Chapter 7 — Electricity and Magnetism

What is electric charge?
Electric charge is a property of some objects that makes them pull or push other objects without touching. We notice charging when a balloon rubbed on hair sticks to a wall or when tiny paper pieces jump to a charged comb. There are two types of charge which we call positive and negative. Like charges push each other away (repel) and opposite charges pull each other together (attract).

How to observe charge
Simple experiments are enough to see charge. Rubbing a plastic comb on dry hair moves small bits of paper. A glass rod rubbed with silk becomes charged and can attract small pieces of paper. When two similar rods rubbed with the same cloth are brought close they repel; if rubbed with different materials they may attract. These easy observations show that charge is transferable and that it acts at a distance.

Conservation of charge idea
Charge can be transferred but total charge is conserved in a closed system. For example, rubbing transfers electrons from one object to another; the net amount of charge in the system remains the same. This concept helps explain why a charged object loses charge when it touches the ground: the charge flows away.

Practical notes
Dry conditions make static effects stronger because water in the air helps charges leak away. Many everyday phenomena such as shocks from door handles or sparks when touching metal in winter are due to static charge.

Definition and basic idea
Materials differ in how easily electric charge moves through them. Those that allow charge to move freely are called conductors; those that prevent charge movement are insulators. Conductors typically have free electrons that can move from atom to atom, while insulators hold their electrons tightly. Knowing which materials are conductors or insulators is important when designing circuits and when taking precautions to stay safe near electricity.

Common examples and why they behave so
Metals such as copper, aluminium, silver and iron are good conductors. Copper is widely used for electrical wiring because it has low resistance and conducts well. Plastics, rubber, glass, dry wood and porcelain act as insulators. The difference comes from atomic structure: in metals, some electrons are loosely bound and form an 'electron sea' that moves easily under an applied force; in insulators, electrons remain bound to atoms and cannot move freely.

Simple experiments to classify materials
Students can test materials using a simple battery-bulb-wires circuit. Insert the test material in a break in the wire: if the bulb lights, the material allows current and is a conductor; if not, it is an insulator. Another test uses a charged object: bring a charged comb near different items and observe whether charge transfers or is felt. Moisture matters: dry wood may be an insulator, but wet wood can conduct because water provides ions and paths for current.

Applications and safety
Conductors are used where electricity must flow: wires, connectors and appliance internals. Insulators are used to protect people: rubber-coated handles, plastic sheathing on wires, porcelain insulators on poles. In electrical systems, proper insulation prevents short circuits and accidental shocks. Understanding conductors and insulators helps choose correct materials when making circuits and following safety rules at home and in school laboratories.

Charging by friction
When two different materials are rubbed, electrons may move from one to the other. The object losing electrons becomes positively charged and the one gaining electrons becomes negatively charged. Rubbing a balloon on hair transfers electrons to the balloon making it negative; hair becomes positive and the two attract each other. This is a simple and common way to create static electricity in class demonstrations.

Charging by contact
In charging by contact a charged object is touched to a conductor. Charges flow until both objects share charge and come to the same potential. For example, touching a negatively charged rod to a neutral metal sphere lets some extra electrons flow to the sphere, giving it a net negative charge. Contact charging is useful when we want to move charge between objects directly.

Charging by induction
Induction rearranges charges in a conductor without direct touch. Bringing a charged object near a neutral conductor pushes like charges away and pulls opposite charges closer, creating a separation. If the conductor is momentarily earthed while the charged object is nearby, the earth allows some charges to leave; when the earth connection is removed and the charged object moved away, the conductor is left with a net charge opposite in sign to the nearby object. Induction is used in some practical devices and explains how charge can be placed on objects safely.

Demonstrations and observations
Use a metal sphere on an insulating stand to show contact charging: touch it with a charged rod and watch an electroscope detect the charge. For induction, bring a charged rod near a metal can, touch the can briefly to earth, then remove the touch and the rod; the can will be charged. Note that humidity and grounding change results; dry conditions show static effects more clearly. These methods build understanding of how charges move and interact in real situations.

What is an electroscope?
An electroscope is a simple instrument used to detect the presence and give a qualitative idea of the amount and sign of electric charge on an object. A common classroom electroscope has a metal cap connected to a metal rod that ends in two thin leaves (gold or aluminium) enclosed in a glass jar. The metal parts conduct charge to the leaves which then show repulsion if they carry like charges.

How it works
When an electroscope is neutral its leaves hang close together. If a charged object is brought near the cap without touching, charges in the electroscope rearrange: opposite charges move toward the cap while like charges move to the leaves, making them diverge. If a charged object actually touches the cap, charge transfers into the electroscope and the leaves diverge more as both leaves gain like charge. The angle by which leaves diverge gives a rough idea of the amount of charge: a larger divergence usually means more charge.

Using the electroscope to find sign of charge
To find whether an unknown object is positively or negatively charged, use a reference charged object of known sign. First charge the electroscope slightly by touching a known positive rod and note the leaves' behaviour. Then bring the unknown object close and observe whether the leaves diverge more, collapse, or change in a way that indicates whether the unknown has the same or opposite sign. Careful sequences of touching and removing help deduce sign by comparing effects.

Limitations and precautions
Electroscopes are qualitative, not quantitative; they show presence and relative amount but do not measure charge value. They are sensitive to humidity because moisture helps charges leak away. Always handle electroscopes gently, avoid strong air currents, and discharge them before storage. Building and using an electroscope helps students visualise invisible electric effects and learn how charges move and interact in experiments.

Defining electric current
Electric current is the movement of electric charge through a conductor. In metallic wires the charge carriers are electrons which drift slowly under the influence of an electric push. The rate of flow of charge is measured as current. In the classroom it is useful to imagine many tiny charges moving along a wire; how many pass a point each second tells us the current. The standard unit of current is the ampere (A), which corresponds to a certain amount of charge passing per second.

Conventional and actual direction
There are two descriptions of direction: the real motion of electrons is from the negative terminal to the positive terminal of a cell. However, by historical convention we draw current direction from the positive terminal to the negative terminal; this is called conventional current. For most circuit diagrams and problems we follow conventional current because that is the accepted standard in textbooks and instruments. When measuring with an ammeter the sign may depend on how the instrument is connected relative to this convention.

How current is produced and flows
A cell or battery creates a potential difference between its terminals by chemical reactions; this difference provides a push that drives charges around a closed circuit. Current flows only when there is a complete path: a bulb lights when the circuit is closed and goes off when opened. The flow through all parts of a simple series circuit is the same because there is only one path for charges to move.

Measuring current and classroom practice
An ammeter measures current and must be placed in series so that all the current passes through it. Typical student experiments use small cells and bulbs; increasing the number of cells in series usually increases current because the push (voltage) increases. Discuss effects such as heating and magnetic effects of current and show how current is linked to everyday devices. Emphasise safe low-voltage practice during experiments.

Understanding voltage
Voltage, also called potential difference, is the energy provided to each unit of charge as it moves through a source such as a cell or battery. It is the electrical 'push' that makes charges move in a circuit. If a cell has a voltage of 1.5 volts, it gives 1.5 joules of energy to each coulomb of charge that moves from its negative to positive terminal through the external circuit. For Class 7, students should grasp that higher voltage means a stronger push and usually a larger current when other conditions are the same.

Cells and batteries
A single chemical cell produces a fixed voltage; a common dry cell gives about 1.5 V. When cells are connected in series, their voltages add so two 1.5 V cells in series give about 3.0 V. Batteries are combinations of cells designed to provide a desired voltage and capacity. In contrast, connecting identical cells in parallel keeps the voltage the same but increases the available current capacity (they last longer under load).

Measuring voltage
A voltmeter measures potential difference between two points and is connected in parallel across the component whose voltage we want to measure. In class experiments measure the voltage across a single cell, across bulbs, and across parts of series and parallel circuits to study how voltage divides. Remember that a voltmeter must have a high resistance so it does not change the circuit significantly when connected.

Practical view and energy
Voltage tells how much energy per coulomb the source supplies. As charges move through resistive elements they lose potential energy which is converted into heat or light. For example, a bulb converts electrical energy into light and heat using the energy supplied by the cell's voltage. Emphasise safe handling: use low-voltage cells for student work and never use mains supply for simple classroom experiments.

Resistance as a concept
Resistance is the property of a material that opposes the flow of electric current. In everyday terms, resistance is like friction for electric charges. If a wire has higher resistance, it is harder for charges to pass through and less current flows for the same applied voltage. Factors that affect resistance include the material of the conductor, its length, thickness (cross-sectional area), and temperature. For Class 7 the qualitative idea that long thin wires resist more and short thick wires resist less is most important.

Observing resistance in experiments
Students can compare currents using wires of different lengths or thicknesses while keeping the same cell voltage. For example, replace a short thick wire with a long thin wire in a simple battery-bulb circuit: the bulb will glow less brightly with the longer or thinner wire. Using different materials also shows the difference: a copper wire conducts much better than a nichrome wire of the same dimensions, so current differs accordingly.

Ohm-like behaviour
Although formal Ohm's law is introduced later, you can show that for many conductors the current increases roughly in proportion to the voltage applied, provided temperature and other conditions remain constant. That is, double the voltage and the current approximately doubles. This proportionality explains why adding another cell in series usually increases the bulb brightness: the increased voltage causes more current to flow through the same resistance.

Everyday implications and safety
Resistance is exploited in devices that produce heat: electric heaters, toasters and filament bulbs use materials with suitably high resistance to convert electrical energy into heat. In wiring, low-resistance conductors like copper minimise energy loss. Emphasise practical safety: a wire that is too thin for the current may overheat due to its resistance and cause fire risks; therefore use appropriate wire sizes and fuses to limit current.

Series circuits explained
In a series circuit the components are connected end-to-end so that there is only one path for current to travel. The same current flows through every component. If one component like a bulb is removed or burns out, the circuit is broken and the whole circuit stops working. In series, adding more bulbs divides the available push (voltage) among them and usually makes each bulb dimmer because the total resistance increases and current falls.

Parallel circuits explained
In a parallel circuit components are connected on separate branches across the same two points. Each component has its own path to the supply, so each bulb receives the full voltage of the source. If one bulb fails in a parallel circuit, the others continue to work because current can still flow through their own branches. Parallel circuits are commonly used in homes so that one appliance does not affect others.

Building and comparing
Students should build both types on a board or with connecting wires to compare behaviour. For example, connect two identical bulbs in series to a single cell and observe their brightness. Then rewire them in parallel with the same cell and compare: bulbs in parallel will be brighter because each gets the full voltage. Use an ammeter to measure currents: in a series circuit the same current flows through all elements; in parallel the total current is the sum of branch currents. Use a voltmeter to measure voltage drops across each component to see how voltage divides in series but remains equal across branches in parallel.

Practical use and safety
Household wiring and most lighting circuits are parallel. Teach students to draw neat circuit diagrams before building and to keep circuits low-voltage for safety. Use switches to open and close branches and show how adding or removing components affects current and brightness. Understanding series and parallel is essential for analysing and designing simple practical circuits.

Observation and cause
When an electric current passes through a conductor, the conductor may heat up. This heating effect occurs because moving charge carriers (electrons) collide with atoms in the conductor and transfer energy to them, causing increased vibration and heating. This simple physical effect is the basis of many everyday devices such as electric irons, heaters, kettles and filament bulbs.

Classroom demonstrations
Safe, supervised demonstrations help students see and feel the heating effect. A filament bulb glowing shows heating to high temperature; a small coil of nichrome wire connected to a low-voltage source will warm up and can be felt carefully (use teacher supervision and low voltage). Another demonstration is to pass current through a wire wound around a nail and observe that the nail may warm slightly as the coil is energized. Emphasise that the heating is related to the amount of current and the resistance of the conductor.

Practical devices using heating
Appliances designed to produce heat use elements with specific resistance so they get hot at typical supply voltages; the heating element converts electrical energy into thermal energy efficiently. Incandescent light bulbs also use the heating effect: the filament becomes so hot that it emits visible light. However, much energy appears as heat and is wasted as compared to more efficient lamps like LEDs.

Safety and protection
Heating can be dangerous if current is too large for a wire: the wire may overheat, melt insulation and cause fires. For this reason circuits include protective devices such as fuses and circuit breakers that open when currents exceed safe values. Students should learn not to short-circuit batteries (directly connecting positive and negative terminals) because very large currents produce rapid heating and possible damage. Always follow teacher instructions and use correct wire sizes and protective devices in experiments.

What is a magnet?
A magnet is an object that exerts forces on certain materials and on other magnets. Permanent magnets retain their magnetism for a long time and are used in compasses, locks, speakers and many devices. Every magnet has two poles: a north-seeking pole (called north) and a south-seeking pole (called south). Like poles repel and unlike poles attract. The forces are strongest near the poles.

Magnetic materials
Materials fall into three basic groups with respect to magnetism. Ferromagnetic materials (iron, steel, nickel, cobalt) are strongly attracted by magnets and can be made into permanent magnets. Paramagnetic materials are weakly attracted; diamagnetic materials are slightly repelled. For young students the useful classification is simply magnetic (iron, steel, some alloys) and non-magnetic (wood, plastic, rubber, glass). Testing items with a magnet helps classify them quickly in the classroom.

How permanent magnets are made and cared for
Permanent magnets are made by aligning magnetic domains (small regions of atoms) in materials such as steel. Strong impacts, high temperatures, or placing magnets with like poles together can reduce magnetism because they disturb or randomise the alignment. Storing magnets with opposite poles together or stacking them carefully helps preserve strength. Soft iron is different: it can be magnetised easily but loses magnetism when the external field is removed; this makes it ideal as a core in electromagnets.

Using magnets safely and effectively
Small magnets can be dangerous if swallowed by children and can damage electronic devices and credit cards. In class activities, use magnets to pick up iron filings or small nails, and use a compass to find pole direction. Discuss everyday uses such as door catches, fridge magnets, and parts in motors and generators. Understanding basic magnetic materials and permanent magnets prepares students for studying magnetic fields and electromagnetic devices.

Magnetic field concept
A magnetic field is the region around a magnet in which magnetic forces act. Although invisible, the field can be represented by magnetic field lines which give both direction and qualitative strength. Field lines close on themselves: they leave the north pole, curve through space and enter the south pole, then pass through the magnet back to the north pole. The closer the lines, the stronger the magnetic field in that region.

How to show field lines
One simple way is to place a bar magnet under a sheet of paper and sprinkle iron filings on top. Tap the paper gently and the filings arrange themselves along the field lines, forming clear curved patterns from north to south. Another method uses a small compass to find the direction of the field at various points: place the compass at several positions, mark the needle direction and then draw smooth curves through the marks to map the lines. These activities help students visualise the otherwise invisible field.

Important properties
Field lines never cross; if they did, there would be two directions of the field at the same point, which is impossible. The tangent to a field line at any point gives the direction of the magnetic field there. For two magnets placed near each other, the combined field lines show interaction: between unlike poles lines join and show strong attraction; between like poles lines bow out and show regions of repulsion. Field patterns also explain how compasses align and how forces act on moving charges and current-carrying wires.

Applications in simple devices
Understanding field lines is useful for explaining how motors, generators and speakers work. Field patterns inside and near coils and magnets determine forces and motion. Classroom mapping of field lines strengthens spatial thinking and prepares students for deeper study of electromagnetism in later classes.

What is an electromagnet?
An electromagnet is a temporary magnet created when electric current flows through a coil of wire. The current produces a magnetic field around the coil and the field magnetises an iron or steel core, making the whole assembly act like a strong magnet. Electromagnets are very useful because their strength can be changed by varying the current or by changing the number of turns in the coil; they can also be switched on and off.

Making an electromagnet
For a simple demonstration, take a soft iron nail and tightly wind many turns of insulated copper wire around it. Connect the ends of the wire to a battery or cell. When the circuit is closed current flows through the coil and the nail becomes magnetised enough to pick up paper clips or small nails. Removing the battery stops the current and the nail quickly loses its magnetism because soft iron does not retain magnetisation strongly.

Factors affecting strength
The strength of an electromagnet depends on the current through the coil, the number of turns of wire, and the core material. Increasing current or adding more turns increases the magnetic field. Using a core of soft iron amplifies the field because the iron concentrates the magnetic lines. For safety, do not use very large currents in classroom coils as wires can heat up and batteries can be damaged by short circuits.

Uses and classroom safety
Electromagnets are used in electric bells, relays, MRI machines, scrapyard cranes and many industrial devices. In class demonstrations show how an electromagnet can pull metal objects and how it releases them when switched off. Emphasise safety: use low-voltage supplies, insulate connections, avoid overheating and never short the coil. Electromagnets demonstrate the fundamental connection between electricity and magnetism and lead naturally into studying motors and generators.

Motors: electricity to motion
An electric motor converts electrical energy into mechanical motion using the force between magnetic fields and electric currents. In a simple DC motor a coil of wire carrying current is placed between the poles of a permanent magnet. The magnetic field interacts with the current in the coil and exerts forces on the sides of the coil. These forces produce a torque that rotates the coil. A commutator or split-ring reverses the current at appropriate moments so the torque continues to rotate the coil in the same direction.

Generators: motion to electricity
A generator works on the reverse principle: when a coil is moved in a magnetic field, the magnetic flux through the coil changes and an electric current is induced. This is how mechanical energy (from wind, water, steam or hand cranking) is turned into electrical energy. Small dynamos on bicycles and hand-cranked torch chargers are simple examples. The basic idea is that motion through a magnetic field can produce a voltage and hence a current in a closed circuit.

Classroom models and demonstrations
Students can build a simple motor using a coil, battery and magnet to see rotation when current flows. A hand-cranked generator connected to a bulb will light the bulb when the coil or magnet is turned. Showing both devices emphasises that motors and generators are closely related: a motor needs electricity to make motion; a generator uses motion to make electricity. Discuss the role of commutators in DC motors and slip rings in AC machines at a conceptual level without detailed mathematics.

Importance and applications
Motors and generators are essential in daily life, powering fans, mixers, pumps, vehicles and the electrical grid. Understanding their basic working helps students appreciate energy conversion and real engineering devices. Emphasise safety: avoid loose clothing near moving parts and use low-voltage models in school practicals to demonstrate the concepts safely.

Currents produce magnetic fields
One important connection is that an electric current produces a magnetic field around the conductor. This field can be observed by placing a compass near a current-carrying wire: the compass needle will turn to align with the circular field around the wire. Winding the wire into a coil concentrates the field and makes it stronger. This idea shows that moving charges create magnetic effects.

Magnetic fields act on currents
Conversely, a magnetic field exerts forces on current-carrying wires. When a wire carrying current is placed in an external magnetic field, it experiences a force which can produce motion. This is the basis of electric motors where forces on different sides of a current loop cause rotation. The mutual action — current creates a field and fields act back on currents — is the foundation of electromagnetism.

Simple classroom activities
Pass a current through a straight wire placed over several compasses to see the needle directions change in a circular pattern around the wire. Make a coil, connect it to a battery and use iron filings or a compass to observe the stronger field. Place a current-carrying wire near a small magnet to feel a small push or pull, demonstrating the force between fields. These experiments help students see the two-way relationship between electricity and magnetism.

Applications and importance
Devices from loudspeakers to relays, solenoids and motors depend on electromechanical interactions. Understanding the link prepares students for later study of electromagnetism and technologies like transformers, generators and electric drives. Emphasise that careful wiring and correct currents control the strength and direction of magnetic effects in practical devices.

Why safety matters
Electricity is very useful but can be dangerous if not handled correctly. Electric shocks, burns and fires are possible when circuits are misused, overloaded or short-circuited. Learning and following simple safety rules prevents accidents both at school and at home. For class practicals always use low-voltage supplies, correct equipment and teacher supervision.

Basic safety rules
Always keep hands dry before touching any electrical device; wet hands lower the resistance of your body and increase the risk of shock. Use insulated wires and tools and keep wiring away from water and wet surfaces. Do not insert metal objects into sockets or touch exposed metal connections. Use proper holders for bulbs and cells; never short-circuit a battery by directly connecting its terminals with a wire because this causes large current, rapid heating and possible damage.

Protective devices and correct wiring
Fuses and circuit breakers protect wiring by opening the circuit if current becomes too large, preventing overheating and fire. Use the correct fuse rating for appliances and avoid overloading a socket with too many plugs. Household wiring is usually in parallel so each appliance receives full voltage and one fault does not switch off everything; still, misuse and damaged insulation can be hazardous. Teach students to report faulty cords, sparks, or heating equipment immediately.

In emergencies
If someone receives a shock, do not touch them if they are still in contact with the live source. Switch off the power supply if possible, or use a dry non-metallic object to separate the person from the source. Call for help and, if trained, apply first aid. Keep emergency numbers handy and practise safe behaviour: do not play with sockets or electrical appliances, and always follow teacher instructions for experiments.