Why does electricity zip happily through a copper wire but refuse to cross the plastic coating around it? The answer comes down to one thing: whether a material has free charges that can move. On this page we’ll sort materials into conductors and insulators, look inside a metal to see why it conducts, and finish with the reason insulators can still crackle with static.
📘 What you need to know
A conductor lets charge flow through it easily — most conductors are metals
An insulator has very few free charges, so charge can’t flow through it easily
Inside a metal: positive ions sit in a fixed lattice, surrounded by a sea of delocalised (free) electrons
Metals conduct well because those delocalised electrons are free to move, and current is their flow
Insulators (rubber, plastic, glass, wood) hold their electrons tightly, so charge can’t drift through
Insulators can still hold static electricity — charge that builds up on the surface
Conductors: the easy path for charge
A conductor is a material that lets charge — usually electrons — flow through it easily. If you want current to travel from A to B, you build the path out of a conductor. The classic examples are all metals:
Silver — the best conductor of all, but pricey
Copper — cheap and excellent, so it’s what most wires are made of
Aluminium — light and good, used in power lines
Steel — strong and conducts reasonably well
Notice the pattern: conductors tend to be metals. That’s not a coincidence — it’s down to how metals are built, which we’ll get to in a moment.
Insulators: the road-block for charge
An insulator is the opposite: a material with very few free charges, so it does not let charge flow through it easily. Common insulators include:
Rubber — wrapped around wires to keep us safe
Plastic — the coating on nearly every cable
Glass and wood — everyday non-metals that block current
Materials aren’t simply “conductor” or “insulator”, though — they sit on a spectrum. Some, like carbon, conduct a bit. Others, like glass and rubber, are near-perfect blockers. Here’s roughly how a few line up:
From copper (superb conductor) all the way to rubber (superb insulator), with everything else in between.
Here’s the trick to remembering which is which: think about the wires in your phone charger. The metal core is the conductor — it carries the current. The plastic coating is the insulator — it stops the current escaping into your hand. Every cable is a conductor and an insulator working as a team.
Why do metals conduct? Look inside
Time to zoom in to the atomic level. In a metal, the atoms give up one or more of their outer electrons. What’s left behind is a neat, fixed grid of positive metal ions. And those freed electrons? They no longer belong to any single atom — they drift around the whole structure. We call them delocalised electrons, and together they form a “sea” that washes through the lattice.
Red = fixed positive metal ions. Blue = delocalised electrons, free to roam. That freedom is what makes a metal conduct.
Now the payoff. Remember that current is the rate of flow of charge. A metal is packed with these free-roaming electrons, all ready to drift. Connect a cell, and they all start shuffling in the same direction — a huge, easy flow of charge. That’s exactly why metals are excellent conductors: they have a vast number of charges that can move.
Metal lattice fixed + ions
plus a sea of free electrons
Electrons drift when pushed
flow of charge = current
Good conductor
Insulators are different at the atomic level: their electrons stay firmly attached to their atoms. With almost no free charges to move, there’s nothing to carry the current, so charge simply can’t flow through.
WE 1
Explain, in terms of their structure, why copper is a good electrical conductor but rubber is an insulator.
Copper — a metal
It has a lattice of positive ions in a sea of delocalised electrons
These electrons are free to move, so charge flows easily → good conductor
Rubber — an insulator
Its electrons are held tightly to their atoms, so there are almost no free charges
With nothing free to move, charge can’t flow through
Free electrons vs bound electronsThe whole answer hinges on one idea: are the charges free to move, or locked in place?
Static electricity on insulators
Here’s a twist. Even though insulators are hopeless at carrying a current, they can still hold static electricity — charge that builds up and sits on the surface without flowing anywhere.
Rub a balloon on your hair and it picks up extra electrons; the balloon now clings to a wall. That charge is stuck on the surface because the insulator won’t let it drift away. But bring that charged insulator into contact with a conductor, and the charge can suddenly move — it transfers across, often as a little spark.
This is why you sometimes get a zap touching a metal door handle after walking across a carpet. Your body (and the carpet) built up static charge as an insulator; the metal handle is a conductor, so the instant you touch it, all that charge rushes across at once. Same physics as the balloon — just a bit more startling.
WE 2
A plastic rod is rubbed with a cloth and gains a negative charge, but the charge does not flow away. State why the charge stays put, and what happens if the rod touches a metal pipe.
Why the charge stays put
Plastic is an insulator — it has almost no free charges, so the extra electrons can’t flow through it. They sit as static on the surface
Touching a metal pipe
Metal is a conductor, so the charge can now move — it transfers from the rod to the pipe
Static stays on insulators; it flows on contact with a conductorInsulators trap charge in place; conductors give it somewhere to go.
💡 Top tips
It all comes down to free charges. Free to move → conductor. Locked in place → insulator.
Conductors are usually metals because of their delocalised electrons.
Use the exact phrase “sea of delocalised electrons” in explain questions — examiners look for it.
Insulators can still hold static charge on their surface, even though they can’t carry a current.
It’s a spectrum, not two boxes — carbon and water fall somewhere in the middle.
⚠ Common mistakes
Saying metals conduct because “electrons move faster” — it’s that they have many free electrons
Forgetting the metal ions are positive and fixed — only the electrons move
Claiming insulators can never hold charge — they can hold static, they just can’t carry a current
Treating “conductor” and “insulator” as absolute — conductivity is a sliding scale
Confusing the ions and electrons in the lattice diagram (ions +, fixed; electrons −, free)
Quick recap: Conductors (mostly metals) let charge flow because they have a sea of delocalised electrons free to move. Insulators (rubber, plastic, glass, wood) have almost no free charges, so current can’t pass — but they can still build up static charge on their surface, which transfers when they touch a conductor.
So metals conduct beautifully — but not perfectly. Even copper puts up a little fight against the flowing electrons, and that resistance is where energy gets turned into heat. Next up in Electric Resistance we’ll see exactly why that happens inside the lattice, and meet the equation that measures it: R = V/I.
Want the atomic picture to stick?
Book a free meeting and we’ll go through conductors, insulators and the whole electron story together.
