Electrostatics

Welcome to your first introduction to the field of electromagnetism! This section will provide a solid foundation in understanding how electricity and magnetism work, and how they relate to one another, both in everyday life and in various technologies.

At the end of this section, you should be able to;

· Explain how the gold leaf electroscope can detect the charge carried by a body.

· Identify electrons as mobile charge carriers.

· Explain how charge carriers in conductors, semiconductors and insulators behave.

· Explain the distribution of charges on surfaces; spherical, pear shaped and sharp points.

· Define charge as a fundamental property of matter (like mass).

· Explain the conservation of charge and its behaviour.

· Differentiate between the two charges (positive and negative).

· Distinguish between magnetic and non-magnetic materials.

· Describe the magnetic field.

· Describe the processes involved in magnetisation and demagnetisation.

Key ideas

· The Gold Leaf Electroscope is a tool used to detect electric charges.

· Mobile charge carriers, such as electrons, create electric currents by moving through conductors.

· Electric charge is a fundamental property of matter that causes it to interact with electric fields.

· Charge distribution refers to how electric charge is spread across an object, while the conservation of charge principle states that charge cannot be created or destroyed, only transferred.

· Magnetic materials are materials attracted to magnets, and non-magnetic materials, which are not.

· The magnetic field is the area around a magnet where its force can be felt.

Lastly, magnetisation is the process of making a material magnetic, and demagnetisation is the process of removing its magnetism.

Electrons Electrons are tiny particles that orbit the nucleus of an atom. They’re also mobile charge carriers, meaning they can move freely within some materials, such as metals, when they gain enough energy to escape their atoms. As they flow, they carry charge with them.

Detecting charge Fig 6.1: The Gold leaf electroscope A gold leaf electroscope is an instrument for detecting and testing small electric charges. This instrument is also used for detecting and measuring static electricity or voltage.

The outside metal disc, made of brass, is connected to a narrow metal plate inside the metal case and a thin piece of gold leaf is fixed to the plate. The complete electroscope is insulated from the body of the instrument.

Fig 6.2: The Gold leaf electroscope When a charged (positive or negative) body is brought near to the metal disc:

· If a negative charge is brought near, electrons in the electroscope are repelled, causing them to move downward to the leaves. This makes both leaves negatively charged, leading to divergence.

· If a positive charge is brought near, electrons from the leaves are attracted to the disc. This makes both leaves positively charged, leading to divergence.

· If the charged object actually touches the metal plate, it will transfer charge to the electroscope and the leaf will remain lifted from the metal rod.

· If a different charged object is then brought near to the electroscope its charge can be determined by observing whether the leaf is repelled further or falls closer the rod.

The leaf can be made to fall again by touching the disc and therefore allowing the built-up charge to dissipate through your body and to the earth (earthing). An earth terminal prevents the case from becoming live.

Activity 6.1: Investigating the gold leaf electroscope

Note: in the absence of practical equipment, you can use the following simulation to perform the experiment:

Materials needed:

· Gold leaf electroscope (or video/picture) · Balloon or plastic rod for charging · Dry cloth or fur for rubbing Procedure:

1. Observe the electroscope and identify its key components (compare to the diagram above).

Fig 6.3: Gold-leaf electroscope with scale

2. Rub the balloon or plastic rod with the dry cloth or fur to generate static electricity.

Fig 6.4: Charging a rubber rod by friction

3. Bring the charged object near the electroscope without touching it.

4. Observe how the metal leaves respond.

5. Consider ways in which you might alter the response of the electroscope and investigate these. Can you make it attract instead of repel? Can you make it repel further from the metal rod?

6. Discussion:

a. Describe the observations and explain the reasons behind the responses.

b. Discuss the concept of electric charge and its detection using the gold leaf electroscope.

Activity 6.2 Charging the Electroscope by Friction

Materials needed:

· Gold leaf electroscope · Glass rod · Silk cloth · Plastic rod · Fur cloth Procedure:

1. Demonstration 1: Glass Rod with Silk:

Fig 6.5: Charging a rubber rod by Friction

a. Rub the glass rod with silk to generate static electricity.

b. Predict the behaviour of the electroscope before charging.

c. Charge the electroscope by touching the glass rod to the metal rod.

d. Observe and record the behaviour of the gold leaves.

2. Demonstration 2: Plastic Rod with Fur:

a. Rub the plastic rod with fur to generate static electricity.

b. Predict the behaviour of the electroscope before charging.

c. Charge the electroscope by touching the plastic rod to the metal rod.

d. Observe and record the behaviour of the gold leaves.

3. Comparison and Discussion:

a. Compare the behaviour of the electroscope for both demonstrations.

b. Discuss the reasons behind the differences in behaviour.

See Annex 6.1 for further explanation.

4. Conclusion:

a. Summarise the key points about charging the electroscope by friction.

b. Emphasise the importance of understanding electric charge and its detection.

Electrical Properties of Materials

Conductors have high electrical conductivity; low resistance and delocalised (free) electrons flow easily. Examples; aluminium, copper, gold.

Semiconductors have medium electrical conductivity, medium resistance and electrons flow with some restriction. However, many semiconductors can be made more or less resistive based on environmental conditions such as brightness and temperature. Examples; silicon, germanium.

Insulators have low electrical conductivity, high resistance and electrons are tightly bound and so they don’t flow easily. Examples; wood, glass, plastics.

Charge carriers are particles that carry electric charge, and their behaviour differs in conductors, semiconductors, and insulators due to differences in their electronic structure and the energy levels of their electrons.

See Annex 6.2 for further information about conductors, insulators and semiconductors.

Balancing of charges and net charge Fig 6.6: net charges of neutral, positively charged and negatively charged objects The first diagram contains 6 negative and 6 positive charges; therefore, their overall charge is (0) zero which implies the material is neutral. The overall charge for the 2ⁿᵈone is 2. That of the 3ʳᵈone is 3.

Behaviour of two bodies of different charges Fig 6.7: Behaviour of two bodies of different charges If an electron is removed from an atom or material a net positive charge builds up in atom or material, and therefore the atom or material is said to be positively charged. When a neutral atom or material gains an electron, it has a net negative charge and hence the atom or material is negatively charged. Thus, when two different material such as glass and silk are put into contact by rubbing, the glass gives up some electron to the silk. Thus, the silk now has a negative charge, while the glass has a deficiency of negative charge which means that it has a positive charge (+).

Example: Transfer of charges between human hair and a comb When you comb the hair, electrons move from the hair to the comb by friction which results in negative charge on the comb and a positive charge on the hair.

Fig 6.8: Electrostatic charging of hair by combing

Activity 6.3: Investigating the movement of charge in insulators Fig 6.9: Balloon rubbed against hair Explore static electricity and its effects through a fun and interactive experiment with balloons.

Materials Needed:

· A balloon (preferably latex, but be aware of those who may have allergies within the classroom) · A smooth, dry surface (like a table or your head) · Optional: a comb or your hair for additional effects Procedure:

1. Inflate the balloon: Blow up the balloon and tie it securely.

2. Charge the Balloon: Rub the balloon vigorously on your hair or piece of wool fabric for about 15-30 seconds. This will create static electricity.

3. Test the Attraction:

a. Bring the balloon close to your hair (or the wool fabric) without touching it. Observe how your hair stands up and becomes attracted to the balloon. Slowly move the balloon away from your hair and observe how far you can go before the attraction stops.

b. You can also hold the balloon near small pieces of paper to see if they get attracted.

c. Press the charged balloon against the smooth wall. Hold it there for a few seconds and then release it. Observe how the balloon sticks to the wall.

4. Discussion:

a. Why did your hair stand up? Use the comb example given above to help form your answer.

b. What is static electricity?

c. Discuss why the balloon sticks to the wall. Explain how the static charge creates an attractive force between the balloon and the wall.

d. Why could a conductor, such as a metal sheet or pan, not be used in the place of the balloon for these experiments?

Safety Tips:

· Ensure that the area is dry to maximize the effects of static electricity.

· Avoid using the activity near sensitive electronic devices as static electricity can cause damage.

Activity 6.4: Investigating the movement of charge in conductors and semiconductors Materials needed · Light bulb · Battery · Wires · Ammeter · Plastic rod · Switch · NTC thermistors (optional) · Beaker (optional) · Kettle (optional) · Water (optional) · Ice (optional) · LDRs (optional) Procedure:

1. Connect the circuit as shown below. Begin by connecting the plastic rod in the place of component X.

Fig 6.10: Simple electric circuit diagram with battery, lamp, and ammeter

2. Close the switch and record the reading on the ammeter.

3. Replace the plastic rod with a piece of metal wire.

4. Close the switch and record the reading on the ammeter.

5. Replace the piece of metal wire with an NTC thermistor.

6. Place the thermistor into a beaker of hot water and record the reading on the ammeter. Now, place it into a beaker of ice-cold water and record the reading on the ammeter.

7. Replace the NTC thermistor with an LDR.

8. Place the LDR under a bright light and record the reading on the ammeter. Now, cover the LDR with your hand and record the reading on the ammeter.

Discussion and conclusions:

Given that the plastic rod is an insulator, the wire is a conductor and the thermistor and LDR are both semiconductors, explain your observations of ammeter reading for each component.

See Annex 6.1 for explanations.

Activity 6.5: Classification of elements based on electrical conductivity Observe, in the periodic table below, the group and period where conductors, semiconductors and insulators are found.

· Conductors (e.g., copper, aluminium) · Semiconductors (e.g., silicon, germanium) · Insulators (e.g., oxygen, nitrogen) Fig 6.11: Periodic table of the elements

Activity 6.6: Classification of materials into conductors, semiconductors and insulators Classify the materials into conductors, semiconductors and insulators

Table 6.1: Table to complete Activity 6.6 Material Conductor Semiconductor Insulator Glass ✓ Metal iron Silicon Germanium Plastics Wood

Activity 6.7: Modelling Charge Flow in Conductors, Semiconductors, and Insulators In small groups, model the behaviour of the charge flowing through a) a conductor, b) a semi-conductor and c) an insulator.

What is Electric Charge?

Electric charge is a basic property of matter (just as mass is a basic property of matter) that causes it to interact with electric and magnetic forces. Imagine it as something that can make objects attract or repel each other, like when you rub a balloon on your hair, and it makes your hair stand up!

Electric charge is measured in Coulombs (C).

Activity 6.8: Mind Mapping Matter and Charge

Objective: Create a mind map to explore and connect the fundamental properties of matter, including electric charge.

Materials Needed:

· Large sheet of paper · Markers or coloured pencils Procedure:

1. Write “Properties of Matter” in the centre of your paper and draw a big circle around it.

2. Think of different properties of matter and electric charge, such as:

a. Attraction to a magnet

b. Electrical conductivity (how well something conducts electricity)

c. Thermal conductivity (how well something conducts heat)

d. Physical state (solid, liquid, gas)

e. Density (how heavy something is for its size)

f. Electric charge (positive and negative charges)

g. Anything else you can think of!

3. Draw lines from the central circle to smaller circles for each property.

4. Draw lines between related properties. For example:

a. Attraction to a magnet can be connected to Electrical conductivity because some magnetic materials are also good conductors.

b. Physical state can be connected to Density because density changes with the state of matter (e.g., ice vs. water).

5. Annotate your diagram with any additional information or detail about the links that you have made between the properties.

Distribution of Charges on Surfaces

Conductors:

When you place a charge on a conductor, the charges spread out evenly throughout the body and across the surface. This is because the charges want to stay as far apart as possible.

Fig 6.12: Examples of conductors Insulators:

When you place a charge on an insulator, it stays in the place where you put it.

The charges don’t move around much and stay put.

Fig 6.13: Examples of insulators Surface Charge Density

1. Surface charge density measures how much electric charge is spread over a certain area of a surface.

2. High Surface Charge Density: When there is a lot of charge packed into a small area, we have a high surface charge density. It’s like having many people squeezed into a small space.

3. Low Surface Charge Density: When the charge is spread over a larger area, the surface charge density is low. It’s like having a few people in a big space where they can spread out.

4. Surface charge density helps us understand how strongly a surface can attract or repel other charges. A high density means stronger effects.

Sharp Edges and Round Surfaces

Sharp Edges:

1. Charges tend to collect at sharp edges or points. This happens because the charges get crowded together at these points, making the surface charge density higher there. It is like having more people crowding into the corners of a room.

Round Surfaces:

1. On smooth, round surfaces, charges spread out more evenly.

2. The surface charge density is lower at any single point, and the charges are more evenly distributed.

Fig 6.14: Charge distribution on isolated conductors Summary:

1. Conductors let charges spread out evenly.

2. Insulators keep charges in one place.

3. Sharp edges collect more charges, creating higher surface charge density.

4. Round surfaces have charges spread out more evenly, leading to lower surface charge density.

Activity 6.9: Exploring Static Electricity and Charge Distribution

Objective: Explore static electricity and understand how different shapes and materials (conductors and insulators) affect the distribution of electric charge.

Materials needed:

· Spherical fruits (e.g., oranges, apples) · Tapered sticks (e.g., pencils, wooden dowels) · Sharp metallic objects (e.g., needles, nails) · Cloth (wool or synthetic) · Small pieces of tissue paper · Worksheet or table for observations Procedure:

1. Gather the materials.

2. Rub the cloth against each object for about 30 seconds.

3. Bring each object close to small pieces of tissue paper and observe what happens.

4. Use a worksheet to record your observations.

5. Fill in details like:

Table 6.2: Table to record observation made in Activity 6.9 Name of Object Shape of Object Material (Conductor or Insulator) Effect of Rubbing (Attraction or Repulsion of Tissue) Discussion questions:

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

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

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

Explain why.

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

Activity 6.10: Detecting charge: Build a Simple Electroscope Objective: Build a simple electroscope and observe how it reacts to charged objects.

Materials Needed:

· A glass jar (clear and clean) · Aluminium foil · Metal wire or unfolded paperclip · Sewing needle or nail · Scissors · Tape/glue · Small piece of cardboard Procedure

1. Take the glass jar and make sure it is clean and dry.

2. Punch a small hole in the centre of the jar’s lid and insert the metal wire or unfolded paperclip through it, securing it with tape or glue on the top side.

3. Cut two small strips of aluminium foil (about 2 cm wide and 4 cm long).

4. Attach these foil strips to the bottom end of the metal wire inside the jar, allowing them to hang freely as “leaves.”

5. Assemble the Electroscope: Close the jar with the lid, ensuring the foil leaves are inside (if not humidity may affect it) and hanging down. The metal wire should stand vertically in the centre.

6. Test the Electroscope: Rub a plastic rod or balloon with wool to charge it, then bring it close to the top of the metal wire without touching it.

Observe how the foil leaves spread apart as they gain the same type of charge and repel each other Fig 6.15: Steps to build a simple electroscope Follow: https://youtu.be/61gr7vAkVAA to watch a video of the electroscope being built.

Questions to think about:

1. What happens to the foil when you bring a charged object near?

2. Does the foil behave differently with a pointed object like a needle?

3. How does the charge build-up affect the electroscope?

Positive and Negative Charges

Everything around us contains tiny particles, some of which are called charges.

There are two types of charges: positive and negative. Positive charges are found in protons, and negative charges are found in electrons.

1. Positively Charged Object: This happens when an object loses electrons and has more protons than electrons.

2. Negatively Charged Object: This happens when an object gains electrons, making it have more electrons than protons.

When two objects have the same type of charge, they push away from each other (repel). When objects have opposite charges, they pull towards each other (attract).

By exploring how different objects interact with each other, we can see these charges in action with the activities below.

So, when we rub an insulator with a dry cloth, does it gain or lose its electrons to the cloth? Does it become positively or negatively charges?

The sequence below is called the triboelectric series. Below can help you predict which material will be positively or negatively charged during rubbing. Materials at the top (e.g., human skin, glass) tend to lose electrons easily and become positively charged. Materials at the bottom (e.g., plastic, Teflon) tend to gain electrons and become negatively charged:

Human skin Glass Hair Nylon Wool Silk Paper Cotton Wood Rubber Polyester Styrofoam Plastic (e.g., PVC) Teflon

Activity 6.11: Creating a mnemonic for the triboelectric series You can make up your own mnemonic of the triboelectric series to help you remember it at all times!

Positive and negative charges in nature

Example: Lightning

During a storm, clouds become charged. The top of the cloud becomes positively charged, and the bottom of the cloud becomes negatively charged.

The ground usually has a positive charge. When the difference between the charges becomes large enough, lightning occurs as electrons move from the cloud to the ground.

Activity 6.12: Simulating lightning Objective: Understand how charge differences create lightning.

Materials needed:

· A balloon · A metal object (like a spoon) · A piece of wool or cloth · A dark room Procedure:

1. Rub the balloon with wool to create a static charge.

2. Turn off the lights so that you are in pitch black conditions.

3. Hold the charged balloon close to the metal object and observe what happens.

4. Note how the balloon’s charge affects the metal object.

Questions:

a. What did you observe when you brought the balloon close to the metal object?

b. How does this mimic the concept of lightning?

5. Create a storyboard of the key stages in the process which cause thunder and lightning.

Activity 6.13: Investigating the interaction of like charges Objective: Explore how similar charges interact.

Materials needed:

· Two charged balloons (rubbed with wool), one tied to a piece of string · A piece of wool or a dry cloth Procedure:

1. Charge the Balloons: Rub both balloons with wool to create static electricity.

2. Test Repulsion: Hold the free balloon near to the balloon which is suspended by a string. Try to place the regions of the balloons which were rubbed with the wool close to one another.

3. Observe and Record: Note if the balloons attract or repel each other and explain why this has happened.

Conservation Of Charge

The principle of conservation of charge is a basic rule in electricity. It says that electric charge cannot be created or destroyed. Instead, it can only move from one place to another. This means that the total amount of charge in a closed system always stays the same. Understanding this principle helps us see how electrical devices work and how charges interact in everyday life.

Principle of Conservation of Charge

The principle of conservation of charge says that electric charge cannot be created or destroyed. It can only move from one place to another. This means the total amount of charge stays the same, even if it moves around.

This means that, in an electric circuit, the amount of charge entering a junction in a particular period of time is equal to the amount of charge leaving the junction in that same period of time. In the diagrams below, Q₁= Q₂+ Q₃and I₁= I₂+ I₃, where I is the current or the ‘rate of flow of charge’. This is known as Kirchhoff’s First Law.

Fig 6.16: Charge and current flow at a junction The above definition of current leads us to the relationship:

Current, I (Amperes, A) = Charge, Q (Coulombs, C)___________________ time, t (seconds, s) I = Q/t

Activity 6.14: Calculating charge and electron flow in electrical circuits

1. An electric car battery charges at a rate of 100 Amps. How much charge is transferred to the battery in 30 minutes?

2. A household circuit has a current of 15 Amps. How much charge flow through the circuit in one hour?

3. An LED uses a current of 20mA. How much charge flows through the LED in 5 minutes?

4. A small electronic device draws a current of 50mA. How long will it take to consume a charge of 1C?

5. For each of the examples above, find the number of electrons which make up the total charge given in the question. (Magnitude of the charge of an electron = 1.6 × 10⁻¹⁹C)

Activity 6.15: Experiment to verify Kirchhoff’s first law Materials needed:

· Resistors or bulbs · Battery · Switch · Connecting wires · Ammeter Procedure:

1. Connect the circuit as shown in the image below.

Fig 6.17: Electrical circuit showing a junction and Ammeters

2. Use the ammeter, measure the current in the positions labelled A1, A2 and A3.

3. Verify whether Kirchhoff’s First Law is supported by this experiment;

does the current entering the junction equal the sum of the currents leaving the junction?

Activity 6.16: Researching timeline of key discoveries in electricity and charge Research and prepare a timeline of the key discoveries in the field of charges and electricity; ensure that you include a description of the discoveries of Franklin, Coulomb, Faraday and Maxwell.

Activity 6.17: Exploring beta decay and conservation laws (extension task) Conduct independent research into beta-minus and beta-plus decay. Focus on:

1. Charge Conservation

2. Lepton Number Conservation

3. The Role of Neutrinos

4. Energy and Momentum Conservation

Review Question 6.1

1. Draw and explain the functions of the parts of a gold leaf electroscope.

2. Why does your hair stand up when you rub a balloon on it?

3. Why does dust sometimes cling to your TV screen?

4. Define mobile charge carriers.

5. Why do manufacturers use copper instead of rubber to produce electric wires?

6. Identify and explain the key electrical properties that differentiate conductors, semiconductors and insulators.

7. How do solar panels work during the day but not at night?

Review Questions 6.2

1. What is the principle of conservation of charge?

2. How do charges behave on the surface of a conductor?

3. Why are people advised to avoid being outdoors with metal objects during a thunderstorm?

4. How does the shape of a conductor affect charge distribution?

5. Explain why lightning rods are pointed at the top rather than flat.

6. A metal has a charge density of 8.4 × 10²⁸free electrons per cubic metre.

If the current in the circuit is 0.3A, and the cross-sectional area of the wire is 2.5 × 10⁻⁶m², what length of wire do the electrons travel, on average, during one second?

Review Questions 6.3

1. How does the magnetic field around a straight current-carrying conductor differ from that around a bar magnet? Use diagrams to support your explanation.

2. Describe the role of magnetisation in the functioning of a hard drive. How does demagnetisation ensure data security during recycling?

3. Explain the process of creating a simple electromagnet and discuss how the magnetisation process differs from that of a permanent magnet?