Slide an iron rod into a solenoid and its magnetic pull leaps by hundreds of times, yet pull the rod out and a different choice of metal would have stayed magnetised, ruining the device. This final lesson of the focus area brings everything together: you will choose the right core material and investigate exactly what makes an electromagnet strong.
You wind a coil around an empty cardboard tube and use it to pick up paper clips. Then you slide an iron nail into the tube.
Before reading: what do you expect to happen to the number of paper clips the coil can hold, and why? Write your prediction and a one-sentence reason.
Why a ferromagnetic core makes an electromagnet
A coil on its own makes a modest field; slide in an iron core and the iron's own atoms line up with the coil's field, adding their magnetism to it and boosting the total enormously.
An electromagnet is a solenoid wound around a ferromagnetic core. The coil's field ($B = \mu_0 n I$) causes the magnetic domains inside the iron to line up with it, so the iron becomes magnetised and adds its own field to the coil's. The result is a magnetic field hundreds of times stronger than the empty coil produced, which is why sliding the nail into the tube lets it hold far more paper clips. Because the field still comes from the current, the electromagnet switches off the moment the current stops (provided the core does not retain its magnetism).
An electromagnet is a solenoid with a ferromagnetic (iron) core. The coil's field aligns the iron's magnetic domains, so the iron adds its own field and the total can be hundreds of times stronger than the bare coil, yet it still switches on and off with the current.
Pause, copy the highlighted statement into your book before moving on.
Did you get this? True or false: inserting a ferromagnetic core into a solenoid greatly increases the magnetic field it produces.
Soft iron versus steel, a secondary-source assessment
We just saw that an iron core boosts the field. That raises a question: does the kind of iron matter? This card answers it → yes, soft and hard magnetic materials behave very differently when you switch off.
The best core magnetises strongly when the current is on but lets go of its magnetism the instant the current stops, and that points to soft iron, not steel.
Assessing core materials (a secondary-source task, comparing manufacturers' data and reference tables) reveals two key properties: how strongly a material magnetises, and whether it retains that magnetism. Soft iron magnetises strongly and loses almost all its magnetism when the field is removed, so it is ideal for an electromagnet you need to switch off, such as a scrapyard crane or a relay. Steel (a hard magnetic material) also magnetises but retains its magnetism, so it stays magnetised after the current stops, useful for permanent magnets but useless for a crane, which is the answer to the hook. Materials like copper or aluminium are not ferromagnetic and barely respond at all.
Soft iron is the best core for a switchable electromagnet: it magnetises strongly but loses its magnetism when the current stops. Steel retains magnetism (good for permanent magnets, bad for switchable ones). Non-ferromagnetic metals barely respond.
Add the highlighted comparison to your notes before the check below.
Quick check: Why is soft iron preferred over steel for the core of a scrapyard crane's electromagnet?
Pattern C, Plan, test, conclude
Build an electromagnet (insulated wire wound on an iron nail, powered by a low-voltage supply) and measure its strength by the number of paper clips it can hold. In your book: (1) choose one factor to investigate (current, number of turns, or core material) and state your hypothesis; (2) list the independent, dependent and at least two controlled variables to make it a fair test; (3) describe how you would vary the factor and record results in a table; (4) write one sentence predicting the trend and explaining it using $B = \mu_0 n I$ or the role of the core.
Odd one out: three of these are ferromagnetic materials suitable for an electromagnet CORE. Which one is not?
Dot point: Conduct a secondary-source investigation to assess the choice of materials used in the core of electromagnets
Current, turns, core, and testing one at a time
We just planned an investigation. That raises a question: what should the results actually show? This card answers it → strength rises with current and turns per metre, and depends strongly on the core.
Everything that increases the coil's field, or improves the core, increases the electromagnet's strength, and the equation $B = \mu_0 n I$ tells you exactly how the coil contributes.
An electromagnet is stronger when: the current is larger (more current, stronger field, since $B \propto I$); there are more turns per unit length ($B \propto n$); and the core is more strongly ferromagnetic (soft iron beats air by hundreds of times). To investigate any one of these fairly, change only that factor while keeping the others constant, and measure strength by a consistent quantity such as the number of paper clips lifted or the force on a sensor. Plotting strength against the chosen factor reveals the relationship, for example, an upward trend with current that matches $B \propto I$. This investigation is the capstone of the focus area, tying the solenoid equation to a real, measurable device.
An electromagnet has $200$ turns wound over a length of $0.25\ \text{m}$ and carries $1.5\ \text{A}$. It is rewound with $300$ turns over the same length and the current is raised to $3.0\ \text{A}$. By what factor does the field strength inside the coil increase?
An electromagnet's strength increases with the current ($B \propto I$), the turns per unit length ($B \propto n$), and the use of a strongly ferromagnetic core like soft iron. Test one factor at a time, controlling the others, and measure strength consistently (e.g. paper clips held).
Pause, copy the highlighted factors into your book before the check below.
Last check: Decide if the statement is true or false.
Increasing the current, adding more turns per unit length, and using a soft-iron core all increase the strength of an electromagnet.
Match the pairs: Match each change to its effect on an electromagnet's STRENGTH.
Dot point: conduct a practical investigation to examine the factors affecting the strength of an electromagnet
A fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
UnderstandBand 5(4 marks) 1. Explain why soft iron is chosen over steel for the core of a switchable electromagnet, and state two factors (other than the core) that affect an electromagnet's strength, with a reason for each.
1 mark: soft iron magnetises strongly · 1 mark: soft iron loses its magnetism when the current stops (whereas steel retains it), so the load is released · 1 mark: increasing the current increases strength ($B \propto I$) · 1 mark: increasing the turns per unit length increases strength ($B \propto n$)
AnalyseBand 5(3 marks) 2. A student researching electromagnet cores finds a data table listing the permeability and the retentivity of iron, steel and nickel. Explain what each of these two properties tells them, and state what value of each they should look for when choosing a core for an electromagnet that must switch on and off.
1 mark: permeability indicates how strongly the material magnetises and therefore how much it multiplies the field, so a HIGH value is wanted · 1 mark: retentivity indicates how much magnetism the material keeps after the current stops, so a LOW value is wanted · 1 mark: the choice is the material combining high permeability with low retentivity, which is soft iron rather than steel
Dot point: Conduct a secondary-source investigation to assess the choice of materials used in the core of electromagnets
AnalyseBand 5(3 marks) 3. A student investigates the factors affecting the strength of an electromagnet by counting how many paper clips it can lift. Identify the independent, dependent and two controlled variables for an investigation into the effect of current, and state one way to improve the reliability of the results.
1 mark: independent variable is the current, dependent variable is the number of paper clips lifted · 1 mark: names two controlled variables, e.g. number of turns, core material, size of paper clips · 1 mark: an improvement, e.g. repeat each measurement several times and average
Dot point: conduct a practical investigation to examine the factors affecting the strength of an electromagnet
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Q1 (4 marks): Soft iron is chosen because it is strongly ferromagnetic, so it magnetises strongly and greatly boosts the coil's field, but it also loses almost all of its magnetism the moment the current is switched off. Steel, by contrast, retains its magnetism after the current stops, so an electromagnet with a steel core would stay magnetised and could not release its load, which is why soft iron is used for switchable devices like cranes and relays. Two further factors that affect strength are the current and the number of turns per unit length: increasing the current increases the field because $B \propto I$, and increasing the turns per unit length increases the field because $B \propto n$ in $B = \mu_0 n I$. Both make the electromagnet stronger.
Q2 (3 marks): Permeability describes how readily the material becomes magnetised by the coil's field, and therefore how much it multiplies the field the electromagnet produces. A core of high permeability can increase the field by a factor of hundreds compared with air, so a high value is what the student wants. Retentivity describes how much magnetism the material keeps once the magnetising current is switched off. For an electromagnet that must release whatever it is holding, retained magnetism is a fault rather than a feature, so a low value is what the student wants. The two properties must therefore be read together: the right core is the material that combines high permeability with low retentivity. Of the three in the table, soft iron meets both conditions, whereas steel has a high retentivity and would stay magnetised after switch-off.
Q3 (3 marks): The independent variable is the current through the coil, which the student deliberately changes, and it would be measured with an ammeter in series. The dependent variable is the strength of the electromagnet, measured indirectly as the number of paper clips it can lift. Controlled variables must include the number of turns on the coil and the core material and dimensions, since both strongly affect the field, and also the size and type of the paper clips used, since a heavier clip would change the count without any change in the magnet. The way the clips are presented to the magnet should also be kept consistent. To improve reliability the student should repeat the count at each current several times, allowing the coil to cool between trials so that its resistance and therefore the current do not drift, and take an average of the repeated counts.
You can now explain how a core multiplies a solenoid's field, choose soft iron for a switchable electromagnet, and plan a fair test of its strength. That completes Electricity and magnetism, these electromagnets reappear powering the motors and generators of Year 12.
↩ Back to focus-area hubAt the start you predicted what the iron nail does to the coil's lifting power. The answer: it greatly increases it, because the coil's field aligns the iron's magnetic domains so the iron adds its own field, often hundreds of times stronger. And on the hook: a soft-iron core lets the crane switch off because it loses its magnetism, while steel would retain it. If either surprised you, revisit Cards 1 and 2.
A short spaced-retrieval set, mixing this lesson with earlier ones. It gets harder to guess and easier to remember.
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