Learning outcomes
- Describe the field of a current-carrying solenoid.
- Explain how an iron core strengthens an electromagnet.
- State factors affecting electromagnet strength.
- Explain relays, bells and lifting magnets.
- Evaluate advantages of switchable magnetism.
3.1 Field of a solenoid
A solenoid is a long coil of insulated wire. When current passes, the circular magnetic fields from individual turns combine to produce a strong field inside the coil. The external pattern resembles that of a bar magnet, with one end acting as north and the other as south.
The field inside a long solenoid is approximately uniform because the lines are straight, parallel and closely spaced. Reversing current reverses the poles. The end polarity can be found using a right-hand grip rule: curl fingers in the direction of conventional current around the turns; the thumb points towards the solenoid’s north pole.
3.2 Strengthening an electromagnet
The magnetic field becomes stronger when current increases, when the number of turns per unit length increases, or when a soft-iron core is inserted. The iron becomes magnetised by the solenoid field and greatly increases the magnetic flux through the coil.
There are practical limits. A large current heats the wire and may damage insulation. More turns require more wire, which increases resistance and mass. Good design balances field strength, heating, power consumption and response time.
3.3 Why soft iron is used
Soft iron has high magnetic permeability, meaning it becomes strongly magnetised in an applied field. It also has low retentivity, so it loses most magnetism when the field is removed. These properties make the electromagnet strong when current flows and weak when current stops.
Steel would retain magnetism and might keep an armature or load attached after switching off. That would be unsafe in a lifting crane and would prevent a relay or electric bell from resetting properly.
3.4 Relay operation
A relay is an electrically operated switch. A small current in a coil magnetises an iron core and attracts a hinged armature. The armature opens or closes contacts in a separate circuit, allowing a low-current control circuit to operate a high-current or high-voltage load.
Relays provide electrical isolation between circuits and can control several contacts. Modern systems may use transistor switches instead, but the electromagnetic relay remains important for understanding how current produces a controllable mechanical force.
3.5 Electric bell and lifting magnet
In an electric bell, current through a coil attracts an iron armature carrying a striker. The movement breaks the contact, current stops, the electromagnet releases, and a spring restores the contact. This rapid make-and-break cycle repeats, causing the striker to hit the gong.
A scrap-yard crane uses a large electromagnet. Switching current on produces a strong field that attracts ferromagnetic scrap; switching off releases it. Control, reversibility and adjustable strength are advantages over a permanent magnet.
3.6 Design and safety
An electromagnet may overheat if current is too large or if it remains energised for too long. Coils need insulated wire, suitable current ratings and sometimes cooling. In a crane, backup systems are needed because a power failure can release the load.
When comparing devices, explain both magnetic principle and mechanical consequence. A statement such as “current makes magnetism” is incomplete unless it connects the field to attraction of an armature, switching contacts or lifting a load.
Worked examples
Increasing strength
A student doubles the turns while keeping current the same and adds a soft-iron core. Both changes increase the field. The exact increase is not necessarily a factor of two because core response and coil geometry matter.
Relay purpose
A temperature sensor provides only a small current. A relay uses this current to close contacts in a separate heater circuit carrying a much larger current.
Practical focus
Investigation
Wind insulated wire around a soft-iron nail and connect it briefly to a low-voltage supply through an ammeter and switch. Count how many paper clips are lifted for different currents or turn numbers. Change one factor at a time, allow cooling, repeat, and average.
Examination guidance
- Name the factor you change and state what is kept constant.
- A soft-iron core strengthens the field and demagnetises quickly.
- For a relay, distinguish the coil circuit from the switched circuit.
- Mention heating when evaluating large currents.
Check your understanding
- Give three ways to increase electromagnet strength.
- Why is steel unsuitable for a relay core?
- What makes an electric bell vibrate repeatedly?
Answers
- Increase current, increase turns per unit length, and insert a soft-iron core.
- It retains magnetism and may not release the armature when current stops.
- The armature repeatedly makes and breaks the coil circuit.
