Magnetic & Heating Effects of Current

Electricity is easiest to understand if you picture it as something flowing. Inside a metal wire there are countless tiny particles called electrons that are not tightly held by any one atom. They are free to drift about. Left alone they wander in random directions and nothing useful happens. Connect the ends of that wire to a cell, and the cell pushes all those free electrons the same way. That steady, one-way drift of charge is what we call an electric current.

Current is measured in amperes, written as A, and is usually given the letter I. An instrument called an ammeter measures it. The ammeter is joined into the loop itself, one after the other with the rest of the circuit, so that the current it is measuring actually flows through it.

A cell is the pump. Inside it, a chemical reaction separates charge and keeps one terminal positive and the other negative. Two or more cells joined end to end, positive of one touching negative of the next, make a battery, which pushes harder than a single cell can. In a circuit diagram a cell is drawn as a pair of lines: the long thin line stands for the positive terminal and the short thick line for the negative terminal.

Long before anyone knew that electrons existed, scientists had already agreed to mark the direction of current as going from the positive terminal, round the outside of the circuit, to the negative terminal. We still use that agreement today and call it the conventional direction of current. The electrons in the wire actually drift the opposite way. Nothing goes wrong because of this. It is simply a label that everybody stuck with, and every circuit diagram you will ever read uses it.

None of this happens unless the path is complete. Charge cannot pile up at a dead end, so every part of the loop, meaning the cell, the wires, the bulb and the switch, has to be joined with no gap anywhere. A switch is just a deliberate, movable gap. Closed, the loop is whole and the bulb lights. Open, the loop is broken and everything stops in the same instant. That is also why a bulb with a broken filament refuses to glow: there is now a gap inside the bulb itself, and the rest of the circuit can do nothing about it.

Materials split into two useful groups here. Conductors such as copper, aluminium, iron and graphite have free charges and let current through easily. Insulators such as rubber, plastic, dry wood and glass hold their charges tightly and block it. That is exactly why an ordinary wire is built the way it is: the copper core carries the current, and the plastic sleeve wrapped round it keeps the current in and keeps your fingers out.

Touch the plug of a phone charger that has been working for an hour and it feels warm. Look at the coil inside a room heater and it glows orange. Both are the same thing happening: when a current passes through a conductor, part of the electrical energy turns into heat. This is the heating effect of electric current, and it happens in every wire, always, sometimes because we want it to and often whether we want it or not.

The reason is simple to picture. The cell keeps pushing free electrons along the wire, but a wire is not an empty tunnel. It is packed with metal atoms, and the moving electrons keep colliding with them. Every collision hands over a little energy and leaves the atoms vibrating harder than before. A metal whose atoms vibrate harder is a metal that is hotter. So the electrical energy the cell supplies ends up, at least partly, as heat in the wire.

Three things decide how much heat you get.

The current. More current means more electrons pushing past every second and many more collisions, so more heat. The rise is steep: doubling the current gives far more than double the heat. This is why a wire that is perfectly safe carrying a small current can become dangerously hot carrying a large one, and it is the single most important idea behind fuses and every other safety device later in this chapter.

The resistance of the wire. Resistance is how strongly a material opposes the flow of current through it, and it is measured in ohms, written with the symbol Ω. A thin wire resists more than a thick one of the same material and length. A long wire resists more than a short one. And, most importantly of all, different materials resist very differently. For the same current, a wire with higher resistance produces much more heat.

The time. Heat keeps building up for as long as the current keeps flowing, which is why an iron left switched on for ten minutes is far hotter than one switched on for ten seconds.

You can see resistance at work in any electric iron or heater. The flexible cord that runs to the socket and the heating element inside are joined one after the other in the same single loop, so exactly the same current passes through both of them. The cord stays cool enough to hold in your hand. The element glows. Nothing about the current is different between the two, and the time is the same for both. Only the resistance is different, and that alone is enough to make one part red hot while leaving the other barely warm.

Unwanted heating is a real cost, not just a curiosity. A phone that heats up while charging, a laptop that warms your lap and an extension board that gets hot when overloaded are all turning electricity you paid for into heat you did not ask for. Engineers fight this by using thick copper wire everywhere that current only needs to be carried rather than converted.

Once you know that heat depends on resistance, the design of everyday appliances starts to make sense. Each part is made of a material picked for one specific job.

The filament lamp. Inside an old-style bulb is a very long, very thin wire wound into a tight coil, called the filament. It is made of tungsten, a metal chosen for one overwhelming reason: it melts only above 3,000 degrees Celsius. Because the filament is long and extremely thin, its resistance is high, so the current heats it fiercely, hot enough to glow white and give out light. Almost any other metal would simply melt at that temperature. The wire is coiled so that a great length of it fits into a tiny space. The glass bulb around it is either emptied of air or filled with an unreactive gas such as argon, so that the scorching filament does not react with oxygen and burn away in seconds. When a bulb finally fuses, it is the filament that has snapped; the loop is now open and no current flows at all.

A filament lamp is a poor lamp and an excellent heater, and that is precisely the problem with it. Most of the electrical energy it draws leaves as heat rather than as light. That is why LED lamps have taken over: they produce light without needing to make anything white hot, so they stay cool and give the same brightness for far less electricity.

Heating appliances. A room heater, an electric iron, a toaster, a kettle, an immersion rod and a geyser all contain a heating element, a coil of wire whose entire job is to get hot. Elements are usually made of nichrome, an alloy of nickel and chromium. Nichrome has two useful properties. Its resistance is far higher than that of copper of the same size, so it heats strongly. And it stands up to being red hot in air for long periods without quickly corroding away. Copper would make a terrible element. Carrying the same current it hardly warms up at all, and if a copper coil were put across the supply in place of the element, its very low resistance would let a dangerously large current through, so it would behave like a short circuit and blow the fuse instead of heating steadily.

Keeping the heat in check. An iron that only ever got hotter would scorch clothes, so it contains a thermostat. The commonest kind uses a bimetallic strip, which is two different metals bonded firmly together. The two metals expand by different amounts when heated, so the strip has no choice but to bend as it warms. At the set temperature it has bent far enough to break the contact and cut off the current. As it cools it straightens again, the contact is remade, and heating restarts. The iron therefore hovers around one temperature instead of climbing without limit. The same trick appears inside a geyser and an electric kettle.

Notice the pattern running through all of these. Copper is used wherever current must simply be carried. A high-resistance alloy is used wherever current must be converted into heat. Choosing the right material for each job is most of the engineering.

The heating effect is not only useful. It is also the basis of the device that stops a house from catching fire.

An electric fuse is a deliberately weak link. It is a short piece of wire, of carefully chosen thickness, made of a metal or alloy that melts at a fairly low temperature. It is joined into the circuit so that all the current has to pass through it. While the current stays within the safe limit, the fuse warms slightly and nothing happens. If the current rises beyond the value the fuse is built for, the fuse wire heats up faster than it can get rid of that heat, melts, and breaks the circuit. Everything switches off before the household wiring or the appliance is damaged. Fuses carry a rating marked on them, such as 5 A or 15 A, and each circuit must be given a fuse matched to the wiring it is protecting.

Two things commonly push the current too high.

Overloading happens when too many appliances are run from one socket or one circuit. Each appliance draws its own current through the same wires, and the total can easily go past what those wires were built to carry. A heater, an iron and a kettle all plugged into a single extension board is the classic way to do it.

A short circuit happens when the insulation between two wires fails and they touch each other directly. The current then takes a very short, very easy path back instead of travelling through the appliance. With almost nothing opposing it, the current becomes enormous in an instant, and an enormous current means violent heating. That is exactly why a short circuit can start a fire within seconds.

A miniature circuit breaker, or MCB, does the same job as a fuse but behaves like a switch rather than a sacrifice. When the current becomes too large, the MCB trips and its lever snaps down to the off position, breaking the circuit. Once the fault has been found and fixed, you simply push the lever back up. A blown fuse, by contrast, has to be thrown away and replaced. Most MCBs actually use both of the effects in this chapter at once: a bimetallic strip that bends when a sustained overload heats it, and an electromagnet that pulls the contacts apart almost instantly during a short circuit.

Earthing guards against a different danger. Many appliances have a metal body. If a live wire inside works loose and touches that body, the body itself becomes dangerous to touch. The third, thicker pin of a three-pin plug connects that body to the earth wire, which runs down to a metal plate buried in the ground. Any leaking current then takes the easy path into the earth instead of through a person, and the sudden large current blows the fuse or trips the MCB, cutting off the supply.

Sensible habits matter as much as the devices do. Never replace a blown fuse with a thick piece of ordinary wire, because that removes the weak link and defeats the entire idea. Never touch switches or appliances with wet hands, since water on the skin makes it a far better conductor. Never poke anything into a socket. And get frayed or cracked cords replaced properly rather than wrapped in tape.

Electricity and magnetism were treated as two separate subjects until a lecture demonstration in 1820. The Danish scientist Hans Christian Oersted noticed that a compass needle lying near a wire twitched the moment current was switched on in that wire, and settled back when it was switched off. A wire is not a magnet, but a wire carrying a current apparently is. That one observation joined two branches of physics together, and every device in the rest of this chapter descends from it.

You can repeat it with almost nothing. Take a compass and let its needle settle along the north to south line. Run a straight piece of insulated wire just above the compass, lying along that same north to south direction so that it is parallel to the needle, and connect the two ends of the wire to a cell through a switch. Close the switch. The needle swings away from north and settles at an angle. Open the switch and it swings straight back. The wire never moved, never touched the needle, and never became warm enough to matter. The only thing that changed was that a current flowed.

Three follow-up tests tell you most of what there is to know.

Reverse the connections to the cell so that the current flows the other way, and the needle deflects to the other side of its resting position. The direction of the magnetic effect depends on the direction of the current.

Use a second cell so that a larger current flows, and the deflection becomes bigger. A stronger current makes a stronger magnetic effect.

Move the compass further away from the wire and the deflection shrinks. The effect is strongest close to the wire and fades away with distance.

What is going on is that a current-carrying wire has a magnetic field around it. Around a long straight wire this field is arranged in closed circles that go round the wire, rather like rings drawn around a pencil, and the compass needle simply lines itself up with whatever field exists at the place where you put it. Sprinkle iron filings on a stiff card with a current-carrying wire passing straight through it, tap the card gently, and the filings settle into those same circular patterns.

The most useful difference from an ordinary bar magnet is that this magnetism can be switched. A bar magnet is magnetic whether you want it to be or not, and it always has the same strength and the same poles. A current-carrying wire is magnetic only while the current flows, is magnetic the opposite way round if you reverse the current, and is as strong or as weak as you choose to make the current. Being able to turn magnetism on, off and around by flicking a switch is exactly what makes it useful inside machines.

The magnetic field around one straight wire is real, but it is feeble. Everything from this point on is about making that effect stronger without needing a bigger and bigger current.

The first step is to bend the wire into a loop. Around a straight wire the circles of magnetic field simply spread outwards and grow weaker, and no part of the wire reinforces any other. Bend the wire into a circle and, at the centre of the loop, every part of it contributes a magnetic effect pointing the same way, so those separate contributions add together. A single loop is therefore noticeably stronger at its centre than a straight wire is beside it.

The second step is to use many loops instead of one. A long coil of insulated wire, wound in many closely spaced turns, is called a solenoid. Each turn adds its own magnetic effect to the same total, so a coil of a hundred turns is far stronger than a single loop carrying the same current. The wire has to be insulated, usually with a thin coat of enamel, so that the current is forced to travel the whole length of the spiral instead of jumping straight across from one turn to the next.

A current-carrying solenoid behaves remarkably like a bar magnet. It has a north pole at one end and a south pole at the other. It attracts iron pins and paper clips. Hung so that it can turn freely, it swings round until it lies roughly along the north to south line. Bring a bar magnet up to one of its ends and it will attract or repel, depending on which pole you offer it. And, unlike a bar magnet, it stops being a magnet the instant you open the switch.

The third step is to put a core inside the coil. Slide a soft iron rod, or even a large iron nail, into the middle of the solenoid and the magnetic effect becomes dramatically stronger, strong enough to pick up a handful of pins using a single cell. A coil wound on a soft iron core in this way is an electromagnet.

The choice of soft iron is deliberate. Soft iron becomes strongly magnetic while the current flows and loses almost all of that magnetism the moment the current stops, which is exactly what a machine needs: a magnet with an off switch. Steel behaves the other way round. It is harder to magnetise in the first place, but once magnetised it holds on to its magnetism afterwards. That property is perfect for making permanent magnets and useless for an electromagnet, which has to be able to let go.

So an electromagnet can be made stronger in three ways: increase the current flowing through the coil, increase the number of turns in the coil, or improve the core by using soft iron and shaping it well. Using thicker wire helps too, but only because thicker wire can safely carry a larger current, which brings you back to the first way.

The electric bell is the classic example of the magnetic effect doing real work, because it needs a magnet that switches itself on and off many times a second. No permanent magnet could manage that.

Inside the bell there is an electromagnet made of coils wound on soft iron cores, a springy iron strip called the armature carrying a small hammer, a metal gong for the hammer to hit, and a contact screw that the strip rests against while the bell is quiet. The circuit runs from the push switch, through the contact screw, through the iron strip, through the coils of the electromagnet, and back.

Press the switch and the sequence goes like this. Current flows through the coils, so the electromagnet becomes magnetic and pulls the iron strip towards it. The hammer on the end of that strip swings across and strikes the gong, giving one sharp note. But as the strip moves it also pulls away from the contact screw, and that breaks the circuit. With no current, the electromagnet instantly loses its magnetism and lets go. The springiness of the strip carries it back to where it started, it touches the contact screw again, the circuit is complete once more, and the whole thing repeats. This make-and-break action goes on many times a second for as long as your finger is on the switch, which is why you hear continuous ringing rather than a single strike. Let go, and it stops at once.

The same idea, magnetism you can switch, turns up everywhere once you start looking.

Huge electromagnets on cranes at scrapyards and steel plants lift heavy loads of iron and drop them exactly where they are wanted, simply by cutting the current. A permanent magnet of that size would be a nightmare, because there would be no way to let go of the load.

Magnetic separators pull iron and steel scrap out of mixed waste travelling on a moving belt, leaving aluminium, plastic and glass behind to carry on.

Loudspeakers and earphones use a coil that is fed with a rapidly changing current and placed close to a permanent magnet. As the current changes, the magnet pushes and pulls the coil to and fro. The coil is fixed to a light paper or plastic cone, so the cone moves with it and shoves the air backwards and forwards, and that moving air is the sound you finally hear.

Relays let a small, safe current in one circuit work an electromagnet that closes the contacts of a second, far heavier circuit, so a delicate switch can control a powerful machine.

And an electric motor works because a current-carrying coil placed in a magnetic field is pushed round and keeps turning. Fans, mixers, pumps, washing machines and toy cars all depend on it.

Look back over the whole chapter and there are really only two ideas, and both belong to the same current: it makes wires hot, and it makes wires magnetic. The bulb, the heater and the fuse all come from the first. The bell, the crane, the loudspeaker and the motor all come from the second. And an MCB, quietly sitting in the box on your wall, uses both at once.