Magnetic Effects of Electric Current · Lesson 5 of 8
Magnetic Field due to a Current in a Solenoid
“A long coil behaves so much like a bar magnet that even the field lines do a double take.”
• Describe the construction of a solenoid. • Compare the magnetic-field pattern of a solenoid with that of a bar magnet. • Explain why the field inside a long solenoid is nearly uniform. • Determine the north and south ends of a current-carrying solenoid. • Explain how a solenoid and soft-iron core form an electromagnet. • Distinguish a current-carrying solenoid from the electromagnet formed with a magnetic core.
A single current-carrying loop produces a useful magnetic field near its centre. Placing many such loops close together along a common axis makes their fields reinforce throughout a long central region. The resulting cylindrical coil is called a solenoid, and its overall field pattern closely resembles that of a bar magnet.
A coil consisting of many closely wound circular turns of insulated copper wire arranged in the shape of a cylinder.
The wire is insulated so neighbouring turns do not make direct electrical contact. Current follows the full length of the winding, and every turn contributes a magnetic field. Because the turns carry current consistently around the same axis, their fields reinforce inside the coil. Outside, many contributions partially cancel or spread out, while the combined pattern forms closed curves from one end to the other.
One end of a current-carrying solenoid behaves as a magnetic north pole and the other as a magnetic south pole. Outside the solenoid, field lines emerge from the north end and curve towards the south end. Inside, they run from the south end towards the north end. The complete lines are closed, just as they are around a bar magnet.
The field lines inside a long solenoid are nearly straight, parallel and equally spaced. Straight parallel lines indicate the same field direction, while equal spacing indicates approximately the same strength. The magnetic field is therefore nearly uniform throughout the central interior region. Close to the ends, the lines spread and bend, so the field there is less uniform.
A magnetic field that has the same direction and magnitude at every point in the region being considered.
| Bar magnet | Current-carrying solenoid |
|---|---|
| Has fixed north and south poles | Has north and south ends determined by current direction |
| Field lines emerge from north and enter south outside | Field lines follow the same outside direction |
| Field exists without an electric circuit | Field exists while current flows |
| Strength is not controlled by a switch | Field can be changed by changing current or turns |
| Internal structure is magnetised material | Field is produced by many current-carrying turns |
View one end of the solenoid and inspect the current around that face. If current appears anticlockwise, that face behaves as a north pole. If current appears clockwise, that face behaves as a south pole. This follows from the right-hand rule: curled fingers follow current and the thumb points towards the solenoid’s north end.
The direction of current controls the polarity. If current is reversed, every turn produces the opposite field direction, so the north and south ends interchange. The basic shape of the magnetic-field pattern remains the same, but all field-line arrows reverse.
A strong field inside a solenoid can magnetise a piece of magnetic material placed in the coil. Soft iron is especially suitable because it becomes strongly magnetised while current flows and loses most of that magnetism when current stops. The current-carrying coil and its magnetised core together form an electromagnet.
A magnet produced by passing current through an insulated coil wound around a magnetic core such as soft iron.
A solenoid is the coil itself, whether or not a core is inserted. An electromagnet is the controllable magnetic arrangement formed when the solenoid’s field magnetises the core. Its strength can be increased by increasing the current, using more turns or using an appropriate soft-iron core. It can be switched off by breaking the circuit, which is a major practical advantage.
| Feature | Ordinary solenoid | Electromagnet |
|---|---|---|
| Essential parts | Current-carrying cylindrical coil | Current-carrying coil with magnetic core |
| Field source | Current in the turns | Current in turns plus magnetised core |
| Typical strength | Useful but lower without a core | Much stronger with soft iron |
| Control | Changed by current and turns | Can be switched and strength can be adjusted |
| Typical use | Producing a controlled field | Lifting, switching or attracting magnetic material |
Problem
Viewed from its left end, current around a solenoid is anticlockwise. Identify that end.
- 1.Curl the fingers of the right hand anticlockwise, following the current.
- 2.The thumb points out through the viewed left end.
- 3.Field lines emerge from a magnetic north pole.
- 4.Therefore the left end is the north pole and the opposite end is the south pole.
Problem
A compass is placed in the central region inside a long solenoid. How will its direction and deflection change as it is moved along the axis without approaching the ends?
- 1.Inside the central region, field lines are nearly parallel, so their direction remains the same along the axis.
- 2.Their equal spacing represents nearly constant field strength.
- 3.The compass therefore keeps approximately the same orientation and comparable deflection through this region.
- 4.Near an end, the field becomes less uniform and the response can change.
Problem
A current-carrying solenoid has a soft-iron rod inserted. The current is then reversed without changing its magnitude. Predict the main changes.
- 1.Reversing current reverses the magnetic field produced by every turn.
- 2.The solenoid’s north and south ends interchange.
- 3.The soft-iron core becomes magnetised in the new direction, so the electromagnet’s poles also interchange.
- 4.If the current magnitude and number of turns stay unchanged, the field strength is approximately unchanged while direction reverses.
Parallel field lines do not mean the field is absent; they represent a common direction. A solenoid does not need a core to produce a field, but adding soft iron creates a much stronger electromagnet. Reversing current swaps poles rather than destroying the field pattern.
Quiz
Which description defines a solenoid?
What do parallel equally spaced lines inside a long solenoid represent?
A solenoid face carries anticlockwise current as viewed from that face. It behaves as
What is the effect of reversing current through a solenoid?
Why is soft iron placed inside a solenoid?
Practice Problems
- Explain why a solenoid resembles a bar magnet. Answer: Its external field lines emerge from a north end and enter a south end, while inside they return from south to north. The complete pattern is similar to that of a bar magnet.
- What evidence in a field-line drawing shows that the field inside a solenoid is uniform? Answer: The lines are straight, parallel and equally spaced, indicating the same direction and nearly the same strength throughout the central region.
- Viewed from one end, current is clockwise. Identify the pole and the internal field direction. Answer: The viewed end is south. Inside the solenoid, the field runs from this south end towards the north end at the opposite side.
- Differentiate a solenoid and an electromagnet. Answer: A solenoid is a cylindrical current-carrying coil. When its field magnetises a core such as soft iron, the coil-core arrangement forms a stronger controllable electromagnet.
- Predict the effects of increasing the number of turns while keeping length and current suitably comparable. Answer: More turns contribute reinforcing fields, so the magnetic field becomes stronger. The pole directions remain unchanged if current circulates in the same sense.
Key Takeaways
• A solenoid is a cylindrical coil of many closely wound insulated turns. • Its magnetic-field pattern resembles that of a bar magnet. • The field inside a long solenoid is nearly uniform and is represented by parallel equally spaced lines. • Current direction determines which end is north and which is south. • Reversing current interchanges the solenoid’s poles. • A soft-iron core placed inside a current-carrying solenoid forms a strong electromagnet. • An electromagnet can be switched and adjusted by controlling the circuit.