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Magnetism is the force exerted by magnets when they attract or repel each other. Magnetism is caused by the motion of electric charges.
As you know, every substance is made up of atoms and inside every atom there are electrons which carry electric charges circling the nucleus. Their movement generates an electric current and causes each electron to act like a microscopic magnet.
In most substances, equal numbers of electrons spin in opposite directions, which cancels out their magnetism. That is why materials such as cloth or paper are said to be weakly magnetic. In substances such as iron, cobalt, and nickel, most of the electrons spin in the same direction. This makes the atoms in these substances strongly magnetic, but magnets.
Magnetic materials are always attracted towards a magnet. However, a magnet may be attracted to or repelled from another magnet.
All magnets have north pole and a south pole. If freely suspended, one pole will point toward the north. The two poles are therefore named the north magnetic pole and the south magnetic pole, or more properly, north-seeking and south-seeking poles, for the attractions in those directions.
Activity 1
You will need:
A bar magnet
a piece of paper
a piece of string
A tub of paperclips or drawing pins
What you will do:
1. Fold the piece of paper so the magnet will sit inside
2. Tie the string to the paper and place the magnet in the paper like the diagram below
3. Hold the string or hang it from a tabletop
4. Leave it hanging until it stops moving
You have made a compass! When it stops moving, the magnet will point roughly North - South.
Image: Openstax (CC BY)
The Earth acts like a large bar magnet with its south-seeking pole near the geographic North Pole. That is why the north pole of your compass is attracted toward the geographic north pole of the Earth. The magnetic pole that is near the geographic North Pole is actually a south magnetic pole!
Confusion arises because the geographic term “North Pole” has come to be used (incorrectly) for the magnetic pole that is near the North Pole. The “North magnetic pole” has been given the wrong name; it should be called the South magnetic pole.
5. Hold the north pole of the magnet close to the north pole of the hanging magnet. What happens?
6. Hold the south pole close to the north pole. What happens?
Bringing a north pole and a north pole together will cause the magnets to push away from each other or repel. They will be pushed in opposite directions. If you bring a north pole and a south pole together, they will attract.
7. Take a magnet and place it in a tub of paperclips or drawing pins. What happens?
Any metal will be attracted to a metal, but not all metals are magnetic. Some materials are magnetic. For example: iron, steel, cobalt, and nickel.
Image: Images-of-elements.com (CC BY)
There are rare earth magnets such as neodymium magnets. These magnets are the most powerful permanent magnets produced commercially and are popular because they are very strong and are small.
Lots of metals are not magnetic. For example: copper and aluminium.
Non-metals such as sand, wood and plastic are not magnetic.
Magnets are used in many different applications:
Magnetic elements on a hard disk help to represent computer data, which is later ‘read’ by the computer to extract information.
Magnets are used inside TVs, sound speakers and radios. The small coil of wire and a magnet inside a speaker transforms the electronic signal to sound vibrations.
Magnets are used inside a generator to transform mechanical energy into electrical energy. In contrast, other kinds of motors use magnets to change electrical energy to mechanical energy.
Electrically charged magnets can help cranes to move large metal pieces.
Magnets are used in filtering machines that separate metallic ores from crushed rocks.
It is also used in food processing industries for separating small metallic pieces from grains etc.
Magnets are used in MRI machines which are used to create an image of the bone structure, organs, and tissues. Even magnets are used to cure cancer.
At home, you use magnets when you stick a paper on the refrigerator in order to remember something. Magnets are also used to keep cupboards closed.
We often use pocket a compass to find out directions when we are on a trek. The pocket compass uses a magnetic needle to point north.
The dark strip on the back of debit and credit cards is magnetic and is used to store data like computers’ hard drives.
Magnets can help collect all the nails which are scattered on the ground after a repair job.
Infinity Learn. (2018). What are the types of magnets? (Standard YouTube licence)
Fun Science. (2021). 6 amazing magnet experiments. (Standard YouTube licence) -
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Most magnets are made of iron or steel because iron is a fairly cheap metal.
In a magnetised piece of iron, there are magnetised domains. Electrons of atoms are grouped into domains in which each domain has the same charge. These domains are lined up and point the same way. This is what makes a magnet a magnet.
Image: Flickr. Mason, D. (CC BY-NC-SA)
Non-magnetic materials do not have lined up domains.
Discovery UK. (2019). Magnets: How it's made. (Standard YouTube licence)
Why are magnets magnetic?
You can magnetise a piece of iron or steel easily.
Activity 1
What you will need:
A piece of steel or a long steel or iron nail
A bar magnet
drawing pins, iron nails or paper clips
insulated copper wire
4.5 V battery
What you will do:
Part 1
1. Check that the nail or piece of steel is not magnetised by dipping it into a dish of the drawing pins.
2. Stroke the piece of steel with the N-pole of the magnet. Start at the marked end and lift the magnet clear at the end of each stroke. Do this about 10 times in the same direction.
How many pins can the steel pick up now?
Place the N-pole of the bar magnet near to the magnetised piece of steel. Does it attract or repel?
The mark on the piece of steel you magnetised will be the north pole of your magnet.
Part 2:
1. Wind about 1 meter of insulated copper wire around a large iron nail. Connect the ends of the wire to the battery as shown in the picture below:
Image: Flickr. Clifford, G. (CC BY-SA)
2. Try and pick up the pins. How many can you pick up?
This is called an electromagnet. Coiling copper around a nail and connecting it to a source of electrical energy creates a non-permanent magnet. These types of magnets are used in cranes, telephones, and fridge doors.
Image: StackExchange (CC BY-SA)
Demagnetisation
Demagnetisation is the process by which a magnet loss its magnetism. The domains get misaligned (disoriented).
A magnet can undergo self-demagnetisation if poorly stored or the process can be influenced externally by giving the dipoles enough energy to overcome the forces holding them in a particular direction.
Demagnetisation can be hastened by any of the following methods:
- Hammering. Hammering a magnet repeatedly while placed in the east-west direction or dropping it violently several times on the hard surface makes it lose most of the magnetism.
- Heating. Heating a magnet until red hot and cooling it suddenly when resting in the east-west direction makes it lose its magnetism.
- Electrical method. Placing a magnet in a solenoid placed in east-west direction and passing an alternating current demagnetises it. This is because alternating current reverses many times per second, disorienting the magnetic dipoles.
TutorVista. (2010). Methods of magnetisation and demagnetisation. (Standard YouTube licence) -
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A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material.
Image: Vecteezy. (PD)
Detecting magnetic fields
A magnetic field is invisible, but it can be detected using a magnetic compass. A compass contains a small bar magnet on a pivot so that it can rotate. The compass needle points in the direction of the Earth’s magnetic field, or the magnetic field of a magnet.
Magnetic fields can be mapped out using small plotting compasses or iron filings.
Activity 1
What you will need:
a bar magnet
a piece of paper
iron filings
What you will do:
1. Place the magnet under the piece of paper.
2. Sprinkle iron filings over the piece of paper above the magnet.
Image: Openstax. BCIT Physics 0312 Textbook. (CC BY)
Iron filings near a magnet act like tiny compass needles, showing the shape of their fields.
Activity 2
What you will need:
a bar magnet
a piece of paper
small compasses
What you will do:
1. Place the plotting compass near the magnet on a piece of paper.
2. mark the direction the compass needle points.
Image: Flickr. Mason, D. (CC BY-NC-SA)
3. Move the plotting compass to many different positions in the magnetic field, marking the needle direction each time.
4. Join the points to show the field lines.
The needle of a plotting compass points to the south pole of the magnet.
Follow the link below:
The direction of magnetic field lines is defined to be the direction in which the north end of a compass needle points.
Magnetic field lines are imaginary lines. Magnetic field lines are a visual tool used to represent magnetic fields. They describe the direction of the magnetic force on a north pole at any given position.
The density of the lines indicates the magnitude of the field. Taking an instance, the magnetic field is stronger and crowded near the poles of a magnet. As we move away from the poles, it is weak, and the lines become less dense.
Images: Xaktly. (CC BY-NC-SA)
The diagrams show these key features:
- the magnetic field lines never cross each other
- the closer the lines, the stronger the magnetic field
- the lines have arrowheads to show the direction of the force exerted by a magnetic north pole
- the arrowheads point from the north pole of the magnet to its south pole
Infinity Learn. (2018), Magnetic field of a bar magnet. (Standard YouTube licence)
Magnetic shielding
Magnetic materials like iron concentrate the magnetic field lines and divert them out from the ends. Using this principle, these magnetic materials can act as magnetic shielding. Magnetic shielding is to prevent surrounding magnetic field lines from reaching the magnetic sensitive equipment (like MRI scanners) whose operation may be affected by the fields.
Sciensesvideos. (2013). Magnetic shielding. (Standard YouTube licence)
Magnetic shields work by redirecting the force lines away from the shielded object. Because of this, the materials used for magnetic shielding have to be able to sustain a strong magnetic field. Besides common materials such as iron, nickel and cobalt, there are several proprietary alloys commercially available that are especially designed for use as magnetic shields.New technologies have provided some new magnetic shielding materials. For instance, nanotechnology has contributed magnetic shield materials that can be applied directly to the component like a coat of paint. While not always practical, superconductors, materials that lose all their electrical resistance at very low temperatures, are excellent magnetic shields.
Physics Lens. (2017). Magnetic shielding demonstrations. (CC BY)
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Our planet is a giant magnet. Current theory suggests that electric currents circulating in its molten core form an electromagnet. This is consistent with evidence suggesting that the north-south direction of the field has reversed a few times in Earth's history. The average strength of the field is strong enough to cause a lightweight magnet like a compass needle to align with the field lines, which are illustrated below.
Image: Xaktly. (CC BY-NC-SA)
Earth’s liquid iron core forms a convection current because it is heated from beneath by the inner core. Because iron is a metal and conducts electricity (even when molten), its motion generates a magnetic field.
Image: Wikipedia Commons (CC BY)
Earth’s magnetic field is defined by north and south poles representing lines of magnetic force flowing into Earth in the northern hemisphere and out of Earth in the southern hemisphere.
Because of the shape of the field lines, the magnetic force is oriented at different angles to the surface in different locations. The tilt, or inclination of magnetic field lines is represented by the tilt of compass needles in the diagram below. At the north and south poles, the force is vertical. The force is horizontal at the equator. Everywhere in between, the magnetic force is at an intermediate angle to the surface.
Image: University of Saskatchewan. Panchuk, K. (CC BY-NC-SA)
Earth’s magnetic field depicted as the field of a bar magnet coinciding with the core. The south pole of the magnet points to Earth’s magnetic north pole. The red and white compass needles represent the orientation of the magnetic field at various locations on Earth’s surface.
Earth’s magnetic field is generated within the outer core by the convective movement of liquid iron, but although convection is continuous, the magnetic field is not stable. Periodically, the magnetic field decays and then becomes re-established.
When it does re-establish, the polarity may have reversed (i.e., your compass would point south rather than north). Over the past 250 million years, there have been hundreds of magnetic field reversals, and their timing has been anything but regular. The shortest ones that geologists have been able to identify lasted only a few thousand years, and the longest one was more than 30 million years.
FuseSchool - Global Education. (2020). Earth and compasses. (CC BY)
Magnetic declination
The difference between the rotational and magnetic poles has a consequence for us when we're using a compass for navigation. In order to align a map properly along the north-south line, we need to account for the fact that our compass will not be pointing exactly to true north, but to magnetic north.
There are only a few locations on Earth where it points exactly to the True (geographic) North. The direction in which the compass needle points is known as Magnetic North, and the angle between Magnetic North and the True North direction is called magnetic declination.
Magnetic declination varies both from place to place, and with the passage of time. As a traveler cruises the east coast of the United States, for example, the declination varies from 20 degrees west (in Maine) to zero (in Florida), to 10 degrees east (in Texas), meaning a compass adjusted at the beginning of the journey would have a true north error of over 30 degrees if not adjusted for the changing declination. The magnetic declination in a given area will change slowly over time, possibly as much as 2-25 degrees every hundred years or so, depending upon how far from the magnetic poles it is.
Complex fluid motion in the outer core of the Earth causes the magnetic field to change slowly with time. This change is known as secular variation. Because of secular variation, declination values shown on old topographic, marine and aeronautical charts need to be updated if they are to be used without large errors.
True North is north, according to the earth’s axis. True north points to the North Pole.
Grid North refers to the direction northwards along the map projection grid lines. This comes from a 3-D object (the earth) being depicted on a 2D object (a map).
Magnetic North is the direction that a compass needle points to. Magnetic north is the direction of the Earth’s magnetic field lines.
Image: Xaktly. (CC BY-NC-SA)
The blue map of the United States shows isogonic lines, along which the angle between magnetic and true north is constant. East of a line passing through Illinois and Alabama (the agonic line), the compass will point to the west of true north, producing what we call a west declination. True north lies 20˚ to the east of where the compass is pointing. To the west of the agonic line, we find east declinations up to 20˚ in the 48 contiguous US states, meaning that true north lies a number of degrees to the west of what the compass says.
Click on the link below to use the interactive map showing declination all over the globe, including Zanzibar:
https://www.magnetic-declination.com/
AR on AR. (2020). Magnetic declination and magnetic inclination. (Standard YouTube licence)
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