 10 cm if the current is doubled at the same time?
B ∝ I/r. Doubling I doubles B, doubling r halves B. So B stays the same: 2 × ½ = 1, the field is still B.
3. A student looks at one face of a coil and sees the current flowing clockwise. What pole is this face? What if the current is reversed?
Clockwise current makes the face a South pole. Reversing the current makes it anticlockwise, so the face becomes a North pole.
4. Give two ways to make a solenoid's magnetic field stronger without changing the coil's length.
Pass more current, add more turns, or put a soft iron core inside. Any two of these.
5. Why does a long solenoid make a uniform field inside but a coil of one turn does not?
In a long solenoid the fields of many turns add up side by side. Inside, the pieces that would bend the lines cancel out, and the lines become straight, parallel and evenly spaced. A single loop is straight only near its centre and curves elsewhere.
Common mistakes
- Drawing field lines that cross or that start and stop in the air. They are always closed loops that never cross.
- Using the left hand for the thumb rule. The field direction rule for a wire is the RIGHT-hand thumb rule (Fleming's LEFT-hand rule is for the force on a wire, the next lesson).
- Saying field lines inside a bar magnet go from N to S. Inside they go from S to N; only outside do they go N to S.
- Thinking a soft iron core makes a permanent magnet. Soft iron loses its magnetism when the current stops; steel keeps it.