KS3 · Physics

Resistance and wire thickness

Same battery, same length of wire, but swap a thin wire for a thick one and more current flows. What can extra metal possibly do?

Try it · same battery, thicker wire

Make the wire thicker. Watch the ammeter.
1×2×3×drag the wire thicker →

Wire thickness: 1/3. Cross-sectional area 1×. Ammeter reading (current) 1 A

Cross-sectional area1×Ammeter reading (current)1 A

Say the thin wire carries 1 amp. A wire’s thickness is measured by its cross-sectional area. Drag to make that area 2, then 3 times as big. The battery stays exactly the same.

Look inside the wire

Inside a copper wire
−negative end+positive end+metal ionelectron++++++++++electrons drift this way

View: Thin wire. Showing 5 layers: Battery leads, Thin wire, Ions in the thin wire, Electrons in the thin wire, Electron drift

View

Layers

Explore

switch wire, then tap the parts

A thin wire has little metal in it, so only a few electrons are there for the battery to push.

Switch between a thin and a thick wire, then tap the parts to see what each one does.

Predict, then check

Resistance, measured in ohms, tells you how hard it is for current to flow through a component.

A wire is swapped for one of the same metal and length with double the cross-sectional area, on the same battery. What happens to the resistance?

Physics · Electricity

Thin wire or thick wire: which carries more?

Two wires of the same length are connected, one after the other, to the same battery. One is thin and one is thick.

Which is closest to what you think is going on?
How sure are you?

How to investigate wire thickness

Step through the method in order.

  4. Repeat three timesTake three readings for each thickness of wire.

Physics · Models of current

Can you trust the model?

People-and-seats and roads-and-cars are two models of a wire. For each feature, does it match a real wire, or give a misleading picture?

Still to sort

Matches a real wire (0)

A strength: it helps us picture something we cannot see.

Where the line is: The feature stands for something that really is in the wire or really happens in it.

Misrepresents a real wire (0)

A limitation: the model oversimplifies it or gets it wrong.

Where the line is: The feature is missing from the model, or behaves differently from the real wire.

7 of 7 still to sort.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Same battery, different wire. Let’s look inside the metal to see why.

What you need to know

  • Current is a flow of charge. In a metal wire it is the electrons that move, drifting from the negative end to the positive end. The metal ions stay fixed.
  • Resistance, measured in ohms, tells you how hard it is for current to flow through a component.
  • A wire’s thickness is its cross-sectional area. With the same battery, a thicker wire has a lower resistance and carries a greater current.
  • Current is proportional to cross-sectional area: three times the area means three times the current. Proportional means one quantity changes as a multiple of the other.

The big picture

A metal wire is a fixed set of positive metal ions with a sea of loosely held electrons moving between them. A thicker wire has more metal, so more electrons are available to carry charge. From the same battery it carries a greater current and has a lower resistance. Doubling the cross-sectional area doubles the current and halves the resistance.

Key points

1A metal wire is fixed positive metal ions plus a sea of loosely held electrons that can move between them.
2A battery makes the sea of electrons drift through the wire. That flow is the current, and only the electrons move.
3A thicker wire has more metal, so more electrons to carry charge: a greater current and a lower resistance from the same battery.
4Doubling the cross-sectional area doubles the number of electrons that can flow and halves the resistance.
5Silver, copper and gold are the best conductors. Nichrome and constantan have a higher resistance, so they suit investigations.

Worked example

Problem

A wire carries a current of 2 A from a battery. It is swapped for a wire of the same metal and length with 3 times the cross-sectional area, on the same battery. What current does the new wire carry?

⚠ Watch out

Picturing the metal ions racing along the wire, or thinking a thin wire “squeezes” the current through faster. The ions stay put, and only the electrons move. A thin wire just has fewer electrons to carry charge.

🧠

Memory hook

Ions stay, electrons go. Thick wire, bigger crowd of electrons, bigger current.

✓

Check yourself

In your own words: what moves when current flows in a copper wire, and what stays still? Then use the word “electrons” to explain why a thicker wire carries more current from the same battery.

Flashcards

(13)
What is resistance?
How hard it is for current to flow through a component. It is measured in ohms.
What is electric current in a metal wire?
A flow of electrons moving between the metal ions. A battery makes the electrons drift from the negative end to the positive end.
What happens to the atoms in a metal such as copper?
Each atom loses an electron and becomes a positively charged metal ion. The ions stay fixed in position.
What is the “sea of electrons”?
The loosely held electrons that can move between the fixed metal ions in a metal.
Thick wire or thin wire: which has the lower resistance with the same battery, and why?
The thick wire. It has more metal, so more electrons can carry a greater current. The thin wire has a higher resistance and a lower current.
What does doubling a wire’s cross-sectional area do?
It doubles the number of electrons that can flow through it, and halves its resistance.
What does “proportional” mean?
One quantity changes as a multiple of the other. Three times the area gives three times the current.
Which metals are the best conductors?
Silver, copper and gold.
Why use nichrome wire to investigate resistance?
Nichrome has a higher resistance than copper, silver or gold for wires of the same thickness, shape and length.
Why switch the current off straight after taking each reading?
So the wire does not overheat.
How do you handle three repeat readings in the investigation?
Calculate the mean, leaving out any anomalous readings.
In the people-and-seats model, what do the seats and the people stand for?
The seats stand for the fixed metal ions and the people stand for the electrons.
What are the limitations of the roads-and-cars model of a wire?
It leaves out the metal ions, it has far fewer cars than there are electrons, and its drivers choose where to go whereas electrons are pushed one way by the battery.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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