GCSE · Physics · AQA · Spec 8463

Series and parallel circuits

Add a second lamp to a circuit and the total resistance must go up, surely? Wire it in series and it does. Wire it in parallel and it goes DOWN.

Physics · Electric circuits

Same two lamps. Two ways to wire them.

Flip between series and parallel and watch the readings. Each lamp is treated as a steady 4 Ω resistance, and the supply is 6 V.

Two lamps · 6 V supply

Dashed line = current flow

6 VL1L2

Voltage

6 V

Total R

8 Ω

Current

0.75 A

Brightness

Dimmer

Series. One path, so all the charge goes through L1 and then L2. The current is 0.75 A wherever you measure it in the loop, so it is the same through both lamps. The 6 V is shared: 3 V across L1 and 3 V across L2. Total resistance: 4 Ω + 4 Ω = 8 Ω.

Watch out: In series, the first lamp does not use up current. The same 0.75 A flows through L1 and through L2. What the lamps share is the p.d.

Sort the rules

Which rule belongs to which arrangement?

Tick the arrangement where each statement is true. The two sets of rules mirror each other, and that is exactly where people swap them.

The current through each component is the same, whatever their resistances
The supply p.d. is shared between the components
Each component has the same p.d. across it, whatever its resistance
The total current is the sum of the currents through the components
Adding another resistor increases the total resistance
The total resistance is less than the smallest single resistance

Why the total resistance changes

?

Reason it through

Why does adding a resistor in series raise the total resistance, while adding one in parallel lowers it?

Link 1 of 4

First link · your turn

Series first. There is only one path. What does a second resistor do to it?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

Circuits for measuring and testing

A test circuit: where each meter goes, and why
cellswitchAammetervariable resistorcomponent under testVvoltmeter

Showing 2 layers: Series loop, Voltmeter branch

Layers

Explore

Tap each part to see why it sits where it does. Then hide the voltmeter.

The voltmeter joins the loop at the two ends of the component: a second path, side by side with it.

This is one series loop with one parallel branch. Tap each part.

Worked example: a series circuit

Problem

A 12 V supply is connected in series with a 4 Ω resistor and an 8 Ω resistor. Find the current in the circuit and the p.d. across each resistor.

Your turn

Find the missing resistor

A 9 V battery, a 6 Ω resistor and an unknown resistor R are joined in series. An ammeter reads 0.5 A. Find R and the p.d. across it.

  1. Start from the totals: 9 V across the whole loop, and 0.5 A through every part of it.
  2. missing step
Which line is step 2?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What you need to know

  • Components can be joined in series (one path) or in parallel (separate paths joined at the same two points). Some circuits have both.
  • Series: the same current flows through each component, the supply p.d. is shared, and Rtotal = R1 + R2 (resistance in ohms, Ω).
  • Parallel: each component has the same p.d., the branch currents add up to the total current, and the total resistance is less than the smallest resistor.
  • You will calculate currents, p.d.s and resistances in series circuits using the total (equivalent) resistance.

The big picture

Components can be joined in series (one path) or in parallel (separate paths joined at the same two points), and some circuits have both. In series, the same current flows through each component, the supply p.d. is shared, and the resistances add (Rtotal = R1 + R2). In parallel, each component has the same p.d., the branch currents add up to the total current, and the total resistance is less than the smallest resistor. That is why an ammeter goes in series and a voltmeter goes in parallel.

Key points

1Count the paths first. Every rule follows from how many routes the charge has.
2Adding a resistor in series adds to the one path, so the total resistance rises. Adding one in parallel adds a path, so the total resistance falls.
3Ammeter in series, so it carries the same current as the component. Voltmeter in parallel across the component, so it has the same p.d.
4Series method: add the resistances, find the current from the supply p.d., use V = I × R for each component, then check the p.d.s add up to the supply.

Worked example

Problem

In a test circuit, a lamp is in series with a variable resistor and a 4.5 V supply. The ammeter reads 0.25 A, and the voltmeter across the lamp reads 1.5 V. Find the lamp's resistance at this current, and the resistance the variable resistor is set to.

⚠ Watch out

Mixing up the two rule sets: giving each series component the full supply p.d., or splitting the current between series components. In series the current is the same through everything and it's the p.d. that is shared. Full p.d. across each component is the parallel rule.

🧠

Memory hook

S for Series: the Same current, and the p.d. is Shared. P for Parallel: the same P.d., and the currents Plus up.

✓

Check yourself

Cover the page. For two resistors, say which quantity is the same and which is shared or added, in series, then in parallel. Then explain in one sentence why a parallel resistor lowers the total.

Flashcards

(13)
What are the two ways of joining electrical components?
In series and in parallel. Some circuits include both series parts and parallel parts.
How can you tell from a circuit diagram whether two components are in series or in parallel?
Count the paths. One path that passes through both means series. Separate paths that join at the same two points means parallel.
Two components are in series. Which quantity is the same for both, and which is shared?
The current is the same through each. The supply p.d. is shared between them.
Two components are in parallel. Which quantity is the same for both, and which adds up?
The p.d. across each is the same. The currents through them add up to the total current.
Write the equation for the total resistance of two components in series, with units.
Rtotal = R1 + R2, with every resistance in ohms (Ω).
Two resistors are joined in parallel. How does their total resistance compare with each one?
It is less than the resistance of the smallest one.
Why does adding a resistor in series increase the total resistance?
There is only one path, so all the charge must pass through every resistor. The extra resistor adds to the resistance of that one path.
Why does adding a resistor in parallel decrease the total resistance?
It opens an extra path. With the same p.d. across each branch, more total current flows, so the total resistance is lower.
Where does an ammeter go in a circuit, and why?
In series, in the loop. Components in series carry the same current, so it reads the current through the component.
Where does a voltmeter go in a circuit, and why?
In parallel, across the component. Components in parallel have the same p.d., so it reads the p.d. across the component.
What is the equivalent resistance of resistors in series?
The single resistance that could replace them and draw the same current from the same supply: their resistances added together.
In a series circuit calculation, how can you check your p.d. answers?
The p.d.s across the separate components must add up to the supply p.d.
In a series test circuit, what is the variable resistor for?
Changing it changes the total resistance, and so the current. You can then test the component at several different currents.

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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