Chapter 14 — Electric Current And Its Effects

What is Electric Current?
Electric current is the flow of electric charge through a conductor. In metals, this flow is due to the motion of free electrons. A current flows only when there is a closed path (circuit) and a source of electrical energy (like a cell or battery) that produces a potential difference (voltage) across the conductor.

Conventional direction and electron flow
By convention, electric current is said to flow from the positive terminal to the negative terminal of a source. In reality, electrons (which carry negative charge) move from the negative terminal to the positive terminal. Both descriptions are used: conventional current (positive → negative) is what most circuit diagrams show, while electron flow is negative → positive.

SI unit and basic formula
The SI unit of electric current is the ampere (A). If a total charge Q (in coulombs, C) passes through a cross-section of a conductor in time t (in seconds), the average current I is:

• I = Q / t
• 1 A = 1 C s⁻¹

What a circuit needs for current to flow

• A source of potential difference (cell or battery)
• A closed conducting path (wires, components)
• A load (lamp, motor, resistor) that uses electrical energy
• A switch to open or close the circuit

How current is measured
Current is measured by an ammeter connected in series with the circuit element whose current is to be measured. The ammeter must have low internal resistance so it does not significantly change the current.

Factors affecting current
For a given source, the amount of current that flows depends on:

• the potential difference (voltage) across the conductor — higher voltage tends to give higher current,
• the material and dimensions of the conductor — good conductors (copper, aluminium) allow more current; longer or thinner wires carry less current,
• temperature — in many conductors, resistance increases with temperature and current decreases for the same voltage.

Relation with resistance and simple law (for many conductors)
Many metallic conductors behave according to Ohm's law (introduced simply at this stage): the current I through a conductor is directly proportional to the voltage V across it when temperature is constant. This is often written as V = I R, where R is the resistance. (Class 7 focus is on the idea that voltage drives current and that the material and size of the conductor resist it.)

Why electric current matters (effects)
Current produces useful effects that are used in everyday devices: the heating effect (electric heaters, filament bulbs), the magnetic effect (electric bells, motors, electromagnets), and the chemical effect (electroplating, electrolysis). Understanding current helps explain how electrical appliances work and how to use electricity safely.

Safety note
Even small voltages can be dangerous if current passes through the human body. Always avoid touching live wires and use proper insulation, fuses, and switches in circuits.

What is an electric circuit? An electric circuit is a closed path made of conductors through which electric charges (current) can flow. For current to flow, the circuit must be complete (closed) and there must be a source of electric energy (a cell or battery).

Basic components

• Cell/battery: provides electric potential (voltage).
• Conducting wires: connect components and allow charge to flow.
• Load (resistor, bulb): device that uses electrical energy (e.g., lamp, motor).
• Switch: opens or closes the circuit to stop or allow current.
• Ammeter: measures current (connected in series).
• Voltmeter: measures voltage across a component (connected in parallel).
• Fuse/earth/insulation: safety components to prevent damage or electric shock.

Open vs closed circuit: In a closed circuit (switch closed), current flows and devices like bulbs light up. In an open circuit (switch open), the path is broken and current stops.

Circuit diagrams and symbols are simple drawings that use standard symbols to show how components are connected. Common symbols: cell (short and long line), battery (group of cells), bulb (circle with cross), switch (break in line), wire (line), ammeter (A in a circle), voltmeter (V in a circle), resistor (zigzag) and ground/earth.

Types of connections (basic idea)

• Series connection: Components connected end-to-end in a single path. Current is same through all components; total resistance increases (R_total = R1 + R2 + ...).
• Parallel connection: Components connected on separate branches across the same two points. Voltage across each branch is the same; total resistance is less than any branch (1/R_total = 1/R1 + 1/R2 + ...).

Conventional current direction is taken from the positive terminal of the cell to the negative terminal (this is the direction used in most circuit diagrams).

Important practical notes: Always connect an ammeter in series and a voltmeter in parallel. Using the wrong connection can damage the instruments. Switches are used to control devices and safety elements (like fuses and earthing) protect circuits from excessive current.

What are conductors and insulators?

Conductors are materials that allow electric charge (usually electrons) to flow easily through them. Examples include metals like copper, aluminium and silver. Insulators are materials that do not allow electric charge to flow freely; they resist the movement of electrons. Examples include rubber, glass, plastic and dry wood.

Why do they behave differently?

• Conductors: In conductors, some electrons (called free or delocalised electrons) are not tightly bound to atoms. When an electric potential (voltage) is applied, these electrons move and form an electric current.
• Insulators: In insulators, electrons are tightly bound to atoms and cannot move freely. This prevents a steady flow of charge when voltage is applied.

Important points:

• Metals are generally good conductors because of many free electrons in their atomic structure.
• Non-metallic solids (rubber, glass, plastic) are good insulators because their electrons are bound tightly to atoms.
• Some substances (like graphite and certain semiconductors) show intermediate behaviour — graphite conducts, diamond (both forms of carbon) does not.
• Temperature affects conductivity: for most metals, resistance increases with temperature; for some semiconductors, conductivity increases with temperature.

Practical uses: Conductors are used for wiring and connections to carry current (e.g., copper wires). Insulators are used to cover wires, handle electrical switches safely, and to separate charged parts (e.g., PVC covering, ceramic insulators on pylons).

Safety note: Even good insulators can become conductive when exposed to very high voltages, wetting or damage. Always follow safety rules around electricity.

What are series and parallel connections?

Series connection: Components (bulbs, resistors, cells) are said to be in series when they are connected end-to-end so that there is only one path for current to flow. The same current passes through each component. If one component in the path breaks, the whole circuit stops working.

Parallel connection: Components are in parallel when they are connected across the same two points so that there are multiple paths for current. Each component has the same voltage across it. If one component fails, the others can still work because current can flow through the remaining branches.

Key characteristics

• Current: In series the current is the same through all components. In parallel the total current is the sum of currents through each branch.
• Voltage: In series the total voltage across the series is the sum of voltage drops across components. In parallel each branch has the same voltage equal to the supply voltage.
• Resistance: Series resistances add; parallel resistances combine to give a smaller equivalent resistance.

Why it matters

Brightness of bulbs, safety and reliability of circuits, and total resistance depend on how components are connected. For example, household wiring uses parallel connections so each appliance gets full voltage and one appliance can be switched off without affecting others. Old Christmas-lights used series wiring so one blown bulb turned the whole string off.

How to identify

• Series: single loop, components arranged one after another, single current path.
• Parallel: multiple loops or branches joining the same two nodes, each component has two common connection points.

Simple numeric idea

Two identical bulbs/resistors of resistance R connected in series give equivalent resistance 2R, so current from a fixed battery reduces. The same two in parallel give equivalent resistance R/2, so current from the battery increases (but split between branches).

Electric current is the flow of electric charge. At Class 7 level we define electric current (I) as the amount of charge (Q) passing through a cross-section of a conductor per unit time (t):

I = Q / t

Its SI unit is ampere (A). The conventional direction of current is from the positive terminal to the negative terminal of a battery.

Potential difference (p.d.) or voltage between two points is the work done to move a unit positive charge from one point to the other. It tells us how strongly charges are pushed through a circuit element. Mathematically:

V = W / Q

Its unit is volt (V).

Instruments to measure:

• Ammeter: measures current in amperes. It must be connected in series with the component whose current you want to measure so all the charge flowing through the component also flows through the ammeter. A good ammeter has very low internal resistance so it does not change the current significantly.
• Voltmeter: measures potential difference in volts. It must be connected in parallel across the two points whose p.d. is to be measured. A good voltmeter has very high internal resistance so it draws very little current and does not change the circuit conditions.

How to measure current (step-by-step):

1. Switch off the circuit before changing connections.
2. Open the circuit at a point in the path of current and insert the ammeter so that current flows through the ammeter (series connection).
3. Turn on the circuit and read the ammeter scale.

How to measure potential difference (step-by-step):

1. Switch off the circuit before changing connections.
2. Connect the voltmeter's two terminals across (in parallel with) the component or two points whose voltage you want to measure.
3. Turn on the circuit and read the voltmeter scale.

Important safety/tips:

• Never connect an ammeter directly across a battery or supply (i.e., in parallel) — it will short the supply because of its low resistance and may get damaged.
• Do not put a voltmeter in series — its high resistance would prevent current from flowing and give wrong readings.
• Multimeters can act as either ammeter or voltmeter; follow correct settings and wiring.

Relation to Ohm's law (simple connection):

For many conductors (ohmic conductors), the potential difference across the conductor is proportional to the current through it:

V = I R

where R is the resistance (unit: ohm, Ω).

Putting it together: To study a circuit practically, students use an ammeter in series to find current values and a voltmeter in parallel to find potential differences, then apply formulas (I = Q/t, V = W/Q, V = IR) to interpret results.

What it is
The heating effect of electric current means that whenever electric current flows through a conductor, some electrical energy is converted into heat energy. This happens because moving electrons collide with the atoms (ions) of the conductor and transfer energy to them. The increased vibrational energy of atoms is felt as heat.

Why it happens (simple explanation)
In a conductor, electric current is a flow of electrons. As electrons move, they bump into fixed positive ions in the metal. These collisions slow electrons down and transfer kinetic energy to the ions, making them vibrate more. Increased vibration raises the temperature of the material.

Factors that affect heating

• Current (I): Heating rises rapidly with current. If current doubles, heating increases by four times (since heat ∝ I²).
• Resistance (R): For a given current, a higher resistance produces more heat. For a fixed voltage source, higher resistance gives less current and so may produce less heat.
• Time (t): Longer time of current flow produces more heat (heat ∝ time).

Joule's law (class 7 level)
The heat produced (H) in a conductor due to current for time t is given by: H ∝ I²Rt. The precise relation commonly used is H = I² R t. Using Ohm's law (V = IR) we can also write H = V I t or H = V² t / R. Here H is heat energy (in joules), I is current (in amperes), R is resistance (in ohms) and t is time (in seconds).

Important points

• Heating effect can be useful (electric iron, kettle) or harmful (overheating, fuse blowing).
• Thin wires with high resistance heat more for the same current and may melt or burn; this principle is used in fuses which protect circuits.
• Incandescent bulbs produce light because the filament gets very hot; much electrical energy becomes heat (and some light).

Safety note
Excessive heating can cause fires or damage. Always use correct fuse ratings, insulation and avoid overloading sockets.

Short Circuit

A short circuit is an unintended low-resistance connection between two points of different potential (for example between the live and neutral wires). Because the resistance of this path is very small, a very large current flows. This sudden large current causes excessive heating, sparks, fires and may damage appliances and wiring.

• Cause: damaged insulation, exposed wires touching, loose connections, or faulty appliances.
• Effect: large current surge, heating, possible fire, and damage to devices.

Electric Fuse

A fuse is a simple protective device put in series with a circuit (usually in the live wire). It contains a thin wire (the fuse element) made of a metal/alloy with a relatively low melting point. When current in the circuit exceeds the fuse's rated value, the heat produced (by the current) melts the fuse wire and opens the circuit, stopping the current flow. This protects wiring and appliances from damage.

• Construction: thin fuse wire inside a non‑combustible casing (glass or ceramic) with end caps to hold contacts.
• Placement: always in series with the live (phase) wire so that when it blows the appliance is disconnected from the supply.
• Rated current: the maximum current the fuse will carry indefinitely without melting (e.g., 1 A, 5 A, 15 A).
• Working principle: Fuse operates due to Joule heating — heat produced in the fuse element is H = I²Rt; if the heat raises the element to its melting temperature, the wire melts and the circuit opens.

Difference between overload and short circuit

• Overload: Many devices together draw current higher than the safe rating; current rises moderately and may take time to blow a fuse.
• Short circuit: A near-zero resistance path causes an immediate, very large current surge; a fuse blows very quickly.

Practical points & safety

• Always replace a blown fuse with one of the correct rated current — never with a higher rating or a shorting link.
• Do not bypass the fuse; it prevents fires and equipment damage.
• Modern homes often use MCBs (Miniature Circuit Breakers) which can be reset; fuses must be replaced after they blow.

What it means
When an electric current flows through a conductor, it produces a magnetic field around the conductor. This phenomenon is called the magnetic effect of electric current. It shows that electricity and magnetism are closely related.

Key experiments and observations

• Oersted's experiment: A compass needle kept near a current-carrying wire gets deflected. This demonstrated that current produces a magnetic field.
• Shape of the magnetic field: For a straight current-carrying wire, magnetic field lines are concentric circles around the wire. For a circular loop, field lines are curved and pass through the center. For a long solenoid (coil), field lines inside are nearly straight and strong (like a bar magnet), while outside they are similar to a bar magnet's field.
• Electromagnet: A soft iron core inside a coil carrying current becomes strongly magnetic. The strength depends on current and number of turns.

Direction of the magnetic field
Use these simple rules to find direction:

• Right-hand thumb rule (for straight wire): If you hold the wire with your right hand so that your thumb points in the direction of current, your fingers wrapped around the wire show the direction of magnetic field lines (circles around wire).
• Right-hand grip rule (for a coil/solenoid): If you curl the fingers of your right hand in the direction of current in the coil, your thumb points to the north pole of the coil (direction of magnetic field inside).
• Fleming's left-hand rule (for force on a current-carrying conductor in a magnetic field): Useful for motors — index finger = direction of magnetic field, middle finger = direction of current, thumb = direction of force (motion).

Why it matters
Magnetic effect of current is the principle behind many devices: electric motors, loudspeakers, electric bells, relays, electromagnets used in cranes, and more. It explains how electricity can produce motion (motors) or create controllable magnets.

Important points

• Stronger current or more turns in a coil makes a stronger magnetic field.
• Magnetic field strength decreases with distance from the wire.
• A soft iron core increases the magnetic field of a coil by becoming magnetised while the current flows.

Simple classroom demonstrations: Move a compass near a current-carrying wire to see deflection; wrap a coil around an iron nail and connect to a battery to pick up paper clips (electromagnet); place a small magnet near a coil with current to observe attraction or repulsion.

What is a solenoid? A solenoid is a long coil of insulated wire wound in many turns (loops). When an electric current passes through the coil, it produces a magnetic field. The field inside a long solenoid is nearly uniform and its outside field resembles that of a bar magnet.

What is an electromagnet? An electromagnet is a solenoid with a soft iron (or similar ferromagnetic) core placed inside it. The iron core strengthens the magnetic field produced by the solenoid. Unlike a permanent magnet, an electromagnet can be switched on and off by controlling the electric current.

How it works (simple explanation):

• Electric current in each turn of the wire produces a small magnetic field. The fields from all turns add up, giving a strong net field.
• Inside the solenoid the field lines are nearly straight and parallel (similar to the field of a bar magnet from south to north inside the magnet), making the region inside act like the strong pole of a magnet.
• Putting a soft iron core inside increases the magnetic field because the iron becomes magnetised by the solenoid and its relative permeability is much greater than air.
• The direction of the magnetic poles of the solenoid can be found by the right-hand rule for coils: if the curled fingers follow the direction of current, the thumb points to the solenoid’s north pole.

Factors affecting strength of an electromagnet/solenoid:

• Current (I): greater current → stronger magnetic field.
• Number of turns (N): more turns → stronger field (for same current).
• Length of the coil (L): for a given N, a shorter coil (larger turns per unit length) increases field inside.
• Core material: soft iron or ferromagnetic cores increase the field (relative permeability μr > 1).

Key differences—solenoid vs electromagnet:

• Solenoid = coil of wire (may have air core). Electromagnet = solenoid + magnetic core that enhances the field.
• Solenoid demonstrates the magnetic effect of current; electromagnet is used where a controllable magnet is needed.

Overview
Electromagnetism: When electric current flows through a coil of wire, a magnetic field is produced around the coil. If the coil is wound on a soft iron core, the core becomes magnetised and the assembly is called an electromagnet. The magnetism exists only when current flows.

Key factors that affect an electromagnet

• Electric current: stronger current → stronger magnetic field.
• Number of turns in the coil: more turns → stronger field.
• Core material: soft iron core increases strength and loses magnetism quickly when current stops.

Applications of electromagnetism
Electromagnets are used where magnetic force needs to be switched on and off or varied. Examples include electric bells (the armature is attracted when current flows), relays and solenoid switches (to open/close circuits), electric motors and generators (interaction of current and magnetic fields produces motion or electricity), loudspeakers (varying current makes a diaphragm move and produces sound), and scrapyard cranes (electromagnets pick up heavy ferrous materials).

Heating effect of electric current (Joule heating)
When current flows through a conductor, collisions between moving charges and the atoms of the conductor produce heat. The amount of heat produced depends on current, resistance, and time. This heating effect is useful in many appliances but can be harmful if uncontrolled (causes fires or melts wires).

Applications of heating effect
Common uses rely on converting electrical energy into heat: electric heaters, electric iron, geysers and water heaters, toasters, electric stoves, filament of incandescent bulbs (glow due to heating), soldering irons, and fuses (designed to melt and break the circuit when excessive current flows).

Safety and control
Because both electromagnetism and heating effects can be hazardous or damaging, devices use fuses, circuit breakers, proper insulation, and earthing. Fuses exploit the heating effect: an oversized current melts the thin fuse wire and breaks the circuit preventing further damage.

Why safety with electricity is important
Electric current is very useful but can be dangerous. It can give electric shocks, cause burns, start fires or damage appliances. Even small currents through the human body can be harmful. Knowing and following safety precautions reduces these risks.

Basic safety rules

• Always keep electrical appliances, plugs and switches dry. Do not touch them with wet hands.
• Do not use damaged or frayed wires. Replace cords with cracked insulation.
• Do not overload a socket. Plugging many high-power devices into one socket may heat the wires and cause a fire.
• Switch off appliances and the main switch before repairing or replacing parts (for example, changing a bulb).
• Use appliances according to their rating. Match the voltage and current ratings on the appliance and fuse or plug.
• Use three-pin plugs with earthing (the third pin). Earthing gives a low-resistance path for fault current and reduces the risk of electric shock.
• Use insulated tools and handles. Avoid inserting metal objects (nails, knives) into sockets or devices.
• Keep electrical appliances away from water sources (sinks, bathtubs) and avoid using extension cords across wet floors.
• During storms or lightning, unplug sensitive electronics to avoid damage from power surges.
• Teach children that they must not play with switches, plugs, sockets or electrical equipment.

Protection devices

• Fuse: A fuse is a thin wire placed in series with a circuit. If the current exceeds the fuse rating, the wire melts and opens the circuit, preventing overheating and fires. Fuses must be replaced with the correct rating.
• Circuit breaker: A reusable switch that automatically opens when current becomes too large. It can be reset after the fault is cleared.
• Earthing: The earth wire provides a safe path for fault current to flow to the ground, preventing the metal body of an appliance from becoming live.
• Residual Current Device (RCD) / Earth Leakage Circuit Breaker (ELCB): Detects small leakage currents to earth and quickly disconnects power to reduce the risk of fatal shock.

What to do in case of an electric shock

• Do not touch the person who is being shocked if they are still in contact with the live source.
• Switch off the mains power immediately, if it is safe to do so. If you cannot switch off power, use a dry wooden stick or non-conducting object to separate the person from the source.
• Call emergency services and seek medical help right away.
• If the person is unconscious and not breathing, trained rescuers may begin CPR after the person is free from the electrical source.

Simple checks and good habits

• Fit three-pin plugs with the correct fuse for appliances (e.g., low-power devices use lower fuse ratings).
• Have regular electrical inspections at home to check wiring, sockets and earthing.
• Use MCBs/RCDs for added protection where available.
• Store and use appliances according to manufacturer instructions and avoid makeshift repairs.

Conclusion
Basic awareness, correct fitting of protective devices (fuses, earthing, circuit breakers), and sensible habits (dry hands, no overloading, prompt repair of faults) keep electricity useful and safe.

When an electric current flows through a device, the electrical energy is converted into other forms of energy depending on the device's construction and purpose. The main types of conversions seen in everyday electrical devices are:

• Electrical → Heat (Joule heating): Current through a resistive element (wire, heating coil) produces heat. This is used in electric heaters, irons and toasters.
• Electrical → Light: In lamps and bulbs electrical energy is converted into light (and some heat). Incandescent bulbs waste much energy as heat, while LEDs convert a larger fraction into light.
• Electrical → Mechanical: Electric motors convert electrical energy into mechanical energy (motion). Examples: fans, mixers, refrigerator compressors.
• Electrical → Magnetic: Current through a coil creates a magnetic field (electromagnet). This magnetic effect is used in bells, relays and loudspeakers.
• Electrical → Chemical: Current can cause chemical changes (electrolysis) or charge batteries: charging a battery stores electrical energy as chemical energy.
• Electrical → Sound: Devices like buzzers and speakers convert electrical energy into sound (often via magnetic/mechanical effects).

Key ideas to remember:

• Most devices convert electrical energy into more than one form (for example, a filament bulb makes both light and heat).
• Energy conversion efficiency tells how much electrical energy becomes useful output (for a bulb, useful output is light). Efficiency = (useful energy out / total electrical energy in) × 100%.
• Joule heating (thermal energy produced by current in a resistor) is useful in heaters but is wasted energy in devices that should produce light or motion.
• Safety devices (fuses, MCBs) protect circuits by stopping current when too large a current would cause dangerous heating.

This topic helps students identify what energy changes occur in appliances at home and how to compare devices (for example, LED vs filament bulbs) using energy consumption and efficiency.