KS3 · Physics

Electrical resistance

One resistor, and the ammeter reads 0.30 A. Add an identical second one. Does the current halve, stay the same, or drop only after the new resistor? Guess first.

Resistance · One cell, one loop

Add a resistor. What happens to the current?

View: 1 resistor. Showing 2 layers: The circuit, One resistor

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Switch between 1, 2 and 3 resistors and watch both ammeters.

One 10 Ω resistor. A1 and A2 both read 0.30 A — the current is the same on both sides of the resistor.

These are example readings for one 3.0 V cell and identical 10 Ω resistors. Current is measured in amps (A); resistance in ohms (Ω).

The used-up idea

Three bulbs in a row. Which one is dimmest?

A cell and three identical bulbs are joined in one series loop: bulb 1 is nearest the cell, then bulb 2, then bulb 3. You close the switch.

Which idea is closest to what you think right now?

Bulb 1 uses up some of the current, so bulb 3 gets the least and is the dimmest.Bulb 1 lights first, because the current gets there first. Bulb 3 lights a moment later.All three light at the same moment and equally brightly, because the current is the same all the way round.If I add a resistor, it matters where I put it: next to the cell, it cuts the current more.

How sure are you?

I'm sureI think soI'm guessing

Bulb 1 uses up some of the current, so bulb 3 gets the least and is the dimmest.

This is a common misconception

This is the most common idea about circuits — and it’s wrong. Picture the circuit as a loop of rope. The cell is a pair of hands pulling the rope round; the rope is the current; each bulb is a hand gripping the rope. A gripping hand doesn’t eat any rope: the same rope comes out of the grip as went in. So bulb 3 gets exactly the same current as bulb 1, and all three are equally bright.

Bulb 1 lights first, because the current gets there first. Bulb 3 lights a moment later.

This is a common misconception

It feels as if the current has to travel round to reach the bulbs one by one. But pull a rope loop and every part of it starts moving at the same moment — there’s no wait for the ‘front’ to arrive. When you close the switch, current flows through all the components at the same time, so the three bulbs light together.

All three light at the same moment and equally brightly, because the current is the same all the way round.

This matches the evidence

Exactly right. In the rope loop, the rope moves at the same speed everywhere and starts moving everywhere at once. Current flows through all the components at the same time, it isn’t used up, and identical bulbs carrying the same current are equally bright.

If I add a resistor, it matters where I put it: next to the cell, it cuts the current more.

This is a common misconception

Grip the rope tighter at any point — near the pulling hands or far away — and the whole loop slows down together. A resistor is the same: it adds to the resistance of the whole circuit, so it lowers the current everywhere equally. In a series circuit, position doesn’t matter.

Components don’t use current up. Each one adds resistance to the whole loop — like a tighter grip on a rope — so the current is lower everywhere, and position doesn’t matter.

Put a number on it

Which component resists more?

Component A needs a potential difference (p.d.) of 3 V across it to make a current of 0.2 A flow through it. Component B needs 6 V to make the same 0.2 A flow. Calculate each resistance. Which component has the higher resistance?

resistance = potential difference ÷ currentResistance is the ratio of the p.d. across a component to the current through it. p.d. is in volts (V), current in amps (A), and resistance comes out in ohms (Ω).
For A: p.d. = 3 V and current = 0.2 ABefore dividing, sort out which number is which. Volts belong to the p.d.; amps belong to the current.
missing step

Which line is step 3?

resistance of A = 0.2 ÷ 3resistance of A = 3 ÷ 0.2resistance of A = 3 × 0.2

Compare with an ammeter

Rank the components

Four components, P, Q, R and S, are placed one at a time in the same circuit with the same cell. Put them in order of resistance, highest first.

1 · Highest resistance4 · Lowest resistance

Component P
Ammeter reads 0.50 A
Component Q
Ammeter reads 0.10 A
Component R
Ammeter reads 0.25 A
Component S
Ammeter reads 0.05 A

Predict, then check

So far the cell has stayed the same. Now change it instead.

A circuit has one cell and one bulb. You add a second cell in series, the right way round. What happens?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Why some circuits carry less current than others — and how to put a number on it.

What you need to know

Resistance is how hard it is for current to flow through a component. It is measured in ohms (Ω).
resistance = potential difference ÷ current. p.d. is in volts (V) and current is in amps (A).
With the same cell, the bigger the current, the lower the resistance.
Adding a resistor or bulb in series adds to the resistance, so the current falls everywhere in the circuit.

The big picture

Resistance is a measure of how hard it is for current to flow through a component. It is measured in ohms (Ω) and is the ratio of the potential difference across a component to the current through it: resistance = p.d. ÷ current. With the same cell, a smaller current means a bigger resistance. Each resistor added in series adds to the resistance, so the current falls everywhere in the circuit — current is never used up, and position doesn’t matter. Adding a cell increases the voltage, so the current increases.

Key points

1A high-resistance component is harder for current to flow through than a low-resistance one.

2Resistance is the ratio of the p.d. across a component to the current through it. Volts ÷ amps = ohms.

3If one circuit needs a higher voltage to make the same current flow as another, it has the higher resistance.

4Adding an identical resistor in series with the first doubles the resistance; with the voltage unchanged, the current halves.

5Current is not used up. In a series circuit a resistor affects the current everywhere equally, wherever it goes.

6The current depends on the voltage as well as the resistance: adding a cell the right way round increases the current, but the number of charges in the wires stays the same.

7To compare components fairly, place each one in turn in the same circuit and change nothing else.

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Worked example

Problem

A resistor is connected to a 12 V supply, so the p.d. across it is 12 V. The current through it is 0.4 A. (a) Calculate its resistance. (b) An identical resistor is added in series, with the same supply. What is the new current?

⚠ Watch out

Reading a bigger current as a bigger resistance. It’s the other way round: with the same cell, the component that lets MORE current through has LESS resistance.

🧠

Memory hook

Read an ohm as ‘volts per amp’: resistance tells you how many volts it takes to push each amp through. The more volts each amp needs, the higher the resistance.

✓

Check yourself

Cover the page. A component has 6 V across it and 0.3 A through it. What is its resistance? A second component on the same cell lets only 0.1 A through: higher or lower resistance?

Flashcards

(14)

What is resistance?

A measure of how hard it is for current to flow through a component.

What is the unit of resistance, and its symbol?

The ohm, symbol Ω.

What are the units of potential difference and current?

p.d. is measured in volts (V); current in amperes, or amps (A).

Write the word equation for resistance.

resistance = potential difference ÷ current

Which meter measures current, and in what unit?

An ammeter, in amps (A).

Same cell, two components: one lets a bigger current through. Which has the higher resistance?

The one with the SMALLER current. Bigger current means lower resistance.

What happens to the current when you add a resistor in series, keeping the same cell?

It decreases — everywhere in the circuit.

You add an identical resistor in series with the first. What happens to the resistance and the current?

The resistance doubles. With the voltage unchanged, the current halves.

In a series circuit, does it matter where you put a resistor?

No. It affects the current everywhere in the circuit equally.

Do bulbs use up current?

No. The current is the same all the way round a series circuit; identical bulbs light together and equally brightly.

In the rope-loop model, what do the pulling hands, the rope and the gripping hands stand for?

Pulling hands = the cell; the rope = the current; gripping hands = the resistance of the components.

What happens when you add a second cell in series, the right way round?

The voltage increases, so the current increases and a bulb gets brighter. The number of charges in the wires stays the same.

Two circuits carry the same current, but one needs a higher voltage. Which has the higher resistance?

The one that needs the higher voltage.

Comparing components fairly: what do you change and what do you keep the same?

Change only the component (the independent variable). Keep the rest of the circuit the same (the control variables).

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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Three bulbs in a row. Which one is dimmest?

Which component resists more?

Rank the components

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