2Magnetic properties of matter_distributing materials2
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2Magnetic properties of matter_distributing materials2

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08.05.2020
2Magnetic properties of matter_distributing materials2
2Magnetic properties of matter_distributing materials2.pdf

Some materials respond to a magnetic fi eld, and we are used to seeing this when some metals, like iron, are attracted to magnets. However, this is only one form of magnetism, ferromagnetism; materials can also be paramagnetic or diamagnetic.


Ferromagnetic materials are strongly attracted by magnetic fi elds and can retain their magnetic properties even after the fi eld is removed. For this reason permanent magnets are made from ferromagnetic materials. Iron, nickel and cobalt are ferromagnetic.

Paramagnetic materials are weakly attracted by magnetic fi elds. This effect is usually so weak that it is not noticeable. After the magnetic fi eld is removed paramagnetic material does not retain its magnetic properties.

Diamagnetic materials are repelled by magnetic fi elds, but the effect is usually very weak. Superconductors can be strongly diamagnetic and this means they can be levitated above powerful magnets.

Extension Questions Q3. What is a domain?

Within a ferromagnetic material there are regions where the atoms have their magnetic moments aligned in the same direction. However, each of these regions aligns in a different direction and so the material has no overall magnetic fi eld. These regions are known as domains and a strong magnetic fi eld can cause these domains to align. If the domains stay aligned, even after the magnetic fi eld is switched off, the material will become a ‘permanent’ magnet.

Striking or heating the material can cause the domains to align randomly again, demagnetising the material. 

Q4. What is the Curie temperature?

If ferromagnetic materials, like iron, are heated above a certain temperature they lose their magnetism. This is known as the Curie temperature, and for iron is around 770°C. If the material is allowed to cool below the Curie temperature it will again be attracted by magnetic fi elds.

• How do electromagnets work?

Any moving charge will generate a magnetic fi eld. For current fl owing in wires these are generally very weak unless the current is very large. However, a larger magnetic fi eld can be produced by winding the wire into a coil. In an electromagnet this can be further increased by adding a core of ferromagnetic material such as iron. Electromagnets are useful as the strength of the fi eld can be controlled by increasing or decreasing the current in the wire or changing the number of turns and, unlike permanent magnets, can be switched on and off. Electromagnets have many applications, including in motors, loudspeakers and powered door locks, such as those used in the central locking systems of cars.

    The magnetic fi eld of a simple electromagnet          Powerful electromagnets can be made by using superconducting wire.

 

This allows very large currents to be used as superconductors have no

• Suggested Film

          - What Are Electromagnets?

resistance and so large currents do not cause heating. Unfortunately superconductors only work at very low temperatures. 

Extension Question

Q5. How are magnets used in particle accelerators?

Electromagnets are often used in particle accelerators. The magnetic fi eld they create is used to make particles follow a curved path. Beams of particles can also be focused using special confi gurations of electromagnets called quadrupoles. These use two north poles and two south poles, which are arranged around the beam so that each pole is directly across from a similar pole. This produces a magnetic fi eld which increases the further the particles are from the centre of the arrangement, and means the particles are pushed into the centre, focusing the beam.

Magnetic Resonance Imaging (MRI) uses powerful magnets to image the human body. It relies on protons in the water molecules within body tissue aligning with a strong magnetic fi eld. When the fi eld is switched off the protons return to their original state, and the energy released when this happens can be detected and used to produce images of tissue inside the body.

Unlike other scans, MRI does not use X-rays and so the patient is not exposed to radiation. However, care has to be taken to check that the patient does not have metal in their body, which could move and cause injury during scanning, or medical implants which could be affected by the magnetic fi elds.

• Suggested Film Extension Question                 - MRI

Q6. What is a SQUID?

Very weak magnetic fi elds can be measured using a SQUID (a Superconducting Quantum Interference Device). A SQUID measures the fl ow of current through a circuit containing two structures known as Josephson junctions (which are made of two superconductors separated by a thin insulator), and the current through this circuit can be affected by magnetic fi elds. SQUIDs can measure magnetic fi elds of only a few millionths of a trillionth of a tesla and can be used to measure the very weak magnetic signals due to electrical currents in the brain. It is possible that in the future SQUIDS may be used to build smaller, cheaper MRI machines which use weaker magnetic fi elds.

Section 3: The Earth’s Magnetic Field

• What causes the Earth’s magnetic fi eld?

The Earth has a magnetic fi eld which protects us from the solar wind, a stream of charged particles which emanate from the Sun. It is caused by the fl ow of molten iron in the Earth’s outer core. The Earth’s magnetic fi eld lets us navigate using a compass, a small magnet which is free to move and so will orient itself in the direction of the fi eld. The end of the magnet, which points north, is called the north pole. However, as opposite poles attract, this means that the magnetic pole which is found in the north is actually the south pole of the Earth’s magnetic fi eld.

The Earth’s magnetic fi eld is caused by the

    Extension Questions                                                                        fl ow of molten iron in the outer core

Q8. Do other planets have magnetic fi elds?

Many of the other planets in the Solar System have a magnetic fi eld. Mars doesn’t, presumably because the required movement in its molten core has ceased. At one point in the past Mars did have a magnetic fi eld, and the reasons it has now been lost are not well understood.

The Moon has almost no magnetic fi eld. The very weak magnetic fi eld it does have appears to be due to rocks in its crust which have become magnetised. It is possible that the Moon did have a magnetic fi eld early in its history when currents were still fl owing in its core.

Q9. What are the Northern Lights?

As charged particles from the Sun reach the Earth they interact with the Earth’s magnetic fi eld. As this magnetic fi eld accelerates the particles, they collide with molecules in the air and excite them. These molecules then emit light and this can cause spectacular light displays in the night sky. These are usually only visible in the far north, where they are known as the Northern Lights or Aurora Borealis, and in the far south, where they are known as the Aurora Australis. Similar effects have been observed on other planets and moons in our Solar System.

Q10. How does a compass work?

A compass contains a magnet which is allowed to turn and align with an external magnetic fi eld. This is done by supporting the magnet at its centre while still allowing it to rotate. The magnet will align with the Earth’s fi eld and, as this is directed from north to south, this will give an indication of the direction of north and south. The north pole of the magnet will point to the Earth’s north magnetic pole.

A simple compass containing a magnet supported at its centre could be diffi cult to read. This is because as the magnet moves it could overshoot the correct position before being pulled back. It would then pass the correct position again and this oscillation could continue for some time. For this reason, compasses are often fi lled with a liquid like water, which reduces the oscillations and causes the magnet to settle far more quickly.

Some materials respond to a magnetic fi eld, and we are used to seeing this when some metals, like iron, are attracted to magnets

Some materials respond to a magnetic fi eld, and we are used to seeing this when some metals, like iron, are attracted to magnets

Extension Questions Q3. What is a domain?

Extension Questions Q3. What is a domain?

Magnetic Resonance Imaging (MRI) uses powerful magnets to image the human body

Magnetic Resonance Imaging (MRI) uses powerful magnets to image the human body

Section 3: The Earth’s Magnetic

Section 3: The Earth’s Magnetic

Extension Questions fl ow of molten iron in the outer core

Extension Questions fl ow of molten iron in the outer core

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2

2Magnetic properties of matter_distributing materials2
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