Quick answer: A solenoid is a long coil of many closely wound turns of insulated copper wire. When current flows through it, the field lines inside are parallel straight lines, which shows the magnetic field is uniform (the same at all points) inside. Outside, the pattern looks just like that of a bar magnet: one end acts as a north pole and the other as a south pole. The field becomes stronger with more turns, more current and a soft iron core, which turns the solenoid into an electromagnet.
What is a solenoid?
A solenoid is a coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. The insulation (usually enamel) stops neighbouring turns from touching, so the current has to travel round every turn.
Each turn behaves like a small current-carrying loop. Because the turns are close together, their magnetic fields add up and reinforce each other inside the coil.
Turns of insulated wire
( ( ( ( ( ( ( ( ( ( )
| |
+----[ battery ]----+
Magnetic field pattern of a solenoid
| Region | Field lines | What it means |
|---|---|---|
| Inside the solenoid | Parallel, straight, equally spaced | Field is uniform (same everywhere inside) |
| Outside the solenoid | Curve from one end to the other | Pattern is like a bar magnet's |
| Direction inside | From the S end to the N end | Lines form closed loops |
| Direction outside | From the N end to the S end | Same as a bar magnet |
Inside a long solenoid the field is strong and uniform. Outside it is much weaker and spreads out, just like the field around a bar magnet. The field lines are continuous closed loops: they leave the north end, go round outside, enter the south end and run through the inside back to the north end.
Key point: Parallel, straight, equally spaced field lines always mean a uniform magnetic field.
Finding the poles: the clock rule
Look straight at one end of the solenoid and note the direction in which the current flows round that face.
| Current at the face you look at | That end is a |
|---|---|
| Clockwise | South pole (S) |
| Anticlockwise | North pole (N) |
A memory aid: draw arrows on the ends of the letters S and N. The arrows on S run clockwise and those on N run anticlockwise.
You can check the same result with the right-hand thumb rule: curl the fingers of your right hand in the direction of the current in the turns, and your thumb points towards the north pole.
Reversing the current reverses the poles.
Factors that affect the strength of the field
Step 1: Increase the number of turns in the coil. Each turn adds its own field, so more turns give a stronger field.
Step 2: Increase the current. A larger current through the same coil gives a stronger field.
Step 3: Place a soft iron core inside the coil. The iron becomes strongly magnetised by the solenoid's field and adds to it, so the field becomes many times stronger.
For a long solenoid, the field inside is proportional to the number of turns per unit length and to the current. (The exact formula is studied in Class 12.)
Electromagnets
A solenoid with a soft iron core is an electromagnet. It is a magnet only while current flows.
Soft iron is used because it magnetises easily and loses its magnetism as soon as the current is switched off. Steel would keep some magnetism, which is why steel is used for permanent magnets instead.
Electromagnets are used in electric bells, relays, cranes that lift scrap iron and loudspeakers.
Solenoid (electromagnet) vs bar magnet
| Feature | Electromagnet | Bar magnet |
|---|---|---|
| Type of magnetism | Temporary: only while current flows | Permanent |
| Strength | Can be changed by changing current or turns | Fixed |
| Poles | Reversed by reversing the current | Fixed |
| Switching off | Can be switched off instantly | Cannot be switched off |
Summary table
| Idea | Key fact |
|---|---|
| Solenoid | Long coil of many closely wound insulated turns |
| Field inside | Uniform: parallel straight lines |
| Field outside | Like a bar magnet |
| Anticlockwise current at a face | That end is N |
| Clockwise current at a face | That end is S |
| Stronger field | More turns, more current, soft iron core |
| Electromagnet | Solenoid with a soft iron core |
Exam tips
- When asked what the parallel field lines inside a solenoid show, write: "The magnetic field is the same at all points inside the solenoid, so it is uniform."
- Draw field lines as closed loops with arrows: N to S outside and S to N inside.
- When naming the poles, state which end you are looking at and the direction of the current there.
Common mistakes
- Saying the field inside a solenoid is zero or weak. It is strongest and uniform inside.
- Drawing field lines that cross. Field lines never intersect.
- Using steel for an electromagnet's core. Steel keeps its magnetism; soft iron does not.
- Forgetting that reversing the current also reverses the poles.
Frequently Asked Questions
What is a solenoid?
A solenoid is a long coil of many closely spaced turns of insulated copper wire wound in the shape of a cylinder.
Why is the magnetic field inside a solenoid uniform?
Inside the coil the fields of all the turns add up along the axis, giving parallel, equally spaced field lines. This shows the field has the same strength and direction everywhere inside.
How do you find the north pole of a solenoid?
Look at one end. If the current flows anticlockwise at that face, the end is a north pole; if it flows clockwise, it is a south pole.
How can the magnetic field of a solenoid be made stronger?
Increase the number of turns, increase the current, or put a soft iron core inside the coil.
Why is soft iron used in an electromagnet?
Soft iron is magnetised strongly while current flows and loses its magnetism almost completely when the current stops, so the magnet can be switched on and off.