AP Syllabus focus: 'A material’s composition affects its magnetic behavior: ferromagnetic materials can remain magnetized, paramagnetic materials interact weakly, and all materials show diamagnetism.'
Materials respond to magnetic fields according to their internal electron structure. This determines whether they become strongly magnetized, weakly attracted, or slightly repelled when an external magnetic field is applied.
Why materials respond differently
The magnetic behavior of a material depends on the arrangement of electrons inside its atoms and on how neighboring atoms interact. Electrons act like tiny magnetic sources. In many materials, these tiny effects cancel because different electrons point in opposite directions. In others, some effects can line up and produce a measurable response.
Two ideas are especially important:
whether the material contains unpaired electrons
whether many atomic magnetic effects can align together
This is why materials with different chemical composition or internal structure can behave very differently in the same magnetic field. Some respond strongly, some only slightly, and some oppose the applied field.
Ferromagnetic materials
A ferromagnetic material responds very strongly to an external magnetic field.
Ferromagnetic material: A material that can become strongly magnetized because many atomic magnetic moments align in the same direction, and it may remain magnetized after the external field is removed.
In a ferromagnetic material, groups of atoms can line up in regions called domains. Within a domain, many atomic magnetic moments point in the same direction. When no external field is present, different domains may point in different directions, so the overall magnetization can be small. When a magnetic field is applied, more domains turn or grow in the field’s direction.
This produces a large magnetic response.
A B–H hysteresis loop for ferromagnetic materials, with key features labeled (saturation, remanence, and coercive field). The loop shape captures the “history dependence” of magnetization and shows why some magnetization can remain even when the applied field returns to zero. Source
It also explains why ferromagnetic materials can be used to make permanent magnets: after the external field is removed, some alignment can remain. Iron, cobalt, nickel, and some alloys show ferromagnetism.
Ferromagnetism is the strongest of the three behaviors discussed here. On AP Physics 2 questions, the key identifying feature is that a ferromagnetic material can be magnetized and can stay magnetized.
Paramagnetic materials
A paramagnetic material interacts with a magnetic field, but much more weakly.
Paramagnetic material: A material that is weakly attracted to an external magnetic field because some atomic magnetic moments tend to align with the field, but the material does not usually remain magnetized.
Paramagnetic behavior occurs when atoms or molecules have unpaired electrons, giving them a net magnetic effect. An external field encourages these magnetic effects to line up with the field. However, thermal motion keeps the alignment from becoming strong or permanent.
As a result:
the attraction is weak
the effect is usually present only while the external field is applied
the material usually does not become a permanent magnet
Aluminum and oxygen are common examples of paramagnetic substances. In everyday life, paramagnetism is often hard to notice because it is much weaker than ferromagnetism.
Diamagnetic materials
A diamagnetic material responds by producing an effect opposite the applied magnetic field.
Diamagnetic material: A material that is weakly repelled by an external magnetic field because the field induces a magnetic effect that opposes the applied field.
Diamagnetism does not depend on permanent alignment inside the material. Instead, the external magnetic field slightly changes the motion of electrons, creating an induced magnetic effect that opposes the field. This opposition causes a very small repulsion.
A crucial AP Physics 2 idea is that all materials show diamagnetism at some level. However, in ferromagnetic and paramagnetic materials, other magnetic effects are usually stronger and hide the diamagnetic effect. In materials that are not ferromagnetic or paramagnetic, diamagnetism becomes the main observable behavior.
Diamagnetic substances are typically only weakly repelled, so the effect is often subtle. Water, copper, and bismuth are examples of materials in which diamagnetism can be observed.
Comparing the three behaviors
These three magnetic behaviors can be compared by asking two questions: how strong is the response, and does it last after the field is removed?
Ferromagnetic: strong attraction; may remain magnetized
Paramagnetic: weak attraction; does not usually remain magnetized
Diamagnetic: weak repulsion; induced only while the field is present
Another useful comparison is the role of composition:
materials with strong cooperative alignment between atomic magnetic moments can be ferromagnetic
materials with unpaired electrons but no strong cooperative alignment are often paramagnetic
materials with no stronger magnetic effect still show diamagnetism
This is why magnetic behavior is a property of the material itself, not just of the external magnet.
What to recognize on AP questions
You should be able to identify each type from a short description.
If a material is strongly attracted to a magnet and can later act like a magnet itself, it is ferromagnetic.
If a material is only slightly attracted and loses the effect when the field is removed, it is paramagnetic.
If a material is slightly repelled by a magnet, it is diamagnetic.
If a question asks which behavior is universal, the answer is diamagnetism.
If a question asks which type can produce permanent magnets, the answer is ferromagnetism.
Careful reading matters. Words like remain magnetized, weakly attracted, and slightly repelled usually point directly to the correct classification.
FAQ
Above the Curie temperature, thermal motion becomes strong enough to disrupt the long-range alignment that makes ferromagnetism possible.
The material then stops behaving as a ferromagnet and usually becomes paramagnetic instead. If it cools below the Curie temperature again, ferromagnetic behavior can return, although the exact magnetization afterward depends on its magnetic history.
Both are ferromagnetic, but they respond differently when magnetized and demagnetized.
Soft ferromagnetic materials magnetize easily and lose magnetization easily.
Hard ferromagnetic materials are harder to magnetize, but once magnetized they resist demagnetization.
This is why soft materials are useful when magnetization must change often, while hard materials are better for permanent magnets.
Oxygen molecules have unpaired electrons, which give each molecule a net magnetic effect.
That allows oxygen to align weakly with an external magnetic field, making it paramagnetic. Many other gases, such as nitrogen, have electron arrangements in which magnetic effects cancel more completely, so they do not show the same paramagnetic behavior.
Bismuth is diamagnetic like many substances, but its diamagnetic response is unusually strong compared with most common materials.
That makes the repulsion easier to observe in demonstrations. It is still much weaker than ferromagnetic attraction, but it is strong enough to make bismuth a useful classroom example of a material that is pushed away by a magnetic field.
“Stainless steel” is a broad category, not a single material. Its magnetic behavior depends on both composition and crystal structure.
Some stainless steels contain structures that allow ferromagnetic alignment, so they are magnetic. Others have structures in which that alignment does not occur, so they are weakly magnetic or effectively nonmagnetic in ordinary situations. This is a good reminder that small changes in composition can produce very different magnetic behavior.
Practice Questions
Three samples are tested near a bar magnet.
Sample A is strongly attracted and remains magnetized after the magnet is removed.
Sample B is weakly attracted only while the magnet is nearby.
Sample C is weakly repelled.
Identify the magnetic classification of each sample.
Sample A is ferromagnetic. (1 mark)
Sample B is paramagnetic. (1 mark)
Sample C is diamagnetic. (1 mark)
A student places three identical rods, X, Y, and Z, into the same external magnetic field.
Rod X becomes strongly magnetized and later attracts small steel pins even after the field is removed.
Rod Y is weakly attracted while in the field but shows no lasting magnetization.
Rod Z is slightly pushed away from the field.
Answer the following:
(a) Classify rods X, Y, and Z as ferromagnetic, paramagnetic, or diamagnetic.
(b) Which rod could be used to make a permanent magnet? Explain why.
(c) Which type of magnetic behavior is present in all materials, even when it is not the dominant effect?
(a)
X is ferromagnetic. (1 mark)
Y is paramagnetic. (1 mark)
Z is diamagnetic. (1 mark)
(b)
Rod X could be used to make a permanent magnet. (1 mark)
Explanation: ferromagnetic materials can remain magnetized after the external field is removed. (1 mark)
(c)
Diamagnetism is present in all materials. (1 mark)
