6.1.8Chemical Reactions
Electromagnets — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
5 min read 5 parts Lesson 18 of 21 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Construction
An electromagnet combines a coil of wire with a soft iron core, so its magnetism is controlled by the current.
Coil
- Coil A wire wound into many turns, which concentrates the magnetic field inside the coil.
Soft iron core
- Soft iron core Magnetically soft iron is magnetised by the field while current flows, and it loses its magnetism when the current stops.
Current source
- Current source A d.c. supply produces a steady magnetic field, and the field exists only while the current is flowing.
Switching
- Switching Switching the current on and off switches the magnetism on and off, which suits switches, bells and relays.
Section 2 of 5Controlling Strength and Polarity
The strength and the direction of the poles can both be changed without moving the magnet.
1
Increasing the current through the coil increases the magnetic field strength of the electromagnet.
2
Adding more turns to the coil increases the field strength for the same current, because the field from each turn adds together.
3
Inserting a soft iron core inside the coil concentrates the field and makes the electromagnet much stronger.
4
Reversing the direction of the current reverses the poles, so the north and south poles of the electromagnet swap over.
Section 3 of 5Comparing with a Permanent Magnet
The core material and the ability to switch the magnetism are the key differences.
Comparing with a Permanent Magnet
Permanent magnet
- Permanent magnet Made from hard steel, it is always magnetic and its magnetism cannot be switched off easily.
Electromagnet
- Electromagnet Made from a coil and soft iron, it is magnetic only while current flows, and its strength can be adjusted.
Field shape
- Field shape Both produce a field with the same shape as a bar magnet's field, with a north pole and a south pole.
Uses
- Uses Electromagnets are used in electric bells, relays and cranes that lift scrap metal, where switching is needed.
| Feature | Permanent magnet | Electromagnet |
|---|---|---|
| Core material | Steel | Soft iron |
| Magnetism on or off | Always on | Switched by the current |
| Strength adjustable? | No | Yes, by current and number of turns |
Section 4 of 5Worked Example
Question Explain why a steel pin is attracted to an electromagnet only while the current is switched on.
- Recall that the soft iron core becomes magnetised only while current flows through the coil.
- A magnetic field is then present, so the steel pin is attracted to the core.
- When the current is switched off the core loses its magnetism, so the pin is no longer attracted.
Answer The electromagnet produces a magnetic field only while current flows, so the attraction depends on the current.
Section 5 of 5Exam Checklist & Tips
6.1.8 – Key Points for the Exam
- Electromagnet A coil of wire around a soft iron core that is magnetic only while a current flows through the coil.
- Soft iron core A core of magnetically soft iron that concentrates the field and loses its magnetism easily.
- Turns
- Individual loops of wire in a coil;
- increasing the number of turns strengthens the field.
- Individual loops of wire in a coil;
- Relay A switch operated by an electromagnet, used to control a separate circuit.
Common Mistakes
- Saying more coils make the electromagnet stronger; the correct phrase is more turns on the coil.
- Using a steel core, which would stay magnetised; soft iron is needed for a switchable electromagnet.
- Claiming that an electromagnet keeps its magnetism when the current is switched off.
- Stating that reversing the magnet reverses the poles of an electromagnet; reversing the current does that.
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