Topic B.5 — Current & CircuitsPaper 1 & 2Free electrons~5 min read
Electrical Conductors & Insulators
We’ve said the cell supplies the “push” and the current is charge on the move. But here’s a question worth pausing on: why does charge flow easily through a copper wire, but refuse to budge through the plastic coating wrapped around it? Let’s find out.
📘 What You Need to Know
A conductor lets charge flow through it easily — most metals (copper, silver, aluminium, steel) are excellent conductors
An insulator has very few free charges, so it resists the flow of current — rubber, plastic, glass and wood are common examples
Metals conduct so well because of a sea of delocalised electrons that can drift freely between the fixed, positive metal ions
In an insulator, electrons are held tightly to their own atoms and simply can’t wander off
Even insulators can carry static charge on their surface — that’s a different story to conducting a current, and we’ll explain why below
Why metals are such good conductors
Picture a metal as a tightly-packed crowd of positive ions standing in neat rows — that’s the lattice. Now imagine that swirling around and between those rows is a loose “cloud” of electrons that don’t belong to any one ion in particular. They’re free to drift wherever there’s a nudge from a potential difference. That’s exactly what a metal looks like on the inside, and it’s why we call metals conductors: give the electron cloud a push, and current flows almost instantly.
Compare that to an insulator. There, every electron is locked onto its own atom, held by a strong bond. There’s no loose “cloud” to push around — so even a decent push from a cell barely gets any charge moving at all.
In a conductor, electrons roam freely between many ions. In an insulator, each electron is bound to just one atom and has nowhere to wander.
A sliding scale, not a strict on/off switch
It’s tempting to think materials are either “conductors” or “insulators” and nothing in between — but really it’s more of a spectrum. Some materials, like carbon, conduct reasonably well without being metals. Others, like pure water, conduct a little (thanks to dissolved ions) but nowhere near as well as copper. Here’s roughly where some familiar materials sit:
Conductivity is a spectrum. Metals sit at one end, insulators at the other, with materials like carbon and water somewhere in the middle.
Quick recap: conductors (mostly metals) have free, delocalised electrons that carry current easily; insulators keep their electrons locked to individual atoms, so current barely flows.
WE 1
You need to connect a lamp to a battery. One wire is bare copper, the other is copper covered in a thick rubber sleeve. Which part carries the current, and which part keeps you safe?
Think about free electrons:
The copper core has plenty of delocalised electrons, so it’s the conductor doing the actual work.
Think about the rubber:
Rubber’s electrons are locked to their atoms, so no current can leak sideways out of the wire.
Copper carries the current; the rubber sleeve is the insulator that keeps it safely contained
WE 2
If insulators barely conduct current, why can rubbing a balloon on your jumper still make it stick to a wall?
Separate the two ideas:
Conducting a current means charge flows all the way through a material.
That’s different from charge simply sitting still on the surface.What’s happening with the balloon:
Rubbing transfers a layer of static charge onto the balloon’s surface — it stays put because the balloon’s electrons can’t move through it to spread out or escape.
The balloon isn’t conducting a current, it’s just holding a fixed patch of static charge on its surface
💡 Top Tips
Link the word “delocalised” to the picture of a free-roaming electron cloud — it’ll help you recall why metals conduct so well
Remember conductivity is a spectrum, not a strict yes/no label — carbon and water sit somewhere in the middle
If a question mentions static electricity, it’s about surface charge, not current flow — don’t confuse the two
⚠ Common Mistakes
Assuming all non-metals are perfect insulators — some, like carbon, conduct reasonably well
Thinking an insulator can never hold any charge at all — it can hold static charge on its surface, it just can’t let current flow through it
Forgetting that it’s the delocalised electrons doing the conducting in a metal, not the fixed positive ions
Up next: Electric Resistance — now we know why some materials conduct better than others, let’s put a number on exactly how much they resist the flow.
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