Magnetostatics

Welcome to this learning area, where you will explore the differences between magnetic materials and non-magnetic materials as an important part of understanding magnetic forces. Magnets are important in various technologies, and knowing which materials are attracted to magnets forms the foundation of knowledge in both science and engineering.

Fig 6.18: Magnetic and non-magnetic materials with a bar magnet When you observe the picture above you will see that some materials are pulled by the magnets while other materials do not attract to the magnets. In the activities ahead, you will see how these materials behave around magnets and learn why magnetism is important in our everyday lives.

It should be noted that the red and blue bar magnet shown in the diagram and often used it school laboratories is a permanent magnet. This means that it has been magnetised (keep reading for more information about how this is achieved) and has remained magnetic.

The materials shown being attracted to the magnet are being temporarily magnetised as they enter the magnetic field of the bar magnet. This is called induced magnetism. However, when they are removed from that field they will demagnetise (hence, two paper clips do not magnetically attract one another).

The force between a permanent magnet and a temporarily magnetised material is always attractive.

Let us move into hands-on activities to see how magnets behave towards certain materials.

Activity 6.18: Test materials with a magnet to determine their magnetic properties.

Objective: Distinguish between magnetic and non-magnetic materials through hands-on experimentation.

Procedure:

1. Gather a variety of items from around your home or classroom. Ensure that you include some different types of metal as well as plastics and other insulators.

2. Use the magnet to test each item by bringing the magnet close to it.

3. Observe which items are attracted to the magnet and which are not.

4. Create a table with two columns: materials “attracted by magnet” and “not attracted by magnet.” Place each item under the appropriate category based on your observations.

Table 6.3: Table to record observations made in activity 6.18 Attracted by magnet Not attracted by magnet

Note: Materials that are attracted by the magnet are known as magnetic materials and those that are not attracted are non-magnetic materials Discussion Questions:

1. Why are magnetic materials are used in devices like compasses, motors, refrigerator doors and generators?

2. Think of situations where non-magnetic materials are necessary, like in electronics or medical equipment. Discuss your findings with your peers.

3. Discuss items used at home or in school that are magnetic Refer to Annex 6.5 for further information on discussions questions.

Magnetic Fields

When you hold a bar magnet in your hand, there is a real area of influence around it, even though you cannot see it. This area is called a magnetic field. When sprinkle iron filings around the bar magnet, the filings align themselves along the magnetic field lines, revealing the field’s shape as shown in the figure below.

Fig 6.19: Magnetic field lines around a bar magnet (with iron filings and diagram representation) The iron filings make the invisible magnetic field visible, giving you a clear understanding of how magnetic field surround magnets and how they affect nearby objects.

To explore this concept further, here are series of activities you can explore.

Activity 6.19: Visualising magnetic fields with iron filings, a compass and a bar magnet Objective: To understand the properties of magnetic fields by visualising and mapping them.

Materials Needed:

· Bar magnets · Iron filings · Compass · A4-sized white paper Procedure: Magnetic Field Mapping

1. Place a bar magnet under a piece of white paper.

2. Gently sprinkle iron filings evenly over the paper and gently tap the paper.

3. Observe how the iron filings align themselves along the magnetic field lines around the magnet.

4. Use a compass to detect which end of the magnet is north and which is south (note: compasses point out of the North Pole and into the South Pole; this is known as the direction of the magnetic field).

5. Draw the observed pattern on a separate sheet of paper, noting the shape of the field lines and the areas in which they appear more and less densely packed. Add directional arrows to the field lines to show the North to South direction.

Fig 6.20: Iron fillings sprinkled on a bar magnet Discussion questions

1. What shape did the iron filings make around the magnet?

2. How do these field lines help you understand the strength of a magnetic field?

3. Why is the compass crucial for determining the direction of the field?

Activity 6.20: Creating and experimenting with an electromagnet Objective: Create a simple electromagnet and explore the factors that affect its strength.

Materials needed:

· Three 1.5V cells · Copper wire · Iron nail · Switch · 20 paper clips Procedure:

1. Wrap the copper wire around the iron nail and connect the ends of the wire to the battery terminals to create an electromagnet. Ensure that there is a switch in series with the electromagnet so that it can be turned on and off; this avoids the risk of overheating.

2. Test your electromagnet by seeing how many small magnetic objects, like paper clips, it can pick up.

3. Experiment with the following variables:

a. Increase the number of wire turns around the nail and observe the effect on the electromagnet’s strength.

b. Increase the number of dry cells to two and then to three and observe how it affects the electromagnet.

Fig 6.21: Electromagnet formed by a coil of wire around a nail connected to a cell Discussion Questions:

1. Why do you think the iron nail makes the electromagnet stronger?

2. What happens if you use wood or plastic, instead of the iron nail?

3. How does adding more wire coils around the nail change the magnet?

Activity 6.21: Visualising Magnetic Fields with Iron Filings and a Bar Magnet Materials needed:

· Copper wire · Sheet of paper or card · Two 1.5V batteries · Switch · Compasses Procedure:

1. Set up a simple circuit with a straight conductor passing vertically through a piece of paper and sprinkle iron filings over the paper or place compasses around the wire as shown in the figures below Fig 6.22: schematic of the experimental setup

2. Close the circuit to allow current to flow through the conductor.

3. Observe how the iron filings align along the magnetic field lines created by the current.

4. Draw the observed pattern and compare it with the magnetic field of the bar magnet.

Discussion Questions

1. What shape did the iron filings make around the vertical wire?

2. What does this tell you about the magnetic field around the wire?

3. What happens to the iron filings when the electricity is turned off?

Magnetisation And Demagnetisation

Welcome to this exciting learning area where you will explore how magnetisation and demagnetisation shape the world around us.

Magnetisation is the process of turning a material into a magnet. This happens when the magnetic domains (tiny regions within the material) align in the same direction, creating a magnetic field that can attract or repel other magnetic objects.

Study carefully the diagrams below and discuss your observations with you colleagues Fig 6. 23: From unmagnetised to magnetised: alignment of magnetic domains Demagnetisation is the process of reducing or completely removing the magnetic properties of a material. This occurs when the alignment of the magnetic domains is disrupted, causing the material to lose its magnetism.

Fig 6.24: From magnetised to unmagnetised: alignment of magnetic domains Let us explore these concepts further with the activities below

Activity 6.22: Demonstrating magnetisation methods Objective: To show how to magnetise an iron nail using three methods:

stroking, electrical, and induction.

Materials needed:

· An iron nail (about 2-3 inches long) · A strong magnet (like a fridge magnet or bar magnet) · A small dry cell (1.5V) · Insulated copper wire · A few small metal objects (e.g. paperclips or pins) Procedure:

Task 1: Stroking Method

1. Place the iron nail flat on a surface.

2. Hold the bar magnet in one hand.

3. Rub the bar magnet along the length of the nail from one end to the other, always in the same direction.

4. Do this 20-30 times.

5. Remove the bar magnet and test the nail by trying to pick up small metal objects.

6. Note how the nail has become magnetised and can pick up metal objects.

Fig 6.25: Magnetising a nail by the stroking method Task 2: Electrical Method

1. Wrap the copper wire tightly around the iron nail, leaving the ends of the wire free. This creates a simple electromagnet.

2. Attach the ends of the wire to the terminals of the battery (one end to the positive terminal and one end to the negative terminal).

3. After a few seconds, disconnect the battery; the wire itself will no longer be magnetic when there is no current passing through it, so any retained magnetism will be due to the iron nail.

4. Test the nail by trying to pick up small metal objects.

5. Observe how the nail has become temporarily magnetised.

Fig 6.26: Magnetising a nail by the electrical method Discussion Questions

1. How did each method affect the iron nail?

2. Which method was the easiest for you to perform? Why?

3. How does each method of magnetisation work to turn the nail into a magnet?

Activity 6.23: Demagnetisation Methods

Task 1: Heating Method

1. Light a candle or use a lighter.

2. Using pliers or tongs, hold the magnetised nail over the flame until it’s hot, but not red-hot.

3. Allow the nail to cool naturally or dip it in water.

4. After cooling, try to pick up small metal objects.

5. Notice how the nail loses its magnetism after heating.

Fig 6.27: Demagnetising a magnet by heating Task 2: Hammering Method

1. Place the magnetised nail flat on a hard surface.

2. Gently tap the nail with a hammer along its length to disrupt the magnetic domains.

3. Try to pick up small metal objects after hammering.

4. Observe how the nail’s magnetism decreases or disappears.

Fig 6.28: Demagnetising a magnet by hammering Task 3: Alternating Current (AC) Method (Under teacher’s guidance)

1. Under supervision, connect the magnetised nail to an AC power source through a coil.

2. Run the AC current through the coil for a few seconds, disrupting the magnetic alignment.

3. Disconnect the nail and test it by trying to pick up small metal objects.

4. Observe how the nail loses its magnetism after exposure to alternating current.

Fig 6.29: Demagnetising a magnet with alternating current (ac) Discussion Questions

1. How did each method affect the iron nail’s magnetism?

2. Which demagnetisation method worked best? Why do you think so?

Activity 6.24: Exploring the application of magnetic fields in navigation Materials needed:

· A small needle · A bar magnet · A piece of cork or foam · A bowl of water.

Procedure:

1. Rub the needle with the bar magnet about 30 times, always in the same direction.

2. Stick the needle through the piece of cork or foam and gently place it on the surface of the water as shown in the figure below Fig 6.30: Simple compass using a magnetised nail

3. Watch as the needle slowly aligns itself to point north. You’ve just made a simple compass!

4. Try turning the bowl or moving it around. Does the needle always point the same way? Why do you think this happens?

5. Take your magnet outside and instruct a peer to walk a certain path by giving them instructions to follow using their magnet, e.g. “walk 200 steps to the East, now walk 30 steps to the South” etc.

Discussion Questions

1. What direction did the needle point after you floated it on the water?

2. Why do you think the needle points in a specific direction, even when you move the bowl?

3. How do you think this simple compass could be useful in finding your way?

Annex 6.1 – Solutions (Conductors, Semi-conductors and Insulators)

Activity 6.2

The glass rod charged using silk will not build up a large charge as there is little friction between the two surfaces as they are rubbed together. Friction is needed to ‘do work’ (transfer energy to) the electrons and hence to enable them to be liberated from one surface to build up on another. It is this separation of electrons which makes the plastic rod charged when rubbed with the fur, where there is significantly more friction.

Activity 6.3

The balloon becomes charged by the application of friction. When this happens, electrons move from your hair to the balloon or vice versa; hence, the hair and the balloon are now oppositely charged. Opposite charges attract, and so your hair sticks to the balloon.

Fig 6.31: Illustration of electrostatic attraction to a wall and repulsion between like charges When the balloon is placed next to a wall, it also sticks. This seems counter intuitive as the wall is not charged, but the atoms within the wall are able to rotate and move in such a way that the positive ‘sides’ of them attract to the negatively charged balloon (or vice versa if the balloon is positive) and the balloon sticks.

These demonstrations would not work using something made of metal instead of a balloon because a metal in a conductor; charge does not build up on its surface in the same way as the electrons are able to flow freely through the object and then down to the ground via your body.

Activity 6.4

Plastic rod: the ammeter reading is zero because plastic is an insulator and does not conduct electricity.

Piece of wire: the ammeter reads a current because metal is a conductor.

Thermistor: the ammeter reading is higher when the thermistor is warm; this is because the semiconducting material inside it liberates more electrons into the conduction band at higher temperatures and so a greater current is measured.

LDR: the ammeter reading is higher when the LCR is under a bright light; this is because the semiconducting material inside it liberates more electrons into the conduction band at higher brightness and so a greater current is measured.

Activity 6.6

Table 6.4:

Material Conductor Semiconductor Insulator

Glass ✓ Metal iron ✓ Silicon ✓ Germanium ✓ Plastics ✓ Wood ✓ Annex 6.2 – Further information (Conductors, Semi- conductors and Insulators) Electronic Properties of materials

1. Conductivity is the ability of a conductor to conduct electricity due to the presence of mobile electrons in the conduction band

2. Resistivity is the inability of an insulator to conduct electricity due to the absence of mobile electrons in the conduction band Conductors Conductors are materials that allow a flow of electric charge through them. They could be described as materials which have higher electrical conductivity and lower resistivity because of completely filled mobile electrons in the conduction band at normal temperature.

Examples of conductors are earth, impure water, human beings, electrolytes, metals such as silver, copper, gold and aluminium. Non-metals such as carbon (graphite) are good conductors. Certain fluids (i.e. liquids and gases) can conduct electricity; mercury is an excellent liquid conductor.

Some liquid ionic compounds are also good conductors because they have charge carriers. These charge carriers are the ions (cations and anions) in the fluid.

Electrolytes are conductors because they exist in ions. For instance, aqueous H₂SO₄is a conductor according to the reaction:

H₂SO₄(aq) ⟶ 2H+ + SO₄ ²⁻Properties of Conductors

1. Conductors have high electrical conductivity and very low electrical resistivity at normal temperature.

2. Conductors have charge carriers. Free electrons are the charge carriers in solid conductors. For fluid conductors, the charge carriers are the ions. The number of charge carriers determines the conductivity. Larger number of charge carriers give higher electrical conductivity and vice versa.

3. Metallic bonding: Most conductors are metals, which have a unique type of bonding where electrons are delocalised and can move freely throughout the material.

4. High thermal conductivity: Conductors are also good at transferring heat.

This is because the free electrons can carry both electrical and thermal energy.

5. Ductility and malleability: Many conductors are easily shaped and formed, which makes them suitable for various applications.

6. Lustrous appearance: Conductors often have a shiny or reflective surface Semiconductors A semiconductor is a material whose electrical conductivity lies between a conductor and an insulator. They can be described as a material which has moderate electrical conductivity and resistivity. Their conductivity can be controlled by various means, such as temperature changes, light exposure, or the addition of impurities (doping).

Temperature Sensitivity: The conductivity of semiconductors increases with temperature. As the temperature rises, more electrons gain enough energy to jump from the valence band to the conduction band, thus increasing conductivity.

This is in contrast to conductors, where conductivity typically decreases with temperature.

Photoconductivity: Semiconductors exhibit photoconductivity, meaning their electrical conductivity increases when exposed to light. This property is exploited in devices like photodiodes and solar cells.

Semiconductors often exhibit nonlinear characteristics, meaning the relationship between current and voltage is not a straight line. This property is crucial for the operation of devices like transistors, which can amplify signals. Semiconductors are the building blocks of integrated circuits (ICs), which are used in virtually all electronic devices, including computers, smartphones, and communication systems.

Insulators An insulator is a material which has a high electrical resistivity and low electrical conductivity. Examples of insulators are non-metals such as glass, plastic, rubber, wood, sand, quartz, Teflon and carbon (diamond).

An insulator is a solid whose valence band is completely filled with electrons but with a completely empty conduction band and a wider forbidden gap. The energy required to raise a valence electron into the conduction band is about 5eV – 6eV.

Properties of Insulators

1. High Electrical Resistance: Insulators have very high electrical resistance, meaning they do not easily allow the flow of electric current. This makes them ideal for protecting against electrical shocks and preventing short circuits.

2. Low Conductivity: Insulators have low electrical and thermal conductivity.

They do not conduct electricity or heat efficiently, which is why they are used to isolate conductors and prevent the transfer of energy.

3. High Dielectric Strength: Dielectric strength refers to the ability of an insulating material to withstand high voltages without breaking down.

Insulators have high dielectric strength, making them effective at withstanding strong electric fields.

4. Thermal Insulation: Insulators are also used to reduce the transfer of heat.

They have low thermal conductivity, meaning they slow down the rate at which heat is transferred from one area to another.

These properties make insulators essential in electrical engineering, construction, and various industrial applications where the control of electricity and heat is critical.

Annex 6.3- Solutions to some activities on Charge

Activity 6.8

Compare your mind map to the one below. Note that the diagram below is just an example and that you may have considered other properties or found other links between them!

Fig 6.32: Concept map of fundamental properties of matter

Activity 6.9

Discussion questions

1. What happens when you rub the cloth on different objects?

Answer: Some objects attract the tissue paper, showing they have built up static electricity.

2. Does the shape of the object affect static electricity?

Answer: Yes, pointy objects can lose charge more easily, while round objects can hold the charge longer.

3. Which objects are better at holding a charge, conductors or insulators?

Answer: Insulators, like wood or plastic, hold charge better because the electrons cannot move freely.

4. What is an example of static electricity in everyday life?

Answer: When your hair sticks to a balloon after rubbing it, that’s static electricity!

Activity 6.12

When the balloon is brought close to the spoon it creates a very small electric current, which we see as a flash of light, as the electrons which have built up on the balloon transfer to the spoon. This mimics the transfer of charge between the clouds and the ground when lightning strikes.

Activity 6.13

When the balloons are charged, they both become charged. Because they are both made of the same material, they both gain the same charge (in this case, negative). Therefore, when they are brought close to one another they repel and move away from one another.

Activity 6.14

1. 100 × 30 × 60 = 180,000C

2. 15 × 60 × 60 = 54,000C

3. 0.02 × 5 × 60 = 6C

4. 1/0.05 = 20 s

5. 1.13 × 10²⁴, 3.38 × 10²³, 3.75 × 10¹⁹, 6.25 × 10¹⁸Activity 6.17 Charge Conservation: The total electric charge stays the same during decay.

In beta-minus decay, a neutron changes into a proton and gives off an electron (called a beta particle) and an antineutrino. In beta-plus decay, a proton changes into a neutron and gives off a positron and a neutrino. Even though particles change, the overall charge remains the same.

Lepton Number Conservation: Just like charge, the lepton number is also kept the same. This means that when particles like electrons or positrons are released, neutrinos or antineutrinos are released too. This keeps the lepton number balanced.

The Role of Neutrinos: Neutrinos were discovered to explain why energy and momentum seemed missing in decay processes. They help balance out the energy and momentum that can’t be seen directly.

Energy and Momentum Conservation: In any decay process, energy and momentum are always conserved. Neutrinos and other particles help make sure that these important properties are balanced.

Annex 6.4 – Further information on Charge A Bit of History The study of electric charge has a long history, starting with ancient discoveries and leading to modern technology. Here are the key moments:

1. Ancient Greece: Thales of Miletus discovered that rubbing amber could attract small objects like feathers. This was one of the first observations of static electricity.

2. Benjamin Franklin (1706-1790): Franklin’s famous kite experiment proved that lightning is a form of electricity. He also introduced terms like “positive” and “negative” charges and invented the terms “battery” and “conductor.”

3. Charles-Augustin de Coulomb (1736-1806): Coulomb measured the forces between electric charges, leading to Coulomb’s law, which explains how charged objects attract or repel each other.

4. Michael Faraday (1791-1867): Faraday’s work on electromagnetism showed how electric charges interact with magnets, leading to inventions like electric motors and generators.

5. James Clerk Maxwell (1831-1879): Maxwell’s equations connected electric and magnetic fields, laying the foundation for modern electromagnetism.

Real-World Applications of Charge

1. Electronics: Electric charge is the fundamental principle behind the operation of electronic devices, from smartphones and computers to televisions and radios.

2. Medical Devices: In medicine, electric charge plays a key role in devices like electrocardiograms (ECGs) that monitor heart activity and in procedures like electrosurgery, where electric charges are used to cut tissue.

3. Industrial Applications: Electrostatic precipitators use electric charges to remove pollutants from industrial exhaust gases. Similarly, electroplating uses electric charges to coat objects with a thin layer of metal.

4. Environmental Protection: Lightning rods use the principle of charge concentration at sharp points to protect buildings by providing a safe path for lightning discharge.

5. Printing and Photocopying: Photocopiers and laser printers rely on electric charges to attract toner particles to paper, creating images and text.

Annex 6.5 – Solutions to some activities on Magnetic Fields

Activity 6.18

Discussion questions

1. Why are magnetic materials used in devices like compasses, motors, refrigerator doors, and generators?

Answer: Magnetic materials are used in these devices because they respond to magnetic fields. In a compass, the magnet helps point to the Earth’s North. Motors and generators rely on magnets to create movement or generate electricity. Refrigerator doors use magnets to keep them closed tightly.

2. Why are non-magnetic materials important in electronics or medical equipment?

Answer: They don’t interfere with magnets, which helps equipment like MRI machines and electronic devices work correctly.

3. What are some magnetic items you use at home or school?

Answer: Examples include fridge magnets, metal tools, and cabinet latches. These items stick to magnets because they are made of magnetic materials.

Activity 6.19

Discussion questions

1. What shape did the iron filings make around the magnet?

Answer: The iron filings formed a pattern of lines that curve from the north pole to the south pole of the magnet, showing the shape of the magnetic field.

2. How do these field lines help you understand the strength of a magnetic field?

Answer: The spacing of the field lines indicates the strength of the magnetic field: closer lines mean a stronger field.

3. Why is the compass crucial for determining the direction of the magnetic field?

Answer: The pattern shown by the iron filings is symmetrical and on its own gives no indication of the north and south poles.

Activity 6.20

1. Why do you think the iron nail makes the electromagnet stronger?

Answer: The iron nail makes the electromagnet stronger because it helps concentrate the magnetic field. Iron is a magnetic material that enhances the magnetic force created by the copper wire.

2. What happens if you use wood or plastic instead of the iron nail?

Answer: If you use wood or plastic, the electromagnet will be weaker or might not work at all because these materials do not strengthen the magnetic field like iron does.

3. How does adding more wire coils around the nail change the magnet?

Answer: Adding more wire coils around the nail makes the electromagnet stronger because each coil has its own magnetic field and adds a further contribution to the overall field.

Activity 6.21

1. What shape did the iron filings make around the vertical wire?

Answer: The iron filings formed concentric circles around the vertical wire, showing that the magnetic field lines wrap around the wire in a circular pattern.

Fig 6.33: Iron filings forming concentric circles around a vertical wire

2. What does this tell you about the magnetic field around the wire?

Answer: This pattern indicates that the magnetic field around a current- carrying wire forms circular loops that encircle the wire. The direction of the field can be determined using the right-hand rule.

Fig 6.34: magnetic field around a current-carrying wire

3. What happens to the iron filings when the electricity is turned off?

Answer: When the electricity is turned off, the iron filings will no longer align in circular patterns around the wire because there is no magnetic field generated by the current. The filings will lose their alignment and return to a random arrangement.

Activity 6.22

1. How did each method affect the iron nail?

Answer: The stroking method gradually magnetised the nail, allowing it to pick up small metal objects. The electrical method made the nail a stronger magnet while the battery was connected, but it retained some weak magnetism when the current was removed.

2. Which method was the easiest for you to perform? Why?

Answer: The stroking method might be the easiest because it requires only a magnet and a nail, without needing additional materials like wires or batteries.

3. How does each method of magnetization work to turn the nail into a magnet?

· Stroking Method: Aligns the magnetic domains in the iron nail in one direction, turning it into a magnet.

· Electrical Method: Creates a magnetic field around the nail when electricity flows through the wire, magnetising the nail.

Activity 6.23

1. How did each method affect the iron nail’s magnetism?

· Heating Method: The nail lost its magnetism after being heated.

· Hammering Method: The nail’s magnetism decreased or disappeared after being tapped.

· AC Method: The nail lost its magnetism after being exposed to alternating current.

2. Which demagnetization method worked best? Why do you think so?

· Best Method: The AC method usually works best because it disrupts the magnetic domains thoroughly and effectively.

· Reason: AC current creates a fluctuating magnetic field that can completely disrupt the alignment of the magnetic domains.

Activity 6.24

1. What direction did the needle point after you floated it on the water?

Answer: The needle pointed north.

2. Why do you think the needle points in a specific direction, even when you move the bowl?

Answer: The needle points north because it aligns with Earth’s magnetic field, which always points toward magnetic south (geographic north).

3. How do you think this simple compass could be useful in finding your way?

Answer: This simple compass helps you find direction by showing which way is north, helping you navigate and determine your location.