Making and removing magnetism
Compare permanent and temporary magnets, then explain the listed ways to magnetise and demagnetise a material.
Section P4.1 · Simple phenomena of magnetism
An original 0652 lesson for the listed magnetisation and demagnetisation methods. Written explanations do not demonstrate laboratory competence.
Before you begin
- Recognise a magnetic field
- Distinguish a permanent magnet from an electromagnet
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which supply produces a steady field direction in a coil?
What you will learn
- Describe methods of magnetisation
- Compare steel and soft iron
- Describe how heating, hammering and alternating current can demagnetise a material
Magnetising a material
A magnetic material can be magnetised by stroking it with a magnet, by placing it in a direct-current coil, or by hammering it while it is aligned with a magnetic field. These methods can help the material's magnetic regions become aligned.
Steel tends to retain magnetism and is useful for permanent magnets. Soft iron is easier to magnetise and demagnetise, so it is useful where a magnetic effect needs to switch on and off.
Demagnetising a material
Heating or hammering can disrupt the alignment that produces a strong overall magnet. An alternating current in a coil repeatedly reverses the magnetic field and can also remove the material's net magnetism. The course describes these ideas; practical equipment must be used only in a supervised school setting.
Keep the methods distinct
Direct current produces a steady field direction; alternating current continually reverses its direction. Do not confuse magnetising a bar with inducing magnetism in a nearby piece of soft iron: induction does not require the material to be rubbed or placed in a current-carrying coil.
Sources for this lesson
Pause and explain
Which material is more suitable for a permanent magnet?
Worked example
A steel bar is placed inside a coil connected to a direct-current supply. What is the intended effect, and which supply type is used to demagnetise a bar in a coil?
Show the worked solution
- Direct current in the coil produces a steady magnetic field.
- The field can magnetise the steel bar.
- An alternating current is the listed coil method for demagnetisation because its field reverses.
Answer The direct-current coil can magnetise the steel bar. An alternating-current coil is used to demagnetise it because the magnetic field repeatedly reverses.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Name two methods that can magnetise a steel bar and identify the current used in the coil method.
Give me a hint
Recall the mechanical method and the steady-field electrical method.
Compare my reasoning
- One method is stroking the bar with a magnet.
- A second method places it in a coil carrying current.
- The syllabus coil method for magnetising uses direct current.
A steel bar can be magnetised by stroking it with a magnet or placing it in a coil carrying direct current. Hammering it while aligned in a magnetic field is another listed method.
Look for these in your work
- Names at least two listed methods
- Identifies direct current for the coil method
- Does not confuse magnetisation with induction
Why is soft iron often chosen for a temporary electromagnet while steel is used for a permanent magnet?
Give me a hint
Compare how readily each material gains and loses magnetism.
Compare my reasoning
- An electromagnet should stop being strongly magnetic when its field is removed.
- Soft iron is easier to magnetise and demagnetise.
- Steel is more likely to retain magnetism after the field is removed.
Soft iron is useful in an electromagnet because it is readily magnetised and demagnetised when the coil field changes. Steel retains magnetism more readily, so it is better suited to a permanent magnet.
Look for these in your work
- Links soft iron to temporary magnetism
- Links steel to retaining magnetism
- Connects material choice to the device's purpose
A steel bar must lose its overall magnetism. Name two listed methods and explain the coil method in terms of field direction.
Give me a hint
Think about disrupting alignment, then compare steady and reversing current.
Compare my reasoning
- Heating or hammering can disrupt the aligned magnetic regions.
- An alternating current in a coil repeatedly reverses the field.
- This changing field can remove the bar's overall magnetism.
Heating or hammering can demagnetise the bar. In a coil, alternating current repeatedly reverses the magnetic field, disrupting the overall alignment; direct current instead gives a steady field direction.
Look for these in your work
- Names two listed demagnetisation methods
- Explains the reversing field
- Distinguishes alternating from direct current
Common mistakes
- Do not confuse direct current with alternating current.
- Steel and soft iron do not retain magnetism equally well.
- Induced magnetism and permanent magnetisation are different processes.
What can you explain now?
Which current type is used in a coil as a method of demagnetisation?
Compare with the explanation
Alternating current.
Its reversing magnetic field can disrupt the material's overall magnetic alignment.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Recall three ways to magnetise a material and three ways to demagnetise it.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.