Magnetism

BACKGROUND INFORMATION ON MAGNETS

History of Permanent Magnets

Lodestones

The history of permanent magnets goes back to ancient times. Records from early Greek, Roman and Chinese civilisations make reference to rare and mysterious stones called lodestones. These lodestones could attract each other and also small pieces of iron in what seemed a magical way and when suspended from a thread, they always pointed in the same direction. We now know that lodestones contain magnetite, an oxide of iron and that they are a naturally occurring magnet having the composition Fe3O4.

Although lodestones were considered an intriguing phenomenon by scientists of the day, they were not really utilised in any constructive way until around 1200 AD with the introduction of the mariners (magnetic) compass. The mariners compass is a device housing a pivoting magnetised needle, which freely and consistently points towards magnetic north. This enables travellers to consistently and safely navigate their way from one place to another.

Many exciting discoveries involving magnets and relating to electricity have been made over the 800 years since the invention of the mariners (magnetic) compass, however strong permanent magnets as we know them today, are only a very recent invention.

You may think that we would have to go back 150 or even 200 years to look at the development of strong permanent magnets, but that is just not the case. In actual fact the history of magnets as we know them today only goes back as far as 1940.

Modern research into magnetism is heading in many different directions involving a vast array of materials, however at this time there are only four types of magnets that are commonly used throughout the world. They are Alnico, Ferrite, Samarium Cobalt and the most recently invented, Neodymium Iron Boron.

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Facts about Magnets

• Magnets only attract certain substances such as iron, cobalt and nickel.
• These substances are known as magnetic substances
• Substances such as lead, copper, aluminium, glass, wood, rubber, etc. are not attracted by a magnet and are known as non-magnetic substances
• Most metals are attracted by a magnet
• The force of attraction is greater near the end of the magnet than in the middle of the magnet
• The places where the force of attraction is strong are known as the poles of the magnet.
• Any magnet always has two poles, namely the North Pole and the South Pole
• When a magnet is suspended in such a way that it can turn around freely, it always comes to rest in a north-south direction
• The north pole of the magnet will always point north (north-seeking pole)
• A compass consists of a magnetic needle which can turn around in a circle freely
• A compass is used to determine direction
• When working with a compass, ensure that there are no magnetic substances in the immediate vicinity - this will cause the magnetic needle to indicate the wrong direction
• Forces of attraction and repulsion exist between magnetic poles
• Like poles repel each other
• Unlike poles attract each other
• Magnetic forces work at a distance
• The further apart the magnetic poles, the weaker the force
• A magnetic field exists around a magnet
• A magnetic field can be indicated by means of a compass
• A magnetic field consists of force lines which run from one pole to the other pole
• A magnet can be destroyed by heat, incorrect packing and rough handling
• A magnetic field exists around a current-bearing conductor
• The magnetic field lines around a conductor are in the form of concentric circles
• The direction of the field lines is the direction in which the north pole of a compass, placed on the lines, points

Learning Intentions

• To develop an understanding of what types of magnets we have
• Identify various shapes of magnets
• Understand how one must care for magnets
• Understand what makes a magnet a magnet

Success Criteria

WE KNOW WE ARE SUCCESSFUL WHEN WE CAN:

• Demonstrate an understanding of the ‘domains’ of magnets
• Demonstrate an understanding that non-magnetic materials have domains ‘in disarray’
• Explain that aligned ‘domains’ in a metal produce a magnet.
• Explain how to store magnets correctly
• Explain how we can damage magnets
• Demonstrate how to pack magnets correctly.

Lesson 1 - Introduction to magnets

1) Find out from the class how many students have a magnet in their home? Discuss where they can be found if the student does not have one as a toy – cupboard, fridge, on toys, etc.

2) Who knows where magnets come from? Has any student background knowledge they would share about the history magnets?

3) Read brief article on History of magnets.

4) Encourage students to investigate – internet, library – the history of magnets and bring the information to the next lesson.

Lesson 2 - The care of magnets

Good quality magnets lose their magnetic property after constant use. If we take care of the magnets then we extend the life of magnets.

Damaging a magnet

1) Magnets are easily destroyed by heat.

2) Constant dropping and banging of magnets also has a detrimental effect of their magnetic properties. (Students can demonstrate this when they make their own temporary magnets)

3) Permanent magnets work because the magnetic fields of each individual atoms are aligned. When you heat it up or drop it you provide energy. The atoms them move/vibrate more with higher energy and will realign themselves in a lower energy state, which is always less ordered according to the principles of entropy. The total effect is that the total magnetic field will be lessened.

The heat/dropping tends to realign the atoms and rather than have them in an ordered pattern they are in disarray – the magnet loses its magnetic property.

Lesson 3 - What makes a magnet a magnet?

1) Magnetic materials have magnetic “domains”. These are clusters of atoms that act like tiny magnets. Usually a piece of metal has very little order in the direction in which they are pointing. In a magnet however these “domains” all point in the same direction forming magnetic lines of force. These lines of force tend to reinforce each other and we have a magnet. When we damage a magnet we cause these “domains” to become haphazard – hence no magnetic strength.

2) Please explain to students that the north pole of magnets is always marked. The marking can take a number of forms – a cut/notch/hollow in the metal. It can have N painted on it or the magnet itself is painted, usually red for the NORTH and blue for SOUTH.

Learning Intention

• Investigate magnetic and non-magnetic materials.
• Investigate the strength of a magnet.
• To find out of if the magnetic field of a magnet can be shielded.

Success Criteria

WE KNOW WE ARE SUCCESSFUL WHEN WE CAN:

· Discuss, identify and explain what substances are magnetic and non-magnetic.

· Explain where the strongest parts of a magnet are found.

· Explain how the magnetic effect of a magnet can be shielded

Lesson 4 - Experiment 1: Magnetic and non-magnetic substances

Note:

A selection of magnetic and non-magnetic objects needs to be pre-prepare for Students to carry out the experiment. A piece of paper should be placed on the desk so that the various materials are not lost.

Method:

1) Take the contents of your container and spread them on your desk.

2) Take one end of your bar magnet and bring it up to each of the objects in turn.

3) Note down in your table which objects are attracted by the magnet.

4) Repeat the activity, this time using the opposite end of the magnet.

5) See if you get the same results.

6) After you have completed the activity, replace all objects.

2) After the experiment students can investigate around the room to find out what materials/substances are magnetic and which are non-magnetic.

3) After all groups have completed their investigation the teacher can question the groups on their findings before reaching the conclusion:

Not all materials are magnetic. Not all metals are magnetic.

4) It is suggest the students together with the teacher draw up a list of which metals are magnetic and which are not.

Lesson 5 - Experiment 2: Are all parts of magnet equally strong?

Note: When Students are asked to carry out experiment 2 ensure that pins are used for the experiment and NOT iron filings.

Method:

1) Spread some pins onto a sheet of paper.

2) Carefully wrap your magnet in some ‘cling’ plastic.

3) Move the bar magnet slowly in the pins/iron filings and remove it carefully.

4) Look at the magnet and see where most of the pins/iron filings are sticking to.

Students need to be warned about where to hold the magnet. Also warn about the sharp points!

After all groups have completed their investigation the teacher can question the groups on their findings before reaching the conclusion:

The strongest parts of a magnet are the poles?

Lesson 6 - Experiment 3: Can the effect of a magnet be shielded?

Note: 1) Ask the students if it possible to shield the invisible force that surrounds a magnet. Allow the groups to discuss this.

2) Should the students decide it is possible, and then ask them what can be used to shield the magnetic effect?

3) Once again let them discuss.

4) Get feed-back from the groups/students and then ask them to do Experiment 3. They need to read through the method carefully before they start.

Conclusion:

The magnetic effect of a magnet can be shielded by:

• Soft iron
• Thick paper/card/wood – basically any material
• The depth of water

Lesson 7 - Experiment 4: Like and unlike poles

Learning Intention

• That magnets have two poles
• That a freely swinging magnet will align itself to the N-S field of the earth
• The earth has a magnetic field and this affects all compasses/magnets.

Success Criteria

WE KNOW WE ARE SUCCESSFUL WHEN WE CAN:

• Discuss and explain the poles on a magnet.
• Explain the magnetic field of the earth and its effect on a compass.

Note:

1) There are a number of ways of doing this experiment. Students can suspend one magnet in a paper/card/copper wire cradle that can be held in the hand/suspended from a retort stand (be careful the stand is usually metal) from the edge of the desk (be careful the metal legs).

2) Encourage the terms –LIKE and UNLIKE poles.

Method:

1) Place one bar magnet into the cardboard cradle that you have.

2) Allow one of the group to hold the cotton thread so that the magnet is able to swing freely.

3) Now bring the north pole of your second magnet towards the one of the end of the suspended magnet.

4) Observe.

5) Now bring the opposite end of your magnet towards the suspended magnet.

6) Observe.

7) This time, taking note of what ends are involved in each instance, repeat the experiment and see if you can write down a Law after the discussion points have been dealt with.

3) After the experiment, student to note their results and come to the conclusion.

Like poles repel, (N-N and S-S)

Unlike poles attract (N-S, S-N)

Lesson 8 - Experiment 5: The directional property of magnets

Note: Be careful of any metal stands close to the magnet as it will affect the freely-swinging magnet.

Lesson 9 - The magnetic field of the earth and a compass

1) This is usually a teacher directed lesson as most students have very little knowledge about the magnetic field of the earth.

Lesson 10 - Experiment 6/7: Magnetic field lines around magnets

WE ARE LEARNING TO:

• Determine the pattern of a magnetic field surrounding all magnets.
• Make a temporary magnet through magnetic induction.

Success Criteria

WE KNOW WE ARE SUCCESSFUL WHEN WE CAN:

• Discuss, explain and draw the magnetic field surrounding a magnet.
• Explain and demonstrate how to make a temporary magnet by means of induction

Note:

1) Please ensure the supports on which the magnet are strong enough to keep the card level.

2) The supports should not be too close together.

3) The supports must be non-magnetic material.

4) The students must sprinkle the iron filings on the card. Using the salt cellar. The filings should be sprinkled sparingly and the card tapped lightly with the pen/pencil. The magnetic field surrounds the magnet in all directions. It is 3-dimensional.

Method for Experiment 6

1) Place four supports around a bar magnet.

2). Place a piece of cardboard on top of the magnet.

3) Sprinkle some iron filings onto the cardboard.

4) Gently tap the cardboard.

5) Observe. Complete the drawing of the magnetic field lines.

6) Repeat the experiment this time place plotting compasses around the magnet.

6) Now arrange two bar magnets as shown in the second drawing and repeat and then complete the third experiment.

7) As a final experiment, place a horse-shoe/u-shaped magnet under the cardboard, sprinkle iron filings and observe.

8) Complete a neat diagram for each result.

Lesson 11 - Experiment 8: Making a magnet using magnetic induction

Note: 1) Permanent magnets are usually made of steel as it retains. Iron is unsuitable because it loses its magnetism rapidly.

2) Permanent magnets are from steel and electrical currents are used to induce the magnetism.

3) For this experiment we will use an iron nail. It will give the students a temporary magnet.

4) The more strokes used increase the strength of the magnet (up to a limit).

5) The original magnet does NOT lose any of its magnetism no matter how many temporary magnets are made.

1) It is important that the student makes contact with the nail and strokes the nail using deliberate wide sweeps. The wide sweep will ensure the nail becomes a temporary magnet. If the student uses short strokes or sweeps back and forth they will never get the nail magnetised.

4) It is very important that you lift the magnet high away from the nail during your sweep.

5) After you have done this for some time, bring the point of the nail towards some iron filings.

6) Observe.

7) Now tap the nail/heat it in a flame for a while.

8) Bring the point of the nail towards some iron filings.

9) Observe.

2) Students can use a compass (suspended magnet) to determine what pole the tip of the nail is. There is a ‘Law” that allows one to determine the pole at the end of the sweep.

“ The pole of the temporary at the end of the sweep will be opposite to the pole used in making the magnet”

Lesson 12 - Experiment 9: Making an own electromagnet.

Learning Intention

• How an electro-magnet works
• Make an electro-magnet
• Determine the effect of an electric current on a magnet (compass)

Success Criteria

WE KNOW WE ARE SUCCESSFUL WHEN WE CAN:

· Make an electro-magnet

· When we can explain how an electro-magnet works

· What factors affect the strength of temporary electro-magnet

· Explain that a current-bearing conductor produces a temporary magnetic field around it.

Please make sure the students wind the insulated wire in one direction only. If this is not done the coils will ‘neutralise’ each other and the electro-magnet will not work.

4) Connect the ends to a battery as shown.

5) Place the tip of the nail into the iron filings once again and observe.

6) While holding steady, disconnect the cell.

7) Observe the result from a short while.

Note:

Some iron filings may adhere to tip of the nail as a result of previous experiments. Students simply wipe the iron filings off and continue with the experiment.

The quantity of iron filings that clings to the tip of the magnet can depend on a number of factors:

• The number of turns - the more turns the student has on the nail, the greater the strength
• The strength of the cell/battery
• The type of nail used.

Lesson 13 - Experiment 10/11: To determine what effect an electric current has on a compass

Note:

The conducting wire must be tightly strung between the wooden filter stands.

The students must place the compass close to the conducting wire.

The students can be encouraged to place the compass needle above, below and alongside the current bearing conductor. In each case a different result will be obtained.

Note:

The behaviour of the compass needle when the current is turned on would indicate that there is a magnetic field around the wire. This magnetic field affects the compass needle. As soon as the current is turned off the magnetic field is no longer present. The magnetic field is a temporary induced field.

Activity