Electric Current and its Effects

Electricity powers almost everything around us, from lights and fans to phones and computers. The flow of electric charge through a conducting path is called an electric current. For a current to flow, there must be a complete, unbroken path for it to travel along. This complete path is called an electric circuit. A basic circuit needs three essential parts: a source of electricity (such as a cell or battery), connecting wires to carry the current, and a device that uses the current (such as a bulb). A switch is usually added to turn the current on and off.

A circuit works only when it is closed (complete) — that is, when there is an unbroken conducting path from one terminal of the cell, through the wires and devices, and back to the other terminal. When the path is broken at any point, for example by an open switch, it becomes an open circuit, and no current flows, so the bulb does not glow. Turning a switch ON closes the circuit; turning it OFF opens it.

Drawing every component as a realistic picture would be slow and confusing, so scientists use simple, standard circuit symbols to represent each part. A cell is shown as one long thin line (the positive terminal) and one short thick line (the negative terminal); a battery is two or more cells joined together. A switch is shown as a small gap with a lever that can be open or closed; a bulb is shown as a circle with a cross or a loop inside; connecting wires are drawn as straight lines; and an ammeter (which measures current) is shown as a circle with the letter A inside. A diagram drawn using these symbols is called a circuit diagram, and it lets anyone understand and build the circuit easily.

The current in a circuit is taken to flow from the positive terminal of the cell, through the external circuit, to the negative terminal — this is called the conventional direction of current.

When a circuit contains more than one device, such as two or more bulbs, the devices can be connected in two different ways: in series or in parallel. The way they are connected changes how the circuit behaves.

In a series circuit, the components are connected one after another in a single loop, so the same current flows through each component in turn. There is only one path for the current. The main feature — and limitation — of a series circuit is that if any one component fails or its connection breaks, the whole circuit becomes open and all the devices stop working. This is exactly what happens in some older sets of decorative lights: when one bulb fuses, the entire string goes dark. Also, when more bulbs are added in series, each bulb glows more dimly, because they share the same current along one path.

In a parallel circuit, the components are connected on separate branches, so the current has more than one path to flow through. Each device gets its own connection across the source. The key advantage is that each device works independently: if one bulb fuses or is switched off, the others keep working because they still have a complete path of their own. Bulbs connected in parallel also glow at their full, normal brightness, because each receives the full supply.

This is why the wiring in homes is done in parallel: every light, fan, and socket is on its own branch, so each can be switched on or off separately without affecting the others, and each runs at full brightness. A series connection, by contrast, is useful where we want all components to be controlled together or where the same current must pass through each — such as in a simple torch where the bulb and switch lie in one loop. Understanding the difference helps us choose the right connection for each purpose.

When an electric current flows through a wire, the wire becomes warm. This warming of a conductor by an electric current is called the heating effect of electric current. The heat is produced because the moving charges have to push their way through the material of the wire, and the material opposes their flow. This opposition to the flow of current is called resistance. The greater the resistance of a wire, the more heat is produced when current flows through it for a given current.

The amount of heat produced depends on three things: the amount of current flowing (more current produces more heat), the resistance of the wire (higher resistance produces more heat), and the time for which the current flows (longer time produces more heat). Thin wires and special alloy wires have high resistance and so heat up strongly, while thick copper wires have low resistance and stay cool.

The heating effect has many useful applications. Many everyday appliances work by deliberately using a high-resistance wire that gets very hot when current passes through it. Examples include the electric heater, electric iron, toaster, electric kettle, geyser (water heater), and hair dryer. The electric bulb also uses this effect: a thin tungsten filament with high resistance gets white-hot and glows, giving off light. These heating elements are usually made of an alloy called nichrome, because nichrome has high resistance, does not melt easily even when red-hot, and does not get spoilt (oxidise) quickly in air.

A very important safety use of the heating effect is the electric fuse. A fuse is a short piece of thin wire made of a metal with a low melting point, connected in the circuit. If too much current flows — for example during a short circuit or overloading — the fuse wire heats up, melts, and breaks the circuit, stopping the current. This protects appliances and wiring from being damaged or catching fire. Modern homes also use devices called MCBs (miniature circuit breakers) that switch off automatically for the same reason.

Electric current does more than produce heat — it also produces a magnetic effect. The Danish scientist Hans Christian Oersted discovered that whenever an electric current flows through a wire, it produces a magnetic field around the wire. This was shown by a simple experiment: when a current-carrying wire is held over a magnetic compass, the compass needle is deflected (turns aside). When the current is switched off, the needle returns to its normal position. This proves that an electric current behaves like a magnet — this is the magnetic effect of electric current.

The magnetic field around a single straight wire is weak. However, if the wire is wound into a coil of many turns (called a solenoid), the magnetic fields of all the turns add up, producing a much stronger and more useful magnetic field, similar to that of a bar magnet. The strength can be increased even further by placing a piece of soft iron inside the coil.

A coil of insulated wire wound around a soft iron core, which behaves as a magnet only when current flows through it, is called an electromagnet. The great advantage of an electromagnet is that it is a temporary magnet: it acts as a strong magnet when the current is on and loses almost all its magnetism the instant the current is switched off. This makes it possible to switch the magnetism on and off and to control its strength.

The strength of an electromagnet can be increased in three ways: by increasing the number of turns in the coil, by increasing the current flowing through it, and by using a soft iron core inside the coil. Electromagnets have many applications: they are used in electric bells, cranes that lift heavy iron and steel scrap, loudspeakers, electric motors, telephones, and maglev trains, and in devices that separate magnetic metals from waste. The ability to control their magnetism makes them far more useful than ordinary permanent magnets for many machines.

The electric bell is a familiar device that puts the magnetic effect of electric current to practical use. It works on the principle of the electromagnet, cleverly arranged so that it rings continuously as long as the switch is pressed. Understanding how it works ties together the ideas of circuits, the magnetic effect, and the electromagnet from this chapter.

The main parts of an electric bell are: an electromagnet (a coil wound on a soft iron core), an armature (a soft iron strip), a hammer attached to the armature, a gong (the metal cup that produces the sound when struck), a contact screw, and a spring that holds the armature. All these are connected in a circuit with a cell and a switch (the push button).

The working can be followed step by step. When the switch is pressed, the circuit is completed and current flows through the coil, turning it into an electromagnet. The electromagnet attracts the soft iron armature, which swings towards it. As the armature moves, the attached hammer strikes the gong, producing a "ring". But the very same movement of the armature breaks the contact at the contact screw, which opens the circuit. With the circuit broken, the current stops, and the electromagnet loses its magnetism. The spring then pulls the armature back to its original position, which remakes the contact and completes the circuit again.

As soon as the contact is remade, current flows once more, the electromagnet attracts the armature again, the hammer strikes the gong again, and the whole cycle repeats — over and over, very rapidly. This make-and-break cycle continues for as long as the switch is kept pressed, so the bell rings continuously. The moment the switch is released, the circuit stays open, the electromagnet has no current, and the bell falls silent. This neat repeating mechanism is why the electric bell is a classic example of the magnetic effect of current in action.