Electricity: Magnetic and Heating Effects

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The six ideas behind electric effects

From current to circuit, and from a glowing heater to a switchable magnet — these six terms join up to explain almost every electrical gadget you use. Tap each one to see what it means.

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The three big ideas

Electric current is the flow of electric charge. It flows only when there is a complete (closed) path from a source such as a cell or battery.
A circuit has a few basic parts: a cell/battery (the source), connecting wires, a switch (to open or close the loop) and a device such as a bulb or motor.
An open switch breaks the loop (no current); a closed switch completes it (current flows).
Conductors (metals like copper, iron) let current pass; insulators (rubber, plastic, dry wood, glass) block it — which is why wires have a metal core inside a plastic cover.

Worked example. Trace the path of current in a simple torch.

Cell → switch → bulb → back to the cell. Pressing the switch closes the loop, current flows through the bulb, and it lights up. Release it and the loop opens, so the torch goes off.

When current passes through a wire that resists it, the wire heats up. More resistance and more current produce more heat.
The filament of a bulb (made of tungsten, with high resistance) gets white-hot and glows, while the thick, low-resistance connecting wires stay cool.
Appliances such as the electric iron, heater, toaster, geyser and immersion rod use coils of high-resistance wire (like nichrome) to make heat.
A fuse is a thin wire of low melting point placed in the circuit; if the current becomes dangerously high it heats up, melts and breaks the circuit, protecting the wiring and appliances. Many homes now use an MCB for the same job.

Worked example. The same current flows through a bulb's filament and its connecting wires. Why does only the filament glow?

The filament has a much higher resistance than the wires, so it releases far more heat for the same current — enough to glow white-hot, while the connecting wires barely warm up.

Common mistake: replacing a blown fuse with a thick copper wire. A thick wire will not melt under a high current, so the safety is lost and the wiring can overheat and catch fire.

A current-carrying wire produces a magnetic field around it. This was first noticed when a compass needle near such a wire was seen to deflect — showing electricity and magnetism are linked.
Winding the wire into a coil concentrates the field and makes it stronger.
An electromagnet is a coil of insulated wire wound on a soft-iron core. It is magnetic only while current flows and loses its magnetism when the current stops. It grows stronger with more turns, more current and a soft-iron core.
Soft iron is used (not steel) because steel would stay magnetised — turning into a permanent magnet that cannot be switched off.
An electric bell uses an electromagnet that repeatedly pulls an iron arm to strike a gong, then releases it — over and over, making it ring.

Worked example. Your electromagnet lifts only 5 pins. How could you make it lift more?

Wind more turns of wire around the core and use a bigger cell (more current). Both increase the magnetic strength, so it can hold more pins.

Common mistake: thinking an electromagnet and a permanent magnet are the same. A permanent magnet is always magnetic; an electromagnet can be switched on and off — which is exactly why it is so useful.

Where you'll meet it

Electric effects at work

Cranes in scrapyards

Giant electromagnets lift heavy iron and steel: switch on the current to grab a load, move it across the yard, and switch off to drop it exactly where it is needed.

The fuse and MCB at home

A fuse or MCB uses the heating effect to protect your home: when too much current flows — from a short circuit or overload — it breaks the circuit before the wiring can overheat.

Bells, fans and mixers

Doorbells, ceiling fans and kitchen mixers all rely on the magnetic effect of current to turn electricity into movement and sound.

Check yourself

Competency quiz

Modelled on the competency-based pattern — MCQ, assertion–reason and a case study, testing whether you can use the ideas, not just recall them.

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Electricity: Magnetic & Heating Effects